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1. Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:

• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes

less losses.

• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that

is why current also reduces up to 3 times and hence less motor burning is caused.

• In addition, starting torque is increased and it prevents the damage of motor winding.

2. State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both

have the same principle of electromagnetic induction, the only difference is that their construction.

Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip

rings and brushes riding against each other, hence it converts the induced emf into dc current for external

load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for

low voltage output rotating armature and stationary magnetic field is used.

3. Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:

a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.

b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC

transmission

c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.

d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current

will naturally tend to zero at some point making the current easier to interrupt.

4. How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and

distribution of energy in electrical form. Design of all power equipments also comes under power

engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid

systems and they might work on off grid systems that are not connected to the system.

5. What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:

• Low-tension cables, which can transmit voltage upto 1000 volts.

• High-tension cables can transmit voltage upto 23000 volts.

• Super tension cables can transmit voltage 66 kV to 132 kV.

6. Why back emf used for a dc motor? highlight its significance.

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a

DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature

rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In

starting, the value of back emf is zero.

7. What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the

rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a

percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

8. Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:

• For the operation of protective devices and for emergency lighting at generating stations and

substations.

• For starting, ignition and lighting of automobiles, aircrafts etc.

• For lighting on steam and diesel railways trains.

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• As a supply power source in telephone exchange, laboratories and broad casting stations.

• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible

9. Explain advantages of storage batteries

Few advantages of storage batteries are mentioned below:

• Most efficient form of storing energy portably.

• Stored energy is available immediately because there is no lag of time for delivering the stored energy.

• Reliable source for supply of energy.

• The energy can be drawn at a fairly constant rate.

10. What are the different methods for the starting of a synchronous motor.

Starting methods: Synchronous motor can be started by the following two methods:

• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor

poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation

is obtained.

• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles

and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage

induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating

continuously as the rotor field is locked with stator revolving field.

11. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines,

refrigerators, rolling mills, lathes, power factor improvement and cranes.

Following motors are used: -

• Vacuum cleaners- Universal motor.

• Phonographic appliances – Hysteresis motor.

• Vending machines – Shaded pole motor.

• Refrigerators – Capacitor split phase motors.

• Rolling mills – Cumulative motors.

• Lathes – DC shunt motors.

• Power factor improvement – Synchronous motors.

12. State Thevenin’s Theorem:

According to thevenin’s theorem, the current flowing through a load resistance

Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e.

the voltage across the two terminals when RL is removed) and sum of load resistance and internal

resistance of the network. It is given by Voc / (Ri + RL).

13. State Norton’s Theorem

The Norton’s theorem explains the fact that there are two terminals and they are as follows:

• One is terminal active network containing voltage sources

• Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to

the constant source of current and it allows giving the parallel resistance.

The Norton’s theorem also explains about the constant current that is equal to the current of the short

circuit placed across the terminals. The parallel resistance of the network can be viewed from the open

circuit terminals when all the voltage and current sources are removed and replaced by the internal

resistance.

14. State Maximum power transfer theorem

The Maximum power transfer theorem explains about the load that a resistance will extract from the

network. This includes the maximum power from the network and in this case the load resistance is being is

equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the

network. This resistance can be viewed by the output terminals and the energy sources can be removed by

leaving the internal resistance behind.

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15. Explain different losses in a transformer.

There are two types of losses occurring in transformer:

• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses.

Two types of iron losses are:

o eddy current loss

o Hysteresis loss.

These losses depend upon the supply voltage, frequency, core material and its construction. As long as

supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or

not. These are also known as constant losses.

• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary

windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary

winding and they are called variable losses. These losses depend upon the loading conditions of the

transformers. Therefore, these losses are also called as variable losses.

16. Explain different types of D.C motors? Give their applications

Different type of DC motors and their applications are as follows:-

• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable

for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe

machines, tools, belt or chain conveyor etc.

• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions

i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in

lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.

• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL

loads without any danger. This motor will therefore find its application in loads having high inertia load or

requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and

punches, coal cutter and winding machines etc.

17. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are

required in a dc machine.

Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it

carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the

current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo

commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must

be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the

brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of

compoles or interpoles, which are series excited by the armature current. These are located in the interpolar

region of the main poles and therefore influence the armature coils only when these undergo commutation.

18. Comment on the working principle of operation of a single-phase transformer.

Working principle of operation of a single-phase transformer can be explained as

An AC supply passes through the primary winding, a current will start flowing in the primary winding. As

a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced

in both the windings. Now, when the load is connected to the secondary side, the current will start flowing

in the load in the secondary winding, resulting in the flow of additional current in the secondary winding.

Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the

windings. The voltage induced in the primary winding is due to its self inductance and known as self

induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back

emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer

works on the principle of electromagnetic induction.

19. Define the following terms: Reliability, Maximum demand, Reserve-generating capacity, Availability

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(operational).

Reliability: It is the capacity of the power system to serve all power demands without failure over long

periods.

Maximum Demand: It is maximum load demand required in a power station during a given period.

Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes

for preventive maintenance is called reserve-generating capacity.

Availability: As the percentage of the time a unit is available to produce power whether needed by the

system or not.

20. Mention the disadvantages of low power factor? How can it be improved?

Disadvantages of low power factor:

• Line losses are 1.57 times unity power factor.

• Larger generators and transformers are required.

• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to

keep voltage drop within prescribed limits.

• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors

will have to carry more current at low power factor. This requires a large conductor size.

21. State the methods of improving power factor?

Methods of improving power factor:

• By connecting static capacitors in parallel with the load operating at lagging power factor.

• A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.

• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere

turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor

can be made to operate on leading power factor like an overexcited synchronous motor.

22. State the factors, for the choice of electrical system for an aero turbine.

The choice of electrical system for an aero turbine is guided by three factors:

• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.

• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with

simpler pitch- changing mechanism or variable speed with fixed pitch blades.

• Utilization of electrical energy output: in conjunction with battery or other form of storage, or

interconnection with power grid.

23. What are the advantages of VSCF wind electrical system?

Advantages of VSCF wind electrical system are:

• No complex pitch changing mechanism is needed.

• Aero turbine always operates at maximum efficiency point.

• Extra energy in the high wind speed region of the speed – duration curve can be extracted

• Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.

24. Explain the terms real power, apparent power and reactive power for ac circuits and also the units

used.

• Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit

of real power is watt. i.e. Expressed as W or kW.

• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of

apparent power is volt- ampere. Expressed as VA or KVA.

• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and

current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no

other unit but expressed in VAR or KVAR.

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25. Define the following: Average demand, Maximum demand, Demand factor, Load factor.

• Average Demand: the average power requirement during some specified period of time of

considerable duration is called the average demand of installation.

• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the

demand, which have occurred during a given period. It is measured accordingly to specifications,

over a prescribed time interval during a certain period.

• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the

rating of the connected load.

• Load Factor: It is defined as the ratio of the average power to the maximum demand.

26. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.

• Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called

forward-resistance.

• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the

diode to the direct current flowing through it.

• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward

characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

27. How does Zener phenomenon differ from Avalanche breakdown?

The phenomenon when the depletion region expands and the potential barrier increases leading to a very

high electric field across the junction, due to which suddenly the reverse current increases under a very

high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur

independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are

caused by Zener Effect. Junctions that experience breakdown above 5v are caused by avalanche-effect. The

Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The

avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

28. Compare JFET’s and MOSFET’s.

Comparison of JFET’s and MOSFET’s:

• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either

depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction

occurs and the gate-current is substantial.

• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small

leakage current.

• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.

• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be

in a comparable MOSFET.

30. Explain thin film resistors and wire-wound resistors

a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating

substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves

of suitable pitch along its length. During this process, the value of the resistance is monitored closely and

cutting of grooves is stopped as soon as the desired value of resistance is obtained.

b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire

wound resistors. These wires are made of materials such as Constantan and Manganin because of their

high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an

insulating material such as baked enamel

31. What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-

lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise

injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the

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amplifying circuitry rejects it being a common mode signal.

CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a

differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain

would be zero.

32. 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

33. 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.

34. 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.

35. How do we specify the rating of a transformer?

Transformer rating is specified in Kilovolt-Ampere (KVA)

36. What are the cooling methods to cool a transformer?

The cooling methods are:

Oil filled self cooled

Oil filled water cooled

Air blast type

37. 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.

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

Yes, no-load primary current

39. 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.

40. 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.

41. What is the difference between earth and neutral?

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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.

42. 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.

43. 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

44. 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

45. What is Hysteresis loss?

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

46. Which is the motor used in fan?

Single-phase induction motor is used in fan

47. 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

48. 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)

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

Shock due to DC is more dangerous than AC

50. What are the practical applications of variable resistance?

Its applications are:

Volume control

Fan speed control

Light dimmer

51. What is slip?

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

52. 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

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53. 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 Ns. This phenomenon is known as crawling of an

induction motor

54. 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

55. 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

56. 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.

