Pretest Module 21 Unit 4 Single-Phase Motors

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Aubrey Evans
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1 Pretest Module 21 Unit 4 Single-Phase Motors 1. What are the four main components of a single-phase motor? Rotor, stator, centrifugal switch, end bells and bearings 2. How is a rotating field created in a single-phase stator? Two windings out of phase with each other 3. Describe the main winding in a stator for a single-phase motor? Turns of large wire set deep in the stator core 4. Describe the auxiliary winding in a stator for a single-phase motor? Smaller wire placed near the top of the core 5. How far apart (physically) are the main and auxiliary windings placed in the stator for a single-phase motor? 90 Electrical degrees 6. What type of rotor does a split-phase induction motor have? Squirrel-cage (Conductive bars short circuited on both ends by conductive rings) 7. What is the purpose of the centrifugal switch in a split-phase induction motor? Remove the start winding from the circuit at approximately 75% of rated speed 8. How does a centrifugal switch operate? When the motor reaches a certain speed, centrifugal force overcomes the tension of the spring and the flyweights move outward opening a set of contacts 9. When does a centrifugal switch open? 75% Rated speed 10. What force will close a centrifugal switch? Spring tension 11. What information is typically found on the nameplate of a single-phase motor? Current rating, voltage rating, horsepower, and insulation class

2 12. Current in the run winding will lag the applied voltage by what angle? (Approximately) Near Current in the start winding will lag the applied voltage by what angle? (Approximately) Small angle closer to What is the typical phase angle between the start and run windings? What are the typical lead markings for the start winding in a single-phase motor? T5 and T8 16. What is the direction of rotation for the rotor in a single-phase motor in relation to the stator field? Same as 17. What two forces react to produce torque in a single-phase motor? Stator flux and rotor flux 18. Why is the start or auxiliary winding removed from the circuit when the motor reaches 75% of the rated speed? The run winding alone produces more torque than both the start and run after the motor reaches rated speed 19. How strong is the starting torque of a split-phase induction motor? Relatively poor 20. What are some applications for a split-phase induction motor? Fan and blower 21. How is a split-phase motor reversed? Change direction of current in start or run winding (typically start) 22. What is the purpose of the capacitor in a split-phase induction motor? Change the angle of displacement between the start and run windings 23. What is the phase angle between the start and run windings when a capacitor is installed in series with the start winding? 90 deg

3 24. After the centrifugal switch opens on a capacitor-start, induction-run motor and a split-phase, induction motor, what is the difference in operation? None 25. What is the maximum starting torque in a capacitor-start induction motor? Up to 450% 26. How does the size of a starting capacitor compare to the size of a run capacitor in a capacitor-start, capacitor-run motor? Starting capacitors are typically times more capacitance 27. What are the main advantages of the capacitor-start, capacitor-run motor? Improved power factor, efficiency, and overload capacity 28. What application is a capacitor-start, capacitor-run motor best suited for? Air compressors 29. What is the unique characteristic of a reluctance motor? Operates at synchronous speed 30. What are salient poles? Poles that project out from the rotor (sections cut out) 31. What are some applications for the reluctance motor? Timing devices and recording instruments 32. How are the terminal leads marked for the main winding of a split-phase motor? T1 and T4 33. What two terminals are switched to reverse a split-phase motor with a dual voltage start and run winding? (T5 and T8) 34. Which type of AC motor is similar in construction to a DC motor? AC Series 35. What is another name for an AC series motor? Universal

4 36. What is the difference between the windings in a DC motor and an AC series motor? Fewer turns to reduce inductive reactance 37. What type of rotor is in an AC series motor? Laminated steel core with the winding slots skewed for smother operation 38. What happens when AC is applied to a simple series DC motor? It will work but has some problems: Large eddy currents, reactance causes poor power factor and reduced motor current, high circulating currents 39. How is efficiency affected when a series DC motor is operated on AC? Much lower 40. What limits the current in an AC series motor? Counter EMF in the armature 41. What precautions should be taken with an AC series motor with regard to load? Direct coupled (No belts) 42. How is the direction of rotation changed for an AC series motor? Reverse current in the armature 43. What are the typical applications for an AC series motor? Portable hand tools and high-speed vacuums 44. How much of the stator pole is typically shaded in a shaded pole motor? 1/3 45. What are the two sets of windings called in a shaded-pole motor? Exciter winding and shaded-pole winding 46. What is the shading coil in a shaded-pole motor connected to? Shorted out 47. What type of rotor is found in a shaded-pole motor? Squirrel winding 48. In what direction does the flux shift across the pole face in a shaded-pole induction motor? Unshaded part to shaded part

5 49. How is the direction of rotation changed in a shaded-pole motor? Reverse the rotor 50. What are some advantages and disadvantages of shaded-pole motors? Cheap and simple to build. Poor starting torque, efficiency, and overload capability 51. What type of rotor is found in hystersis motors? Hardened Steel 52. What is the main advantage of a hystersis motor? Where is it used? Very smooth operation used in sound equipment 53. Which parts of a split-phase motor are not designed for continuous operation? Starting circuit components 54. How are parts not designed for continuous operation disconnected from the circuit in a split-phase motor? Centrifugal switch 55. What are some of the most common reasons for a split-phase motor not starting? Blown fuses, opens in windings, open or short in capacitor, overload, open in centrifugal switch 56. Which parts of an AC series motor require the most maintenance? Commutator, brushes, and brush rigging 57. What color should the surface of a commutator be after a few weeks in a series motor? Light brown 58. What does a light copper color on the surface of a commutator indicate? Brushes too hard 59. Should there be sparking at the brushes of a properly adjusted series motor? Yes minimal 60. What are some problems encountered with brush holders? Loose or misaligned

AC MOTOR TYPES. DESCRIBE how torque is produced in a single-phase AC motor. EXPLAIN why an AC synchronous motor does not have starting torque.

AC MOTOR TYPES. DESCRIBE how torque is produced in a single-phase AC motor. EXPLAIN why an AC synchronous motor does not have starting torque. Various types of AC motors are used for specific applications. By matching the type of motor to the appropriate application, increased equipment performance can be obtained. EO 1.5 EO 1.6 EO 1.7 EO 1.8