EEE Basics

Transformers

Why is transformer flux constant?

When a load is connected to a transformer current (say I₂) flows through secondary coil thus an M.M.F (N₂I₂) is produced, this produces the secondary flux. This flux reduces the main flux induced in the primary & also reduces E.M.F E₁ in the primary. As a result more current is drawn from the supply. This additional current drawn is due to the load component I₂^’ .This I₂' is anti-phase with I₂.This I₂' sets a flux which opposes the secondary flux & helps the main flux. The load component flux neutralises the secondary flux produced by I₂ .The M.M.F N₁I₂ balances N₂I₂.Thus the net flux is always at constant level

Give an example where transformer is used in DC applications

Transformer never work on DC .but it is

Why an induction motor is called a rotating transformer?

The rotor of an induction motor receive electric power by induction in exactly the same way as the secondary of a 2-winding transformer receives its power from the primary. Thus an induction motor can be treated as a rotating transformer i.e. one in which primary winding is stationary but the secondary is free to rotate.

Why is transformer rating in KVA?

Copper loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss depends on volt-ampere (VA) and not on phase angle between voltage and current i.e. independent of load power factor. That is why rating of transformers is in Kva and not in Kw.

How do we specify the rating of a transformer?

Transformer rating is specified in Kilovolt-Ampere (KVA)

What are the cooling methods to cool a transformer?

The cooling methods are:

·                     Oil filled self cooled

·                     Oil filled water cooled

·                     Air blast type

What is Scott- connected transformer?

A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase electric power (2-φ, 90-degree phase rotation) from a three-phase (3-φ, 120-degree phase rotation) source, or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

Does the transformer draw any current when its secondary is open?

Yes, no-load primary current

Is Cu loss affected by power factor? Why?

Yes, Cu loss varies inversely with power factor

Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

What are Instrument Transformers?

Current Transformer and Potential Transformer are Instrument transformers. 

Current Transformer (CT) is a step-up transformer which steps down the current to a known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of current in the line

Potential Transformer (PT) is a step down transformer and steps down the voltage to a known ratio. The primary of this transformer consists of a large number of fine wire connected across the line. The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of the line voltage.

Basic Electrical

What is the difference between earth and neutral?

Neutral is a return current path for equipments and earthing is protection for humans. Earth is a support for neutral. If neutral is not present, the machine we are using will get in full phase and we will get a shock. So earthing is done to avoid such shocks.  

What is the difference between an Insulator and a Dielectric?

Dielectrics are used to store the electric charges, while insulators are used to block the flow of electric charges. All dielectrics are insulators (they don't allow the flow of electric charges through them) but all insulators aren't dielectric because they can't store charges unlike dielectrics.

What are the disadvantages of low power factor?

The disadvantages are:

·                     Large KVA rating of equipment

·                     Greater conductor size

·                     Large copper losses

·                     Poor voltage regulation

·                     Reduced handling capacity of system

Why is earth pin longest and thickest?

More the thickness of the wire/conductor less is the resistance. A thick earth pin provides a less resistant path for the current to be grounded. This is for safety so that the leakage current if any will take the less resistant path from earth pin to ground rather than a person operating an appliance

What is Hysteresis loss?

Ans. Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps

changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase

To compensate for hysteresis losses, heat-treated silicon steel laminations are used

Which is the motor used in fan?

Single-phase induction motor is used in fan

Why do we use starter for a DC motor?

Large DC motors with field windings instead of permanent magnets have a very heavy load when starting. To prevent fuses blowing, a series of small resistors are sequentially switched in series with the motor. As the motor picks up speed, a back EMF in opposition to the applied voltage limits the maximum current. When the motor reaches its running speed, the 'starter' is out of circuit

What is Form Factor?

The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform)

Which is more dangerous-shock due to AC or DC?

Shock due to DC is more dangerous than AC

What are the practical applications of variable resistance?

Its applications are:

·                     Volume control

·                     Fan speed control

·                     Light dimmer

What is slip?

The difference between the synchronous speed N_s and the actual speed N of the rotor is known as slip

What is the purpose of compensating windings?

The function of compensating winding is to neutralize the cross magnetizing effect of armature reaction. In the absence of compensating windings, the flux will be suddenly shifting backward and forward with every change in load

What is Cogging and Crawling?

