DC Motor Quiz MCQ

1. Starters are used with D.C. motors because

(a) these motors have high starting torque
(b) these motors are not self-starting
(c) back e.m.f. of these motors is zero initially
(d) to restrict armature current as there is no back e.m.f. while
starting

2. For starting a D.C. motor a starter is required because

(a) it limits the speed of the motor
(b) it limits the starting current to a safe value
(c) it starts the motor
(d) none of the above

3. The-type of D.C. motor used for shears and punches is

(a) shunt motor
(b) series motor
(c) differential compound D.C. motor
(d) cumulative compound. D.C. motor

4. If D.C. motor is connected across the A.C. supply it will

(a) run at normal speed
(b) not run
(c) run at lower speed
(d) burn due to heat produced in the field winding by eddy currents

5. To get the speed of D.C. motor below the normal without wastage of electrical energy is…………. used.

(a) Ward Leonard control
(b) rheostatic control
(c) any of the above method
(d) none of the above method

6. When two D.C. series motors are connected in parallel, the resultant speed is

(a) more than the normal speed
(b) less than the normal speed
(c) normal speed
(d) zero

7. The speed of a D.C. shunt motor more than its full-load speed can be obtained by

(a) decreasing the field current
(b) increasing the field current
(c) decreasing the armature current
(d) increasing the armature current

8. In a D.C. shunt motor, speed is

(a) independent of armature current
(b) directly proportional to the armature current.
(c) proportional to the square of the current
(d) inversely proportional to the armature current

9. If the speed of a D.C. shunt motor is increased the back e.m.f. of the motor will

(a) decrease
(b) increase
(c) remain same
(d) become zero

10. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

(a) The motor will stop
(b) The motor will continue to run
(c) The armature may burn
(d) The motor will run noisy

11. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

(a) back e.m.f. is equal to armature drop
(b) armature drop is negligible
(c) flux is proportional to armature current
(d) flux is practically constant in D.C. shunt motors

12. In a D.C. shunt motor, under conditions of maximum power, the current in the armature will be

(a) almost negligible
(b) rated full load current
(c) less than full-load current
(d) more than full-load current

13. Which D.C. motor will have least percentage increase of input current for the same percentage increase in torque?

(a) Shunt motor
(b) Series motor
(c) Cumulative compound motor
(d) Separately excited motor

14. Which of the following load normally needs starting torque more than the rated torque?

(a) Blowers
(b) Conveyors
(c) Air compressors
(d) Centrifugal pumps

15. The starting resistance of a D.C. motor is generally

(a) low
(b) around 5000 ohms
(c) 1000 ohms
(d) infinitely large

16. The speed of a D.C. series motor is

(a) proportional to the armature current
(b) proportional to the square of the armature current
(c) proportional to field current
(d) inversely proportional to the armature current

17. In a D.C. series motor if the armature current is reduced by 50%, the torque of the motor will be equal to

(a) 100% of the previous value
(b) 50% of the previous value
(c) 25% of the previous value
(d) 10% of the previous value

18. The current drawn by the armature of D.C. motor is directly proportional

(a) the torque required
(b) the speed of the motor
(c) the voltage across the terminals
(d) none of the above

19. The power mentioned on the name plate of an electric motor indicates

(a) the power drawn in kW
(b) the power drawn in kVA
(c) the gross power
(d) the output power available at the shaft

20. In Ward Leonard method of speed control of a D.C. motor, change in speed of motor is obtained by the

(a) change in armature voltage of D.C. motor
(b) change in the field excitation of the D.C. motor
(c) change in armature current of D.C. motor
(d) change in supply voltage

21. Which D.C. motor has got maximum self loading property?

(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor

22. For the same H.P. rating and full load speed, following motor has poor starting torque

(a) shunt
(b) series
(c) differentially compounded
(d) cumulatively compounded

23.In case of conductively compensated D.C. series motors, the compensating winding is provided

(a) as separately wound unit
(b) in parallel with armature winding
(c) in series with armature winding
(d) in parallel with field winding

24. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?

(a) Starting torque
(b) Operating speed
(c) Full load current
(d) All of the above

25. When the speed of a D.C. motor Increases

(a) back e.m.f. increases and current drawn decreases
(b) back e.m.f. as well as current drawn both increase
(c) back e.m.f. as well as current drawn both decrease
(d) back e.m.f. decreases and current drawn increases

26. As compared to an induction motor, the air gap in a D.C. motor is

(a) less than 50%
(b) between 60% and 90%
(c) same
(d) more

27. Field winding of a D.C. series motor is usually provided with thick wire

(a) to provide large flux
(b) to reduce the use of insulating materials
(c) as it carries large load current
(d) in order to reduce eddy current

28. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is

(a) 4.0
(b) 2.0
(c) 1.0
(d) 0.5

29. The condition for maximum power in case of D.C. motor is

(a) back e.m.f. = 2 x supply voltage
(b) back e.m.f. =(½) x supply voltage
(c) supply voltage =(½) x back e.m.f.
(d) supply voltage = back e.m.f.

30. For which of the following applications a D.C. motor is preferred over an AC. motor ?

(a) Low speed operation
(b) High speed operation
(c) Variable speed operation
(d) Fixed speed operation

 

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