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AC Motor Theory
20 questions By Tony R. Kuphaldt
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Question 1 of 20
Electromechanical watt-hour meters use an aluminum disk that is spun by an electric motor. To generate a constant “drag” on the disk necessary to limit its rotational speed, a strong magnet is placed in such a way that its lines of magnetic flux pass perpendicularly through the disk’s thickness:
The disk itself need not be made of a ferromagnetic material in order for the magnet to create a “drag” force. It simply needs to be a good conductor of electricity.
Explain the phenomenon accounting for the drag effect, and also explain what would happen if the roles of magnet and disk were reversed: if the magnet were moved in a circle around the periphery of a stationary disk.
Reveal answerThis is an example of Lenz’ Law. A rotating magnet would cause a torque to be generated in the disk.
Notes:Mechanical speedometer assemblies used on many automobiles use this very principle: a magnet assembly is rotated by a cable connected to the vehicle’s driveshaft. This magnet rotates in close proximity to a metal disk, which gets “dragged” in the same direction that the magnet spins. The disk’s torque acts against the resistance of a spring, deflecting a pointer along a scale, indicating the speed of the vehicle. The faster the magnet spins, the more torque is felt by the disk.
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Question 2 of 20
Explain what will happen to the unmagnetized rotor when 3-phase AC power is applied to the stationary electromagnet coils. Note that the rotor is actually a short-circuited electromagnet:
Reveal answerThe rotor will rotate due to the action of Lenz’s Law.
Follow-up question: what would happen if the rotor’s coil were to become open-circuited?
Notes:Here, we see a practical 3-phase induction motor. Be sure to thoroughly discuss what is necessary to increase or decrease rotor speed, and compare this with what is necessary to increase or decrease speed in a DC motor.
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Question 3 of 20
Explain what slip speed is for an AC induction motor, and why there must be such as thing as “slip” in order for an induction motor to generate torque.
Reveal answerThe difference between the speed of the rotating magnetic field (fixed by line power frequency) and the speed of the rotor is called “slip speed”. Some amount of slip is necessary to generate torque because without it there would be no change in magnetic flux (dφ)/dt) seen by the rotor, and thus no induced currents in the rotor.
Notes:It is easy enough for students to research “slip speed” in any motor reference book and present a definition. It is quite another for them to explain why slip is necessary. Be sure to allow ample time in class to discuss this concept, because it is at the heart of induction motor operation.
