Complete Guide To Electrical Machine Design

Power System Stabilty : Basic Concepts and Important Terms

2011-05-10T15:04:00.001+05:30

This article deals with the basic concept of power system stabilty and types of power system stability like steady state stability, transient state stability and transient stability limit etc.

Power System Stability

The term stability is closely related with synchronism. Synchronous generators or alternators and synchronous motors have a tendency to remain in synchronism or in step with each other. During system disturbance such as sudden increase in load, sudden switching, power swings etc. The synchronous machines experience oscillations of torque angle about the mean position. However, the synchronous machines have inherent tendency or maintaining synchronism. Loss of synchronism is called loss of stability.

What is stability ?

Ability of synchronous machine or part of a system to develop restoring forces equal to more than disturbing forces so as to remain in synchronism is called stability.
The disturbance may be sudden or the change in load may be very gradual. Accordingly, there are two distinct terms called transient stability and steady state stability.
The term steady state stability refers to ability of a system or its part to respond to small, gradual change in power at a given point of the system. Steady state stability limit is the maximum possible power that can be transferred at a given point of the system without loss of synchronism, with very gradual increase in power.
The term transient stability limit refers to the maximum possible power that can be transferred at a given point of the system without loss of synchronism for given sudden large change in power.
The concept of stability can well be explained by means of two machine system. The system is used as a conceptual aid.

Losses in DC Machines

2011-01-13T07:40:00.005+05:30

This article discuss various types of losses that occurs in DC Machines. It include Copper losses, iron losses, brush losses, mechanical losses, stray-load losses.

Hello Friends,
We know that no system is perfect in this world. Every system has some kind of unwanted loss of input. The form of this drop may vary from system to system but in every system it is called a loss.
Similar is the case with the DC Machines either motor or generator.
The whole input power is never converted into the output power. The difference between input power and output power is called loss.
In a DC Machine losses are classified into five main categories
Copper Losses or Electrical Losses
Core Losses or Iron Losses
Brush Losses
Mechanical Losses
Stray-Load Losses

Copper Losses or Electrical Losses

These losses are the winding losses because these occurs in the winding of the machine. The copper or electrical are present because of the resistance of the winding. Currents flowing through these windings produce ohmic losses (i.e. I²R losses). The windings that may be present addition to Armature winding are the field windings, interpole and compensating windings.

Armature current losses = I_a²R_a, where I_a is Armature current and R_a is Armature Resistance. These losses are about 30% of total full-load losses.
Copper losses in the shunt field of a shunt machine =I_sh²R_sh where I_sh is the current in the shunt field and R_sh is the resistance of the shunt field winding. The shunt regulating resistance is included in R_sh.
Copper loss in the series field of a series machine = I_se²R_se where I_se is the current through the series field winding and R_se is the resistance of the series field winding.
In a compound machine, both shunt and series field losses occur. There losses are about 20% of full load losses.
Copper loss in interpole windings =I_a²R_i where R_i is resistance of interpole winding.
Copper loss in compensating windings =I_a²R_c where R_c is resistance of compensating winding.

Core Losses or Iron Losses

These losses also called magnetic losses. These are of two types viz. Hysteresis and Eddy-current losses. Since DC machines are usually operated at constant speed and constant flux density, these losses are almost constant. These are about 20% of full-load losses.

Brush Losses

There is a power loss at the brush contacts between the copper commutator and the carbon brushes. In practice, thin loss depends upon the brush contact voltage drop and the Armature current I_a. It is given by
P_BD=V_BDI_a
The voltage drop across a set of brushes is approximately constant over a large range of Armature currents. Unless stated otherwise, the brush voltage drop is usually assumed to be about 2V. The brush droploss is, therefore, taken as 2I_a.

Mechanical Losses

The losses associated with mechanical effects are called mechanical losses. They consist of bearing friction loss and windage loss. Windage losses are those associated with overcoming are friction between the moving parts of the machine and the air inside the machine for cooling purposes. These losses are usually very small.

Stray-Load Losses

Stray-load loss consists of all losses not covering above. These are the miscellaneous losses that result from such factors as (i) the distortion of flux because of Armature reaction, (ii) short circuit currents in the coil, undergoing commutation etc. These losses are very difficult to determine. The indeterminate nature of the stray-load loss makes it necessary to assign reasonable value. For most machines stray losses are taken by convention to be 1% of full load output power.
So friends this was all about various losses occurs in a DC Machine and here I think every kind of loss was covered. If any loss is still left, feel free to add a comment.

Linear Induction Motor : Working, Application and Construction

2010-07-11T13:22:00.003+05:30

Linear Induction motor is normally referred as Linear motor. It is a special purpose motor that is used for linear motors. This is interesting development of a conventional induction motor.
It performs linear motion instead of rotary motion as in a conventional induction motor.

Construction of a Linear Motor or Linear Induction Motor

There are several ways and types of construction of a Linear Motor or Linear Induction Motor. The simplest form of construction of a Linear Motor is as simple as a three phase induction motor. It has three phase winding housed in slots in a field system. It is simply the primary winding on a stator in case of an induction motor. This is obtained if we cut the stator of an induction motor from middle.
In case of a moving object like in a train the primary winding is mounted on the body of vehicle.
The rotor is made by aluminium or copper plates in parallel. In order to complete the flux path a ferromagnetic material is placed with the plates. As the primary is on vehicle or object and secondary is in form of plates so they will have unequal length.
For larger distance primary is kept small and for very small and limited distance secondary is kept small.
Normally two sided primary winding is used. In this configuration the two field system, one on either side of secondary are used.