Cogging

The rotor of a squirrel cage motor sometimes refuses to start at all, particularly when the voltage is low. This happens when the number of stator teeth is equal to the number of rotor teeth and is due to the magnetic locking (cogging) between the stator and rotor teeth

Crawling

Induction motors, especially the squirrel cage type, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed N_s. This phenomenon is known as crawling of an induction motor

What are Dummy coils?

These are used with wave winding and are resorted to when the requirements of the windings are not met by the standard armature punching. They are simply to provide the mechanical balance for the armature because an armature having some slots without windings would be out of balance mechanically

What are CT and PT?

Current Transformer (CT)

It is a step-up transformer which steps down the current to a known ratio. The primary of

this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of current in the line

Potential Transformer (PT)

It is a step down transformer and steps down the voltage to a known ratio. The primary of this transformer consists of a large number of fine wires connected across the line. The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of the line voltage

What is a Universal Motor?

A universal motor is defined as a motor which may be operated either on direct or single phase ac supply at approximately the same speed and output.

What is Voltage Regulation?

It is the change in voltage when the load is reduced from rated value to zero, expressed as percentage of the rated load voltage

What are Interpoles used for?

As the polarity of Interpoles is the same as that of the main pole ahead, they induce an emf in the coil (under commutation) which helps the reversal of current. Another function is to neutralize the cross magnetizing effect of armature reaction

What is the principle of DC motor?

The action of DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left Hand Rule and whose magnitude is given by F=BIL Newton

Explain commutation in DC machines

The process by which current in the short circuited coil is reversed due to reversal of current while it crosses the M.N.A. is called commutation. The brief period during which coil remains short circuited is known as commutation period T_C

What are Damper Windings?

It is a winding consisting of several conducting bars on the field poles of a synchronous machine, short-circuited by conducting rings or plates at their ends, and used to prevent pulsating variations of the position or magnitude of the magnetic field linking the poles. It is also known as Amortisseur winding

 
How do you start an Induction motor?

The stator windings when supplied with 3-phase currents produce a magnetic flux, which is of constant magnitude but which revolves at synchronous speed. This revolving flux induces an AMF in the rotor by mutual induction

What are the uses of Servomotors?

These are used in computers, machine tools, process controllers, and robots, radars, tracking and guiding units

What is Regenerative Braking?

Regenerative braking is a system in which the electric motor that normally drives a hybrid or pure electric vehicle is essentially operated in reverse (electrically) during braking or coasting. Instead of consuming energy to propel a vehicle, the motor acts as a generator that charges the onboard batteries with electrical energy that would normally be lost as heat through traditional mechanical friction brakes. As the motor “acts in reverse,” it generates electricity. The accompanying friction (electrical resistance) assists the normal brake pads in overcoming inertia and helps slow the vehicle

What will happen if we give 220 Volt DC supply to a bulb or tube light?

Bulbs (devices) for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element

Which motor has high Starting Torque and Staring current - DC motor, Induction motor or Synchronous motor?

DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but can start the DC series motor without load

What is 2 Phase motor?

A two phase motor is a motor with the starting winding and the running winding having a phase split. E.g. AC servo motor, where the auxiliary winding and the control winding have a phase split of 90 degree

Give two basic speed control schemes of DC shunt motor?

By using flux control method-in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled

Armature control method-in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa)can be varied and since speed is directly proportional to (Eb-IaRa) the speed can be controlled

What is the difference between Synchronous Generator & Asynchronous Generator?

In simple, Synchronous generator supply's both active and reactive power but Asynchronous generator (Induction generator) supplies only active power and absorbs reactive power for magnetizing. These types of generators are used in windmills

Why synchronous generators are used for the production of electricity?

Synchronous machines have capability to work on different power factor(or say different imaginary power varying the field EMF. Hence used for the production of electricity

Enlist types of DC Generators?

DC Generators are classified into two types

·                     Separately excited DC generator

·                     Self excited DC Generator which is further classified into Series, Shunt and Compound (which is further classified into Cumulative and Differential)

What is Automatic Voltage Regulator (AVR)?

AVR is an abbreviation for Automatic Voltage Regulator. It is an important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator

What is Eddy Current Loss?

It is a phenomenon caused by the rate of change in an induced magnetic field. The relative motion causes a circulating flow of electrons or current within the conductor, leading to efficiency loss

What is plugging of an Induction motor?

