DC Motor Multiple Choice Questions and Answers

1. Consider the following statements regarding speed control of dc motors:

1. Ward Leonard method is suitable for constant torque drive.
2. Field control method facilitates speed control below the base speed.
3. Armature resistance control method is more efficient when compared to Ward Leonard method.
4. Field control method is suitable for constant power drives.

Of these statements
(a) 1, 2 and 3 are correct.
(c) 2, 3 and 4 are correct.
(b) 1, 3 and 4 are correct.
(d) 1 and 4 are correct.

2. Consider the following statements regarding the speed control of dc motors:

1. Ward-Leonard method is suitable for constant torque drives.
2. Ward-Leonard method is suitable for constant power drives.
3. Field-control method facilitates speed control below base speed.
4. Armature resistance control method is more efficient than Ward-Leonard method.
5. Field control method is suitable for constant torque drives.
6. Armature resistance control method is suitable for constant torque drives.

From these the correct answer is
(a) 1, 2, 6
(b) 1, 2, 4
(c) 1, 6
(d) 1, 3, 4

3. For a given torque, reducing diverter resistance of a dc series motor

(a) increases its speed but armature current remains the same.
(b) increases its speed demanding more armature current.
(c) decreases its speed demanding less armature current.
(d) decreases its speed but armature current remains the same.

4. A dc series motor is operating at rated speed with rated excitation. It has two resistances R₁ and R₂ connected across the field and the armature respectively. Speeds above the rated speed can be obtained by

(a) increasing R₁ only.
(b) reducing R₂ only.
(c) reducing R₁ and increasing R₂.
(d) increasing R₁ and reducing R₂.

5. If an additional resistance is connected in series with a dc series motor operating at rated speed, the speed of the motor will

(a) reduce.
(b) increase.
(c) remain unaltered.
(d) oscillate around the normal speed.

6. If a resistance is connected across the armature of a dc series motor, its speed for a constant load torque will

(a) increase.
(b) decrease.
(c) remain unaltered.
(d) become zero.

7. Field divertor method of speed control of a dc series motor gives speeds above rated one due to reduction of

(a) field current.
(b) armature current.
(c) line current.
(d) none of these.

8. The field divertor resistance for a dc series motor is of the order of

(a) 0.1 Ω
(b) 10 Ω
(c) 50 Ω
(d) 100 Ω.

9. A dc series motor is running with a divertor connected across its field winding. If the divertor resistance is reduced then the speed of the motor will

(a) decrease.
(b) increase.
(c) become excessively high.
(d) remain unaltered.

10. Two dc series motors are connected in series and are driving the same mechanical load. If the motors are now connected in parallel, the speed will become

(a) one fourth.
(b) half.
(c) double.
(d) 4-times.

11. In series-parallel control of dc series motor, the torque produced in series arrangement will be …………. of that in parallel arrangement.

(a) double
(b) four times
(c) half
(d) one-fourth

12. The speed of a cumulatively compounded dc motor can be controlled by varying

(a) armature voltage.
(b) armature resistance.
(c) shunt field resistance.
(d) any of the above.

13. The function of the field regulator in a dc compound motor is to

(a) limit the armature current.
(b) vary the armature resistance.
(c) control the field current.
(d) none of the above.

14. A cumulatively compounded long shunt motor is driving a load at rated torque and rated speed. If the series field is shunted by a resistance equal to the resistance of the series field, keeping the torque constant

(a) the armature current increases.
(b) the motor speed increases.
(c) the armature current decreases.
(d) the motor speed decreases.

15. As compared to the use of a single series dc motor for electric traction for a given starting time t, the series parallel control using two similar motors with time t/2 for each series and parallel operation would give a saving in starting energy of

(a) 100%
(b) 50%
(c) 25%
(d) zero.

16. The current drawn by a dc motor at starting is

(a) zero.
(b) very low.
(c) normal.
(d) very high.

17. High inrush of current during a starting of a dc motor can efficiently be restricted to a safer value by increasing the …………. resistance.

(a) shunt field
(b) armature circuit
(c) both (a) and (b)
(d) none of these

18. The dc motor draws high current at the starting instant due to absence of back emf. In order to prevent this high inrush of current at start, a resistance is connected

(a) across the field.
(b) across the armature.
(c) in series with the armature.
(d) in series with the field.

19. For efficient operation of a dc motor, the additional resistance inserted in the armature circuit should be

(a) increased in steps.
(b) cut-out in steps.
(c) left in the armature circuit.
(d) removed immediately after start.

20. Consider the following statements regarding the starting of a dc shunt motor:

1. External field resistance should be minimum.
2. External field resistance should be maximum.
3. Reduced voltage should be applied to the armature.
4. Increased voltage should be applied to the armature.

Of these statements
(a) 1 and 3 are correct.
(b) 1 and 4 are correct.
(c) 2 and 3 are correct.
(d) 2 and 4 are correct.

21. Which one of the following statements is correct for the ideal field excitation during starting in case of shunt and compound motors?

(a) Maximum, because motor torque required to overcome friction and load torque.
(b) Maximum, because voltage should be built slowly.
(c) Maximum, because small excitation will lead low operating speed during starting.
(d) It can be anything because it does not matter in case of these motors.

22. Consider the following statements: At starting, the field excitation of a dc shunt motor is kept at its maximum value to reduce

1. sparking at brushes.
2. acceleration time.
3. starting current.
4. voltage dip in supply.

Of these statements
(a) 1 and 2 are correct.
(b) 2, 3 and 4 are correct.
(c) 1, 3, and 4 are correct.
(d) 1, 2, 3, and 4 are correct.

23. In dc shunt motors, at start, the field excitation is kept at maximum value in order to

(a) reduce starting torque.
(b) increase acceleration duration.
(c) reduce armature heating.
(d) avoid voltage dip in the supply mains.

24. A dc shunt motor has external resistances of Ra and Rf in the armature and field circuits respectively. Armature current at starting can be reduced by keeping

(a) R_f maximum and R_a minimum.
(b) R_a maximum and R_f maximum.
(c) Ra minimum and Rf maximum.
(d) R_a maximum and R_f Minimum.

25. If starter is not used with large dc motor for starting, it will draw a starting current which

(a) is many times its rated full-load current.
(b) will produce excessive voltage drop in line.
(c) may damage the armature and commutator.
(d) will produce very low starting torque.
(e) all of the above except (d).

26. The function of a starter in a dc motor is to

(a) control its speed.
(b) increase its starting torque.
(c) limit the starting current to a safer value.
(d) reduce armature reaction effect.
(e) both (b) and (c).
(f) avoid sparking.

27. Direct-on-line (DOL) starters are not suitable for starting large dc motors, because

(a) the starting current will be enormously high, which will cause large voltage drop in the supply mains.
(b) the motor may not start.
(c) the motor may run away.
(d) the starting torque becomes low.

28. A DOL starter is employed for starting motors of ratings up to

(a) 3.75 kW
(b) 7.5 kW
(c) 15 kW
(d) 25 kW

29. In a dc shunt motor, the three point starter

(a) limits the starting current to a safer value.
(b) provides overload protection.
(c) provides no-volt protection.
(d) all of the above.

30. Three point starters are not suitable for applications where speed variation by flux control is required because the motor may

(a) not start.
(b) run away.
(c) stop at very high speed.
(d) stop at very slow speed.

 

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