Working of a Linear Induction Motor

When the primary is excited by a balanced three phase supply, a rotating electromagnetic flux is induced in primary. The synchronous speed of the field is given by the equation :

n_s=2 f_s/p,
Here, f_s is supply frequency in Hz,
p is the number of poles,
n_s is the synchronous speed of the rotation of magnetic field in revolutions per second.
The developed field will results in a linear travelling field, the velocity of which is given by the equation,
v_s=2 t f_s meter per second
here, v_s is velocity of the linear travelling field,
t is the pole pitch.
For a slip of s, the speed of conducting slave in a linear motor is given by
v_r=(1-s)v_s

Application of Linear Induction Motor or LIM

Although these motors are not frequently used. There are only a few instances where the linear motor is used or is utilized in a proper way. It seems that these motors are technically, feasible but due to economical point of view these motors are not frequently used.

However the possible applications of a Linear Induction Motor are listed below :

Application for Stationary Field System
- Automatic sliding doors in an electrical train,
- Metallic belt conveyer,
- Mechanical handling equipment, such as propulsion of a train of tubs along a certain route,
- Shuttle-propelling application.
Applications for the moving field system
- High and medium speed applications have been tried with linear motor propulsion of vehicles with air cushion or magnetic suspension.
- High speed application as a travelling crane motor where the field system is suspended from loist.

Problems Associated with LIM

The Linear Induction Motor is usually taken as an induction motor with three phase balanced excitation, but this comparision is just an approximation. There are certain effects, peculiar to a linear motor, which should be considered for accurate analysis. Of there, two important effects are :
1. Transverse edge effect and
2. End effect

Induction Machines - Basics

2010-07-03T12:58:00.007+05:30

This article includes basics of induction motor, principle of induction motor, construction and design of induction motor including both three phase and single phase induction motor.

Induction Motor : -

An induction motor is a motor that uses the principle of induction of e.m.f. in an inductor due to the effect of flow of current in other coil that is magnetically attached but physically insulated with the coil. For detail on induction please visit Basics of Electromagnetic Induction.
As I said above induction motor is a motor or device that operates on the principle of induction and converts electrical energy into mechanical energy, which is essentially function of a motor.
Basically I have divided motors in four groups for better understanding of the field machine design. These are :

Principle of Operation of Induction Machines

The principle of operation of induction machine is something similar to that of operation of Transformer.
First of all a supply is given to the machine and due to this supply a rotating magnetic field i.e. A field whose magnetic axis of rotation changes with respect to time is produced.
In three phase induction motors this rotating magnetic field is produced due to the simultaneous maximum of phase voltages. It means that voltage in phase R will reach its maximum first say at wt=90 degrees.
Where w is angular frequency. Now the next phase Y is 120 electrical degree lags the R phase so it will reach its maximum at wt=210 degrees and similarly B phase will reach its maximum at wt=330 degrees.
Now as voltage reaches its maximum at different time levels so the current will also maximum at corresponding time values and thus the flux produced will have a rotating magnetic field with axis changing with time.
Due to this flux similar flux will induced in rotor as in secondary of a transformer due to inducion.
This rotor flux will induced e.m.f. in rotor and due to this e.m.f. current will produced as the rotor bars are short circuited. This current will produce the torque and the rotor tends to follow the stator magnetic field, which is rotating. Thus rotor will also start rotating.

Classification of Induction Machines

There are three main criterion for classifying induction machines. There are :

Based on Supply
- Single Phase Induction Motors
- Three Phase Induction Motors
Based on Type of Rotor
- Squerell Cage Induction Motors
- Slip Ring Induction Motors
Form of Energy Produced
- Induction Generator
- Induction Motor

Based on Supply

This classification consider the number of phases in the machine.

Single Phase Induction Machine

If in an Induction machine there is only one phase of supply, the machine is called single phase induction machine.

Three Phase Induction Machines

If the input of electrical motor or output of induction generator has three phases then this is called a three phase induction machine.

Based on Type of Rotor

There are many rotor type in induction machines like squirell cage, slip rings, double cage and deep bar rotors but among above said following two are very important.

Squirell Cage Induction Motors

If the rotor have simply bars on it and there bars are short circuited with the help of end rings, the rotor and hence the machine is called squirell cage type. Due to low cost there are widely used.

Slip Ring Induction Motors

If in an induction motor's rotor external resistances are connected, it is called slip ring type induction motor. It is more costly and only 10% of total induction motors are slip ring type.

Form of Energy Produced

This classification consider the input energy form and output energy form.

Induction Generator

The induction generator is the machine which produce electrical energy from mechanical energy. It is not practically used as a source of electrical energy or power generation but commonly used in cogeneration. It is used where speed is varying and its efficiency of energy conversion is small as compare to synchronous generators.

Induction Motor

Induction motor is the induction machine which convert electrical energy into mechanical energy. It takes the electrical power as input and produce mechanical power along its rotor shaft. There are widely used in industrial and domestic applications.

Synchronous machines - Applications, Advantages and Disadvantages

2010-02-01T09:10:00.001+05:30

Application of synchronous machines, advantages of synchronous machine and disadvantages of Synchronous machines will be discussed in this article.

Application of Synchronous Machines

Synchronous machines are very important machinery in electrical engineering. Following are the important applications of a synchronous motors or machines : -

It is used in power houses and sub-stations in parallel to the bus bars to improve power factor. For this purpose it is run without mechanical load on it and over excited.
In factories having large number of induction motors or transformers operating at lagging power factor, it is used for improving power factor.
It is used to generate electric power at power station, one of the most important application of synchronous machines.
it is used to control the voltage at the end of transmission line by varying its excitation.
It is also used in rubber mills, textile mills, cement factories, air compressors, centrifugal pumps which requiring constant speed.
It is used in motor genreator sets requiring constant speed.

Synchronous motors are mostly use to drive continuously operating and constant speed equipment such as centrifugal pumps, fans, blowers, ammonia and air compressors, motor-generator sets etc.

Advantages of Synchronous Machine

Being power factor improvement appliance synchronous machines posses lots of positive points. Some very important advantages are : -

There motors can be made to operate at a leading power factor and thereby improve the power factor of an industrial plant from laggoi to one that is close to unity.
It gives constant speed from no load to full load.
Electro-magnetic power varies linearly with voltage.
These motors can be constructed with wider air gaps than induction motors, which make then better mechanically.
These motors operates at higher efficiency, especially in the low speed unity power factor range.

