DC MACHINES 11

DC MACHINES MCQ 11

1. The brake test for the determination of efficiency of a dc machine is
a) an indirect method.
b) a regenerative method.
c) a direct method.
d) none of these
ans. c

2. In Swinburne's method for the determination of efficiency of a dc machine
a) the no-load losses are calculated and the copper losses are measured.
b) the no-load losses and measured copper losses are calculated.
c) both the no-load losses and the copper losses are measured.
d) both the no-load losses and the copper losses are calculated.
ans. b

3. In the Kapp's modificaiton of Hopkinson's efficiency test
a) the power losses in the two machine are supplied mechanically.

b) the power output of the generator is dissipated in a resistor.
c) the two machines are mechanically decoupled.
d) the power losses in the two machines are supplied electrically.
ans. d

4. Commutation conditions at full-load for a large dc machine can be checked by
a) the brake test
b) the swinburne's test
c) the Hopkinson's test
d) none of these
ans. c

5. A dc shunt motor is running at rated speed with rated supply voltage. If the supply voltage is halved , then the speed of the motor becomes
a) half of the rated speed
b) double the rated speed
c) slightly less than the rated speed
d) slightly more than the rated speed
ans. c

6. A dc shunt motor is driving a constant torque load without any additional resistance in the armature circuit . If an additional resistance is placed in the armature circuit then the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) becomes zero
ans. b

7. A dc shunt motor is driving a constant torque load with rated excitation. If the field current is halved then the speed of the motor
a) becomes half
b) becomes slightly more than half
c) becomes double
d) becomes slightly less than double.
ans. d

8. A dc shunt motor is running at rated speed with rated excitation, rated voltage  and with an additional resistance in the armature circuit .Speeds less than the rated speed can be achieved by
a) Reducing the supply voltage and increasing the field excitation.

b) Increasing the supply voltage and decreasing the armature circuit resistance

c) Decreasing the field excitation and increasing the supply voltage.
d) Decreasing the armature circuit resistance and decreasing the field excitation.
ans. a

9. In Ward-Leonard method of speed control the direction of rotation of the motor is reversed usually by
a) reversing the connections of the generator field terminals
b) reversing the connection of the generator armature terminals
c) reversing the connections of the motor armature terminals
d) none of these.
ans. a

10. A dc shunt motor is driving a mechanical load at rated voltage and the rated excitation. If the load torque becomes double then the speed of the motor
a) increase slightly
b) decreases slightly
c) becomes double
d) becomes half
ans. b

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