HVAC - CH 17 - Electric Motors

1. Electric motors are employed to turn the prime movers of air, water, and refrigerant, specifically fans, pumps, and compressors. True/False

Answer: True. Explanation: Electric motors are used for these applications in the HVACR industry.

2. A technician's understanding of motor types and their uses is crucial for effective troubleshooting and replacement with the correct motor. True/False

Answer: True. Explanation: This knowledge ensures proper repair and selection of motors for specific jobs.

3. Despite numerous types of electric motors, most of them operate on fundamentally different principles. True/False

Answer: False. Explanation: Most electric motors operate on similar basic principles.

4. The stator in an electric motor is typically the rotating component responsible for generating the magnetic field. True/False

Answer: False. Explanation: The stator is a stationary winding that generates a magnetic field when energized, while the rotor is the rotating part.

5. What material is the rotor of an electric motor primarily made of? A. Copper B. Aluminum C. Iron D. Steel alloys

Answer: C. Explanation: The rotor is made of iron, and can include copper or aluminum bars.

6. Electric motor bearings are designed to increase friction to improve the motor's grip on its shaft. True/False

Answer: False. Explanation: Bearings reduce friction and heat generated by moving parts.

7. The motor housing, end bells, and base primarily serve to provide cooling for the internal motor components. True/False

Answer: False. Explanation: These parts hold the motor components in place and provide a means for secure mounting.

8. Magnets inherently possess two electrical poles, designated as positive and negative. True/False

Answer: False. Explanation: Magnets have two different magnetic poles, north and south.

9. If a stationary horseshoe magnet is used in an example of electric motor operation, it acts as which component? A. Rotor B. Shaft C. Stator D. Bearing

Answer: C. Explanation: In the example, the horseshoe magnet provides the stationary magnetic field, acting as the stator.

10. In a two-pole split-phase motor, the run windings are insulated wires that become an electromagnet when electrical current is applied. True/False

Answer: True. Explanation: The stator's run windings create the electromagnet.

11. In normal 60-cycle operation, the polarity of the electromagnet in a two-pole split-phase motor changes 60 times per second. True/False

Answer: False. Explanation: The polarity changes 120 times per second in 60-cycle operation because current direction changes twice per cycle.

12. A squirrel cage rotor is typically constructed from solid, uninsulated iron bars. True/False

Answer: False. Explanation: A squirrel cage rotor may be constructed of bars, often copper or aluminum, bound on the ends with aluminum.

13. The rotor in an electric motor is directly wired to the power source, similar to how the stator receives its power. True/False

Answer: False. Explanation: The rotor is not wired to the power source; the magnetic field is induced in it.

14. The attracting and repelling action between the poles of the run windings and the rotor creates a static magnetic field that causes the rotor to turn. True/False

Answer: False. Explanation: This action sets up a rotating magnetic field.

15. In the United States, what is the standard frequency of AC power supply? A. 50 hertz B. 120 hertz C. 60 hertz D. 360 hertz

Answer: C. Explanation: AC power in the United States is supplied at a frequency of 60 hertz.

16. In a single cycle of AC power, the direction of current flow changes only once. True/False

Answer: False. Explanation: The direction of current flow changes twice during each cycle.

17. For a motor with two poles, as in the rotating bar magnet example, the rotor will turn 60 times every second. True/False

Answer: True. Explanation: At 60 Hz, with two poles, it will turn 60 times per second, equating to 3600 times a minute.

18. The synchronous speed of a motor, calculated without load, is always higher than its actual operating speed. True/False

Answer: True. Explanation: Internal motor component friction and connected mechanisms add resistance, causing motor slip.

19. What is the calculated synchronous speed in rpm for a four-pole motor operating on a 60 Hz AC power supply? A. 3600 rpm B. 1800 rpm C. 1200 rpm D. 900 rpm

Answer: B. Explanation: Using the formula (Frequency * 120) / Number of poles, (60 * 120) / 4 = 1800 rpm.

20. Motor slip is often expressed as a percentage and represents the difference between the motor’s calculated and actual speeds. True/False

Answer: True. Explanation: This is the definition of motor slip.

21. Excessive motor slip can indicate issues such as improper motor lubrication or defective motor bearings. True/False

Answer: True. Explanation: These are listed as potential causes of excessive motor slip.