Disadvantages of Synchronous Machines

Everything has its own advantages and disadvantages and Synchronous machine is not an exception. Synchronous machines has some disadvantages. Some of noticeable disadvantages or demarits of synchronous machines are as : -

It cannot be used for variable speed job as there is no possibility of speed adjustment.
It requires d. c. excitation which must be supplied from external source.
It cannot be started under loaded condition. Its starting torque is zero.
It has a tendency to hunt.
It is not possible for places where frequent starting is required.
It may fall out of synchronism and stop when over loaded.
Collector rings and brushes are required.

Working of Transformer : - Technical Aspects

2010-01-16T08:42:00.002+05:30

Hello Friends,
In one of my previous post Basics of Transformer, I discussed some basics of transformer. A quick revision is here : -

Static Device - No rotating part.
Can raise or drop voltage level.
Winding and Core are essential parts with a.c. Supply.

In my current post I will discuss technical aspects behind working of a transformer.
A transformer may consists of two windings called primary and secondary windings. Primary winding is the winding that is connected to supply voltage and the secondary winding is the winding that is connected to load.

Working of Transformer : -

As I said a transformer has two windings viz. Primary and secondary winding.
When we apply an alternating voltage say V1 to the primary winding of the transformer, an alternating flux ¤(fi) is set up in the core. This alternating flux links both the windings and induces e.m.f.s E1 and E2 in them according to Faraday's law of electromagnetic induction. The e.m.f. E1 is termed as primary e.m.f. and e.m.f. E2 is termed as secondary e.m.f.
Clearly, E₁=-N₁(d¤/dt),
and E₂=-N₂(d¤/dt)
thus E₂/E₁ = N₂/N₁
Here one thing should be noted that the magnitude of induced voltages depends upon number of turns in both windings. Thus if number of turns in primary are more than that in secondary more voltage will be induced in primary and transformer will be step down kind and if number of secondary is more secondary e.m.f. E₂ will be more and transformer will be said to step up transformer.

E.M.F. Equation of a transformer : -

E.M.F. Equation of any electrical appliance is very important, thus it is also of prime importance for a transformer.
Consider that an alternating voltage V₁ of frequence f is applied to the primary of a transformer. Due to this alternating voltage, alternating flux ¤ produced by the primary can be represented as :
¤ = ¤_m sin wt
The instantaneous e.m.f. e₁ induced in the primary is given by,
e₁ = -N₁(d¤/dt)
=> e₁ = - N₁ {d(¤_m sin wt)/dt}
=> e1 = -wN₁¤_mcos(wt)
=> e1 = -2*pi*f*N₁¤_mcos(wt)
=> e1 = -2*pi*f*N₁¤_msin(wt-90)
It is clear from the above equation that maximum value of induced e.m.f. in the primary is
E_m1 = 2*pi*f*N₁*¤_m
=> E₁ = E_m1/(2)^1/2
=> E_m1 = 4.44 f N₁¤_m.

Types of Synchronous Machines

2009-12-02T07:07:00.003+05:30

Friends,
As you know machines are basically divided into two types viz. Motor and Generator, but synchronous machines operating on general power supply networks may be divided into five types due to their special features.
These five categories are : -

Hydro-Generator
Turbo-Alternator
Engine Driven Generators
Motors
Compensators

Hydro-Generator : -
The Synchronous generators driven by the water turbines are known as hydro-generators or water wheel generators. They have ratings upto 750 MW and are driven at speed ranging from 100 to 1000 rpm. This is due to the salient pole construction of the hydro-generators.

Turbo-Alternator : -

They are driven by steam turbines. Since the efficiency of steam turbine is high at large speeds, the turbo-alternators are designed for speed upto 3000 rpm. As speed of turbo-alternator is high so the rating. It is upto 1000 MW.
Engine Driven Generators : -

There generators are driven by different forms of internal combustion engines at speed upto 1500 rpm and rating upto 20 MW. Generators driven by gas turbines have higher speed and higher power ratings.
Motors : -

Synchronous motor are kind of motors i.e. Device that convert electrical energy into mechanical energy. A synchronous motor may be either plain synchronous machine or synchronous induction machine.
The plain synchronous machine with salient poles is commonly used. Synchronous motors have some definite advantage over induction motors and these include constant speed operation, power factor control and high operating efficiency. But these advantages are on account of high initial capital and more maintainance charges at normal conditions but at high power low speed operation these synchronous motors are cheaper than that of traditional domaistic motors or induction motors. The common application criteria of synchronous motors include constant speed drives for compressors, blowers, fans, low head pumps etc.

Compensators : -

Synchronous compensators are used for control of reactive power in power supply networks. The main concept of improving power factor is to force current in synchronous machine to lead as in capacitor and thus it cancel the inductive components and improve power factor.
Commonly a synchronous compensators which is also called synchronous condenser is designed for ratings upto 100 MVA_r and speed upto 3000 rpm.
The rating is in MVA_r because the are designed to cancel the reactive component.

Now, from above discussion it is clear that the synchronous machines are divided into three types where other machines are divided into two categories i.e. Motors and Generators but synchronous machines has one more type that is Compensators due to power factor control property.
First three classes discussed above i.e. Hydro-generators, Turbo-alternators and Engine driven generators are Generators.

Constructional Features of Synchronous Machines

2009-12-01T11:37:00.004+05:30

Synchronous machines have important role in electrical engineering, so its essential to know the constructional details of synchronous machines.
In my current article I'll discuss the constructional details of synchronous machines.
I have already discussed the types of synchronous machines in my recent articles Types of Synchronous Machines.
Normally Synchronous machines are used for power generation purpose as a generator. So I'll discuss the constructional features of all Synchronous Genreators in details.
The synchronous genreators are of three main types. These are :

Hydro Generators
Turbo Alternators
Industrial Generators

Out of these three first two viz. Hydro-Generators and Turbo-Alternators are very important.
Before discussing these generators two concepts or features are essential to discuss about synchronous machines or specially about synchronous generators. These are

Prime Mover
Run-Away Speed

Prime Movers for Synchronous Generators

The type of construction used for synchronous generators depends upon the type of prime mover. The following types of prime movers are used, generally :

Steam Turbine
Hydraulic Turbine
Diesel Engine

Steam Turbine

The efficiency of steam turbines is high at large speed and therefore synchronous machines driven by steam turbines are high speed machines. The synchronous generators driven by steam turbines are known as " Turbo-Alternators. " The maximum speed of turbo-alternator is 3000 rpm corresponding to 2 poles and 50 Hz frequency.

