Chapter 9 Review Questions

What is magnetism? When two pieces of iron are attracted to each other by physical means or electrical means. Torque is A. Strength that a motor produces by turning. A magnetic field is D. All the above. True or False: A permanent magnet is a piece of material that has been magnetized and can hold its magnetic strength for a reasonable length of time. True. How is an electromagnet produced? Through electricity. Which of the following produces the best electromagnet? soft iron.
Unlike poles of a magnet repel each other and like poles attract each other. What part does polarity play in the operation of an electric motor? It allows the motor to run continuously. What part of a motor produces an inductive magnetic field within itself to facilitate the rotating motion? Rotor and stator. What part does the frequency of alternating current play in the operation of an electric motor? Helps change polarity 120 times a second. What would be the speed of a two-pole motor operating on a 120 volts 60 Hertz power supply? 3450rpm What are the five types of single-phase motors used in the industry? Which of the following correctly lists the motor’s starting torque from lowest to highest? Shaded pole, three phase, permanent split capacitor, capacitor start. Which of the following is a common use of a shaded pole motor? Furnace fan motor. How does a shaded pole motor operate? The shaded poles produce a magnetic field that is out of phase with the magnetic field of the main winding. 16. How can a shaded pole motor be reversed? The stator must be reversed to change the positions of the shaded poles, and this usually means disassembling the motor.
What determines the rotation of a shaded pole motor? Location of shaded pole. 18. Draw a diagram of a three speed, shaded pole motor. What enables a split phase motor to develop enough torque to begin rotation? The method of splitting the phase of incoming power to produce a second phase of power, giving the motor enough displacement to start. What removes the starting winding from the electrical circuit of an open type split phase motor once it reaches 75% of its operating speed? Centrifugal switch. What are the three probable areas of trouble in a split phase motor?

The bearings, windings, and the centrifugal switch. What is the unit of measurement for the strength of a capacitor? microfarad. What is the purpose of the capacitor? To boost the starting torque or running efficiency of a single phase motor. What is the difference between a running and a starting capacitor? Starting capacitors are usually made of plastic and used to assist a single phase motor in starting. A running capacitor has an oil filled case and is mainly used to increase a motor’s running efficiency. List the five capacitor replacement rules.
The voltage of any capacitor used for replacement must be equal or greater than that of the one being replaced. The strength of the starting capacitor replacement must be at least equal to but not more than 20% greater than the one being replaced. The strength of the running capacitor replacement may vary by plus or minus 10% of the strength of the one being replaced. If capacitors are installed in parallel, the sum of the capacitors is the total capacitance. The total capacitance of capacitors in series may be found in the following formula: Explain the operation of a permanent split capacitor motor.
It has two windings, running and starting. A running capacitor is put in series with the starting winding. The capacitor causes the electron to flow through the starting winding to shift it out of phase with the running winding. Therefore, a rotating magnetic field is set up, causing the rotor to turn. How are a PSC motor and a capacitor start capacitor run motor similar? C. Both use starting relays. What are the advantages and disadvantages of using the following types of motors? Shaded pole motor. What are the similarities between an open type split phase motor and a capacitor start motor?
C. Both have a centrifugal switch. Which of the following is an advantage in using a three phase motor? Stronger. Draw a wiring diagram of a capacitor start capacitor run motor: True or false: All starting apparatuses are mounted externally to the hermetic compressor shell. True. What is the purpose in troubleshooting any electric motor? Which of the following is the capacitance of an 88 uf and a 108 uf starting capacitor connected in series?196uf. Which of the following is the capacitance of two 20 uf running capacitors connected in parallel? 20uf.
If a capacitor produces 15 A on a 240 volt supply, which of the following is its microfarad rating? 166 uf. Which of the following capacitors could be used to replace a 35 uf, 370 volt running capacitor? 35uf, 390 volt. Which of the following capacitors could be used to replace a 188 uf, 250 volt starting capacitor? C. 200uf, 250 volts. Which of the following capacitors or combination of capacitors could be used to replace a 45 uf 370 volt running capacitor? 40uf, 440 volt. Which of the following capacitors or combination of capacitors could be used to replace an 88uf 250 volt starting capacitor?
D. 150uf, 250V. Find the common, start, and run terminals of the following hermetic compressor. Briefly explain the procedure for troubleshooting hermetic compressor motors. Electrically taking resistance readings of the windings with a good ohmmeter. What are the electrical failures categories for hermetic compressor motors? Open, shorted, or grounded. What precautions should be taken when checking hermetic compressor motors? Infinity. What would be the highest allowable resistance reading for a grounded compressor motor? Zero.
What are the advantages of using an electronically commutated motor over a PSC motor? Explain the construction of an ECM. The direct current converters convert the alternating current that the power company supplies to direct current, which can then be regulated to vary the speed of the motor. An ECM is a B. Three-phase DC motor. True or False: The resistance readings of the windings of a properly operating ECM should be equal. False. True or False: The line voltage power supply of an ECM should never be disconnected or connected with the power on. True.