D.C. Motors Multiple Choice Questions And Answers In English -01

1. Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
(A) Lenz’s law
(B) Faraday’s law
(C) Coloumb’s law
(D) Fleming’s left-hand rule
Ans: D

2. 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
Ans: D

3. A D.C. series motor is that which
(A) has its field winding consisting of thick wire and less turns
(B) has a poor torque
(C) can be started easily without load
(D) has almost constant speed
Ans: A

4. In D.C. machines fractional pitch winding is used
(A) to improve cooling
(B) to reduce copper losses
(C) to increase the generated e.m.f.
(D) to reduce the sparking
Ans: D

5. Three point starter can be used for
(A) series motor only
(B) shunt motor only
(C) compound motor only
(D) both shunt and compound motor
Ans: D

6. Sparking, is discouraged in a D.C. motor because
(A) it increases the input power con-sumption
(B) commutator gets damaged
(C) both (A) and (B)
(D) none of the above
Ans: B

7. Speed control by Ward Leonard method gives uniform speed variation
(A) in one direction
(B) in both directions
(C) below normal speed only
(D) above normal speed only.
Ans: B

8. Which of the following motors one will choose to drive the rotary compressor ?
(A) D.C. shunt motor
(B) D.C. series motor
(C) Universal motor
(D) Synchronous motor
Ans: D

9. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will
(A) increase
(B) decrease
(C) remain same
(D) become zero
Ans: A

10. These days D.C. motors are widely used in
(A) pumping sets
(B) air compressors
(C) electric traction
(D) machine shops
Ans: C

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
Ans: D

12. If a 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
Ans: D

13. Sparking at the commutator of a D.C. motor may result in
(A) damage to commutator segments
(B) damage to commutator insulation
(C) increased power consumption
(D) all of the above
Ans: D

13. Which of the following motor is preferred for operation in highly explosive atmosphere ?
(A) Series motor
(B) Shunt motor
(C) Air motor
(D) Battery operated motor
Ans: C

14. When two D.C. series motors are connected in parallel, the resultant speed is
(A) more than the normal speed
(B) loss than the normal speed
(C) normal speed
(D) zero
Ans: C

15. Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
(A) Voltage control method
(B) Field control method
(C) Armature control method
(D) All above methods
Ans: C

16. Usually wide and sensitive speed control is desired in case of
(A) centrifugal pumps
(B) elevators
(C) steel rolling mills
(D) colliery winders
Ans: D

17. 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
Ans: A

18. The ratio of starting torque to full-load torque is least in case of
(A) series motors
(B) shunt motors
(C) compound motors
(D) none of the above
Ans: B

19. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
(A) remain unchanged
(6) reduce to one-fourth value
(C) increase four folds
(D) be doubled
Ans: A

20. At the instant of starting when a D.C. motor is put on supply, it behaves like
(A) a highly resistive circuit
(6) a low resistance circuit
(C) a capacitive circuit
(D) none of the above