Hydraulic Turbine

The synchronous generators driven by water turbines are called water wheel generators. The hydraulic turbines are of different types. The type of water turbine depends upon the water head used. Normally used water turbines are :

Water Head	Turbine
400 mtrs and above	Pelton Wheel
Upto 380 mtrs	Francis Turbine
Upto 50 mtrs	Kaplan Turbine

Diesel Engine

There are used as prime movers for synchronous generators of small ratings.

Run-Away Speed

The run-away speed is defined as the which which the prime mover would have, if it is suddenly unloaded when working at its rated load. When the prime mover is working at full load it receives its feed ( water, steam or diesel ) corresponding to full load conditions and therefore when it is suddenly unloaded it tries to race. This is because there is no load on the prime mover while it is receiving its input corresponding to full load. Steam turbine are equipped with a quick acting over speed governor set to trip at 1.1 times the rated speed and therefore the operation of the governor is reliable.

Introduction to Synchronous Machines

2009-12-01T11:36:00.004+05:30

Synchronous motor is very important part of electrical engineering and these are called constant speed machines. Synchronous machines includes alternators and motors which run at a constant speed in synchronism with the alternating current supply to which they are connected.
An alternator is a machine which has a stationary conductor system called stator and a rotating field system called rotor. The arrangement is very helpful to collect heavy currents at high voltages from stationary terminals. Field is light and so can run with high speed.

Basics of Synchronous Machines

Technically a synchronous machine consists of two major parts viz Armature and Field System. The arrangement of fundamental parts of a synchronous machine is such that armature is placed inside the field winding or frame. This construction is similar to the one used for d. c. Machines wherein the armature winding is placed on the rotor and the field system is housed in the stationary stator. This type of construction is used only for low power synchronous machines and is unsuitable for medium and high power synchronous machines.
In large synchronous machines second type of construction is used. This type of construction uses a stationary armature and a revolving field structure. This method is more economical and convenient construction method.
The revolving field system is almost universal because it has the following advantages :

Stationary Armature :
This kind of construction permits the use of stationary armature on which the windings can be easily braced and insulated for high voltage.
Slip Ring Operation :
The operation of slip rings on account of their sliding contracts is unreliable with large currents at high potential differences. The use of slip rings carrying large current at high voltages is, therefore, avoided in the stationary armature construction.

Classification of Synchronous Machines

The synchronous machines are classified into two types based on type of rotor used in construction. These type of synchronous machine are :

Salient Pole Machine
Cylinderical Rotor Machine

Salient Pole Machine

A salient pole synchronous machine has salient or projecting poles with concentrated field winding. The salient pole construction is used for generators driven by hydraulic turbines since these turbines operate at relatively low speed and a relatively large number of poles are required to produce the desired frequency ; the salient pole construction is better adopted mechanically to this situation.

Cylinderical Rotor Machine

A cylinderically rotor synchronous machine have their field winding distributed in the slots. The distributed field winding produces a sinusoidal flux distribution in the air gap. Cylinderical Rotor Construction is used for turbo-alternator which are driven by high speed steam or gas turbines.

Speed control of D. C. Machines

2009-12-01T09:17:00.005+05:30

This article is about speed control of a D. C. Motor. It describe how speed of a D. C. Motor can be controlled.

Hello Friends,
In our current article we will discuss how speed of a D. C. Motor can be controlled i.e. Which factors are controlled to control the speed of the motor ?
From the back e.m.f. Equation of a D. C. Motor we have :
E_b=P¤NZ/60 A
Where
P = Number of poles in the machine
¤ = Flux enclosed to a pole
N = Speed of Machine in r. p. m.
Z = Number of Armature Conductors
A = Number of Parallel Paths in D. C. Motor
Also from equivalent circuit of a D. C. Motor we have
V = E_b+I_aR_a
Here V is applied voltage to the D. C. Motor
E_b is induced back e.m.f. in motor Armature.
I_a is Armature current and
R_a is Armature resistance.
Normally Armature resistance of a D. C. Motor is of the order of 0.5 ohm.
From above two equations we have
E_b=V-I_aR=NP¤Z/60A
Now we have to control N so by changing position of o other variables we have
N=(V-I_aR_a)(60A)/PZ¤
This equation decide what should be changed to change the speed of a D. C. Motor. We know that for shunt motor flux is constant i.e. Independent of Armature current while in series motor the flux induced in D. C. Motor depends upon Armature current, so speed control method of both motors will be different.

Speed Control of D. C. Shunt Motor

We know that in equation
N=(V-I_aR_a)(60A)/PZ¤
The following are constants i.e. Can't be changed

60 A
P
Z
¤

Note : ¤ can be changed in D. C. Shunt motor by adding external variable resistance in field winding.

Speed Control of D. C. Series Motor

Similarly in D. C. Series Motor, We have the same equation
N=(V-I_aR_a)(60A)/PZ¤
The following are constants i.e. Can't be changed

60 A
P
Z

Here ¤ can be changed by simply increasing or decreasing Armature resistance.

Speed Control of D. C. Motor ( Series and Shunt Both )

From above discussion it is clear that following can be changed in both series and shunt motors

Applied Voltage V
Armature Current I_a
Armature Resistance R_a
Flux ¤

Based on the quantity to be controlled the method of speed controlled are named as

Voltage Control Method
Armature Resistance Control.
Field Flux Control

Lets discuss one by one in detail.

