DC MACHINES 12

 

DC MACHINES MCQ 12

1. Direct-on-line starters are not suitable for starting large dc motors because
a) the motor may not start.
b) large voltage drop may occur in the supply.
c) the starting torque becomes very low.
d) the motor may run away.
ans. b

2. For dc shunt motor, speed control by armature resistance variations is best suited for
a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. c

3. For dc shunt motor, speed control by the variation of field flux is best suited for

a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. a

4. If the field circuit of a dc shunt motor running at rated speed gets open-circuited , then immediately after    this the speed of the motor would tend to
a) decrease
b) increase
c) remain unchanged
d) oscillate around the rated speed
ans. b

5. The field's efficiency test of dc series motors overcomes the difficulty of obtaining readings relatively light loads by connecting
a) the series field of the generator in series with the motor armature.
b) the series field of the motor in series with the generator armature.
c) the armature of the generator in series with the motor.
d) the armature of the motor in series with the generator.
ans. a

6. DC motor starters are used
a) to increase the starting torque
b) to limit the starting current
c) both a and b
d) none of these
ans. b

7. For dc shunt motors the field excitation is kept at maximum value during starting to
a) increase acceleration time
b) decrease starting torque
c) reduce armature heating
d) prevent voltage dip in the supply mains
ans. c

8. DC motors should be stopped by opening the line switches and not by forcing the starter handle back to the off position
a) heavy sparking occurs at the brushes.
b) heavy sparking occurs at the first stud of the starting resistance steps.
c) both a and b
d) none of these.

9. A dc shunt motor has two additional resistances R1 and R2 in the field circuit and armature circuit respectively . The starting armature current can be kept to a minimum by keeping
a) R1 maximum R2 maximum
b) R1 minimum R2 maximum
c) R1 maximum R2 minimum
d) R1 minimum R2 minimum
ans. b

10. Three point starters of dc shunt motors are not used in applications where speed variation by field flux control is required because
a) the motor may stop at very high speeds
b) the motor may stop at very low speeds
c) the motor may stop both at very high and at very low speeds.
d) the motor may run away.
ans. a

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