126 130

28. Why series motor cannot be started on no-load?

Answer: Series motor cannot be started without load because of high starting torque. Series

motor are used in Trains, Crane etc.

29. Why ELCB can't work if N input of ELCB do not connect to ground?

Answer: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in

an ELCB, the current will flow through phase and that much current will have to return neutral so

resultant current is zero. Once there is a ground fault in the load side, current from phase will

directly pass through earth and it will not return through neutral through ELCB. That means once

side current is going and not returning and hence because of this difference in current ELCB will

trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault

current will definitely high and that full fault current will come back through ELCB, and there will

be no difference in current.

30. How electrical power is generated by an A.C Generator?

Answer: For the generation of elect power we need a prime mover which supplies mechanical

power input to the alternator, can be steam turbines, or hydro turbines. When poles of the rotor

moves under the armature conductors which are placed on the stator ,field flux cut the armature

conductor ,therefore voltage is generated and is of sinusoidal in nature...due to polarity change

of rotor poles(i,e) N-S-N-S.

31. Why an ac solenoid valve attract the plunger even though we interchanges the terminal?

Will the poles changes?

Answer: Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC

voltage is continuously changing for every half cycle. So, interchanging of terminals in ac system

does not show any difference. That's why the ac solenoid attract the plunger even though its

terminals are interchanged.

32. What is derating? Why it is necessary, it is same for all means for drives, motors, and

cables.

What is Automatic Voltage regulator (AVR)?

Answer: AVR is an abbreviation for Automatic Voltage Regulator. It is 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.

34. What is an exciter and how does it work

Answer: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to

supply the excitation dc voltage to the fixed poles of generator. Rotary exciter is an additional

small generator mounted on the shaft of main generator. If it is dc generator, it will supply dc to

the rotary poles through slip ring and brushes (conventional alternator). if it is an ac exciter,

output of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.ac exciter is

the ac generator whose field winding are stationary and armature rotates. Initial voltage is built

up by residual magnetism. It gives the starting torque to the generator.

35. Difference between a four point starter and three point starter?

Answer: The shunt connection in four point stater is provided separately from the line where as

in three point stater it is connected with line which is the drawback in three point stater

36. Why use the VCB at High Transmission System? Why can’t use ACB?

Answer: Actually the thing is vacuum has high arc quenching property compare to air because in

VCB, the die electric strengths equal to 8 times of air. That y always vacuum used as in HT

breaker and air used as in LT.

37. What is the difference between surge arrestor and lightning arrestor?

Answer: LA is installed outside and the effect of lightning is grounded, whereas surge arrestor

installed inside panels comprising of resistors which consumes the energy and nullify the effect of

surge.

38. Why syn. generators r used for the production of electricity?

Answer: synchronous machines have capability to work on different power factor (or say

different imaginary POW varying the field emf. Hence syn. generators r used for the production

of electricity.

39. Enlist types of dc generator?

Answer: D.C. Generators are classified into two types

1) Separately excited d.c.generator

2) Self-excited d.c.generator, which is further classified into; 1) series 2) shunt and

3) Compound (which is further classified into cumulative and differential).

40. What is the difference between synchronous generator & asynchronous generator?  

Answer: In simple, synchronous generator supply’s both active and reactive power but

asynchronous generator (induction generator) supply’s only active power and observe reactive

power for magnetizing. This type of generators are used in windmills.

41. Give two basic speed control scheme of DC shunt motor?

Answer: 1. 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

2. 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.

42. What is the principle of motor?

Answer: Whenever a current carrying conductor is placed in a magnetic field it produce turning

or twisting movement is called as torque.

43. What is meant by armature reaction?

Answer: The effect of armature flu to main flux is called armature reaction. The armature flux

may support main flux or opposes main flux.

44. What is the Polarization index value? (Pi value) and simple definition of polarization

index?

Answer: Its ratio between insulation resistance (IR) i.e. meggar value for 10min to insulation

resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good

condition it should be Greater than 2.5.

Polarization Index Test (PI) – The PI test measures the ability of the insulation to absorb voltage

over a period of time. This gives an indication of the overall insulation quality of the individual

pieces of insulation in the transformer. This test is usually performed in conjunction with the

Meggar Test. It is measured as ratio of insulation resistance (IR) for 10 minutes to insulation

resistance for 1 minute.

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

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

1. Losses in form of heat will be reduced,

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

3. It also reduces over heating of transformers.

46. What is 2 phase motor?

Answer: A two phase motor is a motor with the starting winding and the running winding have a

phase split. e.g.; ac servo motor. Where the auxiliary winding and the control winding have a

phase split of 90 degree.