Voltage Control Method

From the back e.m.f. Equation E=PNZ¤/60A=V-I_aR
We have N=(V-I_aR_a)*(60A/PZ¤)
If Flux kept constant and P, A, I_a, R_a and Z are already constant so
N=(V-K)*K₁
here K and K₁ are constant.
Now N is directly proportional to applied voltage. So if applied voltage is increased then the speed of the dc machine will increased and vice versa.

Armature Resistance Control.

If we increase armature resistance it will Speed will be reduced and vice versa.

Field Flux Control

Incresing flux in a DC motor will reduce speed of the maachine and vice versa.

Construction and Working of D. C. Machines

2009-12-01T09:15:00.005+05:30

This article is about constructional features of a D. C. Motor. Here I will discuss importance of various constructional part of a D. C. Motor like function of stator, function of rotor and importance of commutator and brushes etc.

Hello Friends,
Before looking into the constructional features and constructional details of a D. C. Motor it is important to learn how a D. C. Motor works.
So here I am discussing some basics and brief description about how a D. C. Motor works.

Working of D. C. Motor

As we know that for rotation of an electrical machine two flux or two different magnetic field systems are essential. In case of D. C. Motor two types of field systems are used. There are

Permanent Magnet + Armture Winding System
Electromagnet + Armature Winding Arrangement

Permanent Magnet System

In this type of arrangement a permanent magnet is placed across a conductor winding of copper wire. The winding is called armature winding and this winding is fed from a D.C. Supply system.
The end point of the conductors of this winding are connected to a rotating system called commutator. The commutator is a mechanical rectifier and it convert supplied D. C. Power into A. C. and vice versa. The commutator rubs against a set of carbon pieces commonly known as the brushes. The brushes make electrical connection with commutator during its rotation, and the set of brushes are electrical connected to the positive and negative leads of the supply system. This type of arrangement allows electricity to flow through the armature winding. The electrical power flowing through the winding creates a magnetic field that interacts with the magnetic field of the permanent magnet mounted on Stationary part of machine called stator and it makes the armature wise spin and finally spins the rotor where armature winding is hosted.
This is the reason why almost all DC motors have armature winding on rotor and stationary windings or permanent magnet for field system on stator.
There are some conductors at commutators which are named as commutator bars and there bars are soldered to armature coils to conduct the current to the coil or winding through the brushes.
At a given instant the commutator and brush arrangement magnetise a given part of the rotor and thus producing magnetic poles around that particular part. Which rotate that part forward and changing position of the rotor and thus changing position of part magnetized by armature winding and brushes-commutator arrangement. This makes the rotor or D. C. Machine rotates continuously.

Construction of D. C. Motor

Now we have learned the working of a D. C. Motor so it is easy now to understand its construction.
The main constructional parts of a D. C. Motor can be classified in three parts and these three are given below :

Main Magnetic Field and Supporting System
Armature Winding and Supporting System
Commutator and Brush Arrangement

These parts are explained below :

Main Magnetic Field and Supporting System

As I told you there must be two separate magnetic fields to produce rotation in any electrical machine. The field system is used to produce main magnetic field. This arrangement may be of two kind :
(a). Permanent Magnet System
(b). Electromagnetic System
In both cases stator is essential.
Stator
A stator is outermost part of a D.C. Motor and it serve following purposes :
i. It provide mechanical support to the machine.
ii. It host magnetic field system as magnetic poles of main magnetic field are mounted on stator.
iii. Most importantly it provide path to magnetic flux with very small reluctance.
Main Field System
As I have already told main field system is of two types. In permanent magnet system, a permanent magnet is mounted on stator and in electromagnet field system an Inductor is mounted on poles on stator and a current is passed through this inductor to produce a magnetic field.

Armature Winding and Supporting System

This system produce auxiliary field system or second field system. Armature winding is hosted on rotor, the rotating part mounted on a shaft and is placed in stator with small air gap.

Commutator and Brush Arrangement

As I told above function of commutator is to magnetise particular part of the rotor and brushes help commutator to perform this function.
The diagram shown below explain proper construction of a D. C. Motor.

Types of D. C. Machines

2009-12-01T09:13:00.010+05:30

D. C. Machines are of four types. Every type D. C. Machines will be discussed here. Only D. C. Motor types viz. Series D. C. Motor, Shunt D. C. Motor, Compound D. C. Motor and Separately Excited D. C. Motor are discussed here.

What is a D. C. Motor ?

A D. C. Motor is an electrical arrangement or machine that take D. C. Input from the supply and produce mechanical power or torque. Technically, the input is given to machine via a commutator and machine produce torque which rotates the rotor.

Classification of D. C. Motor

There are two ways of classification of D. C. Motors.

Classification Based Upon Power Intake
Classification Based Upon Position of Magnetic Field System

Now we will discuss these classification in detail.

Classification Based Upon Power Intake

Based intake power type D. C. Machine is of two types viz. D. C. Motor and D. C. Generator.

Type	Power Input	Power Output
D. C. Generator	Mechanical Power	Electrical Power
D. C. Motor	Electrical Power	Mechanical Power

Classification Based Upon Position of Magnetic Field System

This is major and widely used classification system or type for both D. C. Motor and D. C. Generator. We will discuss this system only for D. C. Motor. Based on this system of classification ( According to location of field system ) , D. C. Motor are of four types :

Separately Excited D. C. Motor
Series D. C. Motor
Shunt D. C. Motor
Compound D. C. Motor

Separately Excited D. C. Motor

In separately excited D. C. Motors, the field system is located on stator and is completely separated from armature and other parts. In separately excited D. C. Motors the magnetic field is produced either by an electromagnet mounted on pole hosted on stator of the motor.

