LOHAR

voltage and current sources

2011-10-05T01:17:00.000-07:00

Voltage and Current Sources

Real voltage sources can be represented as ideal voltage sources in series with a resistance r, the ideal voltage source having zero resistance. Real current sources can be represented as ideal current sources in parallel with a resistance r, the ideal current source having infinite resistance.
In electric circuit theory, an ideal voltage source is a circuit element where the voltage across it is independent of the current through it. A voltage source is the dual of a current source. In analysis, a voltage source supplies a constant DC or AC potential between its terminals for any current flow through it. Real-world sources of electrical energy, such as batteries, generators, or power systems, can be modeled for analysis purposes as a combination of an ideal voltage source and additional combinations of impedance elements

Working on high voltage power lines, Pearl Harbor

Voltage, otherwise known as electrical potential difference or electric tension (denoted ∆V and measured in volts, or joules per coulomb) is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points. Voltage is equal to the work which would have to be done, per unit charge, against a static electric field to move the charge between two points. A voltage may represent either a source of energy (electromotive force), or it may represent lost or stored energy (potential drop). A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; usually a common reference potential such as the ground of the system is used as one of the points. Voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or a combination of all three.

Voltage is the Cause, Current is the Effect

Voltage attempts to make a current flow, and current will flow if the circuit is complete. Voltage is sometimes described as the 'push' or 'force' of the electricity, it isn't really a force but this may help you to imagine what is happening. It is possible to have voltage without current, but current cannot flow without voltage.


Voltage and Current The switch is closed making a complete circuit so current can flow.	Voltage but No Current The switch is open so the circuit is broken and current cannot flow.	No Voltage and No Current Without the cell there is no source of voltage so current cannot flow.

Voltage, V

Connecting a voltmeter in parallel

Voltage is a measure of the energy carried by the charge.
Strictly: voltage is the "energy per unit charge".
The proper name for voltage is potential difference or p.d. for short, but this term is rarely used in electronics.
Voltage is supplied by the battery (or power supply).
Voltage is used up in components, but not in wires.
We say voltage across a component.
Voltage is measured in volts, V.
Voltage is measured with a voltmeter, connected in parallel.
The symbol V is used for voltage in equations.

Voltage at a point and 0V (zero volts)

Voltage is a difference between two points, but in electronics we often refer to voltage at a point meaning the voltage difference between that point and a reference point of 0V (zero volts). Zero volts could be any point in the circuit, but to be consistent it is normally the negative terminal of the battery or power supply. You will often see circuit diagrams labelled with 0V as a reminder.
You may find it helpful to think of voltage like height in geography. The reference point of zero height is the mean (average) sea level and all heights are measured from that point. The zero volts in an electronic circuit is like the mean sea level in geography.

Zero volts for circuits with a dual supply

Some circuits require a dual supply with three supply connections as shown in the diagram. For these circuits the zero volts reference point is the middle terminal between the two parts of the supply. On complex circuit diagrams using a dual supply the earth symbol is often used to indicate a connection to 0V, this helps to reduce the number of wires drawn on the diagram.
The diagram shows a ±9V dual supply, the positive terminal is +9V, the negative terminal is -9V and the middle terminal is 0V.

Connecting an ammeter in series

Current, I

Current is the rate of flow of charge.
Current is not used up, what flows into a component must flow out.
We say current through a component.
Current is measured in amps (amperes), A.
Current is measured with an ammeter, connected in series.
To connect in series you must break the circuit and put the ammeter acoss the gap, as shown in the diagram.
The symbol I is used for current in equations.
Why is the letter I used for current? ... please see FAQ.

1A (1 amp) is quite a large current for electronics, so mA (milliamps) are often used. m (milli) means "thousandth": 1mA = 0.001A, or 1000mA = 1A
The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits.

Voltage and Current for components in Series

Voltages add up for components connected in series.
Currents are the same through all components connected in series. In this circuit the 4V across the resistor and the 2V across the LED add up to the battery voltage: 2V + 4V = 6V.
The current through all parts (battery, resistor and LED) is 20mA.

Voltage and Current for components in Parallel

Voltages are the same across all components connected in parallel.
Currents add up for components connected in parallel. In this circuit the battery, resistor and lamp all have 6V across them.
The 30mA current through the resistor and the 60mA current through the lamp add up to the 90mA current through the battery.

RMS value

2011-10-05T01:07:00.000-07:00

the root mean square (abbreviated RMS or rms), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids. RMS is used in various fields, including electrical engineering.
It can be calculated for a series of discrete values or for a continuously varying function. The name comes from the fact that it is the square root of the mean of the squares of the values. It is a special case of the generalized mean with the exponent p = 2.

The RMS value of a set of values (or a continuous-time waveform) is the square root of the arithmetic mean (average) of the squares of the original values (or the square of the function that defines the continuous waveform).
In the case of a set of

n

values , the RMS value is given by:

The corresponding formula for a continuous function (or waveform)

f (t)

defined over the interval is

and the RMS for a function over all time is

The RMS over all time of a periodic function is equal to the RMS of one period of the function. The RMS value of a continuous function or signal can be approximated by taking the RMS of a series of equally spaced samples. Additionally, the RMS value of various waveforms can also be determined without calculus, as shown by Cartwright.
In the case of the RMS statistic of a random process, the expected value is used instead of the mean.

RMS of common waveforms

Waveform	Equation	RMS
Sine wave
Square wave
Modified square wave
Sawtooth wave
Notes: t is time f is frequency a is amplitude (peak value) {r} is the fractional part of r

Uses

The RMS value of a function is often used in physics and electrical engineering.

Average electrical power

Electrical engineers often need to know the power,

P

, dissipated by an electrical resistance,

R

. It is easy to do the calculation when there is a constant current,

I

, through the resistance. For a load of

R

ohms, power is defined simply as:

However, if the current is a time-varying function,

I (t)

, this formula must be extended to reflect the fact that the current (and thus the instantaneous power) is varying over time. If the function is periodic (such as household AC power), it is nonetheless still meaningful to talk about the average power dissipated over time, which we calculate by taking the simple average of the power at each instant in the waveform or, equivalently, the squared current. That is,

	(where denotes the mean of a function)
	(as R does not vary over time, it can be factored out)
	(by definition of RMS)

So, the RMS value,

I RMS

, of the function

I (t)

is the constant signal that yields the same power dissipation as the time-averaged power dissipation of the current

I (t)

.
We can also show by the same method that for a time-varying voltage,

V (t)

, with RMS value

V RMS

This equation can be used for any periodic waveform, such as a sinusoidal or sawtooth waveform, allowing us to calculate the mean power delivered into a specified load.
By taking the square root of both these equations and multiplying them together, we get the equation

Both derivations depend on voltage and current being proportional (i.e., the load, R, is purely resistive). Reactive loads (i.e., loads capable of not just dissipating energy but also storing it) are discussed under the topic of AC power.
In the common case of alternating current when

I (t)

is a sinusoidal current, as is approximately true for mains power, the RMS value is easy to calculate from the continuous case equation above. If we define

I p

to be the peak current, then:

where t is time and ω is the angular frequency (ω = 2π/T, whereT is the period of the wave).
Since

I p

is a positive constant:

Using a trigonometric identity to eliminate squaring of trig function:

but since the interval is a whole number of complete cycles (per definition of RMS), the

sin

terms will cancel out, leaving:

A similar analysis leads to the analogous equation for sinusoidal voltage:

Where

I P

represents the peak current and

V P

represents the peak voltage. It bears repeating that these two solutions are for a sinusoidal wave only.
Because of their usefulness in carrying out power calculations, listed voltages for power outlets, e.g. 120 V (USA) or 230 V (Europe), are almost always quoted in RMS values, and not peak values. Peak values can be calculated from RMS values from the above formula, which implies V_p = V_RMS × √2, assuming the source is a pure sine wave. Thus the peak value of the mains voltage in the USA is about 120 × √2, or about 170 volts. The peak-to-peak voltage, being twice this, is about 340 volts. A similar calculation indicates that the peak-to-peak mains voltage in Europe is about 650 volts.
It is also possible to calculate the RMS power of a signal. By analogy with RMS voltage and RMS current, RMS power is the square root of the mean of the square of the power over some specified time period. This quantity, which would be expressed in units of watts (RMS), has no physical significance. However, the term "RMS power" is sometimes used in the audio industry as a synonym for "mean power" or "average power". For a discussion of audio power measurements and their shortcomings, see Audio power.

Amplifier power efficiency

The electrical efficiency of an electronic amplifier is the ratio of mean output power to mean input power. The efficiency of amplifiers is of interest when the energy used is significant, as in high-power amplifiers, or when the power-supply is taken from a battery, as in a transistor-radio.
Efficiency is normally measured under steady-state conditions with a sinusoidal current delivered to a resistive load. The power output is the product of the measured voltage and current (both RMS) delivered to the load. The input power is the power delivered by the DC supply, i.e. the supply voltage multiplied by the supply current. The efficiency is then the output power divided by the input power, and it is always a number less than 1, or, in percentages, less than 100. A good radio frequency power amplifier can achieve an efficiency of 60–80%.
Other definitions of efficiency are possible for time-varying signals. As discussed, if the output is resistive, the mean output power can be found using the RMS values of output current and voltage signals. However, the mean value of the current should be used to calculate the input power. That is, the power delivered by the amplifier supplied by constant voltage

V C C

where

I Q

is the amplifier's operating current. Clearly, because

V C C

is constant, the time average of

P input

depends on the time average value of

I out

and not its RMS value. That is,

Root mean square speed

Main article: Root mean square speed

In the physics of gas molecules, the root mean square speed is defined as the square root of the average speed-squared. The RMS speed of an ideal gas is calculated using the following equation:

where

R

represents the Ideal Gas Constant (in this case, 8.314 J/(mol·K)),

T

is the temperature of the gas in kelvins, and

M

is the molar mass of the gas in kilograms. The generally accepted terminology for speed as compared to velocity is that the former is the scalar magnitude of the latter. Therefore, although the average speed is between zero and the RMS speed, the average velocity for a stationary gas is zero.

Root mean square error

Main article: Root mean square error

When two data sets—one set from theoretical prediction and the other from actual measurement of some physical variable, for instance—are compared, the RMS of the pairwise differences of the two data sets can serve as a measure how far on average the error is from 0.
The mean of the pairwise differences does not measure the variability of the difference, and the variability as indicated by the standard deviation is around the mean instead of 0. Therefore, the RMS of the differences is a meaningful measure of the error.

RMS in frequency domain

The RMS can be computed also in frequency domain. The Parseval's theorem is used. For sampled signal:
, where

X (f) = F F T {x (t)}

n

is number of

x (t)

samples.
In this case, the RMS computed in time domain is the same as in frequency domain:

Relationship to the arithmetic mean and the standard deviation

If is the arithmetic mean and

σ x

is the standard deviation of a population or a waveform then:

From this it is clear that the RMS value is always greater than or equal to the average, in that the RMS includes the "error" / square deviation as well.
Physical scientists often use the term "root mean square" as a synonym for standard deviation when referring to the square root of the mean squared deviation of a signal from a given baseline or fit.^{[citation needed]} This is useful for electrical engineers in calculating the "AC only" RMS of a signal. Standard deviation being the root mean square of a signal's variation about the mean, rather than about 0, the DC component is removed (i.e. RMS(signal) = Stdev(signal) if the mean signal is 0).

frequency

2011-10-05T00:57:00.000-07:00

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second.

Waveform Conversion, Part I - Sine to Square

The system designer is often called upon to convert a sine wave from an oscillator, power splitter, or other RF device into a square wave suitable for driving a logic device. There are numerous acceptable techniques and the best choice will depend upon several factors including the operating frequency, available signal power, available DC power, acceptable edge speeds, and the characteristics of the logic family.
The simplest technique is to directly couple the sine wave into the logic input with a suitable bias circuit. The sine wave should be a few volts p-p for reliable triggering. This technique is most suitable for CMOS devices in the older 4000 series or HC-MOS and only requires a coupling capacitor and two resistors to bias the input to VCC/2. This simple bias scheme should be avoided with AC-MOS devices unless the frequency and amplitude are high and the input signal is always present. Slow-moving or noisy inputs can cause multiple triggers on any gate, however, and Schmitt trigger inputs are recommended when the input sine wave is large enough to overcome the hysteresis. As an example, the 74HC14 Schmitt trigger has a hysteresis voltage of 0.9 volts so an input sine wave should be a couple of volts at a minimum. The simple biased-gate scheme is shown below:

In some instances the input signal is too small to drive the logic devices but sufficient power is available to drive a step-up transformer or matching circuit. One unusual approach uses a series RLC circuit to step up the voltage. At the input frequency, the RLC network simply looks like a resistor since the inductor and capacitor are series resonant. The resistor value is selected to limit the current at resonance (typically 100 ohms) and the inductive reactance is selected to give the desired gate voltage. In the example below, the inductor is selected to give about 5 volts of swing for the signal input current. It is reasonably safe to use the input protection diodes in AC logic to clamp the swing since AC devices are quite immune to latch-up and the input protection circuit is quite robust.

An ordinary RF step-up transformer may also be used to achieve the higher voltage swing and fairly broadband transformers may be constructed on ordinary ferrite beads, pot cores or miniature balun cores.
Although these techniques will give reasonably reliable results, especially when Schmitt trigger inputs are used, more sophisticated approaches may be desired to handle slow-moving signals or wide signal level variation. The following simple two-transistor differential amplifier will give a good square wave for a wide range of sine wave inputs and it has sufficient gain to square up the edges of slower input frequencies. The differential amplifier approach avoids transistor saturation which often limits the speed of single-transistor circuits. Fast diodes may be used to prevent saturation in single-transistor amplifiers but the cost and complexity usually exceeds the addition of another transistor.

Numerous integrated circuit solutions are also available. Line receivers are an excellent choice and are truly designed for the job. The input stages are typically differential amplifiers offering fairly high input impedance and good speed. Some line receivers have built-in voltage dividers allowing the inputs to handle voltages far beyond the power supply rails and some have built-in voltage references for biasing the input stage. They are also available in combination with line drivers and many have tri-state control inputs. The possible choices are numerous - the line driver, receiver data books are among the thicker volumes on the engineer's bookshelf. Beware that many line receivers have built-in frequency response roll-off making them unsuitable for squaring higher frequency oscillators.
The MC1489 is a typical quad line receiver which operates from a single 5 volt supply and can handle input signals from-30 to +30 volts - but the threshold is internally set. This device features a built-in threshold hysteresis of about 0.3 volts (0.9 for the MC1489A) with the lower threshold near one volt. A "response control" pin is provided to shift the threshold points or to add signal filtering.
Another example is the SN55182 which is a dual differential line receiver that also operates from a single 5 volt supply. It is designed to respond to small differential voltages riding on fairly large common-mode voltages and noise. The inputs may be biased to switch anywhere within the 15 volt bipolar common-mode input voltage range. The input impedance is a few thousand ohms but a built-in 170 ohm line-termination resistor is provided. This line receiver's frequency response may also be lowered by adding an external capacitor and a strobe input is available to force the output to a high level. Since many line receivers operate from a single supply and can sense below ground, the schematic can be quite simple. The following circuit shows 1/2 of the SN55182 connected to convert a sine wave centered around ground - typical of transformer or power splitter outputs. The only external component indicated is a 0.1 uF power supply capacitor! It may be desirable to add an additional resistor to ground at the input to match a particular source or cable impedance.

High-speed comparators may also be used in a similar manner to line receivers but the resistor biasing must be done externally and some amount of hysteresis should be added in the form of a feedback resistor from the output to the positive input to ensure positive switching. Some comparators have "totem pole" outputs which are specifically designed to drive TTL and CMOS loads and some have open-collector outputs which only need a pull-up resistor to achieve suitable logic levels.
Many prescaler devices are specifically designed to handle small sine wave inputs and all that is required is a DC blocking capacitor. The designer should be aware that some prescalers will "free-run" or oscillate when no input signal is present.
High frequency sine waves can often be directly applied to ECL inputs with the appropriate bias circuitry if the sine wave amplitude falls within the ECL's voltage specifications. Some ECL devices have a reference voltage output pin which may be used to bias the input. Simply connect a resistor from the reference to the input and capacitor couple the input signal.

Waveform Conversion, Part II - Square to Sine

Oscillators with logic compatible square wave outputs are not suitable for driving many RF devices since the characteristic impedance is usually well below the desired source impedance and the odd harmonics can generate undesired intermodulation products. Converting a square wave to a sine wave is usually accomplished with either a low pass or bandpass filter and a resistor network to achieve the desired impedance. An amplifier at either the input or output of the filter may be necessary to achieve sufficient signal amplitude.
The circuit below uses a single resistor and a pi network to generate a 50 ohm sine wave from ordinary CMOS logic. The series resistor is selected to limit the current and to isolate the logic device from the reactive load presented by the pi network. The resistor should be a minimum of 200 ohms for most inexpensive clock oscillators and low-power logic devices and the output sine wave level will be about 4 dBm. If the logic device can supply higher current, a 100 ohm resistor may be used to achieve an output level of 10 dBm. AC devices can drive even heavier loads and >13 dBm outputs are practical with a 68 ohm series resistor. The pi network is selected to match the series resistance to 50 ohms at the operating frequency. The Q of the pi network may be low since a square wave has little second harmonic and the third harmonic is three times the fundamental frequency. A low-Q pi network allows the use of fixed values with no adjustments. Add a DC blocking capacitor in series with the output if the load has a DC path to ground. This capacitor may be left out if the load can tolerate DC current and the decreased efficiency is acceptable.