Plugging is a method of braking used in induction motors.  Plugging involves interchanging the supply to two of the stator phase windings.  This method is used in applications which require immediate stop applications.  When the phase supply is reversed, torque is produced in the opposite direction.  This leads to braking of the electric motor

What is the principle of operation of tube light?

A fluorescent lamp is a glass tube containing some mercury vapour, with a phosphorescent coating on the interior surface of the tube. The mercury vapour is made to glow by using a high voltage across its electrodes that sets off an electric arc discharge in the tube. The discharge can also be described as conducting plasma

The resulting flow of current through the mercury vapour is stable and well-controlled by external ballast, or loading device, which consists of a high-inductance choke-coil that is connected in series with the tube. The continuous discharge glow is invisible to the human eye but it causes the phosphorescent coating on the interior surface of the tube to emit visible light

What are Brushless Alternators?

A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature

What is equalizer ring used for?

An equalizer, or equalizer connection, connects two or more generators operating in parallel at a point where the armature and series-field leads join, thus connecting the armatures in multiple and the series coils in multiple so that the load will divide between the generators in proportion to their capacities

What is a Ground line?

The purpose of the ground line is to provide a continuous, unbroken path to allow a circuit fault current to return to the distribution panel to trip the circuit's over current device

How may the number of parallel paths in an armature be increased?

By increasing the number of magnetic poles

How are brushes connected in DC generator?

Usually, all positive brushes are connected together and all negative brushes together

How can field distortion be remedied?

By using compensating winding which are embedded in the slots in the pole-shoe and are connected in series with the armature

What is meant by Normal Neutral Plane?

It is a plane which passes through the axis of armature perpendicular to the magnetic field of the generator when there is no flow of current through the armature

What is the best way of minimizing eddy currents in an armature?

Lamination is the best way

Should the brushes of a loaded generator be placed in the neutral plane?

No, because the brushes must be advanced by a certain angle (called brush lead) beyond the neutral plane to prevent sparking.

What causes sparking at the brushes?

It is due to the self-induction of the coil undergoing commutation.

What is the standard direction of rotation of the DC generators?

Clockwise when viewed from the end opposite to the driven end

What is meant by voltage build up of a generator?

It means the gradual increase in the generator voltage to its maximum value after the generator is started from rest

How should a generator be started?

It is usually brought up to speed with the help of the driving engine called prime mover

What is the procedure for shunting down the generator?

First, the load should be gradually reduced, if possible, by easing down the driving engine, and then when the generator is supplying little or no current, the main switch should be opened. When the voltmeter reads almost zero, then brushes should be raised from the commutator

Can a generator be reversed by reversing the connections between armature and field coils?

No, because if these connections are reversed, the generator will not build up at all

What is the permissible rise of temperature in a well-designed generator?

27⁰C above the surrounding air

What are causes of hot bearings?

The causes are

·                     lack of oil

·                     belt too tight

·                     armature not centred with respect of pole pieces

·                     bearings too tight or not in line

What causes heating of armature?

The causes are

·                     eddy currents

·                     moisture which almost short-circuits the armature

·                     unequal strength of magnetic poles

·                     Operation above rated voltage and below normal speed.

What is commutator pitch of a 4-pole DC armature having 49 commutator bars?

Y_C = (49+_-1)/2 = 24 or 25.

How may the direction of rotation of a DC motor be reversed?

This can be done by reversing either the field current or current through the armature. Usually, reversal of current through the armature is adopted

What happens if the direction of current at the terminals of a series motor is reversed?

It does not reverse the direction of rotation of motor because current flows through the armature in the same direction as through the field

What will happen if a shunt motor is directly connected to the supply line?

Small motors up to 1 KW rating may be line-started without any adverse results being produced

High rating motors must be started through a suitable starter in order to avoid the huge starting current which will

-                      Damage the motor itself

-                      Badly affect the voltage regulation of the supply line

A DC motor fails to start when switched on. What could be the possible reasons and remedies?

Any one of the following reasons could be responsible:

·                     Open-circuit in controller-should be checked for open starting resistance or open switch or open fuse

·                     Low terminal voltage-should be adjusted to name-plate value.

·                     Overload-should be reduced if possible otherwise larger motor should be installed.

