1. With no commutating poles used the brushes are given backward lead in the dc motor.
a) True
b) False
Explanation: The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor
2. If the critical commutation time is 2 ms but it was observed that the practical commutation took 2.5 ms. This is the case of __________
a) under commutation
b) over commutation
c) critical commutation
d) none of the mentioned
Explanation: Since the time taken is more than the prescribed time limit for the commutation, it is under commutation
3. The coil under going commutation lies along _____ while the coil getting short circuited lies along ____ for a practical dc machine.
a) gna, mna
b) mna, gna
c) gna, gna
d) mna, mna
Explanation: In a practical dc machine, the coil undergoing commutation lies along GNA so as the brushes and the coils which actually get short circuited lies along mna. This lead to sparking as a voltage difference is created at the coils
4. The principle contributors to magneto motive force is/are __________
a) field current and armature current
b) field excitation
c) electromagnetic torque
d) all of the mentioned
Explanation: The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf
5. The absence of compensating winding lead to __________
a) statically induced emf in armature
b) fluctuations in supply
c) reduction in the flux
d) any of the mentioned
Explanation: The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.
6. The number of compensating conductors/pole faces is _______ (where Z is the number of active armature conductors/pole; A is number of parallel paths; Ia is the armature current).
a) Z/A*Ia
b) 2*A*Ia/Z
c) Z*Ia/A
d) Z/2A*Ia
Explanation: Compensating conductors per pole face is Z/A*Ia.
7. A 400-V, 1000-A, lap wound dc machine has 10 poles, 860 armature conductors. The number of conductors in the pole face to give full compensation if pole face covers 70% of pole span is?
a) 3010
b) 4300
c) 2400
d) 2800
Explanation: AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.
8. Each of the following is valid for interpoles except __________
a) they are connected in parallel with the armature so that they carry part of armature current
b) they are small yoke fixed poles spaced in between main poles
c) their polarity, in case of generator is same as that of main poles ahead
d) they automatically neutralize not only reactance voltage but cross-magnetization also
Explanation: All the options are valid for the interpoles except a because they are connected in series not in parallel.
9. Mark the most incorrect. In dc generator, commutation can be improved by __________
a) using interpoles
b) using carbon brushes
c) shifting brush axis in direction of armature rotation
d) none of the mentioned
Explanation: All the given measures are used to eliminate the effect of armature reaction and hence improves commutation also.
10. The commutation process involves basically reversal of current in armature coil as it crosses MNA.
a) True
b) False
Explanation: By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.