1. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils. The commutator pitch for the machine should be ________
a) 1
b) -1
c) +1 and 1
d) none of the mentioned
Explanation: Commutator pitch for the wave connected progressive winding should be 1.
2. The brushes in dc machine are normally placed electrically in interpolar region to ________
a) make an angle of 90 deg. electrical
b) make an angle of 90 deg. mechanical
c) make an angle of 0 deg. electrical
d) none of the mentioned
Explanation: To obtain proper commutation without any sparks, the magnetic neutral axis and geometrical neutral axis should be aligned to each other,such that coils undergoing commutation experience zero emf across them.
3. The armature rotation in the dc machine and the magnetic field are independent of each other and obey laws of conservation of energy.
a) True
b) False
Explanation: The armature rotation does not affect the stored magnetic energy at the steady state of operation.
4. Under commutation results ________
a) sparking at the trailing edge of brush
b) sparking at the leading edge of brush
c) sparking at the middle of brush
d) any of the mentioned
Explanation: Under commutation takes place when the commutation speed is slow and it takes more time than critical defined time for commutation. Because of this the trailing edge of the brush still remains under the commutation.
5. While the operation of dc motor, there are sparkings at the commutation observed. These can be happening due to ________
a) under commutation
b) over commutation
c) too rapid reversal of the current
d) any of the mentioned
Explanation: The over commutation, over commutation or rapid reversal of the current, any of these reasons can lead to sparking at the brushes.
6. If the speed of rotation of a separately excited dc machine is halved and the filed strength is also reduced to half of the original. Then the emf induced in the armature _____________
a) reduces to 1/4th of original
b) reduces to half of original
c) remains same
d) doubles
Explanation: Emf is proportional to speed of rotation and flux.
7. If the speed of rotation for a four pole, 220V dc machine is 1 m/s, flux density as 2T having length of bore as 0.1m and radius of 0.5m. The emf induced under pole faces will be ________
a) 0.2 V
b) 0.1 V
c) 0.2*pi V
d) 220 V
Explanation: E = B*l*v
= 2*0.1*1
= 0.2 Volts.
8. The induced emf in the armature of a 4-pole dc machine is ________
a) directly proportional to speed and field strength applied to it
b) directly proportional to speed
c) directly proportional to field strength applied to it
d) inversely proportional to speed and inversely proportional to field strength applied to it
Explanation: Emf is proportional to speed of rotation and flux.
9. If the armature current is by mistake increased to 50% for the dc machine then ________
a) the induced emf remains unchanged
b) the induced emf doubles
c) the induced emf halves
d) none of the mentioned
Explanation: Emf is proportional to speed of rotation and flux.
10. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. The same machine is then wave wound, the emf induced in the lap wound dc machine to that of wave wound will be ________
a) 2
b) 0.5
c) 1
d) 4
Explanation: E = 1/(P/2)
= 1/(4/2) = 1/2 V.