·                     Excessive friction-bearing lubrication should be checked.

Why does a DC motor sometime run too fast when under load? Give different possible causes and their remedies.

Different possible causes are as under:

·                     Weak field-remove any extra resistance in shunt field circuit

·                     Line voltage too high-reduce it to name-plate value

·                     Brushes back of neutral-set them on neutral

What causes are responsible for over-heating of Commutator in a DC motor?

It could be due either to the brushes being off neutral or being under excessive spring pressure. Accordingly, brushes should be adjusted properly and the spring pressure should be reduced but not to the point where sparkling is introduced

How is magnetic leakage reduced to a minimum in commercial transformers?

This is done by interleaving the primary and secondary windings

Mention the factors on which Hysteresis loss depends?

·                     Quality and amount of iron in the core

·                     Flux density

·                     Frequency

How do changes in supply voltage and frequency affect the performance of an induction motor?

High voltage decreases both power factor and slip, but increases torque. Low voltage does just the opposite. Increase in frequency increases power factor but decreases the torque. However, per cent slip remains unchanged. Decrease in frequency decreases power factor but increases torque leaving per cent slip unaffected as before

What is, in brief, the basis of operation of a 3-phase induction motor?

The revolving magnetic field which is produced when a 3-phase stator winding is fed from a 3-phase supply

Why induction motors are called asynchronous?

Because their rotors can never run with the synchronous speed

How does the slip vary with load?

The greater the load, greater is the slip or slower is the rotor speed

Enumerate the possible reasons if a 3-phase motor fails to start?

Any one of the following reasons could be responsible:

·                     One or more fuses may be blown

·                     Voltage may be too low

·                     The starting load may be too heavy

·                     Worn bearings due to which the armature may be touching field laminae, thus introducing excessive friction

A motor stops after starting i.e., it fails to carry load. What could be the reasons?

Following can be the reasons

·                     Hot bearings, which increase the load by excessive friction

·                     Excessive tension on belt, which causes the bearings to heat

·                     Failure of short cut-out switch

·                     Single-phasing on the running position of the starter

Which is the usual cause of blow-outs in Induction motors?

The most common cause is single-phasing

Can a 3-phase motor be run on a Single-Phase Line?

Yes, it can be. But a Phase-Splitter is essential

What is a meant by a Phase-Splitter?

It is a device consisting of a number of capacitors so connected in the motor circuit that it produces, from a single input wave, three output waves which differ in phase from each other

What is the standard direction of rotation of an Induction motor?

Counter clock wise, when looking from the front end i.e. non-driving end of the motor

How would you reverse the direction of rotation of a capacitor start-induction-run motor?

By reversing either the running or starting-winding leads where they are connected to the lines. Both must not be reversed

What could be the reasons if a split-phase motor fails to start and hums loudly?

It could be due to the starting winding being open or grounded or burnt out.

What could be the reasons if a split-phase motor runs too slow?

Any one of the following factors could be responsible

·                     Wrong supply voltage and frequency

·                     Overload

·                     Grounded starting and running windings

·                     Short-circuited or open winding in field circuit

What are the two types of turbo-alternators?

Vertical and horizontal

What is direct-connected alternator?

One in which the alternator and engine are directly connected. In other words, there is no intermediate gearing such as belt, chain etc. between the driving engine and alternator

What is the difference between direct-connected and direct-coupled units?

In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling

Can a generator be converted into an alternator?

Yes, by providing two collector rings on one end of the armature and connecting these two rings to two points in the armature winding 118⁰ apart

For what service are synchronous motors especially suited?

They are especially suited for high voltage service

Which has more efficiency; synchronous or induction motor?

Synchronous motor

Mention some specific applications of synchronous motors

Its applications are

·                     constant speed load service

·                     reciprocating compressor drives

·                     power factor correction

·                     voltage regulation of transmission lines

What is a synchronous capacitor?

An over excited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a leading current

What could be the reasons if a synchronous motor fails to start?

It is usually due to following reasons

·                     Voltage may be too low

·                     Some faulty connection in auxiliary apparatus

·                     Too much starting load

·                     Open circuit in one phase or short circuit

·                     Field connection may be excessive.

Under which conditions a synchronous motor will fail to pull into step?

The conditions are

·                     No field excitation

·                     Excessive load

·                     Excessive load inertia

Do stepper motors have internal or external fans?