In short we can say that in separately excited D. C. Motors the excitation is electrical separated from armature. The circuit diagram is shown below

Series D. C. Motor

Sometimes in D. C. Motors the magnetic field is produced by an inductor or a solenoid type electrical component connected in series with the armature coil. This arrangement is called Series D. C. Motor.
The circuit diagram is shown below

Shunt D. C. Motor

Sometimes in D. C. Motors the magnetic field is produced by an inductor or a solenoid type electrical component connected in parallel with the armature coil. This arrangement is called Series D. C. Motor.
The circuit diagram is shown below

Compound D. C. Motor

Sometimes in D. C. Motors the magnetic field is produced by an inductor or a solenoid type electrical component connected in both series and parallel with the armature coil. This arrangement is called D. C. Compound Motor.
The circuit diagram is shown below

Connecting Voltmeter, Ammeter and Wattmeter in a circuit

2009-12-01T09:11:00.010+05:30

Voltmeter, Ammeter, Watt meter and Energy meter are four pillars of electrical energy and are most important devices used in electrical engineering field. You can't imagine any application in electrical engineering without voltage, current, energy and power measurement. Voltmeter, Ammeter, Watt meter and Energy meter connections and basics are discussed here.
If you are an electrical engineer than you must know how to connect various electrical instruments in a circuit, Specially measuring instruments. The most common instruments or measuring apparatus you must know about are Voltmeter, Ammeter, Energy meter and Watt meter.

Voltmeter : -

Voltmeter is a device that is used to measure voltage or potential difference across two given points. Essentially a voltmeter is nothing but a galvanometer with infinite resistance connected in series . This makes the resistance of an ideal voltmeter infinite .
Voltmeter is connected in parallel, this is shown below : -

As shown in image a voltmeter is connected in parallel or in shunt configuration, due to voltage division in series.
If we connect it in series high resistance of voltmeter will make the circuit insulating and no current will flow and the system will stop working.

Voltage Division in Series

If two resistances are connected in series the voltage given to the complete system is divided across resistances.
Let us consider two resistances with resistance R₁ and R₂, then total resistance of series configuration will be given by
R = R₁ + R₂
and Voltage across each resistance is give by
V₁ = R₁*V/R,
V₂ = R₂*V/R,
Here,
V₁ is voltage across R₁ and
V₂ is voltage across R₂.
The below Figure shows this more clearly.

Ammeter

: -
Measure current ( general term ) or rate of flow of charge ( Technical meaning ) between two points on a conductor.
An Ammeter is connected in series because it has low resistance. Connection of an ammeter is shown below with circuit diagram : -

Watt Meter

: -
A watt meter Measures power consumed by an apparatus in a given time. Normally watt meter gives reading in watt or kilowatt. Connection of watt meter is shown below : -

Capacitor Calculation for Power Factor Improvement ( induction motor )

2009-11-23T11:18:00.002+05:30

Capacitor calculation or capacitance calculation for a capacitor bank to improve power factor is very important topic for an electrical engineer. Capacitor bank help to reduce reactive power. Here is one numerical problem on capacitor or capacitance calculation for an induction motor.
Numerical Problem : -
A single phase 220 volts, 50 Hz, motor takes a supply current of 50 Amperes at a power factor of 0.5 lagging. The motor power factor has been improved to 0.9 lagging by connecting a condensor in parallel. Calculate the capacitance of the capacitor required ?
Solution : -
Please note that current power factor is 0.5 it indicates that motor is induction motor.
Data extracted from above problem : -
Motor Type : - 1 phase, Induction motor,
Voltage Applied : - 220 volts,
Current from Supply : - 50 Amp,
Supply Frequency : - 50 Hz,
Current Power Factor =cos ¤=0.5 ( lagging ),
¤ = cos^-10.5=60^¤.
Active Component of current, say I_a = I * cos ¤ = 50 * 0.5 = 25 Amp,
One thing you must remember regarding power factor improvemet Improving power factor does not change active power consumed in the appliance.
Since the voltage also same so only current will be changed after power factor improvement. So only total current will be changed but active component will be same i.e. 25 Amp.
Tan ¤ = tan 60 = 1.732,
Reactive component of current, say I_r1 = I tan ¤ = 25 * 1.732 = 43.3 Amp,
Let the capacitor of C capacitance is connected in parallel with motor then,
New Power Factor = 0.9,
cos ¤₁ = 0.9,
tan ¤₁= 0.4843
New reactive component = I tan¤₁ = 25 * 0.4843 = 12.1 Amp,
Thus, current through capacitor = 43.3 - 12.1 = 31.2 Amp,
As we know,
I_c = V/X_c = 2*pi*f*C*V,
C = 451 * 10^-6 F

Introduction to D. C. Machines

2009-11-23T09:22:00.008+05:30

D. C. Machine basics like d. c. Motor, d. c. Generator, modes of operation of d. c. Motors, d. c. Generators are discussed in this post.

Friends,
Just like transformers D. C. Machines are also part of Electrical Engineering. If you are using any kind of stereo, walkman, DVD player or VCR you must observed a very small rotating motor inside the appliance or instrument you are using. This is no doubt a D. C. Motor.
Peoples says that D. C. Machines are old now but I am not agree with this statement.
There are lots of application which can be completed only with D. C. Machines. Traction is one among these.
So Friends, don't think that D. C. Machines are outdated now. Knowledge about D. C. Machine is of equal importance.
So lets start from begining as we have done in Transformer Design.
Modes of Operation : -
As I told earlier d.c. Machines are not outdated. Basically a d.c. Machine can work in three modes : -

Generator Mode
Motor Mode
As a Break

Generator Mode : -
As we know a generator is a device that is used to produce electrical energy or electricity. The main function of a generator is to alter the form of energy. It converts mechanical energy into electrical energy.
In d.c. Generators, the machine is driven by a prime mover with the mechanical power and produce electrical energy.
A d.c. Generator produce d.c. Current as output.
Motor Mode : -
A motor is just opposite in working and principle as compare to d.c. Generator. A motor or d.c. Motor converts electrical energy [d.c. Voltage] into mechanical power or force.
As a Break : -
Break mode is something different from the above said mode. In break mode, the machine ( which functions as a motor before the application of braking action ) works as a generator and the electrical power developed is either pumped back to the supply as in regenerative braking or is dissipated in the machine system as in dynamic braking. Hence in the braking mode the machine deadlerat on account of power supplied or dissipated by it and, therefore, produce a mechanical braking action.
Applications of D. C. Machines : -