The values for the components of the pi network may be found in many references but for reference the reactances for Q = 2 at three resistances are as follows:
R = 200 ohms, C1 = C2 = 100 ohms reactance, L = 100 ohms reactance
R = 100 ohms, C1 = 50 ohms reactance, C2 = 41 ohms reactance, L = 65 ohms reactance
R = 68 ohms, C1 = 34 ohms reactance, C2 = 30 ohms reactance, L = 50 ohms reactance
The nearest values with these reactances should work well and the actual resistor may be a bit lower since the logic device will have some internal resistance. If a DC blocking capacitor is added in series with the output it should be selected to have a very low reactance at the operating frequency - typically a 0.1 uF.
An ordinary NPN transistor makes an excellent power amplifier for achieving output levels above 13 dBm with light loading of the logic device as shown in the figure below:

The above circuit draws about 12 mA from the 15 volt supply while providing 17 dBm at 6 MHz. The pi network is set to a Q of 4 and a step-down ratio of 4:1. Other output networks are acceptable including tuned transformers or other matching networks. A grounded-gate JFET may be used in place of the NPN transistor eliminating the need for the two base bias resistors and the base bypass capacitor. The U310 is an excellent JFET for frequencies up to several hundred MHz. Reduce the series resistor if the output amplitude is too low - the JFET has a source resistance much higher that the NPN emitter resistance.
A "Tee" network may be used in place of the pi network shown above with some advantages. Since the gate is driving a series inductor instead of a grounded capacitor, the harmonic loading is much less and a series resistor can often be avoided entirely. In applications where the source impedance is not particularly critical and the maximum signal level is desired the Tee network can give excellent results. The circuit below shows an inexpensive clock oscillator driving a tee network to provide about 2 volts p-p into 50 ohms. The network reactances are for Q = 2 and a transformation ratio near 2.5. The reactance values for other network Qs and transformation ratios may be found in RF handbooks and application notes - one favorite is AN-267 from Motorola.

A resistor may be added in series with the gate output as before to achieve a good output VSWR and to protect the gate against shorted outputs. A DC blocking capacitor in series with one of the inductors is recommended for most applications to reduce the loading on the gate and to prevent DC from reaching the load. If a DC blocking capacitor is added it should be selected to have a very low reactance at the operating frequency - typically a 0.1 uF.

Two-Diode Odd-Order Frequency Multipliers

It is often necessary to multiply the frequency of low noise oscillators without significantly degrading the phase noise beyond the theoretical 20 log (N). Low noise frequency doublers constructed with Schottky signal diodes are readily available but higher-order multipliers often exhibit high flicker noise and poor noise floors due to the nature of the switching device. An odd-order diode multiplier topology published in RF Design magazine allows the use of low noise Schottky diodes to generate odd-order harmonics with very low excess noise. A new, half-wave version of the frequency multiplier is presented along with component values for constructing a 10 to 30 MHz tripler and a 10 to 50 MHz quintupler. The conversion loss for these multipliers is good considering their passive design and the input return loss may be easily optimized for different input levels. The circuit for the multiplier is shown below:

The input matching network consists of a choke and capacitor which work together to step up the voltage to overcome the diodes' barrier potential and to provide a low impedance to ground for the desired harmonic while preventing the harmonics form exiting the input. This series tank configuration gives the circuit a degree of feedback which helps to maintain a good conversion loss for a range of input levels. Diode, D1 rectifies the input signal resulting in a DC current in L4. The input signal commutates the two diodes with the result that a square wave of current flows in D2. The output network provides a low impedance to ground for the undesired frequencies and directs the desired harmonic to the output. Other networks may be used in the output circuit but the network should shunt undesired harmonics to preserve the fast diode switching and should block the larger, lower frequency harmonics. (Note: A current meter may be inserted in series with the ground leg of L4 to measure the DC diode current when prototyping.)
This basic configuration may be used for a wide range of frequencies with odd-order multiplication factors to 7 or more. Many fast-switching diode types may be used with excellent results and the choice will depend upon the signal levels and the required phase noise performance. Schottky-barrier diodes such as the 1N5711 are a good choice for most multiplier applications since the conversion efficiency is good and the phase noise performance is better than all but the best sources. Ordinary silicon switching diodes such as the 1N914 will give slightly better conversion efficiency for output frequencies up to 100 MHz but the phase noise performance may be significantly less than provided by Schottky diodes.
Figures 1 and 2 show the conversion loss for a 10 MHz input multiplied to 30 and 50 MHz. The conversion loss is quite low considering the multiplication factor and the 3x multiplier compares favorably with many frequency doublers.

C1 and L1 are selected to give good return loss for the level applied to the input of the multiplier and depends somewhat upon the diode type. Figures 3 and 4 show the return loss for various values of C1 and L1 for the two diode types. For example, if the input level is to be 10dBm, curve #2 would be selected since the return loss is near -30dB. The other component values are selected from the following chart:

C3 may be a 15pF trimmer capacitor for both designs and C2 may be a fixed value with a small trimmer in parallel. The Q of the C2-L2 tank is low and fixed components will usually suffice.

The output of these multipliers may be directly connected to an ordinary MMIC amplifier, if more output is necessary. Choose a low noise figure amplifier if the phase noise performance is critical. The multiplier's intrinsic phase noise can be quite good if constructed with low flicker Schottky diodes. Flicker intercept levels as low as -148 dBc have been realized with the noise floor projected to be near -180 dBc. Few oscillators will be degraded beyond theoretical amounts by such performance.

Tuning Range

Mechanical and/or electrical tuning provisions are provided with most crystal oscillators to adjust the frequency for phase-locking or modulation or to compensate for long-term drift. Mechanical tuning is often accomplished with a single or multi-turn trimmer capacitor or inductor in the crystal circuit which is accessed through a hole in the oscillator housing.
Most Wenzel Associates oscillators employ a varactor diode for all tuning and the mechanical tuning is accomplished internally with a precision potentiometer connected to a precision reference voltage. Our tests have shown that the mechanical stability and hysterisis of the highest quality trimmer potentiometers exceeds the stability of the best precision trimmer capacitors. Electrical tuning is accomplished with the same varactor diode in a configuration yielding high tuning linearity. The mechanical tuning voltage from the potentiometer is applied to the cathode of the tuning diode and the electrical tuning voltage is applied to the anode of the diode through a signal isolating network. The electrical tuning may be centered around zero volts so that in the absence of input the oscillator reverts to the mid-point of the electrical tuning.
As an option, the oscillator's internal reference voltage may be brought out to a pin for connecting to a multi-turn potentiometer for fine tuning. Alternately, a clean DC derived from a separate supply may be applied to the electrical tuning pin. If the available supplies are noisy or unstable, it may be desirable to add a zener diode or reference device and a low-pass noise filter. A temperature compensated zener such as the 1N821 is an excellent choice giving good temperature stability and very low noise - in most instances the 22uF filter capacitor shown below may be left out. Reference devices exhibit excellent stability but they often have rather high noise voltage and the additional filtering is recommended.

Here are a few points to consider:

The amount of tuning range that may be provided is a function of the crystal frequency, cut, overtone, and spot size (or motional capacitance). Precision low frequency oscillators using third-overtone SC-cut crystals will have a tuning range of only a couple of PPM whereas large spot size AT-cut fundamental crystals can achieve over a thousand PPM.
The bandwidth and tuning slope of the electrical tuning input may be specified for PLL applications.
At the time of shipment oven oscillators may exhibit a daily aging rate which would quickly consume all of the mechanical tuning range. This rapid aging will decrase significantly within a few weeks of operation at the oven temperature.
Oven oscillators may use high precision, "stiff" crystals since they operate at a single temperature and the tuning only compensates for aging whereas TCXOs must tune far enough to remove temperature effects as well as compensate for aging.
When an oven oscillator's aging pattern has been established (usually an upward drift) the frequency may be offset in the opposite direction during calibration to nearly double the calibration cycle time.
Linearity is usually specified as a percentage of the total tuning deviation from a straight line fit to the tuning curve. A 10% linearity specification would allow a deviation of 100 Hz away from the best straight line fit for an oscillator with a 1 kHz tuning range. A potential problem is that this deviation can occur rapidly at one end of the range as shown below. The slope is significantly lower at the top of the curve despite the fact that the oscillator meets a fairly tight linearity specification. For PLL systems where the tuning slope impacts the loop stability, it may be appropriate to specify the minimum and maximum tuning slope at all points on the curve in addition to the percent linearity.

Using Precision Oscillators with External References

Many instruments employing precision crystal oscillators as an internal frequency reference have provisions for accepting an external reference as well. The selection of internal or external may be made by manually throwing a switch or it may be automatically triggered by the presence of signal at the external reference input connector. In the simplest of cases the manual switch simply selects which signal source to direct to the instrument and terminates the unused source with an appropriate resistor. When the internal reference is an oven oscillator, it is often desirable to have a "signal-kill" input to turn the oscillator off while leaving the oven operating. TCXOs and non-compensated oscillators may be turned off when the external reference is present to avoid potential interference.
Signal-kill inputs typically accept ordinary TTL levels but they may be configured for custom switching levels without difficulty. Most signal-kill inputs are designed to float to the "on" state when not connected and internal RC circuitry protects the oscillator from damaging voltages or interfering RF signals which might be present on the control line. The circuit shown below will detect the presence of an external reference greater than 1 volt p-p and generate a logic "low" suitable for driving most signal-kill inputs. This control signal could also be used to gate the external input to the instrument's circuitry. The circuit is designed to exhibit a fairly high input impedance so proper cable termination is assumed elsewhere in the instrument.

If the internal oscillator is expected to have better short-term stability than the external reference it may be desirable to phase-lock the internal oscillator to the reference. The locked oscillator will exhibit the good short-term stability of the internal crystal oscillator and the good long-term stability of the external reference. A simple first-order loop may be made with a tri-state bus device or analog switch and a few additional parts. The basic concept is to drive the tri-state control with the external reference signal and pass the internal oscillator signal through one of the tri-stated gates so that the reference signal samples the internal oscillator signal. The output of the tri-state gate is low-pass filtered to remove the RF frequencies and connected to the electrical tuning of the internal oscillator. The tri-state control line is biased such that the gate output "floats" when the external reference is absent. This high-impedance state allows the tuning line to be pulled by a high-value resistor connected to a tuning potentiometer when no external reference is present. The 74HC244 has two sets of four tri-state buffers and is a good choice since the four gates not being used may be employed to buffer the signals.
The following circuit uses the same technique using only two CMOS devices, a 4069 hex inverter and a 4016 or 4066 analog switch. The frequency trimmer potentiometer controls the frequency when no reference is present. The circuit also includes an LED driver to indicate the presence of the external reference signal. Component values are not particularly critical and optimum values will depend upon the application.

The circuit as shown is a first-order loop with most precision oscillators. For example, a 10 MHz oven oscillator might have a tuning sensitivity of 1Hz per volt and, with a 15 volt supply, the phase slope is about 3 volts per radian. The bandwidth of the first-order loop is the product of the tuning sensitivity and the phase slope (1 Hz/volt x 3 volts/rad. = 3 Hz) which is well below the roll-off frequency of the 10k and 10nF network. If the oscillator has a much higher tuning sensitivity, the bandwidth will be proportionally higher and a second-order response may be desired to reduce external reference noise. Although the traditional lossy-integrator op-amp type-2 circuit could be added, additional provisions will be necessary to control the frequency in the absence of the reference since the integrator will tend to wander to one of the supply rails. An alternative way to achieve a second-order response while preserving the automatic tuning switchover is to add an additional passive filter. This type-1 second-order loop is easily achieved by adding a 100k or larger series resistor to the output. Another resistor in series with a capacitor is now added to ground at the output to add an additional roll-off. It may be tempting to simply increase the 10nF capacitor to achieve a lower bandwidth but the damping may become unacceptably low. In fact, if the oscillator has a very wide tuning range, the 10nF may be too large for sufficient damping. (In most reference applications, the first-order loop as shown is usually fine since phase error is not important and the lock bandwidth is not critical.)
Notes:

The circuit above is operating at the frequency limits of the devices at 10 MHz but the 15 volt supply gives about 12 volts of tuning voltage swing without amplification. HCMOS devices will improve the high frequency performance but the lower 5 volt supply will give a smaller tuning voltage range (under 5 volts). An amplifier could be added for oscillators requiring more tuning voltage swing. To calculate the bandwidth of such a system, multiply the amplifier gain by 1.5 to get the amplified phase slope (the 5 volt squarewave will give a 1.5 volt/radian phase slope) and then multiply by the tuning sensitivity of the oscillator.
The reference input may require a resistor to ground on the reference side of the 100nF capacitor to properly terminate the input.
The circuit may be built with tri-state gates but if they are non-inverting gates then a different bias scheme will be required. Remember to bias the external reference gates so that they turn off the tri-state control when no signal is present.
The oscillator tuning input must be a high impedance (megohms). Add a voltage follower to drive lower impedance inputs.

hydraulic pump

2011-09-30T03:46:00.000-07:00

Hydraulic pump Rexroth A4VSO250

Gearpump with external teeth

Gearpump with internal teeth

A gerotor (image does not show intake or exhaust)

Fixed displacement vane pump

Principle of screw pump

Axial piston pump, swashplate principle

Radial piston pump

Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.
Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement (flow through the pump per rotation of the pump) cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.
Gear pumps (with external teeth) (fixed displacement) are simple and economical pumps. The swept volume or displacement of gear pumps for hydraulics will be between about 1 cm³ (0.001 litre) and 200 cm³ (0.2 litre). These pumps create pressure through the meshing of the gear teeth, which forces fluid around the gears to pressurize the outlet side. For lubrication, the gear pump uses a small amount of oil from the pressurized side of the gears, bleeds this through the (typically) hydrodynamic bearings, and vents the same oil either to the low pressure side of the gears, or through a dedicated drain port on the pump housing. Some gear pumps can be quite noisy, compared to other types, but modern gear pumps are highly reliable and much quieter than older models. This is in part due to designs incorporating split gears, helical gear teeth and higher precision/quality tooth profiles that mesh and unmesh more smoothly, reducing pressure ripple and related detrimental problems. Another positive attribute of the gear pump, is that catastrophic breakdown is a lot less common than in most other types of hydraulic pumps. This is because the gears gradually wear down the housing and/or main bushings, reducing the volumetric efficiency of the pump gradually until it is all but useless. This often happens long before wear causes the unit to seize or break down.
Total head and flow are the main criteria that are used to compare one pump with another or to select a centrifugal pump for an application. Total head is related to the discharge pressure of the pump. Why can't we just use discharge pressure? Pressure is a familiar concept, we are familiar with it in our daily lives. For example, fire extinguishers are pressurized at 60 psig (413 kPa), we put 35 psig (241 kPa) air pressure in our bicycle and car tires.For good reasons, pump manufacturers do not use discharge pressure as a criteria for pump selection. One of the reasons is that they do not know how you will use the pump. They do not know what flow rate you require and the flow rate of a centrifugal pump is not fixed. The discharge pressure depends on the pressure available on the suction side of the pump. If the source of water for the pump is below or above the pump suction, for the same flow rate you will get a different discharge pressure. Therefore to eliminate this problem, it is preferable to use the difference in pressure between the inlet and outlet of the pump.

The manufacturers have taken this a step further, the amount of pressure that a pump can produce will depend on the density of the fluid, for a salt water solution which is denser than pure water, the pressure will be higher for the same flow rate. Once again, the manufacturer doesn't know what type of fluid is in your system, so that a criteria that does not depend on density is very useful. There is such a criteria and it is called TOTAL HEAD, and it is defined as the difference in head between the inlet and outlet of the pump.

You can measure the discharge head by attaching a tube to the discharge side of the pump and measuring the height of the liquid in the tube with respect to the suction of the pump. The tube will have to be quite high for a typical domestic pump. If the discharge pressure is 40 psi the tube would have to be 92 feet high. This is not a practical method but it helps explain how head relates to total head and how head relates to pressure. You do the same to measure the suction head. The difference between the two is the total head of the pump.

Figure 25

The fluid in the measuring tube of the discharge or suction side of the pump will rise to the same height for all fluids regardless of the density. This is a rather astonishing statement, here's why. The pump doesn’t know anything about head, head is a concept we use to make our life easier. The pump produces pressure and the difference in pressure across the pump is the amount of pressure energy available to the system. If the fluid is dense, such as a salt solution for example, more pressure will be produced at the pump discharge than if the fluid were pure water. Compare two tanks with the same cylindrical shape, the same volume and liquid level, the tank with the denser fluid will have a higher pressure at the bottom. But the static head of the fluid surface with respect to the bottom is the same. Total head behaves the same way as static head, even if the fluid is denser the total head as compared to a less dense fluid such as pure water will be the same. This is a surprising fact.

Hydraulic Specialists, Inc. known as HSI was founded in 1979. With over 200 years of combined hydraulic experience and is proudly celebrating its 32nd year in business. HSI remanufactures hydraulic components such as pumps, motors, valves, presses and power units. We handle all lines of OEM's such as Oilgear, Vickers, Rexroth, Racine, Denison, HPM, Eaton, Hagglunds, Sunstrand to name a few. We offer our customers an alternative to going back to the OEM which tend to have long lead times and high pricing. We remanufacture current units but our niche is in the obsolete units. HSI also has an "Open Door" Policy... If you would like to come and tour our facilities or watch your unit being evaluated, assembled or tested we invite you to do so....

Hydraulic pump sits at idle dumping over a dump valve to unload the pump. Clamp command is given and the dump valve is energized to prevent oil flowing directly to the tank. Once the clamps have been completed and pressure is at the maximum, the pressure switch actuates and shuts off the clamp solenoid and dump valve. Therefore, unloading the oil from the pump to tank. The clamps maintain pressure because of a P.O. check installed in the valve stack. If the P.O. check leaks or there is excessive leakage the hydraulic pump can be set up to perform a recharge though this is often not required. If a recharge is required the PLC will control the logic and dual setting pressure switches are needed for safety check. The first pressure switch set point will control the recharge point and the second will be the pressure at which will fault the machine if reached during the machining cycle. If the system recharges more then lets say 3 times in a minute, the system alarms out due to excessive cycling which can all be set up in the PLC. Unclamp and clamp circuits can both work under this concept though a recharge is not often needed for the unclamp. Single actuating and double actuating fixtures can be different to save costs, but this type of design setup should work for just about any fixture you would ever want to put on it.