No, because the heat generated in the stator winding is conducted through the stator iron to the case which is cooled by natural conduction, convection and radiation

Any disadvantage of having more phases?

Advantages are

·                     More leads have to be brought out from the motor

·                     More interconnections are required to the drive circuit

·                     More switching devices are needed.

Will there be any harm if the rotor of a hybrid stepper motor is pulled out of its stator?

Yes. The rotor will probably become partially demagnetized and, on reassembling, will give less holding torque

What are the advantages and disadvantages of Compact Fluorescent Lamps?

The advantages are

·                     CFLs are up to four times more efficient than incandescent bulbs

·                     While initially they may cost more, CFLs are less expensive in the long run as they last much longer than incandescent bulbs

·                     CFLs are highly versatile and can be used in any setting that you would normally use incandescent bulbs

The disadvantages are

·                     CFLs when used outdoors need to be covered and protected from the elements. They are also sensitive to temperature

·                     While CFLs are supposed to last about 10,000 hours, turning them on and off too frequently can reduce that lifetime substantially

·                     CFLs are not suitable for focused or spot lights or where narrow beams of light are required. They are meant only for ambient light.

Give an example of integrating instruments

Simple example for integrating instrument is energy meter that is used in houses which indicates the summation of readings (units) consumed

Can a series motor be started without any load?

No, because if a DC series motor is started without load, the armature winding (winding that doesn't spin) current will be at a minimum, resulting in the motor going to maximum speed / runaway, which can cause things to fly apart. As Armature current increases, the speed of the motor will decrease. The armature current will be forced to increase / speed of the motor will be forced to decrease as more and more load is applied

What is specified in synchronous watts?

It is the unit of torque an is defined as

T_sy = Rotor input in watts/2*pi*N_s

What is Phase Swinging?

When a synchronous motor is used for driving a varying load, then a condition known as phase swinging or hunting may occur. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines)

Are single phase induction motors self-starting?

In single phase induction motor, there is no self starting as initially torque is zero.
but we can make it self-stating by adding an extra winding known as starting or auxiliary winding and space the two by 90 degrees. Due to this, the two currents will produce revolving flux what will make the induction motor self starting

How to conserve electricity at homes?

Electricity can be conserved by following methods

·                     Switch off lights, fans and other gadgets when not required

·                     Do not leave your computer and TV on standby

·                     Set the AC in economy mode so that it switches off when the room cools to the required temperature

·                     Buy energy efficient home appliances

Basic Electronics

What is an oscillator?

An oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in many electronic devices. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games

Why positive feedback is given to the oscillator?

Positive feedback is used in oscillators to keep them running or oscillating. An oscillator is basically an amplifier which has a feedback path from the output back to the input. This permits a portion of the output signal to get back to the input to "keep things going" in stage - to keep things oscillating. The feedback must be positive so that it will "contribute" to the stage being able to keep oscillating. If the feedback was negative, it would serve to damp the oscillation and to "kill" the oscillator

Does oscillator has an input?

No. In an oscillator, positive feedback is used to supply its own input signal for its operation.

What are freewheeling diodes?

A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode) is a diode used to eliminate flyback, the sudden voltage spikes seen across an inductive load when its supply voltage is suddenly reduced or removed

What is Phase Margin?

Phase margin (PM) is the difference between the phase, measured in degrees, of an amplifier's output signal (relative to its input) and 180°, as a function of frequency

What is modulation?

Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted

What are the uses of PLL circuits?

PLL circuits are now frequently being used to demodulate FM signals, AM demodulators, FSK decoders, two-tone decoders and motor speed controls

What is a Low Pass filter?

A low-pass filter is an electronic filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cut off frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in audio applications

What are the uses of Schmitt trigger circuits?

Schmitt triggers are typically used in open loop configurations for noise immunity and closed loop configurations to implement function generators

What is the one main difference between UPS & inverter ?

Uninterrupted power supply is mainly used for short time, means according to UPS VA it gives backup. UPS is of two types: Online and Offline. Online UPS has high Volt and Amp for long time backup with high DC voltage. But UPS start with 12v DC with 7 amps. But inverter starts with 12v, 24v DC to 36v DC and 120 Amp to 180 Amp batteries with long time backup

What is an UPS?

