AC MACHINES 4

AC MACHINES

1. Tappings of a transformer are provided

a) at the phase end of the lv side.
b) at the phase end of hv side.
c) at the neutral end of hv side.
d) at the middle of hv side.

ans. d

2. Tappings are usually provided on the HV winding of a transformer because it

a) is easily accessible physically
b) has to handle low currents
c) has a large number of turns which allows smoother variation of voltage
d) all of the above

ans. d

3.In a constant voltage transformer (CVT), the output voltage remains constant due to

a) capacitor
b) input inductor
c) saturation
d) tapped winding's

ans. c

4. The rating of transformer _____ when the supply frequency is increased

a) remains unchanged
b) decreases
c) increases
d) None of these

ans. c

5. Distributed winding and short chording employed in ac machines will result in

a) increase in emf and reduction in harmonics
b) reduction in emf and increase in harmonics
c) increase in both emf and harmonics
d) reduction in both emf and harmonics

ans. d

6. It is desirable to eliminate the 5th harmonic from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of

a) 30 degrees
b) 36 degrees
c) 72 degrees
d) 18 degrees

ans. b

7. A 4-pole, 3-phase, double layer winding is housed in a 36 slot stator for an ac machine with 60 degrees phase spread. Coil span is 7 slot pitches. The number of slots in which top and bottom layers belong to different phases is

a) 24
b) 18
c) 12
d) 0

ans. a

8. The armature of a single-phase alternator is completely wound with T-single turn coils distributed uniformly. The induced voltage in each turn is 2V (RMS). The emf of the whole winding is

a) 2 T volts
b) 1.11 T volts
c) 1.414 T volts
d) 1.273 T volts

ans. d

9. A 3-phase alternator is wound with a 60-degree phase-spread armature winding and develops 300 kVA. If the armature is reconnected utilizing all the coils for single-phase operation with a phase spread of 180 degrees, the new rating of the machine is

a) 100 kVA
b) 200 kVA
c) 250 kVA
d) 300 kVA

ans. b

10. The phase sequence of the three-phase alternator will reverse if

a) the field current is reversed keeping the direction of rotation the same.
b) the field current remains the same but eh direction of rotation is reversed.
c) the field current is reversed and the number of poles is doubled.
d) the number of poles is doubled without reversing the field current.

ans. b