Variable flow Compensator hydraulic systems

This type of system is generally a higher flow pump and much more costly. This system generally requires an oil cooler and flow control valves in the valve stacks, due to higher flow rates. These type of systems are also generally lower pressure systems, typically 2000 PSI or less.
Basic operation
These pumps have basically pistons attached to a plate. When there is a need for flow, the plate shifts increasing the stroke of the pistons and therefore it outputs more flow. This system always runs at full system pressure. When there is no oil flow there is no load because the plate with the pistons is vertical and there is no load. A relief valve is provided for safety but should never see any oil going over it in normal operation if set correctly. This type of system works well because it keeps constant pressure on the system and does not rely on P.O. checks holding pressure at the clamps. With this system you should still have P.O. checks incase the pump shuts off during the machining cycle.

Air over oil Pump systems

These systems are the cheapest of all systems. Depending on the brand and design you may have problems. They are very simple low volume/flow systems. They generally work well when using 2-4 valve stacks that are sequenced and do not require high volume of oil.
Basic operation
Air is supplied to the system. Air will cycle an intensifier type cylinder and push oil from a reservoir into the hydraulic system. Pressure is adjusted by adjusting the incoming air flow.

High Pressure coolant systems buying considerations, efficiency, and coolant system applications

High pressure coolant can be very beneficial for almost any job, some much more then others. The benefits of the high pressure coolant usually out weigh the initial cost when it comes to tooling savings, cycle time and finish. Tooling will last longer, speeds will be increased especially when drilling or boring. High pressure coolant will keep the part from heating up by transferring the heat into the coolant, away from the tool and part. Therefore, holding tighter tolerances, keeping your chips consistent and heat free. Learn to turn, mill, and drill faster with high pressure coolant.

Benefits of high pressure coolant

Often drastic cycle time reduction (20-70%)
Increased speeds in hole production
Eliminate heat related failure of insert
Reduced chip welding and "built-up edge" in aluminum machining
Better chip control in Low Carbon Steels
Improved quality and speed of Threaded Holes
Reduce or eliminate work hardening from peck drilling
Improved tapping in tough materials
Increased life of expensive Custom Tooling
Improved Auto-loading by eliminating chip problems
Increased tool life in Abrasive Materials

The basic hydraulic pump system
The basic unit will contain a .5-1.0 GPM pump which will be sufficient for the majority of fixtures unless you are using larger volume of clamps or multiple operations at once. The size of the motor depends on the pressure needed - 1,3,5(HP) horse power. The pump reservoir size is usually a 1, 3 or 5 gallon. The pump system should also contain an oil low level and HI-temperature switch for protection. Pump specs for volume can be calculated by figuring out the total volume of the clamps and figuring the speed at which the chamber fills up assuming .5 gal per minute.
The hydraulic valving for a double acting fixture (can also be used for single acting)
Directional control valves- Poppet (Least expensive,preferred for low flow small systems) or spool type (can handle more contaminants in oil)
P.O. check valves on A+B
Tapping Block for pressure switches on A and B
Pressure switches on A+B (Sometimes only A port I'll explain later)
Dump valve
Pressure reducing valve (not usually needed)
Flow controls (not usually needed- used usually with small volume work supports)

Hydraulic pressure switch considerations

Hydraulic pressure switches may not be necessary on B port if it is a single actuating fixture. If it is a single actuating fixture with a robot load, I would recommend them to confirm that there is no residual pressure left on the clamp side. Then causing the robot to collide with the clamps.
Also, if it is a manual operation only the PLC may run the unclamp solenoid for a specified amount of time instead of waiting for the unclamp pressure switch to be made. However; if there is any safety concerns during the unclamp cycle where the operator can be injured, this may not be a good idea.

Hydro electricity

2011-09-27T06:46:00.000-07:00

The Gordon Dam in Tasmania is a large hydro facility, with an installed capacity of 430 MW.

Renewable energy

Biofuel Biomass Geothermal Hydroelectricity Solar energy Tidal power Wave power Wind power

Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO₂) than fossil fuel powered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006.^[1] This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable sources

Hydropower has been used since ancient times to grind flour and perform other tasks. In the mid-1770s, French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. By the late 19th century, the electrical generator was developed and could now be coupled with hydraulics. The growing demand for the Industrial Revolution would drive development as well. In 1878 the world's first hydroelectric power scheme was developed at Cragside in Northumberland, England by William George Armstrong. It was used to power a single light bulb in his art gallery. The old Schoelkopf Power Station No. 1 near Niagara Falls in the U.S. side began to produce electricity in 1881. The first Edison hydroelectric power plant, the Vulcan Street Plant, began operating September 30, 1882, in Appleton, Wisconsin, with an output of about 12.5 kilowatts. By 1886 there were 45 hydroelectric power plants in the U.S. and Canada. By 1889 there were 200 in the U.S. alone.

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Hydroelectricity is electricity that is made by the movement of water. It is usually made with dams that block a river or collect water that is pumped there. When the water is "let go" the huge pressure behind the dam forces the water down shafts that lead to a turbine, this causes the turbine to turn, and electricity is produced.

The energy of falling water has been used by humans for thousands of year

Advantages of hydroelectricity:

The way the electricity is produced does not harm the environment as much as fossil fuels like oil or coal. Hydroelectricity is immense and safe and produces no waste. Hydroelectricity can be made very quickly. This makes it useful for times when demand is high. Water that has been stored in a dam can be "let go" when needed, so the energy needed can be made quickly. Also hydroelectricity can not run out as long as there is a good water supply. Once the dam is built the electricity is free, no waste or pollution produced and electricity can be generated constantly also there is then a lot of extra energy to save and use.

Disadvantages of hydroelectricity

The building of large dams to hold the water can damage the environment. In 1983 Australian government stopped the Tasmanian state government from building a dam on the Gordon River in Tasmania after a huge public protest. The dam would have flooded the beautiful Franklin River. The Three Gorges Dam in China will be the world's largest hydroelectricity project. The dam has flooded a huge area, meaning that 1.2 million people have had to be moved. Scientists are concerned about many problems with the dam, such as pollution, silt, and the danger of the dam wall breaking.

Hydro-electricity uses the energy of running water to make electrical energy. Hydro-electric stations are built where there is running water.
Most hydro-electric stations are located in dams where water is stored. But hydro-electric stations can also be built on rivers and near waterfalls.
The flow of the water from the dam can be controlled by opening and closing gates or pipes to the dam. The dam wall also creates a high water level and this increases the pressure in the pipes which takes the water to the turbine
To make hydro-electric power, water from rain or melting snow is collected and stored in a lake or a dam. A large pipe carries the water from the dam or lake to the turbine. The pressure of the water pushes against the blades of the turbine and make it spin. The rotating turbine is connected to a generator which makes the electrical energy or electricity.
The electricity then travels through transformers and transmission lines to homes and factories.
Hydro-electricity is a renewable source of energy.
Hydro-electricity is clean power. It produces no waste, and doesn't produce any greenhouse gases.
The most famous hydro-electric power station in Australia is the Snowy Mountains Hydro-Electric Scheme. Go here to read about it:
Hydro comes from a Greek word meaning water.

electric car

2011-09-24T03:41:00.000-07:00

The REVAi/G-Wiz i electric car charging from an on-street station in London.

An electric car is an automobile which is propelled by electric motor(s), using electrical energy stored in batteries or another energy storage device. Electric cars were popular in the late-19th century and early 20th century, until advances in internal combustion engine technology and mass production of cheaper gasoline vehicles led to a decline in the use of electric drive vehicle. The energy crises of the 1970s and 80s brought a short lived interest in electric cars, but in the mid 2000s took place a renewed interest in the production of electric cars due mainly to concerns about rapidly increasing oil prices and the need to curb greenhouse gas emissions. As of early 2011 series production models available in some countries include the Tesla Roadster, REVAi, Buddy, Mitsubishi i MiEV, Th!nk City, and Nissan Leaf.

Electric cars have several potential benefits as compared to conventional internal combustion automobiles that include a significant reduction of urban air pollution as they do not emit harmful tailpipe pollutants from the onboard source of power at the point of operation (zero tail pipe emissions); reduced greenhouse gas emissions from the onboard source of power depending on the fuel and technology used for electricity generation to charge the batteries; and less dependence on foreign oil, which for the United States, other developed and emerging countries is cause of concerns about their vulnerability to price shocks and supply disruption. Also for many developing countries, and particularly for the poorest in Africa, high oil prices have an adverse impact on their balance of payments, hindering their economic growth.

For electric vehicles other than battery powered road vehicles, see electric vehicle. For passenger electric vehicles, see Electric car.

Citroën Berlingo Electrique vans of the ELCIDIS goods distribution service in La Rochelle, France

A battery electric vehicle, or BEV, is a type of electric vehicle (EV) that uses chemical energy stored in rechargeable battery packs. BEVs use electric motors and motor controllers instead of, or in addition to, internal combustion engines (ICEs) for propulsion.
A battery-only electric vehicle or all-electric vehicle derives all its power from its battery packs and thus has no internal combustion engine, fuel cell, or fuel tank. Sometimes, all-electric vehicles are referred to as BEVs (although a plug-in hybrid is also a battery electric vehicle).

electric charge

2011-09-24T03:19:00.000-07:00

he coulomb, symbol C, is the SI unit of electric charge, and is defined in terms of the ampere: 1 coulomb is the amount of electric charge (quantity of electricity) carried by a current of 1 ampere flowing for 1 second. It is also about 6.241506×1018 times the charge of an electron. It is named after Charles-Augustin de Coulomb (1736-1806).
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two negatively charged objects. Positively charged objects and negatively charged objects experience an attractive force. The SI unit of electric charge is the coulomb (C), although in electrical engineering it is also common to use the ampere-hour (Ah). The study of how charged substances interact is classical electrodynamics, which is accurate insofar as quantum effects can be ignored.
The SI unit of quantity of electric charge is the coulomb, which is equivalent to about 6.242×10¹⁸ e (e is the charge of a proton). Hence, the charge of an electron is approximately −1.602×10⁻¹⁹ C. The coulomb is defined as the quantity of charge that has passed through the cross section of an electrical conductor carrying one ampere within one second. The symbol Q is often used to denote a quantity of electricity or charge. The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer.
After finding the quantized character of charge, in 1891 George Stoney proposed the unit 'electron' for this fundamental unit of electrical charge. This was before the discovery of the particle by J.J. Thomson in 1897. The unit is today treated as nameless, referred to as "elementary charge", "fundamental unit of charge", or simply as "e". A measure of charge should be a multiple of the elementary charge e, even if at large scales, charge seems to behave as a real quantity. In some contexts it is meaningful to speak of fractions of a charge; for example in the charging of a capacitor, or in the fractional quantum Hall effect.

Charge

there are two kinds of charge, positive and negative
like charges repel, unlike charges attract
positive charge comes from having more protons than electrons; negative charge comes from having more electrons than protons
charge is quantized, meaning that charge comes in integer multiples of the elementary charge e
charge is conserved

Probably everyone is familiar with the first three concepts, but what does it mean for charge to be quantized? Charge comes in multiples of an indivisible unit of charge, represented by the letter e. In other words, charge comes in multiples of the charge on the electron or the proton. These things have the same size charge, but the sign is different. A proton has a charge of +e, while an electron has a charge of -e.
Electrons and protons are not the only things that carry charge. Other particles (positrons, for example) also carry charge in multiples of the electronic charge. Those are not going to be discussed, for the most part, in this course, however.
Putting "charge is quantized" in terms of an equation, we say:
q = n e
q is the symbol used to represent charge, while n is a positive or negative integer, and e is the electronic charge, 1.60 x 10^-19 Coulombs.

The Law of Conservation of Charge

The Law of conservation of charge states that the net charge of an isolated system remains constant.
If a system starts out with an equal number of positive and negative charges, there¹s nothing we can do to create an excess of one kind of charge in that system unless we bring in charge from outside the system (or remove some charge from the system). Likewise, if something starts out with a certain net charge, say +100 e, it will always have +100 e unless it is allowed to interact with something external to it.
Charge can be created and destroyed, but only in positive-negative pairs.
Table of elementary particle masses and charges:

Electrostatic charging

Forces between two electrically-charged objects can be extremely large. Most things are electrically neutral; they have equal amounts of positive and negative charge. If this wasn¹t the case, the world we live in would be a much stranger place. We also have a lot of control over how things get charged. This is because we can choose the appropriate material to use in a given situation.
Metals are good conductors of electric charge, while plastics, wood, and rubber are not. They¹re called insulators. Charge does not flow nearly as easily through insulators as it does through conductors, which is why wires you plug into a wall socket are covered with a protective rubber coating. Charge flows along the wire, but not through the coating to you.
Materials are divided into three categories, depending on how easily they will allow charge (i.e., electrons) to flow along them. These are:

conductors - metals, for example
semi-conductors - silicon is a good example
insulators - rubber, wood, plastic for example

Most materials are either conductors or insulators. The difference between them is that in conductors, the outermost electrons in the atoms are so loosely bound to their atoms that they¹re free to travel around. In insulators, on the other hand, the electrons are much more tightly bound to the atoms, and are not free to flow. Semi-conductors are a very useful intermediate class, not as conductive as metals but considerably more conductive than insulators. By adding certain impurities to semi-conductors in the appropriate concentrations the conductivity can be well-controlled.
There are three ways that objects can be given a net charge. These are:

Charging by friction - this is useful for charging insulators. If you rub one material with another (say, a plastic ruler with a piece of paper towel), electrons have a tendency to be transferred from one material to the other. For example, rubbing glass with silk or saran wrap generally leaves the glass with a positive charge; rubbing PVC rod with fur generally gives the rod a negative charge.
Charging by conduction - useful for charging metals and other conductors. If a charged object touches a conductor, some charge will be transferred between the object and the conductor, charging the conductor with the same sign as the charge on the object.
Charging by induction - also useful for charging metals and other conductors. Again, a charged object is used, but this time it is only brought close to the conductor, and does not touch it. If the conductor is connected to ground (ground is basically anything neutral that can give up electrons to, or take electrons from, an object), electrons will either flow on to it or away from it. When the ground connection is removed , the conductor will have a charge opposite in sign to that of the charged object.

The parallel between gravity and electrostatics

An electric field describes how an electric charge affects the region around it. It's a powerful concept, because it allows you to determine ahead of time how a charge will be affected if it is brought into the region. Many people have trouble with the concept of a field, though, because it's something that's hard to get a real feel for. The fact is, though, that you're already familiar with a field. We've talked about gravity, and we've even used a gravitational field; we just didn't call it a field.
When talking about gravity, we got into the (probably bad) habit of calling g "the acceleration due to gravity". It's more accurate to call g the gravitational field produced by the Earth at the surface of the Earth. If you understand gravity you can understand electric forces and fields because the equations that govern both have the same form.
The gravitational force between two masses (m and M) separated by a distance r is given by Newton's law of universal gravitation:

A similar equation applies to the force between two charges (q and Q) separated by a distance r:

The force equations are similar, so the behavior of interacting masses is similar to that of interacting charges, and similar analysis methods can be used. The main difference is that gravitational forces are always attractive, while electrostatic forces can be attractive or repulsive. The charge (q or Q) plays the same role in the electrostatic case that the mass (m or M) plays in the case of the gravity.
A good example of a question involving two interacting masses is a projectile motion problem, where there is one mass m, the projectile, interacting with a much larger mass M, the Earth. If we throw the projectile (at some random launch angle) off a 40-meter-high cliff, the force on the projectile is given by:
F = mg
This is the same equation as the more complicated equation above, with G, M, and the radius of the Earth, squared, incorporated into g, the gravitational field.
So, you've seen a field before, in the form of g. Electric fields operate in a similar way. An equivalent electrostatics problem is to launch a charge q (again, at some random angle) into a uniform electric field E, as we did for m in the Earth's gravitational field g. The force on the charge is given by F = qE, the same way the force on the mass m is given by F = mg.
We can extend the parallel between gravity and electrostatics to energy, but we'll deal with that later. The bottom line is that if you can do projectile motion questions using gravity, you should be able to do them using electrostatics. In some cases, you¹ll need to apply both; in other cases one force will be so much larger than the other that you can ignore one (generally if you can ignore one, it'll be the gravitational force).

ampere's law

2011-09-24T02:54:00.000-07:00

The magnetic field in space around an electric current is proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source. Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.

In the electric case, the relation of field to source is quantified in Gauss's Law which is a very powerful tool for calculating electric fields.

Ampere's Law Applications	Index Ampere's law Currents as magnetic field sources

The magnetic field at a distance r from a very long straight wire, carrying a steady current I, has a magnitude equal to

(31.)

and a direction perpendicular to r and I. The path integral along a circle centered around the wire (see Figure 31.1) is equal to

(31.2)

Here we have used the fact that the magnetic field is tangential at any point on the circular integration path.