22. The start winding in an electric motor is designed to be electrically connected for the entire duration of the motor's operation. True/False

Answer: False. Explanation: The start winding is typically disconnected once the rotor approaches its rated speed.

23. Compared to the run winding, the start winding generally has more turns and is wound with a smaller diameter wire. True/False

Answer: True. Explanation: This design feature is described for the start winding.

24. What is the term for the twisting power of the motor shaft? A. Amperage B. Voltage C. Torque D. Resistance

Answer: C. Explanation: Torque is defined as the twisting power of the motor shaft.

25. A refrigeration compressor must have a low starting torque because system pressures typically equalize during the off cycle. True/False

Answer: False. Explanation: A refrigeration compressor must have a high starting torque to start under heavy loads. Systems with capillary tubes may use low starting torque, but TXV systems require greater starting torque.

26. Locked-rotor amperage (LRA) is typically about half of the full-load or rated-load amperes (FLA or RLA) for a motor. True/False

Answer: False. Explanation: LRA is typically about five times the FLA/RLA.

27. The inrush current at motor start-up is of long duration and significantly affects wire sizing for the motor. True/False

Answer: False. Explanation: The inrush current is of short duration and does not figure in the wire sizing for the motor.

28. A fan motor requires a great deal of starting torque to overcome significant pressure differences when beginning operation. True/False

Answer: False. Explanation: A fan motor requires only enough starting torque to overcome friction, as there is no pressure difference when it starts.

29. Residences in the United States are typically furnished with a single-phase power supply. True/False

Answer: True. Explanation: This is a common characteristic of residential power supplies.

30. A typical residential electrical panel can furnish both 115-V and 230-V service. True/False

Answer: True. Explanation: This allows for operation of various appliances.

31. What is a common voltage for large loads such as electric heat or motors in commercial buildings and factories when using three-phase power? A. 115-V single-phase B. 277-V single-phase C. 460-V three-phase D. 560-V three-phase

Answer: C. Explanation: 460-V three-phase is listed as a common option for large loads.

32. Using 460-V three-phase power in a commercial building requires larger wire sizes compared to using 230-V for the same load, increasing installation costs. True/False

Answer: False. Explanation: 460-V reduces wire size, reducing materials and labor costs because it uses half the current of 230V for the same load.

33. A step-down transformer is utilized in a system with 460-V three-phase power to provide lower voltage circuits for small fans and office equipment. True/False

Answer: True. Explanation: This allows for various voltage needs within the same system.

34. Some single-phase motors are designed to operate in dual voltage modes, often 115 V or 208 V to 230 V. True/False

Answer: True. Explanation: Dual-voltage motors are mentioned for single-phase applications.

35. When a dual-voltage motor operates in the high-voltage mode (e.g., 230 V), its two run windings are typically wired in parallel. True/False

Answer: False. Explanation: For high-voltage mode, the run windings are wired in series.

36. The direction of rotation of a single-phase motor can be reversed by changing the connections in the motor terminal box to reverse the current flow through the start winding. True/False

Answer: True. Explanation: This is a method for reversing rotation.

37. Split-phase motors are typically used for operating fans in the fractional horsepower range due to their high starting torque. True/False

Answer: False. Explanation: They have a medium amount of starting torque and are used for fans.

38. What is a common normal operating range for split-phase motors in rpm? A. 900 rpm and 1200 rpm B. 1725 rpm and 1750 rpm C. 1800 rpm and 3600 rpm D. 3450 rpm and 3500 rpm

Answer: C. Explanation: The normal operating ranges given are 1800 rpm and 3600 rpm (synchronous speeds).

39. If a split-phase motor loaded to the point where its speed falls below 1725 rpm (for an 1800 rpm motor), its current draw will decrease. True/False

Answer: False. Explanation: The current draw will climb above the rated amperage.

40. The speed of a split-phase motor is solely determined by the method of wiring its motor poles. True/False

Answer: False. Explanation: It's determined by both the number of motor poles and the method of wiring.

41. All split-phase motors require their start windings to remain connected to the circuit for an extended period to ensure smooth operation. True/False

Answer: False. Explanation: Start windings must be disconnected quickly to prevent overheating.

