Electrotechnik

How a Fluorescent Light Works - Schematic Animation

2012-02-10T10:06:00.000-08:00

Water cooled busbars

2012-02-10T07:16:00.000-08:00

Water cooled bus bars are used in induction heating applications where the bus bar carrying currents is subjected to extreme temperature. The currents used in induction heating is usually very high of the order of kilo amperes.

Water cooled bus bars are also used to build power electromagnets which require very high currents. Such electromagnets are generally used for research purposes.

In such hot environments, the bus bar which is carrying heavy current should be continually cooled to prevent it from melting due to the external heat. Demineralized water is circulated through holes drilled through the bus bars.

The temperature of the water should be maintained below the boiling point of water. This is because the water can flash to steam above the boiling point and the high pressure created can damage the bus bar.

Corrosion is a issue in water cooled bus bars. The water quality should be constantly monitored to prevent corrosion.

Turbo Alternators

2012-02-09T07:13:00.000-08:00

Turbo Alternators are alternators coupled to turbines. Turbo alternators run at high speeds and develop large quantities of power usually of the order of hundreds of Megawatts. The rotor of the Turbo alternator is usually made of many pieces connected together. Turbo generators usually have cylindrical rotors. The speed of the rotor is around 3000 rpm (50 Hz) or 3600 (60 Hz).

Turbo alternators are extremely sensitive machines as they operate at very high speed. The slots for the rotor windings are milled on the rotor shaft. The rotor of the turbo alternator does not have damper windings as the prime mover used is usually a turbine which provides a consistent and steady torque unlike the diesel engine which provides pulsating torque. The wedges used to secure the rotor winding to the slots are made of steel and function as a damper winding as well.

Cooling is a major issue in Turbo generator, given the enormous amount of heat generated. Air, Hydrogen and water have been employed to transfer the heat away from the generator.

What is Pull-in torque and Pull-out Torque in synchronous machines

2012-02-08T07:09:00.000-08:00

A synchronous motor is not self starting. Thus, its speed has to be brought near the rated rpm before it can run on its own.

There are many methods of starting the synchronous motors such as the use of an external pony motor, amortisseur windings which start the motor as an induction motor and the use of VFDs.

When the speed of the synchronous motor reaches 97% of the nominal speed, the dc supply to rotor of the synchronous motor is switched on. This produces a "pull-in" torque which helps the rotor poles to catch the imaginary stator poles and run in synchronism.

The "pull out" torque of the synchronous motor is the torque which can cause the motor to slip and step out of synchronism. In some cases, the rotor misses a pole and catches the next stator pole. In many other cases, the rotor runs like the rotor of an induction motor and never gains synchronism. This can result in serious damage to the rotor. Hence, synchronous motors should be provided with a reliable "pull-out" protection

Nitrogen in Transformers

2012-01-12T08:55:00.000-08:00

Oil filled Transformers usually have a conservator which maintains the level of oil. The conservator has a layer of air on top of the oil. This air communicates with the atmosphere through a breather containing a desiccant,usually, silica gel.

In some transformers, the tank of the transformer is filled with a layer of nitrogen above the oil in which the windings and the core are immersed. Nitrogen, being an inert gas, reduces the risk of a fire hazard. It also protects the transformer oil and the transformer insulation from reacting with oxygen in the air and deteriorating.

In transformers containing nitrogen, the nitrogen is led into the space above the conservator through a pressure reducing valve. When the oil level in the transformer rises and the transformer breathes out, the nitrogen inside is released into the atmosphere. When the oil levels fall due to a reduction in ambient temperature or the load, fresh nitrogen is released into the conservator through the cylinder and the valves.

Vacuum Circuit Breakers

2012-01-09T06:12:00.000-08:00

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry. Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber. The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc. The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A. When the arc is initiated during contact separation, metallic vapour is produced from the contacts. If the contacts are of soft metal, very little vapour is produced. While this ensure quick arc extinction. The current waveform can get chopped and lead to high voltage transients. If too soft metals are used in the contacts, the arc would cause erosion of the material. Hence, the contacts are made of materials which are neither too soft or hard. Common materials used are alloys Copper-bismuth or copper-chrome.

C Rating and battery capacity.

2011-12-23T06:33:00.000-08:00

The capacity of a battery is indicated in Ampere-hours (Ah). For instance, 200Ah, 80 Ah, etc. The rate at which the battery can be discharged is indicated by the C rating.

Batteries are usually rated as C/10, c/8, etc. a battery with a 200 Ah capacity and a C/10 rating indicates that the battery can supply 20 A for 10 hours.

The capacity of a battery varies inversely with the discharge rate. Batteries usually have different discharge rates for a fixed capacity.