Figure 31.1. Magnetic field generated by current. Any arbitrary path can be thought of as a collection of radial segments (r changes and [theta] remains constant) and circular segments ([theta] changes and r remains constant). For the radial segments the magnetic field will be perpendicular to the displacement and the scaler product between the magnetic field and the displacement is zero. Consider now a small circular segment of a trajectory around the wire (see Figure 31.2). The path integral along this circular segment is equal to
(31.3)

Figure 31.2. Path integral along a small circular path. Equation (31.3) shows that the contribution of this circular segment to the total path integral is independent of the distance r and only depends on the change in the angle [Delta][theta]. For a closed path, the total change in angle will be 2[pi], and eq.(31.3) can be rewritten as
(31.4)

This expression is Ampere's Law:

" The integral of B around any closed mathematical path equals u₀ times the current intercepted by the area spanning the path "

Example: Problem 31.5

Six parallel aluminum wires of small, but finite, radius lie in the same plane. The wires are separated by equal distances d, and they carry equal currents I in the same direction. Find the magnetic field at the center of the first wire. Assume that the currents in each wire is uniformly distributed over its cross section.

A schematic layout of the problem is shown in Figure 31.3. The magnetic field generated by a single wire is equal to

(31.5)

where r is the distance from the center of the wire. Equation (31.5) is correct for all points outside the wire, and can therefore be used to determine the magnetic field generated by wire 2, 3, 4, 5, and 6. The field at the center of wire 1, due to the current flowing in wire 1, can be determined using Ampere's law, and is equal to zero. The total magnetic field at the center of wire 1 can be found by vector addition of the contributions of each of the six wires. Since the direction of each of these contributions is the same, the total magnetic field at the center of wire 1 is equal to

(31.6)

Figure 31.3. Problem 31.5

31.2. The solenoid

A solenoid is a device used to generate a homogeneous magnetic field. It can be made of a thin conducting wire wound in a tight helical coil of many turns. The magnetic field inside a solenoid can be determined by summing the magnetic fields generated by N individual rings (where N is the number of turns of the solenoid). We will limit our discussion of the magnetic field generated by a solenoid to that generated by an ideal solenoid which is infinitely long, and has very tightly wound coils.
The ideal solenoid has translational and rotational symmetry. However, since magnetic field lines must form closed loops, the magnetic field can not be directed along a radial direction (otherwise field lines would be created or destroyed on the central axis of the solenoid). Therefore we conclude that the field lines in a solenoid must be parallel to the solenoid axis. The magnitude of the magnetic field can be obtained by applying Ampere's law.

Figure 31.4. The ideal solenoid. Consider the integration path shown in Figure 31.4. The path integral of the magnetic field around this integration path is equal to
(31.7)

where L is the horizontal length of the integration path. The current enclosed by the integration path is equal to N ^. I₀ where N is the number of turns enclosed by the integration path and I₀ is the current in each turn of the solenoid. Using Ampere's law we conclude that

(31.8)

or

(31.9)

where n is the number of turns of the solenoid per unit length. Equation (31.9) shows that the magnetic field B is independent of the position inside the solenoid. We conclude that the magnetic field inside an ideal solenoid is uniform.

Example: Problem 31.14

A long solenoid of n turns per unit length carries a current I, and a long straight wire lying along the axis of this solenoid carries a current I'. Find the net magnetic field within the solenoid, at a distance r from the axis. Describe the shape of the magnetic field lines.

The magnetic field generated by the solenoid is uniform, directed parallel to the solenoid axis, and has a magnitude equal to

(31.10)

The magnetic field if a long straight wire, carrying a current I' has a magnitude equal to

(31.11)

and is directed perpendicular to the direction of r and I'. The direction of B_wire is therefore perpendicular to the direction of B_sol. The net magnetic field inside the solenoid is equal to the vector sum of B_wire and B_sol. Its magnitude is equal to

(31.12)

The angle a between the direction of the magnetic field and the z-axis is given by

(31.13)

Example: Problem 31.15

A coaxial cable consists of a long cylindrical copper wire of radius r₁ surrounded by a cylindrical shell of inner radius r₂ and outer radius r₃ (see Figure 31.5). The wire and the shell carry equal and opposite currents I uniformly distributed over their volumes. Find formulas for the magnetic field in each of the regions r < r₁, r₁ < r < r₂, r₂ < r < r₃, and r > r₃.

The magnetic field lines are circles, centered on the symmetry axis of the coaxial cable. First consider an integration path with r < r₁. The path integral of B along this path is equal to

(31.14)

The current enclosed by this integration path is equal to

(31.15)

Applying Faraday's law we can relate the current enclosed to the path integral of B

(31.16)

Therefore, the magnetic field is B is equal to

(31.17)

Figure 31.5. Problem 31.15. In the region between the wire and the shell, the enclosed current is equal to I and the path integral of the magnetic field is given by eq.(31.14). Ampere's law states then that
(31.18)

and the magnetic field is given by

(31.19)

In the third region (r₂ < r < r₃) the path integral of the magnetic field along a circular path with radius r is given by eq.(31.14). The enclosed current is equal to

(31.20)

The magnetic field is therefore equal to

(31.21)

The current enclosed by an integration path with a radius r > r₃ is equal to zero (since the current in the wire and in the shell are flowing in opposite directions). The magnetic field in this region is therefore also equal to zero.

31.3. Motion of charges in electric and magnetic fields

The magnetic force acting on particle with charge q moving with velocity v is equal to

(31.22)

This force is always perpendicular to the direction of motion of the particle, and will therefore only change the direction of motion, and not the magnitude of the velocity. If the charged particle is moving in a uniform magnetic field, with strength B, that is perpendicular to the velocity v, then the magnitude of the magnetic force is given by

(31.23)

and its direction is perpendicular to v. As a result of this force, the particle will carry out uniform circular motion. The radius of the circle is determined by the requirement that the strength magnetic force is equal to the centripetal force. Thus

(31.24)

The radius r of the orbit is equal to

(31.25)

where p is the momentum of the charged particle. The distance traveled by the particle in one revolution is equal to

(31.26)

The time T required to complete one revolution is equal to

(31.27)

The frequency of this motion is equal to

(31.28)

and is called the cyclotron frequency. Equation (31.28) shows that the cyclotron frequency is independent of the energy of the particle, and depends only on its mass m and charge q.
The effect of a magnetic field on the motion of a charged particle can be used to determine some of its properties. One example is a measurement of the charge of the electron. An electron moving in a uniform magnetic field will described a circular motion with a radius given by eq.(31.25). Suppose the electron is accelerated by a potential V₀. The final kinetic energy of the electron is given by

(31.29)

The momentum p of the electron is determined by its kinetic energy

(31.30)

The radius of curvature of the trajectory of the electron is thus equal to

(31.31)

Equation (31.31) shows that a measurement of r can be used to determine the mass over charge ratio of the electron.
Another application of the effect of a magnetic field on the motion of a charged particle is the cyclotron. A cyclotron consists of an evacuated cavity placed between the poles of a large electromagnet. The cavity is cut into two D-shaped pieces (called dees) with a gap between them. An oscillating high voltage is connected to the plates, generating an oscillating electric field in the region between the two dees. A charged particle, injected in the center of the cyclotron, will carry out a uniform circular motion for the first half of one turn. The frequency of the motion of the particle depends on its mass, its charge and the magnetic field strength. The frequency of the oscillator is chosen such that each time the particle crosses the gap between the dees, it will be accelerated by the electric field. As the energy of the ion increases, its radius of curvature will increase until it reaches the edge of the cyclotron and is extracted. During its motion in the cyclotron, the ion will cross the gap between the dees many times, and it will be accelerated to high energies.
Up to now we have assumed that the direction of the motion of the charged particle is perpendicular to the direction of the magnetic field. If this is the case, uniform circular motion will result. If the direction of motion of the ion is not perpendicular to the magnetic field, spiral motion will result. The velocity of the charged particle can be decomposed into two components: one parallel and one perpendicular to the magnetic field. The magnetic force acting on the particle will be determined by the component of its velocity perpendicular to the magnetic field. The projection of the motion of the particle on the x-y plane (assumed to be perpendicular to the magnetic field) will be circular. The magnetic field will not effect the component of the motion parallel to the field, and this component of the velocity will remain constant. The net result will be spiral motion.

31.4. Crossed electric and magnetic fields

A charged particle moving in a region with an electric and magnetic field will experience a total force equal to

(31.32)

This force is called the Lorentz force.

Figure 31.6. Charged particle moving in crossed E and B fields. Consider a special case in which the electric field is perpendicular to the magnetic field. The motion of a charged particle in such a region can be quit complicated. A charged particle with a positive charge q and velocity v is moving in this field (see Figure 31.6). The direction of the particle shown in Figure 31.6 is perpendicular to both the electric field and the magnetic field. The electric force acting on the particle is directed along the direction of the electric field and has a magnitude equal to

(31.33)
The magnetic force acting on the charge particle is directed perpendicular to both v and B and has a magnitude equal to

(31.34)
The net force acting on the particle is the sum of these two components and has a magnitude equal to

(31.35)
If the charged particle has a velocity equal to

(31.36)
then the net force will be equal to zero, and the motion of the particle will be uniform linear motion. A device with crossed electric and magnetic fields is called a velocity selector. If slit are placed in the appropriate positions, it will transport only those particles that have a velocity defined by the magnitudes of the electric and magnetic fields.

Figure 31.7. Current in a magnetic field. A technique used to determine the density and sign of charge carriers in a metal is based on the forces exerted by crossed E and B fields on the charge carriers. The diagram shown in Figure 31.7 shows a metallic strip carrying a current in the direction shown and placed in a uniform magnetic field with the direction of the magnetic field being perpendicular to the electric field (which generates the current I). Suppose the charge carriers in the material are electrons, than the electrons will move in a direction opposite to that of the current (see Figure 31.7). Since the magnetic field is perpendicular to the electric field, it is also perpendicular to the direction of motion of the electrons. As a result of the magnetic force, the electrons are deflected downwards, and an excess of negative charge will be created on the bottom of the strip. At the same time, a deficit of negative charge will be created at the top of the strip. This charge distribution will generate an electric field that is perpendicular to the external electric field and, under equilibrium conditions, the electric force produced by this field will cancel the magnetic force acting on the electrons. When this occurs, the internal electric field, E_in, is equal to the product of the electron velocity, v_d, and the strength of the magnetic field, B. As a result of the internal electric field, a potential difference will be created between the top and bottom of the strip. If the metallic strip has a width w, then the potential difference [Delta]V will be equal to

(31.37)
This effect is called the Hall effect.
The drift velocity of the electrons depend on the current I in the wire, its cross sectional area A and the density n of electrons (see Chapter 28):

(31.38)
Combining eq.(31.38) and eq.(31.37) we obtain the following expression for [Delta]V

(31.39)
A measurement of [Delta]V can therefore be used to determine n.

31.5. Forces on a wire

A current I flowing through a wire is equivalent to a collection of charges moving with a certain velocity v_d along the wire. The amount of charge dq present in a segment dL of the wire is equal to

(31.40)
If the wire is placed in a magnetic field, a magnetic force will be exerted on each of the charge carriers, and as a result, a force will be exerted on the wire. Suppose the angle between the direction of the current and the direction of the field is equal to [theta] (see Figure 31.8). The magnetic force acting on the segment dL of the wire is equal to

(31.41)
The total force exerted by the magnetic field on the wire can be found by integrating eq.(31.41) along the entire wire.

Figure 31.8. Magnetic force on wire.

Example: Problem 31.33

A balance can be used to measure the strength of the magnetic field. Consider a loop of wire, carrying a precisely known current, shown in Figure 31.9 which is partially immersed in the magnetic field. The force that the magnetic field exerts on the loop can be measured with the balance, and this permits the calculation of the strength of the magnetic field. Suppose that the short side of the loop measured 10.0 cm, the current in the wire is 0.225 A, and the magnetic force is 5.35 x 10^-2 N. What is the strength of the magnetic field ?

Consider the three segments of the current loop shown in Figure 31.9 which are immersed in the magnetic field. The magnetic force acting on segment 1 and 3 have equal magnitude, but are directed in an opposite direction, and therefore cancel. The magnitude of the magnetic force acting on segment 2 can be calculated using eq.(31.41) and is equal to

(31.42)
This force is measured using a balance and is equal to 5.35 x 10^-2 N. The strength of the magnetic field is thus equal to

(31.43)

Figure 31.9. Current loop in immersed in magnetic field.

31.6. Torque on a current loop

If a current loop is immersed in a magnetic field, the net magnetic force will be equal to zero. However, the torque on this loop will in general not be equal to zero. Suppose a rectangular current loop is placed in a uniform magnetic field (see Figure 31.10). The angle between the normal of the current loop and the magnetic field is equal to [theta]. The magnetic forces acting on the top and the bottom sections of the current loop are equal to

(31.44)
where L is the length of the top and bottom edge. The torque exerted on the current loop, with respect to its axis, is equal to

(31.45)

Figure 31.10. Current loop placed in uniform magnetic field. Using the definition of the magnetic dipole moment u, discussed in Chapter 30, eq.(31.45) can be rewritten as

(31.46)
where

(31.47)
Using vector notation, eq.(31.45) can be rewritten as

(31.48)
where the direction of the magnetic moment is defined using the right-hand rule.
The work that must be done against the magnetic field to rotate the current loop by an angle d[theta] is equal to - [tau] d[theta]. The change in potential energy of the current loop when it rotates between [theta]₀ and [theta]₁ is given by

(31.49)
A common choice for the reference point is [theta]₀ = 90deg. and U([theta]₀) = 0 J. If this choice is made we can rewrite eq.(31.50) as

(31.50)
In vector notation:

solenoids

2011-09-24T01:47:00.000-07:00

Solenoid

A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. The term solenoid refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space (where some experiment might be carried out).

In short: the magnetic field inside an infinitely long solenoid is homogeneous and its strength does not depend on the distance from the axis, nor on the solenoid cross-sectional area.
This is a derivation of the magnetic flux density around a solenoid that is long enough so that fringe effects can be ignored. In the diagram to the right, we immediately know that the flux density vector points in the positive z direction inside the solenoid, and in the negative z direction outside the solenoid. We see this by applying the right hand grip rule for the field around a wire. If we wrap our right hand around a wire with the thumb pointing in the direction of the current, the curl of the fingers shows how the field behaves. Since we are dealing with a long solenoid, all of the components of the magnetic field not pointing upwards cancel out by symmetry. Outside, a similar cancellation occurs, and the field is only pointing downwards.
Now consider the imaginary loop c that is located inside the solenoid. By Ampère's law, we know that the line integral of B (the magnetic flux density vector) around this loop is zero, since it encloses no electrical currents (it can be also assumed that the circuital electric field passing through the loop is constant under such conditions: a constant or constantly changing current through the solenoid). We have shown above that the field is pointing upwards inside the solenoid, so the horizontal portions of loop c doesn't contribute anything to the integral. Thus the integral of the up side 1 is equal to the integral of the down side 2. Since we can arbitrarily change the dimensions of the loop and get the same result, the only physical explanation is that the integrands are actually equal, that is, the magnetic field inside the solenoid is radially uniform. Note, though, that nothing prohibits it from varying longitudinally, which in fact it does.
In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid.

Magnetic field and vector potential for finite continuous solenoid

Magnetic field line and density created by a solenoid with surface current density

A finite solenoid is a solenoid with finite length. Continuous means that the solenoid is not formed by discrete coils by a sheet of conductive material. We assume the current is uniformly distributed on the surface of it, and it has surface current density K. In cylindrical coordinates:

The magnetic field can be found by vector potential. the vector potential for a finite solenoid with radius a, length L in cylindrical coordinates is is:

where

The

K (m)

E (m)

, and

Π(n, m)

are complete elliptic integral of first, second, and third kind.
By using

the magnetic flux density is:

Hydraulics

2011-09-22T21:33:00.000-07:00

Hydraulics and other studies

Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control, and transmission of power by the use of pressurized liquids. Hydraulic topics range through most science and engineering disciplines, and cover concepts such as pipe flow, dam design, fluidics and fluid control circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement, river channel behavior and erosion.
Free surface hydraulics is the branch of hydraulics dealing with free surface flow, such as occurring in rivers, canals, lakes, estuaries and seas. Its sub-field open channel flow studies the flow in open channels.

DC sevomotors

2011-09-22T21:25:00.000-07:00

Dc servo motors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these servo motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens servo motor response to command signals. Servo motors include permanent-magnetic, printed-circuit, and moving-coil (or shell) dc servo motors. The rotor of a shell dc servo motor consists of a cylindrical shell of copper or aluminum wire coils which rotate in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core. The servo motor features a field, which is provided by cast AlNiCo magnets whose magnetic axis is radial. Servo motors usually have two, four, or six poles.
Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.
A servomechanism, or servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.
A common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. This procedure is one widely used application of control theory.
Speed control via a governor is another type of servomechanism. The steam engine uses mechanical governors; another early application was to govern the speed of water wheels. Prior to World War II the constant speed propeller was developed to control engine speed for maneuvering aircraft. Fuel controls for gas turbine engines employ either hydromechanical or electronic governing.
Positioning servomechanisms were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a leadscrew or a linear motor to give linear motion.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)
In industrial machines, servos are used to perform complex motion.
DC Servo Motor Selection: The first selection approach is to choose a servo motor large enough for a machine that has already been designed; the second is to select the best available servo motor with a specific feature and then build the system around it; and the third is to study servo motor performance and system requirements and mate the two.
The final servo motor system design is usually the least sophisticated that meets the performance specifications reliably. Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances. This says that the servo designer must define the system carefully, establish the servo motor's performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servosystem and choose a servo motor type.

electric dipole

2011-09-22T21:14:00.000-07:00

Dipole Moment

The electric dipole moment for a pair of opposite charges of magnitude q is defined as the magnitude of the charge times the distance between them and the defined direction is toward the positive charge. It is a useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances between the charges are too small to be easily measurable. It is also a useful concept in dielectrics and other applications in solid and liquid materials.

Applications involve the electric field of a dipole and the energy of a dipole when placed in an electric field.

