AC Motor Theory
AC Electric Circuits
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.
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:
What would we have to do in order to reverse the rotation of this three-phase induction motor?
Explain your answer. Describe how the (simple) solution to this problem works.
If a copper ring is brought closer to the end of a permanent magnet, a repulsive force will develop between the magnet and the ring. This force will cease, however, when the ring stops moving. What is this effect called?
Also, describe what will happen if the copper ring is moved away from the end of the permanent magnet.
A technique commonly used in special-effects lighting is to sequence the on/off blinking of a string of light bulbs, to produce the effect of motion without any moving objects:
What would the effect be if this string of lights were arranged in a circle instead of a line? Also, explain what would have to change electrically to alter the “speed” of the blinking lights’ “motion”.
If a set of six electromagnet coils were spaced around the periphery of a circle and energized by 3-phase AC power, what would a magnetic compass do that was placed in the center?
Hint: imagine the electromagnets were light bulbs instead, and the frequency of the AC power was slow enough to see each light bulb cycle in brightness, from fully dark to fully bright and back again. What would the pattern of lights appear to do?
If a closed-circuit wire coil is brought closer to the end of a permanent magnet, a repulsive force will develop between the magnet and the coil. This force will cease, however, when the coil stops moving. What is this effect called?
Also, describe what will happen if the wire coil fails open. Does the same effect persist? Why or why not?
These two electric motor designs are quite similar in appearance, but differ in the specific principle that makes the rotor move:
Synchronous AC motors use a permanent magnet rotor, while induction motors use an electromagnet rotor. Explain what practical difference this makes in each motor’s operation, and also explain the meaning of the motors’ names. Why is one called “synchronous” and the other called “induction”?
An interesting variation on the induction motor theme is the wound-rotor induction motor. In the simplest form of a wound-rotor motor, the rotor’s electromagnet coil terminates on a pair of slip rings which permit contact with stationary carbon brushes, allowing an external circuit to be connected to the rotor coil:
Explain how this motor can be operated as either a synchronous motor or a “plain” induction motor.
Many single-phase “squirrel-cage” induction motors use a special start winding which is energized only at low (or no) speed. When the rotor reaches full operating speed, the starting switch opens to de-energize the start winding:
Explain why this special winding is necessary for the motor to start, and also why there is a capacitor connected in series with this start winding. What would happen if the start switch, capacitor, or start winding were to fail open?
Some AC induction motors are equipped with multiple windings so they may operate at two distinct speeds (low speed usually being one-half of high speed). Shown here is the connection diagram for one type of two-speed motor:
There are six terminals on the motor itself where the connections are made:
The motor’s datasheet will specify how the connections are to be made. This is typical:
Speed φ-A φ-B φ-C Left open Shorted together
Low 1 2 3 4,5,6
High 4 5 6 1,2,3
Explain why the motor runs at half-speed in one connection scheme and full speed in the other. What is going on that makes this possible?
This electric motor was operating just fine, then one day it mysteriously shut down. The electrician discovered two blown fuses, which he then replaced:
When the on/off switch was closed again, the motor made a loud “humming” noise, then became quiet after a few seconds. It never turned, though. Upon inspection, the electrician discovered the same two fuses had blown again.
If you were asked to help troubleshoot this electric motor circuit, what would you recommend as the next step?
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