An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails

Principles of Power Systems

What are the equipments used in sub- station?

The elements of a sub- station are

·                     Primary power lines

·                     Ground wire

·                     Overhead lines

·                     Transformer for measurement of electric voltage

·                     Disconnect switch

·                     Circuit breaker

·                     Current transformer

·                     Lightning arrester

·                     Main transformer

·                     Control building

·                     Security fence

·                     Secondary power lines

What is the difference between Relay and Fuse?

A relay is a switch; it is not usually meant to open under fault conditions (high current).

A fuse is a fault protection device. If a short circuit develops, large currents will flow, and the fuse link will melt, causing the circuit to open

What is PLCC?

Power line communication or power line carrier (PLC), also known as power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or broadband over power lines (BPL) are systems for carrying data on a conductor and are also used for electric power transmission. A wide range of power line communication technologies are needed for different applications, ranging from home automation to Internet access

Why is alternating current used for transmission?

Ans. AC is used for transmission because transformers can step it up to high voltage for long distance, and down to low voltage for local distribution and use. The output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC

Also, it is much easier to change the voltage of AC electricity for transmission and distribution and the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivalent of DC transmission
AC transmission provides a number of technical advantages. When a fault in the network occurs, large fault current flows.  In an AC system, this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt

What is corona?

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as Corona

What is skin effect?

The tendency of alternating current to concentrate near the surface of a conductor is known as Skin Effect

Why is voltage stepped up before transmission?

Ans. Voltage is stepped up before transmission due to the following reasons:

·                     Reduces volume of conductor material

·                     Increases transmission efficiency

·                     Decreases percentage line drop

What are standard Transmission and Distribution voltages?

Primary Transmission: 66 kV, 132 KV, 220 kV and 400 kV

Secondary Transmission: 33 kV

Primary Distribution: 11 kV, 3-phase, 3-wire

Secondary Distribution: 400 V between two phases (3-phase, 4-wire) and 230 V between any one phase and neutral

Explain Transmission and Distribution systems

Electric-power transmission system is the system used for bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. The different systems of transmission are:

·                     DC system

·                     Single phase AC system

·                     Two phase AC system

·                     Three phase AC system

The part of power system which distributes electric power for local use is known as Distribution System. The different distribution systems are:

·                     AC Distribution system

-                    Primary distribution system

-                    Secondary distribution system

·                     DC Distribution system

-                    2 wire DC system

-                    3 wire DC system

What are Lightning Arrestors?

A Lightning Arrestor is a device used in electrical power systems to protect the insulation on the system from the damaging effect of lightning

Explain the principle of operation of a Relay and a Circuit breaker

RELAY: An electromagnetic relay works on two principles

·                     Electromagnetic attraction

·                     Electromagnetic induction

The force of attraction produced has two components-one is constant and the other is fluctuating at a frequency, twice the natural frequency. This force in terms of current is represented as-

F= K₁ – K₂ * I²

K₁=K₂ * I²

Since K₁ and K₂ both are constants, so the current I will be constant. This is the condition when the relay is at the verge of operation and F=0.

CIRCUIT BREAKER: Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms

What is a Bus bar?

A Bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. They form a link between the incoming and outgoing circuits. The size of the bus bar determines the maximum amount of current that can be safely carried

What is Arc formation?

When a short circuit occurs, a heavy current flows through the contacts of the circuit breaker and they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between the contacts is sufficient to ionise the medium. The ionised air or vapour acts as a conductor and an arc is struck between the contacts.

What is 3-Phase supply?

It is a type of poly phase system and is the most common method used by grids worldwide to transfer power. In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times

Why do we require protection against lightning?

The lightning surges may cause serious damage to the expensive equipments in the power system (e.g. generators, transformers etc.) either by direct strokes on the equipment or by the strokes on the transmission lines that reach the equipment as travelling waves

What is a Sub-station?

The assembly of apparatus used to change some characteristics (e.g. voltage, AC to DC, frequency, power factor) of the electric supply is called a sub-station

What is grading of cables?

The process of achieving uniform electrostatic stress in the dielectric of cables is called grading of cables

What is transmission cable made up of?

The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper, varnished cambric and polyvinyl chloride.

Explain the purpose of fuse

When a short circuit or overload occurs, the current through the fuse increases beyond its rated value which raises the temperature and fuse element melts (or blows out), disconnecting the circuit protected by it

What is ACSR cable and where we use it?