Electric dipole transition is the dominant effect of an interaction of an electron in an atom with the electromagnetic field.
Following , consider an electron in an atom with quantum Hamiltonian

H 0

, interacting with a plane electromagnetic wave

Write the Hamiltonian of the electron in this electromagnetic field as

Treating this system by means of time-dependent perturbation theory, one finds that the most likely transitions of the electron from one state to the other occur due to the summand of

W (t)

written as

Electric dipole transitions are the transitions between energy level in the system with the Hamiltonian

H 0 + W D E (t)

.
Between certain electron states the electric dipole transition rate may be zero due to selection rule, and then the transitions between such levels are approximated by higher-order transitions.
The next order summand in

W (t)

is written as

and describes magnetic dipole transitions.
Even smaller contributions to transition rates are given by higher electric and magnetic multipole transitions

electri current

2011-09-22T21:07:00.000-07:00

Electric current is a flow of electric charge through a medium. This charge is typically carried by moving electrons in a conductor such as wire. It can also be carried by ions in an electrolyte, or by both ions and electrons in a plasma.
The SI unit for measuring the rate of flow of electric charge is the ampere, which is charge flowing through some surface at the rate of one coulomb per second. Electric current is measured using an ammeter.

Electric current is the rate of charge flow past a given point in an electric circuit, measured in Coulombs/second which is named Amperes. In most DC electric circuits, it can be assumed that the resistance to current flow is a constant so that the current in the circuit is related to voltage and resistance by Ohm's law. The standard abbreviations for the units are 1 A = 1C/s.
The unit of electric charge is the Coulomb (abbreviated C). Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Charge is quantized as a multiple of the electron or proton charge:

The influence of charges is characterized in terms of the forces between them (Coulomb's law) and the electric field and voltage produced by them. One Coulomb of charge is the charge which would flow through a 120 watt lightbulb (120 volts AC) in one second. Two charges of one Coulomb each separated by a meter would repel each other with a force of about a million tons! The rate of flow of electric charge is called electric current and is measured in Amperes.
In introducing one of the fundamental properties of matter, it is perhaps appropriate to point out that we use simplified sketches and constructs to introduce concepts, and there is inevitably much more to the story. No significance should be attached to the circles representing the proton and electron, in the sense of implying a relative size, or even that they are hard sphere objects, although that's a useful first construct. The most important opening idea, electrically, is that they have a property called "charge" which is the same size, but opposite in polarity for the proton and electron. The proton has 1836 times the mass of the electron, but exactly the same size charge, only positive rather than negative. Even the terms "positive" and "negative" are arbitrary, but well-entrenched historical labels. The essential implication of that is that the proton and electron will strongly attract each other, the historical archtype of the cliche "opposites attract". Two protons or two electrons would strongly repel each other. Once you have established those basic ideas about electricity, "like charges repel and unlike charges attract", then you have the foundation for electricity and can build from there.
From the precise electrical neutrality of bulk matter as well as from detailed microscopic experiments, we know that the proton and electron have the same magnitude of charge. All charges observed in nature are multiples of these fundamental charges. Although the standard model of the proton depicts it as being made up of fractionally charged particles called quarks, those fractional charges are not observed in isolation -- always in combinations which produce +/- the electron charge.
An isolated single charge can be called an "electric monopole". Equal positive and negative charges placed close to each other constitute an electric dipole. Two oppositely directed dipoles close to each other are called an electric quadrupole. You can continue this process to any number of poles, but dipoles and quadrupoles are mentioned here because they find significant application in physical phenomena.

DC motors

2011-09-22T21:01:00.000-07:00

A DC motor is an electric motor that runs on direct current (DC) electricity.

Main article: Brushed DC electric motor

This is a brushed DC electric motor generating torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets. Torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a force known as Lorentz force. The commutator consists of a split ring 80 degree shows the effects of having a split ring.

This is a brushed DC electric motor generating torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets. Torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a force known as Lorentz force. The commutator consists of a split ring 50 degree shows the effects of having a split ring of different amount of split.

This is a brushed DC electric motor generating torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets. Torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a force known as Lorentz force. The commutator consists of a split ring 20 degree shows magnetic force exerted on coil only when contact is establish, momentum makes the coil to continue to rotate when there is no current.

The brushed DC electric motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent or electromagnets), and rotating electrical magnets.
Like all electric motors or generators, torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.

Brushless

Brushless DC motors use a rotating permanent magnet or soft magnetic core in the rotor, and stationary electrical magnets on the motor housing. A motor controller converts DC to AC. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motor

Principles of operation

In any electric motor, operation is based on simple electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field. As you are well aware of from playing with magnets as a kid, opposite (North and South) polarities attract, while like polarities (North and North, South and South) repel. The internal configuration of a DC motor is designed to harness the magnetic interaction between a current-carrying conductor and an external magnetic field to generate rotational motion.
Let's start by looking at a simple 2-pole DC electric motor (here red represents a magnet or winding with a "North" polarization, while green represents a magnet or winding with a "South" polarization).

Every DC motor has six basic parts -- axle, rotor (a.k.a., armature), stator, commutator, field magnet(s), and brushes. In most common DC motors (and all that BEAMers will see), the external magnetic field is produced by high-strength permanent magnets¹. The stator is the stationary part of the motor -- this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotate with respect to the stator. The rotor consists of windings (generally on a core), the windings being electrically connected to the commutator. The above diagram shows a common motor layout -- with the rotor inside the stator (field) magnets.

The geometry of the brushes, commutator contacts, and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnet(s) are misaligned, and the rotor will rotate until it is almost aligned with the stator's field magnets. As the rotor reaches alignment, the brushes move to the next commutator contacts, and energize the next winding. Given our example two-pole motor, the rotation reverses the direction of current through the rotor winding, leading to a "flip" of the rotor's magnetic field, driving it to continue rotating.
In real life, though, DC motors will always have more than two poles (three is a very common number). In particular, this avoids "dead spots" in the commutator. You can imagine how with our example two-pole motor, if the rotor is exactly at the middle of its rotation (perfectly aligned with the field magnets), it will get "stuck" there. Meanwhile, with a two-pole motor, there is a moment where the commutator shorts out the power supply (i.e., both brushes touch both commutator contacts simultaneously). This would be bad for the power supply, waste energy, and damage motor components as well. Yet another disadvantage of such a simple motor is that it would exhibit a high amount of torque "ripple" (the amount of torque it could produce is cyclic with the position of the rotor).

So since most small DC motors are of a three-pole design, let's tinker with the workings of one via an interactive animation (JavaScript required):

You'll notice a few things from this -- namely, one pole is fully energized at a time (but two others are "partially" energized). As each brush transitions from one commutator contact to the next, one coil's field will rapidly collapse, as the next coil's field will rapidly charge up (this occurs within a few microsecond). We'll see more about the effects of this later, but in the meantime you can see that this is a direct result of the coil windings' series wiring:

There's probably no better way to see how an average DC motor is put together, than by just opening one up. Unfortunately this is tedious work, as well as requiring the destruction of a perfectly good motor.
Luckily for you, I've gone ahead and done this in your stead. The guts of a disassembled Mabuchi FF-030-PN motor (the same model that Solarbotics sells) are available for you to see here (on 10 lines / cm graph paper). This is a basic 3-pole DC motor, with 2 brushes and three commutator contacts.

The use of an iron core armature (as in the Mabuchi, above) is quite common, and has a number of advantages². First off, the iron core provides a strong, rigid support for the windings -- a particularly important consideration for high-torque motors. The core also conducts heat away from the rotor windings, allowing the motor to be driven harder than might otherwise be the case. Iron core construction is also relatively inexpensive compared with other construction types.
But iron core construction also has several disadvantages. The iron armature has a relatively high inertia which limits motor acceleration. This construction also results in high winding inductances which limit brush and commutator life.
In small motors, an alternative design is often used which features a 'coreless' armature winding. This design depends upon the coil wire itself for structural integrity. As a result, the armature is hollow, and the permanent magnet can be mounted inside the rotor coil. Coreless DC motors have much lower armature inductance than iron-core motors of comparable size, extending brush and commutator life.

Diagram courtesy of MicroMo The coreless design also allows manufacturers to build smaller motors; meanwhile, due to the lack of iron in their rotors, coreless motors are somewhat prone to overheating. As a result, this design is generally used just in small, low-power motors. BEAMers will most often see coreless DC motors in the form of pager motors.

Again, disassembling a coreless motor can be instructive -- in this case, my hapless victim was a cheap pager vibrator motor. The guts of this disassembled motor are available for you to see here (on 10 lines / cm graph paper). This is (or more accurately, was) a 3-pole coreless DC motor.
I disembowel 'em so you don't have to.

DC motor operation:

electrical energy produced

2011-09-22T04:27:00.000-07:00

The moving or falling water travels through a turbine, causing the turbine blades to spin.

This in turn, is used to drive a generator (in simple terms, this is a magnet that spins inside a coil of copper wire).

The rotating magnetic field produced by the spinning magnet induces an electrical current in the copper wire hence generating electricity.

The first ever public electric power distribution system appeared in the late 19th century and mostly supplied direct currents, with multiple output voltages, or under the form of alternating current, which reverses the polarity of the current every 20 milliseconds. However, the energy demand and the advantages that derive from the use of standard voltage outputs quickly impose the alternating current as the preferred type for production, transport and distribution.

Most of the electric energy production process takes place in power plants burning fossil fuel, such as coal, natural gas or crude oil, which accounts for the massive amounts of greenhouse gases emitted daily in the Earth's atmosphere. Heat produced during fuel burning is used to evaporate large amounts of water and create steam that powers a steam turbine, turning the energy extracted during burning fuel into mechanical work. By connecting a mechanical generator to the turbine's drive shaft, the mechanical work can be turned efficiently into electric energy.

However, the electric energy cannot be directly supplied to the consumer in this form, thus it must undertake certain transformations in order to meet the required standard, which state that the electric current must have parameters ranging in certain imposed values. For example, in Europe,

most of the countries use alternating electric current that has a nominal voltage of 220 volts and an oscillating frequency of 50 hertz, while the United States prefer an electric current having 110 volts, and a frequency of 60 hertz.

The frequency of the alternating current is usually determined by the construction of the generator, however voltage of the current represents a key aspect in the transport and distribution of the electric energy, thus it is uneconomical and unpractical to build generators that produce different voltages.

Instead, transformers are used to vary the voltage of the electric current in the required domain. Transformers consist basically of two electric circuits formed of two coils, separate electrically by an inductive coupling used to transfer energy between the two. There are two primary types of electric transformers: high-voltage transformers and low-voltage transformers. High-voltage transformers are used to increase the voltage output, while the low-voltage ones are used to decrease it.

The electric energy transport is usually greatly affected by the loss of power in long conductors, as a result of the presence of electrical resistance. Even the best electrical conductors having low electrical resistance can produce enormous power loss over relative small distances. To solve this problem, the electrical current's voltage is being increased with the help of high-voltage transformers, meaning that for a voltage increase of a 10 factor, the current intensity will drop by the same amount, thus reducing the power loss by over one hundred times.

Most of the outlets present in our houses have a mono-phase current configuration, consisting of two electrodes, one used as ground and the other to supply the alternating electric current. However, the transport systems mostly imply the use of three-phase currents that consist of four metal conductors, three for alternating electric current supply, while most of the time the fourth conductor is used as ground, though this is not necessarily required.

The electric currents through the three-phase metal system are out of phase by 120 solid degrees and voltages between the lines usually measure an average of 220 volts.

The use of fossil fuel for electric energy production has determined massive emissions of greenhouse gases; however, in the last decades, attention shifted towards alternative energy sources, such as wind power, solar power and, last but not the least, nuclear power that is quickly becoming the base of electric power production.

transformer oil

2011-09-22T04:09:00.000-07:00

Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Explanation

The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of thousands of KVA) may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.
Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.
Oil filled transformers with a conservator (an oil tank above the transformer) tend to be equipped with Buchholz relays. These are safety devices that detect the build up of gases (such as acetylene) inside the transformer (a side effect of corona or an electric arc in the windings) and switch off the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.
The flash point (min) and pour point (max) are 140 °C and −6 °C respectively. The dielectric strength of new untreated oil is 12 MV/m (RMS) and after treatment it should be >24 MV/m (RMS).

Transformer Oil Testing

Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there are contaminations caused due to chemical interactions with windings and other solid insulations, catalyzed by high operating temperature. As a result the original chemical properties of transformer oil changes gradually, rendering it ineffective for its intended purpose after many years. Hence this oil has to be periodically tested to ascertain its basic electrical properties, and make sure it is suitable for further use or necessary actions like filtration/regeneration has to be done. These tests can be divided into:

Dissolved gas analysis
Furan analysis
PCB analysis
General electrical & physical tests:

Color & Appearance
Breakdown Voltage
Water Content
Acidity (Neutralization Value)
Dielectric Dissipation Factor
Resistivity
Sediments & Sludge
Interfacial Tension
Flash Point
Pour Point
Density
Kinematic Viscosity

Transformer oils (insulating oils) are dielectric oils used in oil-filled transformers, some high voltage capacitors and circuit breakers, fluorescent lamp ballasts.

Oil-filled transformers are more efficient, more reliable more compact, more quiet and with longer lifespan than dry transformers.

Transformer oils have the following functions:

Prevention of electrical discharges between the transformer coils: corona and arc (insulating).
Removal of heat generated by the transformer (cooling). Transformer oil forced by natural convection circulates through a radiator carrying the heat out of the transformer.

Transformer oils (insulating oils) have replaced toxic PCB (polychlorinated biphenyl).

Mineral oils, vegetable lubricants (soybean oil, vegetable based esters) or synthetic lubricants (polydimethylsiloxane) are used as basics for transformer oils.

Properties of some transformer oils

(Materials Data)

Mineral transformer oil
Silicon transformer oil
Synthetic transformer oil

electrical symbols

2011-09-22T03:46:00.000-07:00

Electrical symbols are small pictures or images (also known as pictograms) which you may see in a wide variety of places. If you’ve ever worked on building electronic devices of your own (using circuit boards for example) then you will need to know which different component to use in your project. Usually, when you try to interpret a circuit diagram, all of the electrical symbols will have their own image. If you are unfamiliar with the electronic images, then you can use this website as a reference for them.

Where might you see these symbols?
There are a number of places where electrical symbols can be found, including project magazines and on a variety of websites. In some places you can print our your own circuit boards in order to create your own machines or gadgets.
What symbols are the most common to find in electrical projects?
Electric bell
The electric bell is found in a normal house doorbell, and when activated it makes a ringing sound (a sound that you are probably very used to!)
Buzzer
An electrical buzzer is similar to the bell, but instead of a single tone o rbell sound it makes a constant buzz noise.
Loudspeaker
A loudspeaker can take digital input and turn it into analogue sound waves. One of the most important parts of a wide range of electrical products like iPods, TVs and telephones.

Some Electrical Symbols

Description	Picture	Symbol

a connecting wire


a bulb


a switch


a battery


a motor


a buzzer

Electrical Symbols

Transformer Winding

2011-09-21T06:34:00.000-07:00

In the good old times it was a matter of fact that every electronic hobbyist or technician would wind himself any power transformers he needed, and rewind any that burned out. Unfortunately, nowadays transformer winding is fast becoming a lost art, and I have seen many people despair about where to find some very specific transformer, or pull their hair out about the cost of having one professionally wound to specifications.
Since I started in electronics, as a 12 year old boy, I have always wound my own transformers. I started using the basic, but useful instructions provided in The Radio Amateur's Handbook of the time, and later I came to better understand how transformers work, which enabled me to optimize a given transformer for the intended application.
Following a request by many readers of my web site, I've added this page, which is complementary to the previously published Transformers and coils. You should first read (and understand!) that page, before trying to design any transformer. Then come to this more practically-oriented page, to learn some tricks and hints about the design process, and about hands-on winding.
This page addresses mainly single-phase power transformers in the power range from about 1 watt to 10,000 watts, operating at line frequencies, but much of what's described here can be applied to a wide range of other transformers too.
Let's start with the materials. To make a typical transformer, you need the iron laminations for the core, enameled copper wire of several different diameters for the windings, a bobbin (or some material to make one), insulating material to apply between wire layers, between windings, around the whole winding assembly, and on exposed wires, and in most cases it's also a good idea to use an impregnation varnish.
The photo here shows several stacks of iron E-I laminations, two coils of wire (with cardboard protecting the wire from damage), one roll of thick, stiff Pressspan, another roll of NMN laminate (we will soon see what that is), two small bundles of spaghetti for wire protection, and a can of transformer varnish. Add to this some glue, cotton straps, ropes, adhesive tape, terminals, bolts, angle iron, and other small material, and that's it.
All these materials are sold by companies specializing in transformers and parts for transformers. Enameled wire is also sold by many other distributors, but is usually cheapest at the places that sell it together with the other materials. You will have to dig into the phone book or some other directory to find these companies, since they don't usually have a shiny nice store in the downtown shopping mall!

Transformer iron is an alloy of iron with silicon and some other minor components. It's characterized by a relatively high permeability, very high saturation flux density, relatively low hysteresis loss, and relatively high specific resistance. This latter factor, along with the practice of using the material in thin, insulated sheets, reduces the power losses produced by eddy currents.
The most common shape of these sheets is shown at right. It's the classic "economy E-I" shape. Why it's called E-I should be pretty obvious when looking at the photo! But the explanation for "economy" might be a bit more elusive: It's because at the exact proportions shown in the photo, the I's are nothing else than the cutouts to make the windows in the E's, when two E's are cut facing each other! This allows stamping E's and I's out of a large steel sheet, without any wasting of material, except for the little round bits cut out of the bolt holes. By the way, small laminations often don't have such bolt holes, and such cores are held together by clamps instead of bolts, or even welded.
The lamination in the photo is a large one, as the comparison with my hand shows. It's an E80 (the center leg is 80mm wide), typically used for transformers in the 3 to 10 kilowatt range!
In any E-I lamination you are likely to encounter, the center leg is twice as wide as each of the other parts. This is because the entire magnetic flux has to go through the center leg, but then splits up, with one half of the flux returning through each of the side legs. If you ever come across a lamination that has all three legs of the same width, then you are looking at a lamination intended for three phase transformers!
Such an economy E-I lamination like shown here has completely fixed proportions, beyond the rule above, that stem from the need to cut the I out of the winding window of two E's facing each other: If the center leg is 2 units wide, then the window is 1 x 3 units, the total E is 6 x 4 units, the I is 1 x 6 units, and so on.