Traction : -
This is the best example to realize the importance of d.c. Machines. Traction is the field where d.c. Motors has countable applications. Nowadays induction motors are also used in traction but d.c. Series Motors are admirably suited to traction applications like inter-city and rapid transit trains on account of its high starting torque, matching torque speed characteristics, where the torque decreases as the speed increases and the ability of a number of series motors operating in parallel on one locomotive to draw current at any given speed varying from that of any other motor within close limit.
Driver for process Industry : -
The control of speed of d.c. Motors can be expressed to close limits with a high degree of precision. This aspect is particularly useful in process driver and name winding machinery.
Battery Driven Vehicles : -
In modern world when environment is a big issue, battery driven bikes, cars and yo-bikes are of first importance. The primary source of supply to road driven vehicles with electrical fuel is essentially a d.c. Battery. In all vehicles including cars, buses, trucks etc. d.c. Battery is a source of secondary supply so d.c. Batteries are essential for vehicles whether it is electrical operated or petrol operated.
Home Based Appliances : -
Battery driven miniature d.c. Motors and d.c. Batteries are used in home based domestic appliances like shaving razors, fridger or refrigerator, tape recorders, mp3, mobile phones etc.
Automatic Control : -
The advent of automatic control has again brough the use of d.c. Machines into limelight. Small motors designed with high energy permanent magnets, encapsulated epoxy resin winding and electronic commutation are cheaper and highly reliable is compared with a.c. Counterpart. There machines together with power transistors are used in feedback control system.
D.C. Transmission : -
Are you suprised with this. Yes you heard right. D.C. Transmission system is practical now. You have often obsesed your house supply system it is a.c. System. But for transmitting high power d.c. System is widely used. The main reason behind this is it produce small power losses due to absense of skin effect. Which reduce resistance of the transmission line.
It is all about the basics of d.c. Machines.

Biot Savart's Law

2009-11-23T07:54:00.001+05:30

Biot-savart's law is discussed here.
Friends,
We have already discussed Faraday's law of electro-magnetic induction. It was all about the induction of emf in an electric circuit. Now we will discuss Biot Savart's law.
Biot Savart Law is also a very important concept from the point of view of Electric Machine Design.
The law deals with the force produced on account of interaction between a magnetic field and a current carrying conductor.
According to Biot Savart's law the electromagnetic force is give by the formula : -
F_e=B*l*i*sin (@)
The meaning of various symbols in above formula can be given as,
F_e= Net Electromagnetic force produced in Newtons
B = flux density, in Wb/m² or in Tesla ;
l = length of the conductor in meter ;
i = current carried by the conductor in Ampere ;
@ = angle between the direction of current and the direction of magnetic field.
The direction of the force produced is perpendicular to both current and magnetic field.

Faraday's Law of Electro-magnetic Induction

2009-11-23T07:24:00.003+05:30

Friends,
To study the principles and operations of various electric machines It is very important to understant some basic laws. Faraday's law of electro-magnetic induction is one among such law.
This law deals with the emf induced in an electric circuit and you know every machine works on the principle of emf induction except d.c. Machines.
Faraday's law of electro-magnetic induction states that the emf induced in a circuit is equal to the rate of change of flux linkages.
Flux linkage is give my product of N¤( number of turns of coil*phi, flux linking with all of them ).
This is theoritical explanation but in most of the cases this flux phi, does not link with all the turns or in other words all turns do not link with the same flux. Under such conditions the summation of the products of magnetic flux with complete turns of the magnetic circuit gives the total value of flux linkage.
Now according to Faraday's law of electro-magnetic induction if there is any change in flux linking with a conductor, or induction coil, an emf will be induced and magnitude of this induced emf is given by : -
e [ directly proportional to ] d(flux linking)/dt.
I think you are suprised by this result but don't be excited it is correct explanation for Faraday's law of electro-magnetic induction.
I know you are thinking about the negative sign but that is not a part of Faraday's law. That negative sign is explained by Lenz law. According to lenz law the direction of induced emf is always in opposition to that of main or parent emf. This is the reason we put a negative sign to calculate the magnitude and direction of the induced emf.
After applying both laws viz. Lenz and Faraday's law the exact formula will be
e [ directly proportional to ] - d(flux linking)/dt.
Now when you learn basics about Faraday's law of electromagnetic induction I would like to tell you the causes of emf induced in an electric circuit according to Faraday's law of electromagnetic induction.
The emf in a circuit is induced by change in flux linking ( discussed above ) and this flux linking can be changed by any of the following actions or by a combination of these : -

The coil is stationary with respect to flux and the flux varies in magnitude with respect to time.
The flux is constant with respect to time and is stationary and the coil moves through it.
Both the changes mentioned above occur together i.e. The coil moves through a time varying field but relative motion should not be zero.

> If coil is stationary and flux is time varying, an emf called transformer emf or pulsational emf is produced.
> If the flux is stationary w.r.t. time and conductor is moving the emf induced is give by the relation : -
e = B*l*v volts here v is linear velocity of conductor in m/s. In such case the generated emf is called motional emf. So friends this was all about the Faraday's law of electromagnetic induction or emf generation.

Basics of Transformer

2009-11-22T14:40:00.003+05:30

Friends,
In this post I'll discuss some basics of transformers.
What is Transformer : -
Friends, you must observed the black and bulky box hanged on the electric pole, probably near your home or at a distance from your home.