42. The centrifugal switch is the most common method for disconnecting start windings in hermetic motors. True/False

Answer: False. Explanation: It is most common in open motors, as the arc it produces would harm refrigerant in hermetic motors.

43. At approximately what percentage of its rated speed does a centrifugal switch typically disconnect the start winding from the circuit? A. 50% B. 65% C. 75% D. 90%

Answer: C. Explanation: The centrifugal switch operates at approximately 75% of rated speed.

44. If a centrifugal switch fails to open its contacts, the motor will draw too much current, potentially activating an overload device. True/False

Answer: True. Explanation: This failure mode leads to high current draw and motor protection activation.

45. A capacitor-start motor is fundamentally different from a split-phase induction motor, having only one winding for both starting and running. True/False

Answer: False. Explanation: It is basically the same as a split-phase induction motor, having two distinct windings for starting and running.

46. In a capacitor-start motor, a start capacitor is wired in parallel with the start windings to enhance starting torque. True/False

Answer: False. Explanation: The start capacitor is wired in series with the start windings.

47. In an inductive circuit, the current typically leads the voltage. True/False

Answer: False. Explanation: In an inductive circuit, the current lags the voltage.

48. The capacitor in a capacitor-start motor is designed to remain in the circuit during the entire running operation to improve efficiency. True/False

Answer: False. Explanation: The capacitor is not designed for continuous use and must be switched out after the motor starts.

49. Capacitor-start motors are sometimes referred to as CSIR motors, standing for "Capacitor-Start-Induction-Run." True/False

Answer: True. Explanation: This acronym reflects their operational characteristics.

50. What type of motor is one of the most efficient used in refrigeration and air-conditioning equipment and often used with belt-drive fans and compressors? A. Shaded-pole motor B. Split-phase motor C. Permanent split-capacitor (PSC) motor D. Capacitor-start, capacitor-run motor

Answer: D. Explanation: Capacitor-start, capacitor-run motors are described as very efficient and suitable for belt-drive applications.

51. In a capacitor-start, capacitor-run motor, both the run and start capacitors are wired in series with the start winding, and in parallel to one another. True/False

Answer: True. Explanation: This configuration provides high starting torque and efficient running.

52. The microfarad rating of the run capacitor in a capacitor-start, capacitor-run motor is generally much higher than that of the start capacitor. True/False

Answer: False. Explanation: The run capacitor's microfarad rating is much lower.

53. When the start switch opens in a capacitor-start, capacitor-run motor, the run capacitor is also taken out of the circuit. True/False

Answer: False. Explanation: The run capacitor and start winding remain in the circuit after the start capacitor is removed.

54. If a run capacitor fails due to an open circuit, the motor may still start, but the running amperage will typically be about 10% too low. True/False

Answer: False. Explanation: The running amperage will be about 10% too high.

55. A permanent split-capacitor (PSC) motor utilizes a start switch to disconnect its start winding once the motor reaches speed. True/False

Answer: False. Explanation: PSC motors do not use a start switch; both windings and the run capacitor are continuously active.

56. Which motor type is described as having very low starting torque, limiting its use to low-starting-torque applications? A. Capacitor-start motor B. Three-phase motor C. Permanent split-capacitor (PSC) motor D. Split-phase motor

Answer: C. Explanation: PSC motors are noted for their very low starting torque.

57. In a multispeed PSC motor, decreasing the electrical resistance of the motor winding typically decreases the motor speed. True/False

Answer: False. Explanation: As resistance decreases, motor speed increases.

58. For 60-Hz operation, a 6-pole PSC motor would have a rated synchronous speed of 1800 rpm. True/False

Answer: False. Explanation: A 6-pole motor would have a synchronous speed of 1200 rpm (60 * 120 / 6).

59. A significant advantage of the PSC motor in fan applications compared to a split-phase motor is its very soft start-up, which reduces noise. True/False

Answer: True. Explanation: This soft start-up is a desirable characteristic mentioned for PSC motors.

60. Shaded-pole motors are known for their high starting torque and excellent efficiency, making them suitable for heavy-duty applications. True/False

Answer: False. Explanation: They have very little starting torque and are not as efficient as PSC motors, used only for light-duty applications.