Not all laminations follow the "economy" proportions, though. Here is an example of a lamination that comes in one piece, instead of being divided into an E and an I, and that has the windows proportionally much larger than the E-I lamination shown above. Such a lamination is a bit more expensive to make, because the steel cut from the windows is wasted, unless the manufacturer can find some other use for it. But being able to accomodate a much large winding assembly, it has some advantages in certain cases.
These "non-economy" laminations were quite usual in Europe, many years ago, but nowadays copper is so much more expensive than steel, that transformers are usually designed to use more steel and less copper. And for that goal, the economy lamination is very well suited. So you won't very often come across a lamination like this, unless you are restoring antique equipment.

The laminations should be thin, and reasonably well insulated from each other, to reduce eddy currents to an insignificant value. Typical thicknesses vary from 0.2 to 0.5mm, but higher frequency transformers (audio) use much thinner ones, while extremely large transformers might use slightly thicker ones.
The insulation is often applied at the factory that makes the big rolls of steel sheet, even before stamping the E's and I's. Different kinds of insulation are used: A thin oxide layer, a thin layer of enamel, or any of several chemical processes. Antique transformers sometimes even used very thin paper!
When I was young, patient and overly eager to do things right, I painted each and every E and I for my transformers, using diluted transformer varnish, to make a thin, nice layer. The photo shows the steel for a 200 watt transformer, drying. Later, getting old and lazy, I noticed that the layer of rust on old, recycled laminations is more than enough insulation, and that the very thin and imperfect insulation that comes on new laminations is enough too, even if it takes only a light scratch with the multimeter's test probe to puncture it and get through to the steel. We don't need perfect insulation between the sheets! We only need enough resistance to reduce eddy currents to a low level.
Transformer steel is not all born alike. Manufacturers will provide data sheets about their products (often on their web sites), where you can see what they offer. There are usually many grades, with vastly different loss characteristics. At a given flux density and frequency, a good material might have ten times less loss than a cheap material! So it pays to look, investigate, and decide intelligently what to buy. Thinner sheets normally have lower loss, and the rest of the secret lies in the exact alloy. In any case, you need to know what material you have, to be able to make a meaningful transformer design!
Some transformer steel is grain-oriented. That means that when rolling the steel sheets, a process is used to align the crystalline grains in the direction of the rolling. This kind of material has particularly good behavior when the magnetic flux is aligned with the direction in which the sheet was rolled, but is worse than standard material in the perpendicular direction. Such grain-oriented material is ideal for toroidal cores, which are made by coiling up a long strip of steel, but is not a large improvement for E-I laminations, because in these a significant portion of the material has to work with the flux perpendicular to the rolling direction.

Enamelled copper wire comes in many different diameters, and with several different kinds of enamel. The diameters vary from less than that of a hair, to about that of a child's finger. Different standards exist for the wire diameter. A very common one is American Wire Gauge, shortened to AWG, which is used in much of the world. Britain has its own standard, and in many countries the wire is specified simply by its diameter in millimeters.
Thick wires usually are coated with a sort of enamel that is very tough, an excellent insulator, highly heat-resistant, highly resistant to solvents, and that clings to copper even better than dirt does to children! This enamel is usually yellowish clear, so that the wire coated in it looks mostly copper-colored, but many exceptions exist. To solder the ends of these wires, it's necessary to scrape off the enamel, using a sharp knife or similar tool. This procedure would be too difficult with a thin, fragile wire, so that these thin wires are instead covered with an enamel that has most of the same characteristics of the other one, except the heat resistance: It will melt and turn into solder flux at a temperature a common soldering iron easily achieves! This allows easily soldering these wires, without previously stripping them. But transformers using this latter kind of wire enamel cannot survive temperatures as high as those using only the former kind of wire enamel. The red wire on the right side in this photo has this kind of enamel. But be careful with colors! The clear wire on the extreme left side also has solderable enamel, while the dark violet one in the middle is of the non-melting variety!
The thickness of the enamel layer depends on the wire thickness, the manufacturer, and can sometimes be chosen: Some manufacturers will offer the wire with seeral different thicknesses of enamel. In any case, the diameter specified by a certain AWG number refers to the copper diameter, so that the complete wire, with enamel, will be slightly thicker than what the AWG standard tells!
Here is a wire table for AWG wire. It shows the AWG number, the diameter in millimeters excluding the enamel, the approximate typical total diameter including the enamel (but this varies somewhat), the cross sectional copper area in square millimeters, the area of the square of window space occupied by that wire in a transformer (including the enamel, of course), the current carrying capacity at a typical, average value of current density, the resistance in ohms per meter, and finally how many meters of that wire come in one kilogram, because enamelled wire is usually bought by weight, not length.
This table has wires from AWG #1 to #40, and for the thickest ones I didn't calculate all data. But you should be aware that there are wires exceeding this range! The thinnest I have ever used was #46. It breaks when you blow at it! The photo here shows a #39 wire lying on a #7 wire. The hairy thing below is my floor carpet. Note that even this #39 wire is not much thicker than the hairs of this carpet!
It's interesting to note that every three AWG numbers, the cross sectional area exactly doubles. Any deviation from this in my table is due to approximation errors.

Modern transformers of small to moderate size are usually wound on plastic bobbins. Here you can see simple ones. Some bobbins have pins or terminals molded into them, others have one or two divisions. Some don't have the slits for terminals, which the ones shown here do have.
Typically for a given size of E-I laminations, bobbins will be available in two or three sizes, accomodating different numbers of steel sheets. So you can vary the amount of steel in your transformer not only by choosing the lamination size, but also the height of the lamination stack!

Here is a little transformer using a divided (or split) bobbin. This is very practical, because it completely separates the primary from the secondary winding, making it much easier to achieve the degree of insulation required for safety. More about that later.

This article is about the electrical device. For the toy line franchise, see Transformers. For other uses, see Transformer (disambiguation).

Pole-mounted distribution transformer with center-tapped secondary winding. This type of transformer is commonly used in the United States to provide 120/240 volt "split-phase" power for residential and light commercial use. Note that the center "neutral" terminal is grounded to the transformer "tank", and a grounded conductor (right) is used for one leg of the primary feeder.

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (V_s) is in proportion to the primary voltage (V_p), and is given by the ratio of the number of turns in the secondary (N_s) to the number of turns in the primary (N_p) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making N_s greater than N_p, or "stepped down" by making N_s less than N_p.
In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.
Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer. The secondary current arises from the action of the secondary EMF on the (not shown) load impedance.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in the directions indicated, given the primary current and voltage in the directions indicated (each will be alternating current in practice).

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where V_s is the instantaneous voltage, N_s is the number of turns in the secondary coil and Φ is the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for V_s and V_p gives the basic equation for stepping up or stepping down the voltage

N_p/N_s is known as the turns ratio, and is the primary functional characteristic of any transformer. In the case of step-up transformers, this may sometimes be stated as the reciprocal, N_s/N_p. Turns ratio is commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.

Ideal power equation

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:

giving the ideal transformer equation

Transformers normally have high efficiency, so this formula is a reasonable approximation.
If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio. For example, if an impedance Z_s is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (N_p/N_s)²Z_s. This relationship is reciprocal, so that the impedance Z_p of the primary circuit appears to the secondary to be (N_s/N_p)²Z_p.

Detailed operation

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages V_P and V_S measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF". This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Practical considerations

[ Leakage flux

Leakage flux of a transformer

Main article: Leakage inductance

The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings. Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the poer supply. It is not directly a power loss (see "Stray losses" below), but results in inferior voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Transformers are therefore normally designed to have very low leakage inductance. Nevertheless, it is impossible to eliminate all leakage flux because it plays an essential part in the operation of the transformer. The combined effect of the leakage flux and the electric field around the windings is what transfers energy from the primary to the secondary.
In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced to a transformer's design to limit the short-circuit current it will supply.Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as electric arc welders.
Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings
Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if the "per-unit" inductance of two transformers is the same (a typical value is 5%), they will automatically split power "correctly" (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice the current)

Design of sub-stations:

2011-09-18T04:37:00.000-07:00

A 50 Hz electrical substation in Melbourne. This is showing three of the five 220 kV/66 kV transformers, each with a capacity of 185 MVA

A 115 kV to 41.6/12.47 kV 5 MVA 60 Hz substation with circuit switcher, regulators, reclosers and control building at Warren, Minnesota

A substation is a part of an electrical generation, transmission, and distribution
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Electric power may flow through several substations between generating plant and consumer, and its voltage may change in several steps.
A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.

SF₆ 110 kV instrumental current transformer TGFM series, Russia

Elements of a substation
A:Primary power lines' side B:Secondary power lines' side
1.Primary power lines 2.Ground wire 3.Overhead lines 4.Transformer for measurement of electric voltage 5.Disconnect switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester 9.Main transformer 10.Control building 11.Security fence 12.Secondary power lines

Substations generally have switching, protection and control equipment, and transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may
Substations generally have switching, protection and control equipment, and transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation.
Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.
Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution.

Earthing and Bonding

The function of an earthing and bonding system is to provide an earthing system connection to which transformer neutrals or earthing impedances may be connected in order to pass the maximum fault current. The earthing system also ensures that no thermal or mechanical damage occurs on the equipment within the substation, thereby resulting in safety to operation and maintenance personnel. The earthing system also guarantees eqipotential bonding such that there are no dangerous potential gradients developed in the substation.

In designing the substation, three voltage have to be considered.

1. Touch Voltage: This is the difference in potential between the surface potential and the potential at an earthed

equipment whilst a man is standing and touching the earthed structure.

2. Step Voltage: This is the potential difference developed when a man bridges a distance of 1m with his feet

while not touching any other earthed equipment.

3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the earthing grid.

Substation Earthing Calculation Methodology

Calculations for earth impedances and touch and step potentials are based on site measurements of ground resistivity and system fault levels. A grid layout with particular conductors is then analysed to determine the effective substation earthing resistance, from which the earthing voltage is calculated.

In practice, it is normal to take the highest fault level for substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for.

To determine the earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained.

Earthing Materials

1. Conductors: Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves

usually have a cross-sectional area of 95 square millimetres, and they are laid at a shallow depth

of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground

earthing ring is usually provided, to which all metallic substation plant is bonded.

2. Connections: Connections to the grid and other earthing joints should not be soldered because the heat generated

during fault conditions could cause a soldered joint to fail. Joints are usually bolted, and in this case, the

face of the joints should be tinned.

3. Earthing Rods: The earthing grid must be supplemented by earthing rods to assist in the dissipation of earth fault

currents and further reduce the overall substation earthing resistance. These rods are usually made of

solid copper, or copper clad steel.

4. Switchyard Fence

Earthing: The switchyard fence earthing practices are possible and are used by different utilities. These are:

(i) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then

bonded to the grid at regular intervals.

(ii) Place the fence beyond the perimeter of the switchyard earthing grid and bond the fence to its

own earthing rod system. This earthing rod system is not coupled to the main substation earthing

grid.

Layout of Substation

The layout of the substation is very important since there should be a Security of Supply. In an ideal substation all circuits and equipment would be duplicated such that following a fault, or during maintenance, a connection remains available. Practically this is not feasible since the cost of implementing such a design is very high. Methods have been adopted to achieve a compromise between complete security of supply and capital investment. There are four categories of substation that give varying securities of supply:

Category 1: No outage is necessary within the substation for either maintenance or fault conditions.
Category 2: Short outage is necessary to transfer the load to an alternative circuit for maintenance or fault conditions.
Category 3: Loss of a circuit or section of the substation due to fault or maintenance.
Category 4: Loss of the entire substation due to fault or maintenance.

Different Layouts for Substations

Single Busbar

The general schematic for such a substation is shown in the figure below.

With this design, there is an ease of operation of the substation. This design also places minimum reliance on signalling for satisfactory operation of protection. Additionally there is the facility to support the economical operation of future feeder bays.

Such a substation has the following characteristics.

Each circuit is protected by its own circuit breaker and hence plant outage does not necessarily result in loss of supply.
A fault on the feeder or transformer circuit breaker causes loss of the transformer and feeder circuit, one of which may be restored after isolating the faulty circuit breaker.
A fault on the bus section circuit breaker causes complete shutdown of the substation. All circuits may be restored after isolating the faulty circuit breaker.
A busbar fault causes loss of one transformer and one feeder. Maintenance of one busbar section or isolator will cause the temporary outage of two circuits.
Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.
Introduction of bypass isolators between busbar and circuit isolator allows circuit breaker maintenance facilities without loss of that circuit.

Mesh Substation The general layout for a full mesh substation is shown in the schematic below.

The characteristics of such a substation are as follows.

Operation of two circuit breakers is required to connect or disconnect a circuit, and disconnection involves opening of a mesh.
Circuit breakers may be maintained without loss of supply or protection, and no additional bypass facilities are required.
Busbar faults will only cause the loss of one circuit breaker. Breaker faults will involve the loss of a maximum of two circuits.
generally, not more than twice as many outgoing circuits as infeeds are used in order to rationalise circuit equipment load capabilities and ratings.

One and a half Circuit Breaker layout The layout of a 1 1/2 circuit breaker substation is shown in the schematic below.

The reason that such a layout is known as a 1 1/2 circuit breaker is due to the fact that in the design, there are 9 circuit breakers that are used to protect the 6 feeders. Thus, 1 1/2 circuit breakers protect 1 feeder. Some characteristics of this design are:

There is the additional cost of the circuit breakers together with the complex arrangement.
It is possible to operate any one pair of circuits, or groups of pairs of circuits.
There is a very high security against the loss of supply.

Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

Spatial Separation

Earth Clearance: this is the clearance between live parts and earthed structures, walls, screens and ground.
Phase Clearance: this is the clearance between live parts of different phases.
Isolating Distance: this is the clearance between the terminals of an isolator and the connections thereto.
Section Clearance: this is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works.

Separation of maintenance zones Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

1. The provision of a section clearance

2. Use of an intervening earthed barrier

The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

A section clearance is composed of a the reach of a man, taken as 8 feet, plus an earth clearance.
For the voltage at which the earth clearance is 8 feet, the space required will be the same whether a section clearance or an earthed barrier is used.

HENCE: Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance

Separation by section clearance = 2.44m + Earth clearance

For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.
The height of the platform is taken as 1.37m below the highest point of work.

Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as is the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts, when isolated, either by section clearances or earth barriers.

Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts.

There are at least three such electrical separations per phase that are needed in a circuit: 1. Between the terminals of the busbar isolator and their connections.

2. Between the terminals of the circuit breaker and their connections.

3. Between the terminals of the feeder isolator and their connections.

Components of a Substation

The substation components will only be considered to the extent where they influence substation layout.

Circuit Breakers

There are two forms of open circuit breakers:

1. Dead Tank - circuit breaker compartment is at earth potential.

2. Live Tank - circuit breaker compartment is at line potential.

The form of circuit breaker influences the way in which the circuit breaker is accommodated. This may be one of four ways.

Ground Mounting and Plinth Mounting: the main advantages of this type of mounting are its simplicity, ease of erection, ease of maintenance and elimination of support structures. An added advantage is that in indoor substations, there is the reduction in the height of the building. A disadvantage however is that to prevent danger to personnel, the circuit breaker has to be surrounded by an earthed barrier, which increases the area required.
Retractable Circuit Breakers: these have the advantage of being space saving due to the fact that isolators can be accommodated in the same area of clearance that has to be allowed between the retractable circuit breaker and the live fixed contacts. Another advantage is that there is the ease and safety of maintenance. Additionally such a mounting is economical since at least two insulators per phase are still needed to support the fixed circuit breaker plug contacts.
Suspended Circuit Breakers: at higher voltages tension insulators are cheaper than post or pedestal insulators. With this type of mounting the live tank circuit breaker is suspended by tension insulators from overhead structures, and held in a stable position by similar insulators tensioned to the ground. There is the claimed advantage of reduced costs and simplified foundations, and the structures used to suspend the circuit breakers may be used for other purposes.

Current Transformers CT's may be accommodated in one of six manners:

Over Circuit Breaker bushings or in pedestals.
In separate post type housings.
Over moving bushings of some types of insulators.
Over power transformers of reactor bushings.
Over wall or roof bushings.
Over cables.

In all except the second of the list, the CT's occupy incidental space and do not affect the size of the layout. The CT's become more remote from the circuit breaker in the order listed above. Accommodation of CT's over isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations.

Isolators

These are essentially off load devices although they are capable of dealing with small charging currents of busbars and connections. The design of isolators is closely related to the design of substations. Isolator design is considered in the following aspects:

Space Factor
Insulation Security
Standardisation
Ease of Maintenance
Cost

Some types of isolators include:

Horizontal Isolation types
Vertical Isolation types
Moving Bushing types

Conductor Systems

An ideal conductor should fulfil the following requirements:

Should be capable of carrying the specified load currents and short time currents.
Should be able to withstand forces on it due to its situation. These forces comprise self weight, and weight of other conductors and equipment, short circuit forces and atmospheric forces such as wind and ice loading.
Should be corona free at rated voltage.
Should have the minimum number of joints.
Should need the minimum number of supporting insulators.
Should be economical.

