Unit 33 Delmar’s Standard Textbook of Electricity
1. What are the three general classifications of split-phase motors? Resistance-start
induction-run, capacitor-start induction-run, capacitor-start capacitor-run
2. How many phases are required to produce a rotating magnetic field? 2
3. How many degrees (physically) out of phase are the start and run windings in a
split-phase motor? 90
4. How are the start and run windings identified? (Visually) Start is smaller wire and
placed near top of stator core. Run winding is larger wire and placed in bottom of
stator core
5. What two factors determine synchronous speed in the stator? Number of poles
and frequency
6. What factors determine the starting torque of a resistance-start, induction-run
motor? Stator field strength, rotor field strength, and phase angle between start
and run windings
7. What is the approximate phase angle between the start and run windings in a
resistance-start, induction-run motor? 35-40
8. Where is the centrifugal switch connected in a resistance-start, induction-run
motor? In series with the start winding
9. Why are starting relays used instead of a centrifugal switch on refrigeration and
air-conditioning equipment? Hermetically sealed
10. What is the advantage of a hot-wire starting relay? Both starting relay and
overload protection
11. Briefly, how does a current-relay operate? Senses current flow using magnetic
field
12. What is a thermistor and what is unique about it? Resistance changes dramatically
with temperature changes
13. Does a thermistor open the circuit to the start winding? Explain. No, it remains in
the circuit but the thermistor limits the current to a very small value
14. What is the phase relationship between the induced rotor voltage and the rotor
current for a squirrel-cage rotor? Almost 90 degrees (High Inductive reactance)
Fix
15. What is the direction rotation if the start winding is kept out of the circuit when
power is applied to a split-phase motor? Either direction with a little push
16. Where is a capacitor connected in a capacitor-start, induction-run motor? Series
with the centrifugal switch and start winding
17. Why is the starting torque much higher in a capacitor-start, induction-run motor
than it is in a resistance-start, induction-run motor? Start and run windings are full
90 degrees out of phase
18. What precaution should be taken with a capacitor-start, induction-run motor?
Start no more than 8 times per hour
19. Which terminals are typically switched to reverse a dual-voltage, split-phase
motor? T5 and T8
20. Where is the start winding connected for high-voltage when you have a dualvoltage run winding and a single-voltage start winding? Parallel with one of the
run windings
21. How is the direction of rotation determined for a motor without starting it?
Clockwise from rear T1 connected to T5
22. Which motor is designed so the start winding is not removed from the circuit?
Capacitor-start capacitor-run
23. What type of capacitor is typically used for a capacitor-start, capacitor-run motor?
Oil-filled
24. What is the benefit of leaving the capacitor connected in the motor circuit after
starting? Improve efficiency and power factor
25. How does a potential starting relay operate? Measures the voltage across the start
winding and opens a contact when it is high enough. (75% rated speed)
26. What are the advantages of the shaded-pole induction motor? Simplicity and long
life
27. What are the most common applications of shaded-pole motors? Fans and
blowers
28. At what point in the sine wave is there no voltage induced in the shaded-pole?
Peak value
29. How many poles does a shaded-pole induction motor typically have? Four or six
30. How would you describe the torque of a shaded-pole motor? Low
31. How is a shaded-pole motor reversed? (Practically) It isn’t (Can be by reversing
the rotor but typically isn’t)
32. How is the speed changed in a multi-speed fan motor? Rotary switch
33. What does the selector switch control in a multi-speed fan? Amount of inductance
in series with the run winding
34. What types of loads are best suited to multi-speed motors? Low torque (Fans and
blowers)
35. What are the three types of repulsion motors? Repulsion, repulsion-start
induction-run, and repulsion-induction
36. What principle does a repulsion motor operate on? Like poles repelling
37. How is the rotor and brush assembly on a repulsion motor different from a DC
armature and brush assembly? Brushes of a repulsion motor are shorted together
38. How is the number of poles for a rotor in a repulsion motor determined? Same as
the stator
39. Where should the brushes be placed in relation to the poles for maximum torque?
15 deg away
40. How is the direction of rotation changed for a repulsion motor? Move brushes 15
degrees to the other side of the poles
41. What is the single-phase motor with the highest starting torque? Repulsion
42. What are the two types of repulsion-start, induction-run motors? Brush-riding and
brush-lifting
43. How do the two types of repulsion-start, induction-run motors become induction
run? (What action makes it happen) Shorting of the commutator
44. How is a repulsion-induction motor different from a repulsion-start, induction-run
motor? Squirrel-cage windings are added to the armature
45. What is the advantage of the squirrel-cage winding in the rotor of the repulsioninduction motor? Much better speed characteristics
46. Where are single-phase synchronous motors typically used? Clock motors, timers
and recording instruments
47. What is another name for a single-phase synchronous motor? Hysteresis
48. What is the principle of operation for a stepping motor? Like magnetic poles repel
49. What determines the amount of angular rotation achieved each time a stepping
motor is stepped? Number of teeth machined into the stator
50. What is a permanent-magnet induction motor? Stepping motor
51. Which single-phase motor can be maintained in the stalled condition without
damage to the motor? Stepping motor
52. Which single-phase motor has a starting current that is the same as the running
current? Stepping motor
53. What is another name for the Universal Motor? AC Series
54. What is the purpose of the compensating winding in a universal motor? Counter
the inductive reactance of the armature winding
55. Why is a universal motor “Universal”? Can be operated on either AC or DC
56. What are the two methods of connecting the compensating winding for a
universal motor? Conductive compensation and inductive compensation
57. How would you rate the speed regulation of a universal motor? Poor
58. How is a universal motor reversed? Change the direction of current to the
armature with respect to the field current