61. What physical feature of a shaded-pole motor helps it start by providing an induced current and a rotating field? A. Main capacitor B. Centrifugal switch C. Shading coils D. Potential relay

Answer: C. Explanation: Shading coils are small starting windings that facilitate starting.

62. The direction of rotation on most shaded-pole motors can be reversed by simply changing the electrical connections at the terminal box. True/False

Answer: False. Explanation: Rotation reversal typically involves disassembling the motor and turning the stator over.

63. Three-phase motors usually range from about 1 hp up to thousands of horsepower. True/False

Answer: True. Explanation: This wide range of horsepower is characteristic of three-phase motors.

64. Three-phase motors require no start assist components like starting windings or capacitors because they have inherent low starting torque. True/False

Answer: False. Explanation: They require no start assist because they have high starting torque.

65. A 3600-rpm three-phase motor will have three sets, each with four poles, for a total of 12 poles. True/False

Answer: False. Explanation: A 3600-rpm motor would have three sets of two poles (total of six).

66. Three-phase power supply consists of three single-phase power supplies that are generated 90 electrical degrees out of phase with each other. True/False

Answer: False. Explanation: They are 120 electrical degrees out of phase.

67. To reverse the rotation of a three-phase motor, one must switch all three motor leads in a specific sequence. True/False

Answer: False. Explanation: Switching any two motor leads will reverse the rotation.

68. When a three-phase fan rotates in the wrong direction, it will move approximately half as much air as intended. True/False

Answer: True. Explanation: This is a practical consequence of incorrect rotation.

69. What type of bearings would be preferred in a motor application where noise minimization is a critical factor? A. Ball bearings B. Roller bearings C. Sleeve bearings D. Tapered bearings

Answer: C. Explanation: Sleeve bearings are suggested for quieter operation compared to ball bearings.

70. A hermetically sealed motor is designed to operate in a dry, inert gas atmosphere, not a refrigerant vapor atmosphere. True/False

Answer: False. Explanation: Hermetic motors operate in a refrigerant vapor atmosphere.

71. Materials used in the construction of a hermetic compressor motor, such as winding coatings and papers, must be compatible with the system's refrigerant and oil. True/False

Answer: True. Explanation: Compatibility is crucial to prevent deterioration.

72. Hermetic single-phase compressors are commonly manufactured up to what horsepower rating? A. 1 hp B. 3 hp C. 5 hp D. 7.5 hp

Answer: C. Explanation: Single-phase hermetic compressors are usually manufactured up to 5 hp.

73. In a hermetic motor, a spark from disconnecting the start winding is permissible as it does not harm the refrigerant. True/False

Answer: False. Explanation: A spark will cause the refrigerant to deteriorate and is therefore not allowed.

74. The terminal box on the outside of a hermetic compressor typically houses two motor terminals for the run and start windings. True/False

Answer: False. Explanation: It houses three terminals: run, start, and a line common to both.

75. What insulating material was most popular for hermetic motor terminals for many years, but could become brittle and leak if it became too hot? A. Ceramic B. PTFE C. Neoprene D. Silicone

Answer: C. Explanation: Neoprene was popular but had this drawback.

76. Potential relays are mainly used with single-phase capacitor-start, capacitor-run motors that require relatively low starting torque. True/False

Answer: False. Explanation: They are used for motors needing relatively high starting torque.

77. A potential relay consists of a low-resistance coil and a set of normally open contacts. True/False

Answer: False. Explanation: It has a high-resistance coil and normally closed contacts.

78. In a potential relay, the high-resistance coil is typically wired between which two terminals? A. 1 and 2 B. 2 and 5 C. L and M D. L and S

Answer: B. Explanation: The coil is wired between terminals 2 and 5.

79. When power is applied to a compressor motor using a potential relay, the run and start capacitors are in parallel with one another, but both are in series with the start winding. True/False

Answer: True. Explanation: This specific wiring provides high starting torque.

80. The voltage generated across the start winding of a turning motor, referred to as induced voltage or BEMF, typically opposes line voltage and can exceed 400-V AC in a 230-V circuit. True/False

Answer: True. Explanation: BEMF can be significantly higher than line voltage.

81. The "pick-up voltage" of a potential relay refers to the BEMF value at which the contacts between terminals 1 and 2 open, typically occurring at about 3/4 motor speed. True/False

Answer: True. Explanation: This is the definition and timing of pick-up voltage.