This bulky structure or electrical appliance is called transformer.
The above figure can give you a good idea of transformer, if you haven't seen real one !
If you haven't seen a transformer yet, look at any of the opened electrical appliance box like Stabilizer, T.V. Set, radio etc you will found a square with small coils wrapped on a iron or steel like structure. This is noting but a transformer.
Transformer is a device used to convert alternating voltage from one level to another level.
Here you must remember that it
works on a.c. Voltage only. A transformer cann't work on d.c. Supply. A transformer can convert high voltage into low voltage and low voltage into high voltage, but it cannot change the working frequency of the system although it changes current in the system. The step up transformer converts low voltage in high voltage, converts high current into low current value.
Further, if you are using transformer to decrease voltage level it will increase current in the circuit. Similarly if you use transformer for isolation in a system it will not change current and voltage in the system.
This is against Ohm's law but this is true and is called Transformer's law. It is due to the inductance of the windings.
Working of Transformer : -
Transformer works on the principle of mutual induction. Mutual induction is the phenomenon of induction of electro-motive-force or e.m.f. in an inductor when it is placed in the magnetic field produced by the magnetic flux due to another current carrying inductor. This is the working principle of transformer.
Types of Transformer : -
Basically Transformers are divided into two categories : -

Step-up Transformer
Step-down Transformer

Step-up Transformer is a transformer that is used to increase voltage level of any a.c. wave while step-down decrease voltage level and thus increases current level in the system.
In my next post I will discuss various types of transformers depending upon other classifications.
Till then, bye bye and take care.
Have a nice day.

Electric Charge

2009-11-22T11:34:00.002+05:30

In electrical engineering everything depends upon electric charge. Electric charge and concepts of electric charge are discussed here.

I have already discussed advantages of electrical energy over other forms of energy. So its time to move on to electrical engineering's basic concepts.
Friends, electricity is all about the flow of current and its effects so we will discuss about the origin of current and obviously that is charge. So lets start with charge.
Electric Charge : -
Electric charge is property of a matter or substance due to which electric force is experienced between two particles. Basically electric charge is just an additional property of electrons or protons in an atom that give rise to electric force between them.
You are familiar with mass or weight of a body, weight of the body give rise to force of attraction between two particles similarly electric charge give rise to another force of attraction or repulsion called electrostatic force.
I said force of repulsion or attraction because electrostatic forces are of two types depending upon the type of charge present on two bodies.
Types of Charges : -
Charges are of two types : -

Positive Charges
Negative Charges

Positive Charges : -
If atoms of a particle loose electron then the particle acquires positive charge. It is denoted by a positive sign on the atomic symbol.
For Example, after loosing two electrons Calcium atom is represented as Ca²⁺.
Negative Charges : -
Similarly, if atom(s) of a particle gain electron then the particle acquires negative charge. It is denoted by a negative sign on the atomic symbol. For Example, after gaining two electrons Oxygen is represented by O^2-.
Properties of Electric Charge : -
Additive in nature : -
As we know that mass of a particle is algebric sum of its constituent particles similarly total charges distributed on the different parts of the object is algebric sum of individual charge and this property is called additive nature of charge.
Quantization of electric charge : -
The charge on an particle is integer multiple of elementry charge that is 1.6*10^-/+19. This is called quantization of charge.
Conservation of electric Charge : -
Just like the total energy of an isolated system, electric charge also remains constant this property of charge is called Conservation of electric charge. In other words " Charge can neither be created nor be destroyed, it can be transfered from one body to another. "
Force between two charges : -
As I said earlier that charges on two bodies causes some kind of electrostatic force this force is similar to gravitational force. The main difference is in nature of the force. In gravitational force only attractive force is produced while in electrostics both kind of attractive and repulsive force can be observed. If we are placing two like charges repulsive or negative force will be observied similarly if we are placing two unlike charges attractive or positive force will be observed.
So Friends, This was all about the Electric Charge. In next article we will discuss about the basics of electric field.

Why only Electrical Energy ?

2009-11-22T08:20:00.001+05:30

Hello Friends,
There are number of forms of energy available still we are using electrical energy. This is the fact that prove the need of electrical machinery. Why we have to study electrical machine design ? There are number of other kind of machinery available like mechanical machinery, solar appliances etc. but still we prefer electrical appliances. Why ?
If I am not wrong this will be the first question anyone will ask you when you speak about electrical machine design.
I hope after reading this post you will be able to answer this question efficiently.
Lets start from begining
Energy : -
Energy is anything that help us to do any task. Without energy you cannot do anything. For every single action or activity in life we need energy. Some very common example of task or activities are reading, writing, walking, talking etc even for sleeping we need energy.
Sources of Energy : -
Now, when it is clear that we need energy for every task then you may think about the sources of energy. From where energy come ?
You may be aware about the famous quote " Energy is neither be created nor be destroyed. " Then from where it comes ?
Energy simply changes its forms of availability.
Carbohydrates are most common sources of energy for human beings and these carbohydrates are synthesized in plants containing chlorophill and the reaction is called Photo Synthesis as photo (means light) is essential for this section.
This carbohydrates is formed by plants and CO₂ and water H ₂O are raw materials for carbohydrates synthesis. So it is clear that no one is creating energy but we get energy from CO₂ and water indirectly.
Similarly Sun light, Heat energy, Wind energy etc are another forms or sources of energy.
Then it comes to Electricity.
Do you count how many times you switch any electric appliance ? Positively no. This shows how widely electrical energy or electricity is prefered in our daily life. This is not a co-incidence but there are strong reason behind this.
Why Electrical Energy only : -
Electrical energy is prefered over the hundreds of forms of energy available now a days. It is not possible to compare every form of energy with electrical energy so I will tell you some most appropriate reason thats why electrical energy is widely used.
Very Clean : -
This is the most environmental form of energy. Environmental related problems are very serious in present world. Everyone is worried about Global warning, ozone dissociation and other environmental problems. Electrical energy is completely free from these problems and its use doesn't cause any kind of pollution. The reason behind this is very simple it is all about the internal movement of electrons in a particle so no damage to environment. However you may say Thermal plants are source of serious heat dissipation to nearby atmosphere but I would like to update your knowledge about this. Hydro power plants are replacing thermal plants nowadays due to this reason and even if a thermal plant is installed proper precautions are taken to dissipate the heat and harmful gases, if any.