ACSR means Aluminium conductor steel reinforced. This conductor is used in transmission & distribution

What will happen when power factor is leading in distribution of power?

If there is high power factor, i.e. if the power factor is close to one

·                     Losses in the form of heat will be reduced,

·                     Cable becomes less bulky and easy to carry, and very cheap to afford, &

·                     It also reduces over heating of transformers.

What are the faults in a three phase line?

The types of faults in a three phase line are

·                     Phase-To-Earth Fault

·                     Phase-To-Phase Fault

·                     Phase-To-Phase-To-Earth Fault

·                     Three-Phase Fault

·                     Three-Phase-To-Earth Fault

·                     Phase-To-Pilot Fault

·                     Pilot-To-Earth Fault

What is the approximate power generation capacity in India?

Sector	MW
State Sector	83,563.65
Central Sector	56,572.63
Private Sector	42,553.34
Total	1,82,689.62

Is corona more in AC or DC?

Corona Loss p = .00241[(f+25)/$]*[root of (r/d)]*square of[Vp-Vo]

f= supply frequency    

$= air density correction factor

Vp = operating voltage

Vo= critical disprutive voltage

r= radius of conductor

d= distance of separation for fine weather

Now for DC f=0, there is a certain power loss by corona but less than AC corona loss.

What is current chopping?

Current chopping in CB is the incident of arc current interruption before normal current zero is reached. If this current is enough to deliver a voltage which is greater than restriking voltage then the arc will continue again. But gradually the current falls and certainly the voltage also reduces and finally falls down the restriking voltage and ultimately arc is extinguished. It results in voltage oscillation in the line which affects the electrical equipments

Which circuit breaker uses current chopping circuit?

Vacuum circuit breaker uses current chopping circuit

What is SCADA?

SCADA (supervisory control and data acquisition) generally refers to Industrial Control Systems (ICS): computer systems that monitor and control industrial, infrastructure, or facility-based processes

Why do we use a wave trap?

Line trap also is known as Wave trap. It helps in trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/ tele-protection panel in the substation control room (through coupling capacitor and LMU). The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation Bus bars

Which are preferred - Underground or Overhead cables?

Although many low voltage, local electric distribution lines are buried underground, almost all high-voltage electric transmission lines are tried to be placed as overhead cables due to the cost difference

What is an IDMT relay?

It is an Inverse Definite Minimum Time relay. In IDMT relay, operating is inversely proportional to fault current and also a characteristic of minimum time after which this relay definitely operates

What is a diversity factor?

Diversity Factor = Sum of Individual Maximum Demand/ Maximum Demand on Power Station

Electrical Machine Design

What is the basic structure of a rotating electrical machine?

It mainly consists of the following circuits

·                     Magnetic circuit

·                     Electric circuit

·                     Dielectric circuit

·                     Thermal circuit

·                     Mechanical parts

What is Specific Electric Loading?

The number of Armature (or Stator) Ampere Conductors per Metre of Armature (or Stator) periphery at the air gap is known as Specific Electric Loading

What is Specific Magnetic Loading?

The average flux density over the air gap of a machine is known as Specific Magnetic Loading

What are the various types of ratings of a machine?

The different types of ratings are

·                     Continuous duty

·                     Short time duty

·                     Intermittent periodic duty

·                     Intermittent periodic duty with starting

·                     Intermittent periodic duty with starting and braking

·                     Continuous duty with intermittent periodic duty

·                     Continuous duty with starting and braking

·                     Continuous duty with periodic speed changes

List some ventilating systems

The different ventilating systems are

·                     Radial ventilating system

·                     Axial ventilating system

·                     Combined radial and axial

What is a Runaway speed?

It is defined as the speed which the prime mover would have, if it is suddenly unloaded when working at its rated load

What are the modes of Heat Dissipation?

The heat can be dissipated by

·                     Conduction

·                     Convection

·                     Radiation

What is a Field Form Factor?

It is the ratio of the average flux density over the pole pitch to the maximum flux density. It is also known as flux distribution factor

What is a Window Space Factor?

It is defined as the ratio of copper area in the window of the total window area

What are the different types of cores?

Different Types of cores are

·                     Rectangular core

·                     Square core

·                     Stepped core