The most suitable material for the conductor system is copper or aluminium. Steel may be used but has limitations of poor conductivity and high susceptibility to corrosion. In an effort to make the conductor ideal, three different types have been utilized, and these include:

Flat surfaced Conductors
Stranded Conductors
Tubular Conductors

Insulation

Insulation security has been rated very highly among the aims of good substation design. Extensive research is done on improving flashover characteristics as well as combating pollution. Increased creepage length, resistance glazing, insulation greasing and line washing have been used with varying degrees of success.

Power Transformers

EHV power transformers are usually oil immersed with all three phases in one tank. Auto transformers can offer advantage of smaller physical size and reduced losses. The different classes of power transformers are:

o.n.: Oil immersed, natural cooling
o.b.: Oil immersed, air blast cooling
o.f.n.: Oil immersed, oil circulation forced
o.f.b.: Oil immersed, oil circulation forced, air blast cooling

Power transformers are usually the largest single item in a substation. For economy of service roads, transformers are located on one side of a substation, and the connection to switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take precaution against the spread of fire. Hence, the transformer is usually located around a sump used to collect the excess oil. Transformers that are located and a cell should be enclosed in a blast proof room.

Overhead Line Terminations

Two methods are used to terminate overhead lines at a substation.

Tensioning conductors to substation structures or buildings
Tensioning conductors to ground winches.

The choice is influenced by the height of towers and the proximity to the substation. The following clearances should be observed:

VOLTAGE LEVEL	MINIMUM GROUND CLEARANCE
less than 66kV	6.1m
66kV - 110kV	6.4m
110kV - 165kV	6.7m
greater than 165kV

Classification of sub-stsations:

2011-09-18T04:30:00.000-07:00

There are several ways of classifying sub-station. However the two most important way of classifying them are.

According to service requirement
According to constructional features

According to service requirement:

According to service requirement sub-station may be classified into.

Transformer sub-station

Those sub-station which change the voltage level of electrical supply are called TIF s/s.

Switching sub-station :

These sub-station simply perform the switching operation of power line.

Power factor correction S/S :

These sub-station which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s.

Frequency changer S/S :

Those sub-stations, which change the supply frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization

Converting sub-station :

Those sub-station which change a.c. power into d.c. power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.

Industrial sub-station:

Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.

According to constructional features :

According to constructional features, the sub-station are classified as :

1)Outdoor Sub-Station :-

For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to installed the equipment indoor.

2)Indoor Sub-station :-

For voltage upto 11KV, the equipment of the s/s is installed indoor because of economic consideration. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltage upto 66KV

3)Under ground sub-station :-

In thickly populated areas, the space available for equipment and building is limited and the cost of the land is high. Under such situations, the sub-station is created underground.The design of underground s/s requires more careful consideration.

The size of the s/s should be as minimum as possible.
There should be resonable access for both equipment & personal.
There should be provision for emergency lighting and protection against fire.
There should be good ventilation

4) Pole-mounted sub-station :-

This is an outdoor sub-station with equipment installed overhead on H.pole or 4-pole structure. It is the cheapest from of s/s for voltage not exceeding 11KV(or 33KV in some cases). Electric power is almost distributed in localities through such sub-station. The 11KV line is connected to the T/F (11KV/440V) through gang isolator and fuses. The lighting arresters are installed on the H.T. Side to protect the sub-station from lighting strokes. The T/F step down voltage to 400 V, 3 phase, 4 wire supply. The voltage between any two lines is 400 V & between line & neutral is 230V. The oil ckt breaker installed on the L.T. side automatically isolates the mounted sub-station. T/F are generally in the event of fault generally 200KVA T/F is used.

There are mainly three types of sub-stations.
1.step up/down substations-where stepping up or stepping 
down of voltage has done.
2.power factor improvement sub station-where power factor 
is improved by installing capacitor banks or synchronous 
condensors.
3.switching sub stations- Where switching action like 
switching on or off has done for maintenance purposes for 
other reasons.

Transformer: three phase

2011-09-18T04:18:00.000-07:00

Introduction:

Three phase transformers are used throughout industry to change values of three phase voltage and current. Since three phase power is the most common way in which power is produced, transmitted, an used, an understanding of how three phase transformer connections are made is essential. In this section it will discuss different types of three phase transformers connections, and present examples of how values of voltage and current for these connections are computed.

Three Phase Transformer Construction:

A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.

Three-Phase Transformer Connections:

There are only 4 possible transformer combinations:

Delta to Delta - use: industrial applications

Delta to Wye - use : most common; commercial and industrial

Wye to Delta - use : high voltage transmissions

Wye to Wye - use : rare, don't use causes harmonics and balancing problems.

Three-phase transformers are connected in delta or wye configurations. A wye-delta transformer has its primary winding connected in a wye and its secondary winding connected in a delta (see figure 1-1). A delta-wye transformer has its primary winding connected in delta and its secondary winding connected in a wye (see figure 1-2).

Figure 1-1: Wye-Delta connection

Figure 1-2: Delta-Wye connection

Delta Conections:

A delta system is a good short-distance distribution system. It is used for neighborhood and small commercial loads close to the supplying substation. Only one voltage is available between any two wires in a delta system. The delta system can be illustrated by a simple triangle. A wire from each point of the triangle would represent a three-phase, three-wire delta system. The voltage would be the same between any two wires (see figure 1-3).

Figure 1-3:

Wye Connections:

In a wye system the voltage between any two wires will always give the same amount of voltage on a three phase system. However, the voltage between any one of the phase conductors (X1, X2, X3) and the neutral (X0) will be less than the power conductors. For example, if the voltage between the power conductors of any two phases of a three wire system is 208v, then the voltage from any phase conductor to ground will be 120v. This is due to the square root of three phase power. In a wye system, the voltage between any two power conductors will always be 1.732 (which is the square root of 3) times the voltage between the neutral and any one of the power phase conductors. The phase-to-ground voltage can be found by dividing the phase-to-phase voltage by 1.732 (see figure 1-4).

Figure 1-4:

Connecting Single-Phase Transformers into a Three-Phase Bank:

If three phase transformation is need and a three phase transformer of the proper size and turns ratio is not available, three single phase transformers can be connected to form a three phase bank. When three single phase transformers are used to make a three phase transformer bank, their primary and secondary windings are connected in a wye or delta connection. The three transformer windings in figure 1-5 are labeled H1 and the other end is labeled H2. One end of each secondary lead is labeled X1 and the other end is labeled X2.

Figure 1-5:

Figure 1-6 shows three single phase transformers labeled A, B, and C. The primary leads of each transformer are labeled H1 and H2 and the secondary leads are labeled X1 and X2. The schematic diagram of figure 1-5 will be used to connect the three single phase transformers into a three phase wye-delta connection as shown in figure 1-7.

Figure 1-6:

Figure 1-7:

The primary winding will be tied into a wye connection first. The schematic in figure 1-5 shows, that the H2 leads of the three primary windings are connected together, and the H1 lead of each winding is open for connection to the incoming power line. Notice in figure 1-7 that the H2 leads of the primary windings are connected together, and the H1 lead of each winding has been connected to the incoming primary power line.
Figure 1-5 shows that the X1 lead of the transformer A is connected to the X2 lead of transformer c. Notice that this same connection has been made in figure 1-7. The X1 lead of transformer B is connected to X2, lead of transformer A, and the X1 lead of transformer B is connected to X2 lead of transformer A, and the X1 lead of transformer C is connected to X2 lead of transformer B. The load is connected to the points of the delta connection.

Open Delta Connection:

The open delta transformer connection can be made with only two transformers instead of three (figure 1-8). This connection is often used when the amount of three phase power needed is not excessive, such as a small business. It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).

Figure 1-8: Open Delta Connection

Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).
The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.

Closing a Delta:

When closing a delta system, connections should be checked for proper polarity before making the final connection and applying power. If the phase winding of one transformer is reversed, an extremely high current will flow when power is applied. Proper phasing can be checked with a voltmeter at delta opening. If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate 0 volts. If one phase winding has been reversed, however, the voltmeter will indicate double the amount of voltage.
It should be noted that a voltmeter is a high impedance device. It is not unusual for a voltmeter to indicate some amount of voltage before the delta is closed, especially if the primary has been connected as a wye and the secondary as a delta. When this is the case, the voltmeter will generally indicate close to the normal output voltage if the connection is correct and double the output voltage if the connection is incorrect.

Overcurrent Protection for the Primary:

Electrical Code Article 450-3(b) states that each transformer 600 volts, nominal or less, shall be protected by an individual overcurrent device on the primary side, rated or set at not more than 125% of the rated primary current of the transformer. Where the primary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher standard rating shall be permitted. Where the primary current is less than 9 amps, an overcurrent device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current is less than 2 amps, an overcurrent device rated or set at not more than 300% shall be permitted.

Definitions:

Let

x

be the instantaneous phase of a signal of frequency

f

at time

t

Using this, the waveforms for the three phases are

where

V P

is the peak voltage and the voltages on L1, L2 and L3 are measured relative to the neutral.Balanced loads

Generally, in electric power systems, the loads are distributed as evenly as is practical between the phases. It is usual practice to discuss a balanced system first and then describe the effects of unbalanced systems as deviations from the elementary case.

Star connected systems with neutral

This refers to a system with a resistive load

R

between each phase and neutral

Constant power transfer

An important property of three-phase power is that the power available to a resistive load, , is constant at all times.

To simplify the mathematics, we define a nondimensionalized power for intermediate calculations,

Hence (substituting back):

since we have eliminated

x

we can see that the total power does not vary with time. This is essential for keeping large generators and motors running smoothly. Actually, the load need not be resistive for achieving a constant instantaneous power since, as long as it is balanced or the same for all phases, it may be written as

so that the peak current is

for all phases and the instantaneous currents are

Now the instantaneous powers in the phases are

Using angle subtraction formulae:

which add up for a total instantaneous power

Since the three terms enclosed in square brackets are a three-phase system, they add up to zero and the total power becomes

No neutral current

For the case of equal loads on each of three phases, no net current flows in the neutral. The neutral current is the sum of the phase current.

We define a non-dimensionalized current, :

Hence also

Since we have shown that the neutral current is zero we can see that removing the neutral core will have no effect on the circuit, provided the system is balanced. Such connections are generally used only when the load on the three phases is part of the same piece of equipment (for example a three-phase motor), as otherwise switching loads and slight imbalances would cause large voltage fluctuations.

Transmision & Distribution

2011-09-15T02:53:00.000-07:00

Electricity is the main source of power in our daily lives. Convenience and cleanliness are among the most important advantages of using electricity. It is simple to access this virtually unlimited supply of energy at any home in Hong Kong. You simply plug an appliance into the mains supply, turn on the switch and run the appliance as long as you wish. Unlike burning fuels, electricity would not produce any undesirable waste or emission that pollutes the area of use. Without the danger of gas leakage or the storing of inflammable fuels, electricity is also one of the safest power sources.


Fig. 1 Thomas Edison's greatest contribution was perhaps his economically viable model for generating and distributing electric power.	Fig. 2 The fantastic night view of Hong Kong is made possible by an effective electrical transmission and distribution system. (Photo courtesy of HEC)

With the extensive use of electricity, and the wide geographical distribution of users, an effective transmission and distribution system is essential. It is this extensive network that enables electricity to reach almost every family in Hong Kong from the city centre to remote areas. The history of electricity transmission can be dated back to 1883, when Thomas Edison first introduced an economically viable model for generating and distributing electric power. Edison's greatest achievement was perhaps not the invention of the light bulb or any other single application, but the universally applicable electricity transmission system which has lit up the whole world.

Modern electrical transmission and distribution systems are the result of conscientious efforts and design skills of engineers to ensure high energy efficiency and safety. High energy efficiency means the loss of power through transmission is minimized. In this module, the theories and the design of transmission and distribution systems will be discussed in detail.

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Transmission of electricity

Electricity is transmitted mainly through overhead lines or underground cables. Due to the resistance of the transmission wires, there is always some loss of power through the heating effect of current. The electricity transmission systems must be designed in ways which reduce this loss as much as possible.

High transmission voltage

Fig. 3 High voltage transmission lines on a transmission substation

Electricity generated in power stations is raised to a very high voltage for transmission. This is an essential way to reduce heat loss. Do you know why?

Consider electrical power transmitted at voltage V and current I . The power transmitted P is given by

P = IV

(1)

For a fixed amount of electrical power transmitted, a higher voltage would give a smaller current. To see this relationship more clearly, one can write

I	=	P

		V

(2)

Thus for a fixed power transmitted P, the current I is inversely proportional to the voltage V. When the current flows, the power loss by heat P_loss on a segment of wire of resistance R is given by

P_loss = I²RElectric power transmission or "high-voltage electric transmission" is the bulk transfer of electrical energy, from generating power plants to substations located near population centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become high-voltage transmission networks. In the US, these are typically referred to as "power grids" or just "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection; The Eastern Interconnection and the Electric Reliability Council of Texas (or ERCOT) grid.
Historically, transmission and distribution lines were owned by the same company, but over the last decade or so many countries have liberalized the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business.
Transmission lines mostly use three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used only for very long distances (typically greater than 400 miles, or 600 km); submarine power cables (typically longer than 30 miles, or 50 km); or for connecting two AC networks that are not synchronized.^{[citation needed]}
Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in long distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.
A key limitation in the distribution of electricity is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated system of control is therefore required to ensure electric generation very closely matches the demand. If supply and demand are not in balance, generation plants and transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout, such as occurred in California and the US Northwest in 1996 and in the US Northeast in 1965, 1977 and 2003. To reduce the risk of such failures, electric transmission networks are interconnected into regional, national or continental wide networks thereby providing multiple redundant alternate routes for power to flow should (weather or equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.

Diagram of an electrical system.

Main article: History of electric power transmission

New York City streets in 1890. Besides telegraph lines, multiple electric lines were required for each class of device requiring different voltages.

In the early days of commercial electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882, generation was with direct current (DC), which could not easily be increased in voltage for long-distance transmission. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits.
Due to this specialization of lines and because transmission was so inefficient that generators needed to be near their loads, it seemed at the time that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located nearby their loads.
In 1886 in Great Barrington, Massachusetts, a 1 kV alternating current (AC) distribution system was installed. That same year, AC power at 2 kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase alternating currents. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key advantage to the Westinghouse Company in offering a complete alternating current power system for both lighting and power.

Nikola Tesla's Alternating current polyphase generators on display at the 1893 World's Fair in Chicago. Tesla's polyphase innovations revolutionized transmission.

Regarded as one of the most influential electrical innovations, the universal system used transformers to step-up voltage from generators to high-voltage transmission lines, and then to step-down voltage to local distribution circuits or industrial customers. By a suitable choice of utility frequency, both lighting and motor loads could be served. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC to be provided where needed. Generating stations and loads using different frequencies could be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency, a lower cost for the consumer and increased overall use of electric power.
By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost.^[3]
The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 25 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt.
Voltages used for electric power transmission increased throughout the 20th century. By 1914, fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV.
The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, where large electrical generating plants were built by governments to provide power to munitions factories. Later these plants were connected to supply civil loads through long-distance transmission.

3D Laser Scan - LiDAR - Electric Towers at D Street Bridge, Petaluma

With LiDAR we captured the towers and the powerlines in complete safety from over 200 feet away. Electrical substations, cellular towers, powerlines, tree interference, clash detection, all this can be achieved remotely......

Alabama Electric Co-op Employee Disaster Relief Fund

The storms are over and the lights are coming back on, but the electric co-op family still needs your help. This video from Alabama Electric shows before and after images of Tuscaloosa, along with line crews working to restore......

Avista Mock Electrical Accident April 2010

In mid-April 2010, Avista Utilities, Spokane, Washington, staged a mock electrical accident in a rural area outside of Addy, Washington. This video, designed for Avista employees, captured events on site. Avista Utilities provides electric service to 355,000 customers. ...

LineScout - 2010 Edison Award Winner

From VisuallyIntelligent: Hydro-Quebec and BCTC received the 2010 Edison Award from the Edison Electric Institute in the International Affiliate category for their partnership on the field deployment and enhancement of a high performance robot called LineScout......

Baltimore Gas & Electric on 2010 Blizzards

From Constellation Energy: BGE reviews the back-to-back blizzards that dumped feet of snow on the BGE service area in central Maryland in 2010. Hats off to BGE's customers for being prepared and patient as the company worked to restore power......

Safety Rap

From Dominion Virginia Power: Dominion Lineman Dennis McDade teaches safety with a rap that he and his fellow lineman crew have performed at various industry events around the country ......

power station

2011-08-10T03:30:00.000-07:00

A power station (also referred to as a generating station, power plant, or powerhouse) is an industrial facility for the generation of electric energy.
At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available and on the types of technology that the power company has access to.

History

The world's first power station was built by Sigmund Schuckert in the Bavaria town of Ettal and went into operation in 1878 The station consisted of 24 dynamo electric generators which were driven by a steam engine. It was used to illuminate a grotto in the gardens of Linderhof Palace.
The first public power station was the Edison Electric Light Station, built in London at 57, Holborn Viaduct, which started operation in January 1882. This was an initiative of Thomas Edison that was organized and managed by his partner, Edward Johnson. A Babcock and Wilcox boiler powered a 125 horsepower steam engine that drove a 27 ton generator called Jumbo, after the celebrated elephant. This supplied electricity to premises in the area that could be reached through the culverts of the viaduct without digging up the road, which was the monopoly of the gas companies. The customers included the City Temple and the Old Bailey. Another important customer was the Telegraph Office of the General Post Office but this could not be reached though the culverts. Johnson arranged for the supply cable to be run overhead, via Holborn Tavern and Newgate.

Thermal power stations

Rotor of a modern steam turbine, used in power station.

