Electrical Machines Quiz MCQ Questions

1. Between Field’s test and Hopkinson’s test the main common thing is that both

(a) use negligible power
(b) are regenerative tests
(c) need two similar mechanically coupled motors
(d) need two electrically coupled series motors

2. The use of armature divertor in the rheostatic method of speed control for a D.C. shunt motor makes the method

(a) less expensive
(b) less wasteful
(c) suitable for rapidly changing loads
(d) unsuitable for changing loads

3. The series parallel system of speed control of D.C. series motors widely used in traction work gives a speed range of about

(a) 1:10
(b) 1 : 8
(c) 1: 8
(d) 1 : 4

4. If conditions for maximum power for a D.C. motor are established the efficiency of the motor will be

(a) less than 50%
(b) 60 to 70%
(c) 80 to 90%
(d) 100%

5. In which of the following tests only one motor is required

(a) Brake test
(b) Hopkinson’s test
(c) Field’s test
(d) Swinburne’s test

6. In Field’s test for series motors, one motor drives the other machine as

(a) cumulative compound generator
(b) differential compound generator
(c) separately excited generator
(d) series generator

7. A motor for punching machine is usually subjected to

(a) no load
(b) continuous part toad
(c) continuous full load
(d) intermittent load

8. Which of the following is not necessarily the advantage of D.C. motors over A.C. motors ?

(a) Batter speed control
(b) Low cost
(c) High starting torque
(d) Wide speed range

9. The armature shaft of a D.C. motor must be able to withstand

(a) any unbalanced magnetic pull on the armature core
(b) twisting strains due to transmission of torque
(c) bending moment due to the weight of the armature and commutator
(d) all of the above

10. In an overloaded motor main danger arises due to

(a) winding getting overheated
(b) bus-bars getting heated
(c) starter getting damaged
(d) bearings getting overheated

11. A wide and very sensitive speed control is usually required in case of

(a) elevators
(b) steel rolling mills
(c) colliery winders
(d) all of the above

12. Regenerative braking on D.C. shunt motors is used when

(a) the load has overhauling characteristics
(b) the load is variable
(c) the load also acts as a braking force
(d) the load is constantly decreasing

13. Which motor should be used for centrifugal pumps?

(a) Series motor
(b) Shunt motor
(c) Either of the above
(d) None of the above

14. Which of the following methods is most effective in finding out the no load losses in a large D.C. shunt motor ?

(a) Field’s test
(b) Block rotor test
(c) Swinburne’s test
(d) Hopkinson’s test

15. The main disadvantage of Hopkinson’s test for finding efficiency of the shunt D.C. motors is that it

(a) needs one motor and one generator
(b) require two identical shunt machines
(c) requires full-load power
(d) ignores any change in iron loss

16. Which losses can be determined by performing the retardation test ?

(a) Friction losses
(b) Eddy current losses
(c) Stray losses
(d) Copper losses

17. The generated e.m.f. and the current are in the opposite direction in case of

(a) D.C. generators
(b) D.C. motors
(c) D.C. generators and D.C. motors
(d) none of the above

18. Hopkinson’s test is conducted at

(a) no load
(b) part load
(c) low load
(d) full load

19. Which of the following tests can be conducted on all types of D.C. machines ?

(a) Hopkinson’s test
(b) Running down test
(c) Block rotor test
(d) Field’s test
(e) Brake test

20. In case of a shunt motor if the supply voltage is increased by 10% which of the following will decrease?

(a) full load current
(b) full load speed
(c) starting torque
(d) none of the above

21. Which of the following, in a D.C. motor can sustain the maximum temperature rise?

(a) Commutator
(b) Armature windings
(c) Slip-rings
(d) Field windings

22. Speed control by the variation of flux, in a D.C. shunt motor, will give

(a) constant torque drive
(b) variable torque drive
(c) constant power drive
(d) variable power drive

23. A brake test on D.C. motors is usually restricted to

(a) small horse power motors
(b) variable speed motors
(c) high speed motors
(d) open frame type motors

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

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

25. 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

26. A three point starter is considered suitable for

(a) shunt motors
(b) shunt as well as compound motors
(c) shunt, compound and series motors
(d) all D.C. motors

27. Small D.C. motors up to 5 H.P. usually have

(a) 2 poles
(b) 4 poles
(c) 6 poles
(d) 8 poles

28. A shearing machine has cyclic load consisting of intermittent light and heavy loads. Which of the following D.C. motor will be suitable for this purpose?

(a) Series motor
(b) Shunt motor
(c) Cumulative compound motor
(d) Differential compound

29. 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

 

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