82. If the pick-up voltage generated by BEMF is below the minimum rating for a potential relay, the contacts between terminals 1 and 2 will never open, causing high amp draws. True/False

Answer: True. Explanation: This results in the start capacitor remaining in the circuit.

83. The "drop-out voltage" of a potential relay is the BEMF voltage required to initially open the contacts. True/False

Answer: False. Explanation: It's the BEMF voltage that must be generated across the relay coil to hold the contacts open once they have been picked up.

84. Current relays are primarily used on single-phase, high-horsepower motors requiring very high starting torque. True/False

Answer: False. Explanation: They are used on fractional horsepower motors requiring low starting torque.

85. In a current relay, the coil is short in length and large in diameter, designed to carry the full-load current of the motor. True/False

Answer: True. Explanation: This describes the characteristics of a current relay coil.

86. Where is the coil of a current relay typically wired? A. Between terminals L and S B. Between terminals L and M C. Between terminals M and S D. Between terminals 1 and 2

Answer: B. Explanation: The coil is wired between terminals L (line) and M (main winding).

87. When power is applied to a motor with a current relay, the start winding initially experiences the locked-rotor amperage because the contacts between L and S are normally open. True/False

Answer: False. Explanation: The start winding does not experience LRA initially because the contacts are normally open. The run winding and relay coil are at LRA.

88. After the motor starts and the current draw of the run winding decreases, the magnetic field in the current relay's iron core strengthens, keeping the contacts closed. True/False

Answer: False. Explanation: The magnetism decreases, allowing spring pressure or gravity to open the contacts.

89. Positive temperature coefficient resistors (PTCRs) vary their resistance inversely with temperature; as temperature increases, resistance decreases. True/False

Answer: False. Explanation: As temperature increases, their resistance increases, which is why they have a positive temperature coefficient.

90. What specific material are PTCRs made from? A. Silicon carbide B. Pure semiconducting ceramic C. Gallium arsenide D. Germanium

Answer: B. Explanation: PTCRs are made from a very pure semiconducting ceramic material.

91. PTCRs can replace current and potential relays in HVACR systems, offering simpler wiring and fewer moving parts. True/False

Answer: True. Explanation: They are presented as a modern alternative for starting devices.

92. A PTCR is typically wired in series with the run capacitor and in parallel with the start winding in a permanent split capacitor (PSC) motor. True/False

Answer: False. Explanation: It's wired in parallel with the run capacitor and in series with the start winding.

93. At room temperature, a PTCR has very high resistance, effectively shunting out the run capacitor and limiting initial current flow. True/False

Answer: False. Explanation: At room temperature, its resistance is very low, acting like a straight wire and shunting out the run capacitor.

94. When a PTCR increases in temperature due to high current flow, its resistance greatly increases, causing it to act as an open circuit in less than one second. True/False

Answer: True. Explanation: This rapid increase in resistance is its operational characteristic.

95. Some ice machine manufacturers use a hot gas solenoid valve energized for up to 45 seconds prior to compressor starting to help equalize system pressures. True/False

Answer: True. Explanation: This practice aids in easier starting for the compressor.

96. To troubleshoot a PTCR, one should visually inspect for damage, then immediately check its resistance with an ohmmeter while it's still hot from operation. True/False

Answer: False. Explanation: One should wait at least 10 to 15 minutes for the PTCR to cool down to room temperature before measuring resistance.

97. Two-speed compressor motors typically achieve different speeds by switching between two-pole and four-pole motor operations. True/False

Answer: True. Explanation: This is the mechanism described for two-speed operation.

98. What component typically controls the automatic changeover between speeds in a two-speed compressor motor? A. Current relay B. Potential relay C. Space temperature thermostat D. Centrifugal switch

Answer: C. Explanation: The thermostat and proper compressor contactor manage this.

99. A large three-phase compressor motor may have a winding thermostat for each winding, allowing for multiple points of protection. True/False

Answer: True. Explanation: This provides enhanced motor protection.

100. For a three-phase compressor motor, the resistance across each winding is typically different, allowing for varied torque characteristics. True/False

Answer: False. Explanation: The resistance across all three windings is the same.