Electrical Machines (Special Machines)

1. A single-stack, 4-phase, 6-pole VR stepper motor will have a step angle of
(a) 15º
(b) 30º
(c) 45º
(d) 90º

2. In a three-stack 12/8-pole VR motor, the rotor pole pitch is
(a) 15º
(b) 30º
(c) 45º
(d) 60º

3. A three-stack VR stepper motor has a step angle of 10º. What is the number of rotor teeth in each stack ?
(a) 36
(b) 24
(c) 18
(d) 12

4. If a hybrid stepper motor has a rotor pitch of 36º and a step angle of 9º, the number of its
phases must be
(a) 4
(b) 2
(c) 3
(d) 6

5. What is the step angle of a permanent-magnet stepper motor having 8 stator poles and 4 rotor poles ?
(a) 60º
(b) 45º
(c) 30º
(d) 15º

6. A stepping motor is a .................. device.
(a) mechanical
(b) electrical
(c) analogue
(d) incremental

7. Operation of stepping motors at high speeds is referred to as
(a) fast forward
(b) slewing
(c) inching
(d) jogging

8. Which of the following phase switching sequence represents half-step operation of a VR
stepper motor ?
(a) A, B, C, A .......................
(b) A, C, B, A .......................
(c) AB, BC, CA, AB .......................
(d) A, AB, B, BC .......................

9. The rotational speed of a given stepper motor is determined solely by the
(a) shaft load
(b) step pulse frequency
(c) polarity of stator current
(d) magnitude of stator current.

10. A stepper motor may be considered as a .................. converter.
(a) dc to dc
(b) ac to ac
(c) dc to ac
(d) digital-to-analogue

11. The rotor of a stepper motor has no
(a) windings
(b) commutator
(c) brushes
(d) all of the above.

12. Wave excitation of a stepper motor results in
(a) microstepping
(b) half-stepping
(c) increased step angle
(d) reduced resolution.

13. A stepper motor having a resolution of 300 steps/rev and running at 2400 rpm has a pulse
rate of— pps.
(a) 4000
(b) 8000
(c) 6000
(d) 10,000

14. The torque exerted by the rotor magnetic field of a PM stepping motor with unexcited stator is called ..................torque.
(a) reluctance
(b) detent
(c) holding
(d) either (b) or (c)

15. A variable reluctance stepper motor is constructed of .................. material with salient poles.
(a) paramagnetic
(b) ferromagnetic
(c) diamagnetic
(d) non-magnetic

16. Though structurally similar to a control transmitter, a control receiver differs from it in
the following way :
(a) it has three-phase stator winding
(b) it has a rotor of dumbell construction
(c) it has a mechanical damper on its shaft
(d) it has single-phase rotor excitation.

17. The control ........................ synchro has three phase winding both on its stator and rotor.
(a) differential
(b) transformer
(c) receiver
(d) transmitter

18. Regarding voltages induced in the three stator windings of a synchro, which statement is
false ?
(a) they depend on rotor position.
(b) they are in phase.
(c) they differ in magnitude.
(d) they are polyphase voltages.

19. The low-torque synchros cannot be used for
(a) torque transmission
(b) error detection
(c) instrument servos
(d) robot arm positioning.

20. Which of the following synchros are used for error detection in a servo control system ?
(a) control transmitter
(b) control transformer
(c) control receiver
(d) both (a) and (b).

21. For torque transmission over a long distance with the help of electrical wires only, which of the following two synchros are used ?
(a) CX and CT
(b) CX and CR
(c) CX and CD
(d) CT and CD.

22. The arrangement required for producing a rotation equal to the sum or difference of the
rotation of two shafts consists of the following coupled synchros.
(a) control transmitter
(b) control receiver
(c) control differential transmitter
(d) all of the above.

23. Which of the following motor would suit applications where constant speed is absolutely
essential to ensure a consistent product ?
(a) brushless dc motor
(b) disk motor
(c) permanent-magnet synchronous motor
(d) stepper motor.

24. A switched reluctance motor differs from a VR stepper motor in the sense that it
(a) has rotor poles of ferromagnetic material
(b) rotates continuously
(c) is designed for open-loop operation only
(d) has lower efficiency.

25. The electrical displacement between the two stator windings of a resolver is
(a) 120º
(b) 90º
(c) 60º
(d) 45º

26. Which of the following motor runs from a low dc supply and has permanently magnetized
salient poles on its rotor ?
(a) permanent-magnet d.c. motor
(b) disk d.c. motor
(c) permanent-magnet synchronous motor
(d) brushless d.c. motor.

27. A dc servomotor is similar to a regular d.c. motor except that its design is modified to cope with
(a) electronic switching
(b) slow speeds
(c) static conditions
(d) both (b) and (c).

28. One of the basic requirements of a servomotor is that it must produce high torque at all
(a) loads
(b) frequencies
(c) speeds
(d) voltages.

29. The most common two-phase ac servomotor differs from the standard ac induction motor
because it has
(a) higher rotor resistance
(b) higher power rating
(c) motor stator windings
(d) greater inertia.

30. Squirrel-cage induction motor is finding increasing application in high-power servo
systems because new methods have been found to
(a) increase its rotor resistance
(b) control its torque
(c) decrease its intertia
(d) decouple its torque and flux.

1. a 2. c 3. d 4. a 5. b 6. d 7. b 8. d 9. b 10. d 11. d 12. b 13. c 14. d 15. b 16. c 17. a 18. d 19. d 20. d 21. b 22. d 23. c 24. b 25. b 26. a 27. d 28. c 29. a 30. d.


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