Main article: Thermal power station

In thermal power stations, mechanical power is produced by a heat engine that transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. Not all thermal energy can be transformed into mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses by-product heat for the desalination of water.
The efficiency of a steam turbine is limited by the maximum temperature of the steam produced and is not directly a function of the fuel used. For the same steam conditions, coal, nuclear and gas power plants all have the same theoretical efficiency. Overall, if a system is on constantly (base load) it will be more efficient than one that is used intermittently (peak load).
Besides use of reject heat for process or district heating, one way to improve overall efficiency of a power plant is to combine two different thermodynamic cycles. Most commonly, exhaust gases from a gas turbine are used to generate steam for a boiler and steam turbine. The combination of a "top" cycle and a "bottom" cycle produces higher overall efficiency than either cycle can attain alone.

Classification

CHP plant in Warsaw, Poland.

Geothermal power station in Iceland.

Coal Power Station in Tampa, United States.

Thermal power plants are classified by the type of fuel and the type of prime mover installed.

By fuel

Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine. A coal-fired power station produces electricity by burning coal to generate steam, and has the side-effect of producing a large amount of carbon dioxide, which is released from burning coal and contributes to global warming. About 50% of electric generation in the USA is produced by coal fired power plants
Nuclear power plants^[6] use a nuclear reactor's heat to operate a steam turbine generator. About 20% of electric generation in the USA is produced by nuclear power plants.
Geothermal power plants use steam extracted from hot underground rocks.
Biomass-fuelled power plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
Solar thermal electric plants use sunlight to boil water, which turns the generator.

HydroelectricityDams built to produce hydroelectricity impound a reservoir of water and release it through one or more water turbines, connected to generators, and generate electricity, from the energy provided by difference in water level upstream and downstream.

Pumped storage

A pumped-storage hydroelectric power plant is a net consumer of energy but can be used to smooth peaks and troughs in overall electricity demand. Pumped storage plants typically use "spare" electricity during off peak periods to pump water from a lower reservoir or dam to an upper reservoir. Because the electricity is consumed "off peak" it is typically cheaper than power at peak times. This is because the "base load" power stations, which are typically coal fired, cannot be switched on and off quickly so remain in service even when demand is low. During hours of peak demand, when the electricity price is high, the water pumped to the high reservoir is allowed to flow back to the lower reservoir through a water turbine connected to an electricity generator. Unlike coal power stations, which can take more than 12 hours to to start up from cold, the hydroelectric plant can be brought into service in a few minutes, ideal to meet a peak load demand. Two substantial pumped storage schemes are in South Africa, one to the East of Cape Town (Palmiet) and one in the Drakensberg, Natal.

solar

2011-08-10T03:00:00.000-07:00



Electricity from:
Solar Energy

The ultimate source of much of the world's energy is the sun, which provides the earth with light, heat and radiation. While many technologies derive fuel from one form of solar energy or another, there are also technologies that directly transform the sun's energy into electricity.

The sun bathes the earth in a steady, enormous flow of radiant energy that far exceeds what the world requires for electricity fuel.

Since generating electricity directly from sunlight does not deplete any of the earth's natural resources and supplies the earth with energy continuously, solar energy is a renewable source of electricity generation. Solar energy is our earth's primary source of renewable energy.
There are two different approaches to generate electricity from the sun: photovoltaic (PV) and solar-thermal technologies.

Initially developed for the space program over 30 years ago, PV, like a fuel cell, relies upon chemical reactions to generate electricity. PV cells are small, square shaped semiconductors manufactured in thin film layers from silicon and other conductive materials. When sunlight strikes the PV cell, chemical reactions release electrons, generating electric current. The small current from individual PV cells, which are installed in modules, can power individual homes and businesses or can be plugged into the bulk electricity grid.
Solar-thermal technologies are, more or less, a traditional electricity generating technology. They use the sun's heat to create steam to drive an electric generator. Parabolic trough systems, like those operating in southern California, use reflectors to concentrate sunlight to heat oil which in turn creates steam to drive a standard turbine.

Two other solar-thermal technologies are nearing commercial status. Parabolic dish systems concentrate sunlight to heat gaseous hydrogen or helium or liquid sodium to create pressurized gas or steam to drive a turbine to generate electricity. Central receiver systems feature mirrors that reflect sunlight on to a large tower filled with fluid that when heated creates steam to drive a turbine.

Solar Electricity Basics

The three most common types of solar-electric systems are grid-intertied, grid-intertied with battery backup, and off-grid (stand-alone). Each has distinct applications and component needs.

Grid Intertied Solar-Electric Systems
Also known as on-grid, grid-tied, or utilityinteractive (UI), grid-intertied solar-electric systems generate solar electricity and route it to the electric utility grid, offsetting a home’s or business’ electrical consumption and, in some instances, even turning the electric meter backwards. Living with a grid-connected solar-electric system is no different than living with grid power, except that some or all of the electricity you use comes from the sun. In many states, the utility credits a homeowner’s account for excess solar electricity produced. This amount can then be applied to other months when the system produces less or in months when electrical consumption is greater. This arrangement is called net metering or net billing. The specific terms of net metering laws and regulations vary from state to state and utility to utility. Consult your local electricity provider or state regulatory agency for their guidelines.
The following illustration includes the primary components of any grid interie solar electric system. See our Solar Electric System Components section for an introduction to the function(s) of each component.

See also the following Home Power feature articles:
Energy Smarts-Efficiency Gains + Solar Electricity
Creating A Brighter Future
Getting Off the Lifetime Utility Payment Plan: Grid-Connected PV

Grid-Intertied Solar-Electric Systems with Battery Backup
Without a battery bank or generator backup for your gridintertied system, when a blackout occurs, your household will be in the dark, too. To keep some or all of your electric needs (or “loads”) like lights, a refrigerator, a well pump, or computer running even when utility power outages occur, many homeowners choose to install a grid-intertied system with battery backup. Incorporating batteries into the system requires more components, is more expensive, and lowers the system’s overall efficiency. But for many homeowners who regularly experience utility outages or have critical electrical loads, having a backup energy source is priceless.
The following illustration includes the primary components of any grid intertied solar electric system with battery backup. See our Solar Electric System Components section for an introduction to the function(s) of each component.

See also the following Home Power feature articles:

Eight Years of Solar Electricity: and Counting...
Walking the Talk: Energy Group Gets Solarized

Off-Grid Solar-Electric Systems
Although they are most common in remote locations without utility grid service, off-grid solar-electric systems can work anywhere. These systems operate independently from the grid to provide all of a household’s electricity. That means no electric bills and no blackouts—at least none caused by grid failures. People choose to live off-grid for a variety of reasons, including the prohibitive cost of bringing utility lines to remote homesites, the appeal of an independent lifestyle, or the general reliability a solar-electric system provides. Those who choose to live off-grid often need to make adjustments to when and how they use electricity, so they can live within the limitations of the system’s design. This doesn’t necessarily imply doing without, but rather is a shift to a more conscientious use of electricity.
The following illustration includes the primary components of any off grid solar electric system. See our Solar Electric System Components section for an introduction to the function(s) of each component.

See also the following Home Power feature articles:
Postmodern PV Pioneers
Solar Comfort in the Idaho Wilderness: Off-Grid PV
All Creatures Under the Sun—My Solar Powered Barn
Green Half-Acre: Off-Grid Country Living – In the City

System Components

Electrical

2011-08-08T22:17:00.000-07:00

The word is from the New Latin ēlectricus, "amber-like", coined in the year 1600 from the Greek ήλεκτρον (electron) meaning amber (hardened plant resin), because static electricity effects were produced classically by rubbing amberElectricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge.Electrical phenomena have been studied since antiquity, though advances in the science were not made until the seventeenth and eighteenth centuries.

Adding microprocessors to field instruments has transformed them into data acquisition systems and transmission terminals. Now technicians can use well-designed software to see what is happening inside plant instrumentation.

When asset management software is used, devices that need service can signal the operator or maintenance shop before they fail, lowering maintenance or repair costs and reducing the risk of unscheduled shutdown.

Instrumentation technicians responsible for reliable operation of thousands of field devices commonly receive calls from operators who are having trouble controlling a piece of production equipment, a reactor for example. If the plant is equipped with state-of-the-art asset management software, the technician can quickly determine which devices are associated with that reactor and which ones could be causing problems. Rather than going into the plant, the technician can evaluate each smart device from a personal computer in the instrument shop. For example, a quick computer check of the condition of each field device serving that reactor might reveal a travel deviation alert from one control valve, indicating a significant difference between the valve set point and its actual position--a situation requiring attention. The technician is spared long hours of checking out individual devices on the plant floor and knows exactly what must be done to correct the problem.
Although the poorly functioning valve in the reactor is hypothetical, the solution to this common problem is not. Up to 60 percent of instrument maintenance labor dollars are spent on devices where no problem exists or for routine checks to verify the condition of properly functioning devices. These requests often occur because an operator has no means of checking on the health and validity of field instrumentation and therefore calls the instrument shop when a problem is suspected. The most time-consuming and expensive service calls are those that conclude with no problem found.
Now, instrumentation technicians have a way to see what is happening inside plant instrumentation by using well-designed software in conjunction with intelligent field devices and new standard communications. These technologies allow technicians to

Make sure field devices deliver optimum performance
Use predictive maintenance to maximize service and repair resources
Perform configurations and calibrations in half the normal time
Document maintenance as required by industry regulators.

From a broader perspective, the integration and use of information acquired from intelligent field devices eliminates unexpected shutdowns, reduces downtime, and improves overall equipment performance.
Intelligent field devices
The process control system is designed for tight control of valves, motors, heaters, etc., in real time to manage feed stocks and make quality products. However, to increase the reliability and maintainability of plant instrumentation and process equipment and to lower the mean time between failures and time to accomplish repairs, a vast amount of information is needed about the condition and status of field equipment. The increase in smart instruments throughout the process industry makes it both practical and economical to use all the information they generate. It is estimated that 30 times more information is available from smart instruments than the simple variables required for process control.
Consider a smart pressure transmitter. Beyond providing basic pressure data, it can produce information relative to an overpressure condition (which can lead to inaccurate readings), an overtemperature condition (which can cause premature failure), a loss of signal, a stuck signal, and more. In the case of a control valve, the number of times the valve has cycled is a key indicator of how much work it has done and can be used in predicting its useful life. As the reactor example mentioned previously suggests, the internal position of a valve stem versus where it is supposed to be is critically important to the reliable operation of that piece of equipment. Smart valves routinely provide this kind of information.
The accuracy of field measurements and the reliability of field instrumentation are influenced by internal conditions that can be reported only by intelligent devices, and those conditions can have a direct impact on the availability or reliability of the process itself. Demand to make use of such data is growing. The acquired information must be made available well beyond the process control system.
Communicating the data
Automation architectures are evolving to deliver information from intelligent field devices, acting as information servers, around the control system to computers where the information can be used for diagnostic and maintenance purposes, for reliability analysis, for purchasing and inventory control systems, and for the overall management of plant assets.
General-purpose field communications protocols capable of transmitting large volumes of information for these purposes are currently in use. Today, the Highway Addressable Remote Transducer (HART) protocol offers the broadest range of user benefits and is supported by a wide range of vendors. Unlike proprietary communications technologies that lock users into field devices from a single manufacturer, the HART protocol is an open communications standard that works alongside any control system without interrupting the flow of process data.
Whereas HART transmits both analog and digital signals, the emerging Foundation Fieldbus protocol will be used with all-digital systems. Profibus is another new protocol under development. These sophisticated protocols are capable of carrying complex messages. Their use enables technicians to examine an instrument's self-diagnostics and also run extensive diagnostics programs on each device.
Using the information
For field data to be turned into useful knowledge, a reliable method of receiving, processing, and presenting it is required. Maintenance personnel require information on the condition of equipment, while operations personnel want other information and purchasing or inventory control requires something different. Each group needs information tailored to specific requirements. For example, the enhancement most desired by maintenance supervisors is seamless access to maintenance-related data. Advanced software applications are performing that function now.
Monitoring the intelligent devices installed in a plant and viewing their self-diagnostics probably are the most common functions of the instrumentation technician. Most smart instruments provide extensive information about their own health. Access to on-line device status provides a way to monitor and ensure proper device performance. The technician calls up the information device by device to see if any faults are flagged.
Automatic alert monitoring also is available to automatically scan devices on a user-determined schedule. If device problems exist, the information is posted to an alert monitor list. If no specific problems are found but something is suspected, it may be possible, depending on the software package, to use on-demand diagnostics. Certain operating parameters may be changed slightly to see if the device responds. For example, safety valves might be moved slightly to determine that the valve is not stuck and actuator pressures are sufficient to move the valve.
Automatic scanning of devices requires a higher level of sophistication. Such scans may uncover devices that need immediate attention, or they might generate lists of devices due for calibration. Some software is designed to interface with computerized maintenance management systems that track maintenance schedules and alert technicians when maintenance is due. Sophisticated software that identifies a need for instrument calibration combined with intelligent calibrators may also be able to automatically perform configurations and calibrations in a fraction of the time required for conventional calibrations.
Advanced, maintenance-oriented software permits maximum use of the information transmitted through a general-purpose protocol. The more advanced software receives the data, organizes it into open databases, and makes it available to other applications within the organization. Predictive maintenance is one of the key attributes of such systems. When the software makes a prediction about the expected service life of any piece of equipment, knowledgeable decisions can be made as to when repair or replacement will cause the least disruption to production.
Decisions such as run until failure, continue to run at reduced load, run until a scheduled shutdown, or repair immediately are based on highly reliable information, including the importance of the piece of equipment to the process. This approach helps focus limited resources on problems that warrant attention rather than wasting time on devices expected to continue working properly. In this way, instruments and control systems can be maintained in a high state of continued reliability.
What to look for
It is not difficult to obtain a communications and software package capable of accessing information generated by smart instruments, but not all software is created equal. First, potential buyers should be sure that nothing done with this ancillary information degrades the performance of the control system. Users of heavily loaded control systems may want to reserve all remaining control system capacity for future process control needs. Solutions available from various vendors of HART-compatible devices will carry the information through multiplexing, through intrinsic safety barrier panels, and through intelligent termination panels where information can be accessed by computers without using control system communications and computation resources.
However, data acquisition by itself does not provide the cost and time-saving benefits most maintenance managers are seeking. A flexible, maintenance-oriented, modular software platform that is both expandable and scalable must be found. Such software can provide access to a single device or to highly integrated client/server solutions for large multi-instrument plants.
In addition, the software should use device descriptions (DD), which are essentially files of field device attributes residing in the host application. The DD is essential to the interoperability of devices, allowing users to choose those that best meet their needs, regardless of manufacturer. As each new device is introduced, the description of attributes and capabilities is loaded into the host, providing complete data about each device in the plant. The DD technology is fundamental to the HART and Fieldbus protocols.
To be most useful, the software must accommodate the broadest range of field devices. It should support virtually all current HART and future Fieldbus devices, not just those manufactured by one company. In other words, the company should choose a technology to accommodate the widest range of applications needed to meet current and future plant and business needs. A number of good solutions are available. However, many tend to focus on a specific vendor's field instrumentation, which does not generally solve the problem of the large multi-instrument user.
Finally, it must be possible to integrate data from the field management software with any computerized maintenance management system the plant is running. An integrated solution is essential to realize the greatest value from information derived from intelligent field devices, transported through a state-of-the-art data highway, and processed in sophisticated, maintenance-oriented software.
Estimates show that one-third of all dollars spent on maintenance are wasted because of unnecessary work or ineffective practices. This trend can be reversed through the application of information generated by intelligent field devices and acceptance of the concept of predictive maintenance. The result can be a significant contribution to a plant's profitability.
It is not necessary to totally restructure maintenance practices. A plant can start small and expand the use of process floor information as needs dictate and budgets allow. The key is to begin building a database from the smart devices now in operation and expand the use of the available information as the number of such devices grows. A scalable and expandable platform can grow into a plantwide system that supports the reliability and maintainability of all field instrumentation. MT

LOHAR

voltage and current sources

Voltage and Current Sources

Voltage is the Cause, Current is the Effect

Voltage, V

Voltage at a point and 0V (zero volts)

Zero volts for circuits with a dual supply

Current, I

Voltage and Current for components in Series

Voltage and Current for components in Parallel

RMS value

RMS of common waveforms

Uses

Average electrical power

Amplifier power efficiency

Root mean square speed

Root mean square error

RMS in frequency domain

Relationship to the arithmetic mean and the standard deviation

frequency

Waveform Conversion, Part I - Sine to Square

Waveform Conversion, Part II - Square to Sine

Two-Diode Odd-Order Frequency Multipliers

Tuning Range

Using Precision Oscillators with External References

hydraulic pump

Variable flow Compensator hydraulic systems

Air over oil Pump systems

High Pressure coolant systems buying considerations, efficiency, and coolant system applications

Benefits of high pressure coolant

The basic hydraulic pump system

Hydraulic pressure switch considerations

Hydro electricity

Disadvantages of hydroelectricity

electric car

electric charge

Charge

The Law of Conservation of Charge

Electrostatic charging

The parallel between gravity and electrostatics

ampere's law

Ampere's Law Applications

solenoids

Magnetic field and vector potential for finite continuous solenoid

Hydraulics

DC sevomotors

electric dipole

Dipole Moment

electri current

DC motors

Brushless

electrical energy produced

transformer oil

Explanation

Transformer Oil Testing

electrical symbols

Transformer Winding

Induction law

Ideal power equation

Detailed operation

Practical considerations

[ Leakage flux

Design of sub-stations:

Classification of sub-stsations:

Transformer: three phase

Definitions:

Star connected systems with neutral

No neutral current

Transmision & Distribution

power station

History

Thermal power stations

Classification

By fuel

Pumped storage

solar

Electrical

Adding microprocessors to field instruments has transformed them into data acquisition systems and transmission terminals. Now technicians can use well-designed software to see what is happening inside plant instrumentation.