57. 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

58. 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

59. 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

60. 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 TC

61. 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

62. 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

63. What are the uses of Servomotors?

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

units

64. 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

65. 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

66. 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

67. 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

68. 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

69. 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

70. 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

71. 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)

72. 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

73. 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

74. What is plugging of an Induction motor?

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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

75. 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

76. 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

coilsand a rotating armature (power coils). The main alternator uses the opposite configuration with a

rotating field and stationary armature

77. 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

78. 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

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

By increasing the number of magnetic poles

80. How are brushes connected in DC generator?

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

81. 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

82. 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

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

Lamination is the best way

84. 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.

85. What causes sparking at the brushes?

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

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

Clockwise when viewed from the end opposite to the driven end

87. 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

88. How should a generator be started?

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

89. 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

90. 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

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

270C above the surrounding air

92. 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

93. 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.

94. 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

95. 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

96. 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

1. Damage the motor itself

2. Badly affect the voltage regulation of the supply line

97. 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.

98. 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

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

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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

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

This is done by interleaving the primary and secondary windings

101. Mention the factors on which Hysteresis loss depends?

Quality and amount of iron in the core

Flux density

Frequency

102. 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

103. 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

104. Why induction motors are called asynchronous?

Because their rotors can never run with the synchronous speed

105. How does the slip vary with load?

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

106. 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

107. 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

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

The most common cause is single-phasing

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

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

110. 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

111. 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

112. 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

113. 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.

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114. 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

115. What are the two types of turbo-alternators?

Vertical and horizontal

116. 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

117. 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

118. 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

119. For what service are synchronous motors especially suited?

They are especially suited for high voltage service

120. Which has more efficiency; synchronous or induction motor?

Synchronous motor

121. 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

122. What is a synchronous capacitor?

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

leading current

123. 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.

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

The conditions are

No field excitation

Excessive load

Excessive load inertia

125. 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

126. Any disadvantage of having more phases?

Advantages are

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More leads have to be brought out from the motor

More interconnections are required to the drive circuit

More switching devices are needed.

127. 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

128. 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.

129. 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

130. 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

131. 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)

132. 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

133. 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

134. 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

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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

135. Does oscillator has an input?

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

136. 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

137. 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

138. 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

139. 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

140. What is a Low Pass filter?

A low-pass filter is an electronic filter that passes low-frequency signals but attenuates (reduces

theamplitudeof) 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

141. What are the uses of Schmitt trigger circuits?

Schmitt triggers are typically used in open loop configurations for noise immunity and closed

loopconfigurations to implement function generators

142. 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

143. 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, typicallymains

power, fails

144. 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

145. What is the difference between Relay and Fuse?

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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

146. 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

147. Why is alternating current used for transmission?

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

148. What is corona?

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission

line is known as Corona

149. What is skin effect?

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

150. Why is voltage stepped up before transmission?

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

Reduces volume of conductor material

Increases transmission efficiency

Decreases percentage line drop

151. 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

152. Explain Transmission and Distribution systems

Electric-power transmission system is the system used for bulk transfer of electrical energy, from

generatingpower 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

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Secondary distribution system

DC Distribution system

2 wire DC system

3 wire DC system

153. 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

154. 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

155. What is a Bus bar?

A Bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution

board,substationor 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

156. 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.

157. 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

158. 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

159. 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

160. What is grading of cables?

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

161. What is transmission cable made up of?

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The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper,

varnished cambric and polyvinyl chloride.

162. 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

163. What is ACSR cable and where we use it?

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

164. 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.

165. 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

166. 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

167. 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.

168. 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

169. Which circuit breaker uses current chopping circuit?

Vacuum circuit breaker uses current chopping circuit

170. 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

171. Why do we use a wave trap?

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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

172. 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

173. 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

174. What is a diversity factor?

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

175. 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

176. 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

177. What is Specific Magnetic Loading?

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

178. 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

179. List some ventilating systems

The different ventilating systems are

Radial ventilating system

Axial ventilating system

Combined radial and axial

180. 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

181. What are the modes of Heat Dissipation?

The heat can be dissipated by

Conduction

Convection

Radiation

182. What is a Field Form Factor?

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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

183. What is a Window Space Factor?

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

184. What are the different types of cores?

Different Types of cores are

Rectangular core

Square core

Stepped core