1. A duplex lap wound armature is used in six pole dc machine with six brush set. The number of current paths in this machine is?
a) 12
b) 6
c) 3
d) 2
Explanation: The number of parallel paths for a lap wound machine is number of parallel paths. As it is a duplex winding, it will have 6*2 = 12 parallel paths.
2. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils has simplex winding. The number of current paths is?
a) 2
b) 6
c) 3
d) 4
Explanation: The number of parallel path for a wave wound machine is equal to two.
3. For a four pole, 200 V dc shunt generator having armature resistance of 1 ohms. The current flowing in the armature is 10 A. What is the back emf generated if the terminal voltage of 220 V supply is fed?
a) 230 Volts
b) 210 Volts
c) 190 Volts
d) 200 Volts
Explanation: E = V+I*ra
= 220+10*1
= 230 Volts.
4. For a 4 pole 200 V, 20 A dc generator running at the rated rpm with no load will be ________
a) zero no load losses
b) 4000 W no load losses
c) negligible no load losses
d) none of the mentioned
Explanation: As there is no load and assuming no armature resistance, the no load losses should ideally be zero.
5. Which of the following cannot reduce the terminal voltage of a dc shunt generator?
a) Commutation
b) Armature reaction
c) Armature ohmic losses
d) Any of the mentioned
Explanation: Commutation ideally does not reduce the terminal voltage of a dc machine. If it takes place properly at desired timings, the losses can be avoided. Armature reaction, ohmic losses contribute to the losses in the terminal voltage.
6. What is the principle of torque production in a dc machine?
a) Lorentz’s law
b) Lenz’s law
c) Faraday’s law
d) Self inductance
Explanation: Though all the mentioned laws contribute towards the torque generation, but primarily its the Lorentz’s forces which initiate and define the torque direction.
7. The torque induced beyond the pole shoes in the DC machine is?
a) 0
b) 2/pi *phi *i
c) 4/pi *phi *i
d) none of the mentioned
Explanation: The magnetic field vanishes at the pole terminals and thus the cross product with the magnetic field yields zero.
8. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. Suddenly the switch is closed at t=0, what is observed in the circuit?
a) Current will flow but zero induced EMF
b) Current will not flow and zero induced EMF
c) Current will not flow but EMF is induced
d) Current will flow and EMF will also be induced
Explanation: Initially the induced emf in the armature is zero. But the current will flow as there is terminal source voltage.
9. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What will be the magnitude of the following current at t=0+?
a) 400A
b) 200A
c) 0
d) any of the mentioned
Explanation: I = (V-E)/r
= (120-0)/0.3
= 400 A.
10. If the torque induced is zero in the dc machine, it can be said that ____________
a) current is zero
b) flux can be zero
c) current or flux=0
d) any of the mentioned
Explanation: The torque at steady state of operation is zero as the emf induced is equal to the source voltage. Also the the very much starting the current will be zero which will not let the emf to get induced.