Combined Instrument Transformers

2011-12-21T12:30:00.000-08:00

Combined Instrument Transformers are special measuring transformers which combine the functions of a current and a voltage transformers. They are generally used where space is a constraint. Typical scenarios are when measuring transformers need to be added in a functioning substation.

Combined instrument Transformers usually have multiple secondaries for both current and voltage measurement. These specialized transformers have been used upto 300 kV.

What are Primary and Secondary Cells ?

2011-12-20T06:05:00.000-08:00

Primary cells are cells in which the chemical reaction which produces the power is not reversible. That is, the cell cannot be charged. A common example is the dry cell. Primary cells are cheaper and find application in radios, flash lights, torches, etc.

Secondary cells are those cells in which the chemical reaction can be reversed by passing an electric current in the reverse direction. The active material of the battery is reformed and the battery is recharged.

Rod Earthing

2011-11-30T06:28:00.001-08:00

Rod Earthing is an extremely popular form of Earthing as they are easy to install and retrofit. Rod earthing involves driving metal rods made of copper, copper clad steel or stainless steel.

The Rods used for earthing are usually about a metre long with a diameter of around 12 mm. The rods should be driven to a depth of around 2.5 metres. Usually the rods have threaded sections on one end so that the rods can be driven one on top of another to achieve the required resistance.

The Rods can be hammered into the ground. This would be possible in sandy soil. If the soil is hard, a Earthing rod driver which is a tool, usually pneumatic, which drives the earth rods into the soil is used.

Capacitance Grading

2011-11-29T05:32:00.001-08:00

Capacitance Grading is a method of distributing the electrostatic stress across the insulation of cables by using materials with different dielectric strength. When a cable is live, the insulation around the conductor is in a state of electrostatic stress.

This stress is high near the centre and reduces towards the periphery. This uneven electrostatic stress can cause the failure of insulation. To prevent this kind of catastrophic failure, the insulation is graded. The permittivity of the insulating material is made to vary inversely as the distance from the centre.

It is not possible to have a single material which has a permittivity which varies depending on the distance. Hence, the insulation is made of a number of layers of insulation made from different materials.

Each of these materials has a different value of permittivity. This ensures that the insulation is approximately uniform throughout the cross section of the insulation.

Rated Voltage Factor of Potential Transformers

2011-11-24T21:27:00.001-08:00

The Rated Voltage Factor of Potential Transformers is the maximum limit of excess voltage up to which the voltage transformer can maintain its rated characteristics. It is mentioned in percentage of the nominal voltage.

For instance a Rated voltage factor of 1.2 would mean that the Potential Transformer can maintain its rated characteristics up to 120% of the nominal voltage.

The following table lists the common Rated Voltage factors in Potential Transformers for different connections

Rated voltage factor	Rated time	Primary Winding Connection
1.2	Continuous	Between phases in a network Between transformer star-point and the earth in any system
1.2 1.5	Continuous for 30 seconds	Between the phase and earth in a system whose neutral is effectively earthed
1.2 1.9	Continuous for 30 seconds	Between the phase and earth in a system whose neutral is non-effectively earthed with automatic fault tripping
1.2 1.9	Continuous for 8 hours	Between the phase and earth in a system whose neutral is isolated without automatic fault tripping or in a system with resonant earthing without automatic fault tripping

Universal Motor - An Overview

2011-11-22T20:25:00.001-08:00

Universal Motors are motors which run on both AC as well as DC power. These motors find wide application in household electric appliances such as mixers, blenders. The small size, high power and speed of universal motors make them ideal in these applications. The Universal Motor is the motor of choice for washing machines as they can be used to agitate the drum by easily reversing direction.

A universal motor is a dc series motor which usually has some modifications which enable it to run efficiently on AC as well. Running the motor on AC leads to eddy currents being developed in the core of the motor. Hence, the core and poles of universal motors are made of laminated sheets. The windings of the motors are made thicker to reduce the reactance.

Like all DC machines, universal motors suffer from the effects of commutation such as sparking. Hence, universal motors are used mainly below 1000W. The high speed of the universal motor enables it to deliver a higher power even while having a small size. Universal motors are usually kept coupled to the loads such as gears or blowers as they tend to run at very high speeds on no load conditions.

Speed Control is usually done by Electronic means.

Intersheath Grading

2011-11-18T06:42:00.001-08:00

Intersheath Grading is a method of ensuring that the voltage gradient across the insulation of a cable does not become so steep as to cause the failure of the insulation. The insulation of a cable is subjected to constant electrostatic stress. This electrical stress is dependent on the voltage of the conductor. The electrostatic stress needs to be uniform across the insulation. Uneven electrostatic stresses can result in failure of the insulation.

Intersheath Grading is a method of creating uniform voltage gradient across the insulation by means of separating the insulation into two or more layers by thin conductive strips. These strips are kept at different voltage levels through the secondary of a transformer. This ensures that all parts of the insulation are exposed to relatively the same stress.

Single Circuit and Double Circuit Transmission Lines

2011-11-12T21:46:00.000-08:00

Transmission lines which carry three phase power are usually configured as either single circuit or double circuit. A single circuit configuration has three conductors for the three phases. While a double circuit configuration has six conductors (three phases for each circuit).

Double Circuits are used where greater reliability is needed. This method of transmission enables the transfer of more power over a particular distance. The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view. However, running two circuits in close proximity to each other will involve inductive coupling between the conductors. This needs to be taken into account when calculating the fault level and while designing the protection schemes.

Double circuit transmission lines usually contain bundled conductors with the conductors placed as far as possible to minimize inductance.

Proximity Effect

2011-11-11T21:40:00.000-08:00

Proximity Effect refers to the phenomenon where the resistance of one conductor in a transmission line or a bus bar decreases due to the magnetizing flux from another conductor in close proximity.

When two conductors are placed close to each other the varying magnetic flux caused by one conductor induces currents in the other conductor. This increases the current density in the other conductor and results in increased impedance.

The Proximity Effect increases with frequency. Conductors are usually spaced with a sufficient distance to minimize the Proximity effect.

A comparison of Overhead and Underground Transmission

2011-11-09T21:40:00.000-08:00

Electric Power needs to be carried over long distances from the point of generation to the point of consumption. This Transmission is done either through overhead lines or underground cables. Each of these two methods of transmission has its own advantages and disadvantages.

Overhead Transmission lines are cheaper as the insulation cost is lesser and the conductor material cost is lesser too. They also have better heat dissipation.

However, they have significant disadvantages. Overhead lines are vulnerable to lightning strikes which can cause interruption. Overhead lines use bare conductors and can cause damage if they break. They are considered to be unsightly as they mar the scenery of the landscape. The maintenance cost of overhead lines is more and the voltage drop in overhead lines is more.

Underground transmission due to cables is costlier than overhead transmission as the ground needs to be excavated. This can be difficult when passing though geographic obstructions such as hills, marshes and rivers. Special trenches need to be constructed when passing through loose soil. Besides, heat dissipation in underground cables is an issue. Hence, the conductors have to be thicker. The insulation required for the cables is expensive. Hence, it is difficult to use underground cables for voltages at HV levels (> 33 kV).

Underground cables may have to be rerouted to accommodate other underground structures such as pipelines, sewage lines, etc. It is necessary that the routes of underground lines are clearly marked with sign boards to prevent accidents when excavations are carried out for other reasons at a later date.

Transposition of Conductors

2011-10-25T04:59:00.000-07:00

Transposition of Conductors refers to the exchanging of position of conductors of a three phase system along the transmission distance in such a manner that each conductors occupies the original position of every other conductor over an equal distance.

When conductors are not transposed at regular intervals, the inductance and capacitance of the conductors will not be equal.

When conductors such as telephone lines are run in parallel to transmission lines, there is a possibility of high voltages induced in the telephone lines. This can result in acoustic shock or noise. Transposition greatly reduces this undesired phenomenon.

In practice, however, conductors are not transposed in the transmission lines. The transposition is done in the switching stations and the substations.

XLPE Cables

2011-09-13T07:41:00.000-07:00

XLPE stands for Cross linked Polyethylene. It is a long chain polymer whose chains are cross-linked. XLPE is a widely used material for use in cable insulation. XLPE can be used from LV cables to cables with voltages upto 275 kV.

The power factor of XLPE cable is very close to one. XLPE cables have smaller charging currents and lesser dielectric loss.

XLPE cables do not require the protective metallic sheath. They are lighter as compared to other cables and are easier to lay.

Tin Plated Copper

2011-09-09T07:43:00.000-07:00

Copper used in bus bars is sometimes plated with Tin. This is because copper oxidises in the presence of air. The oxides of copper fall off the surface, this exposes more copper and further oxidation takes place.

These oxides increase the contact resistances in electrical contacts causing localized heating. This heating causes further oxidation. Thus, the copper is steadily eroded.

Tin plating is used protect the copper from corrosion. Tin does not react easily with either air or water. Tin is a soft metal which can be easily polished to give a shiny finish. Tin Plated contacts are also used in the contacts of isolators and switches.

Flexible Busbars

2011-09-07T08:34:00.000-07:00

Flexible bus bars are used in Low voltage installations. The are used in place of cables, particularly where parallel runs of cables need to be drawn to supply equipment in a particular location.

Flexible bus bars are made of flexible strips of electrolytic copper which is usually tin-plated. These strips are enclosed in a PVC insulation. These bus bars can slide over each other which makes them flexible. These bus bars can be bent to a considerable extent. The elimination of parallel runs of cables reduces costs.

Flexible bus bars have better heat dissipation than conventional rigid bus bars. Hence, they can carry higher currents for the same cross section.

Since these bus bars are insulated, the clearance from the ground can be lesser as compared to normal bus bars.

Transformer Oils.

2011-09-06T02:23:00.000-07:00

Oil is an important component of most transformers. Oil serves to dissipate the heat generated in the core. It also serves to provide dielectric isolation between the windings and the transformer body. Transformer oils need to be stable at high temperatures with excellent dielectric properties.

Traditionally, mineral oils have been used in transformers. The downside of mineral oil is that they are easily combustible causing transformer fires.

Polychlorinated Biphenyls, a synthetic compound was used as a replacement for mineral oil as it is not inflammable. However, it is highly toxic and carcinogenic, not biodegradable and can cause damage to the environment if leaked. The use of PCBs is now banned in many countries.

Research has led to the development of many types of transformer oil which are fire-resistant and non-polluting. Some of these oils are based on esters which are naturally available in vegetable oil or on synthetic esters. These oils though expensive are environment-friendly, fire resistant and have the require dielectric properties. Their high cost is offset by the elimination of the need for building expensive vaults for the transformer to contain any fire.

Book of the Week

2011-08-16T11:08:00.000-07:00

Electric Motors and Drives: Fundamentals, Types and Applications (3rd Edition)
by Austin Hughes
ISBN-10: 0750647183 | ISBN-13: 978-0750647182

This is a useful and interesting book which covers fairly all the topics in the field of motors and drives. The concepts are explained clearly and the mathematics is kept to a minimum. Topics start from the basics and and cover almost all aspects of the topic. The topics covered include dc motors, induction motors theory and types as well as stepper motors. Starting methods for various motors are also adequately addressed. The book also covers drives for dc and ac motors. It also includes a selection guide for drives. Though there pictures and diagrams, more pictures could have been added.

In all, a comprehensive book which covers all the important topics at a relatively low price. This book would be ideal for industry professionals as well as students of electrical engineering.

Permanent Magnet Synchronous Generators

2011-08-12T08:29:00.000-07:00

Permanent Magnet Synchronous Generators are becoming the generators of choice in the wind turbine industry. They are being increasingly used in place of induction generators (induction machines which run above the synchronous speed). They are advantageous over induction generators as they have a higher efficiency. Besides, they do not need a magnetizing current which needs to be fed from the grid.

The normal synchronous generator has a wound rotor with poles. These poles in the rotor are excited by means of a dc current. In a permanent magnetic Synchronous generator, the wound rotor poles are replaced with permanent magnets.

The machine is often directly coupled to the wind turbine or through gears. The AC output of the synchronous generator which has a variable frequency depending on the turbine speed is converted into DC. This DC is converted into a sinusoidal AC voltage at the system frequency. This voltage is then fed to the grid.

Permanent Magnet Synchronous Generators (PMSG) are costlier as the permanent magnets are made of rare earth metals. NeFeB, an alloy of Neodymium, Iron and Boron is used to make these magnets. The use of permanent magnets in the rotors minimizes the losses in the rotor and improve efficiency.

Lockout /Tagout

2011-08-11T07:38:00.000-07:00

Lockout Hasp

Lockout/Tagout system is a set or procedures which prevent the accidental energization of electric equipment during maintenance. Lockout Kits contain many types of locking systems such as 2 - Hinged Single-Pole Breaker Lockouts Breaker Lockouts which can be used to lock a breaker against operation.

Wall Switch Lockouts prevent the operation of wall mounted switches. Plug Lockouts prevent plugs from being connected to power sockets. Lockout Hasps have multiple locking facility. When more than one person is working on a machine, say, a transformer. Each individual needs to place a lock on the Hasp. This prevents the machinery from being energized until all people have finished their work.

Lockout Padlock with unique key

The Lockout kit also contains a set of padlocks. Each padlock has its own unique key.

Lockout for Plugs

When it is not possible to lock out an equipment, Tag out procedures should be followed. Conspicuous tags should be placed. The Tag or the lock should only be removed by the person placing it

Lockout for MCB

All installations should have clear Lockout/Tagout policies. Employees should be trained on proper implementation of these systems

Lockout Tag