There are probably more single phase induction motors in use today than the total of all the other types put together.
It is logical that the least expensive, lowest maintenance type of ac motor should be used most often. The single phase ac induction motor fits that description.
Unlike polyphase induction motors, the stator field in the single-phase motor does not rotate. Instead it simply alternates polarity between poles as the ac voltage changes polarity.
Voltage is induced in the rotor as a result of magnetic induction, and a magnetic field is produced around the rotor. This field will always be in opposition to the stator field (Lenz’s law applies). The interaction between the rotor and stator fields will not produce rotation, however. The interaction is shown by the double-ended arrow in the illustration below, view (A). Because this force is across the rotor and through the pole pieces, there is no rotary motion, just a push and/or pull along this line.
Rotor currents in a single phase ac induction motor.
Now, if the rotor is rotated by some outside force (a twist of your hand, or something), the push-pull along the line in the illustration above, view (A), is disturbed. Look at the fields as shown in view (B). At this instant the south pole on the rotor is being attracted by the left-hand pole. The north rotor pole is being attracted to the right-hand pole.
All of this is a result of the rotor being rotated 90º by the outside force. The pull that now exists between the two fields becomes a rotary force, turning the rotor toward magnetic correspondence with the stator. Because the two fields continuously alternate, they will never actually line up, and the rotor will continue to turn once started. It remains for us to learn practical methods of getting the rotor to start.
There are several types of single-phase induction motors in use today. Basically they are identical except for the means of starting. In this chapter we will discuss the split-phase and shaded-pole motors; so named because of the methods employed to get them started. Once they are up to operating speed, all single-phase induction motors operate the same.
One type of induction motor, which incorporates a starting device, is called a split-phase induction motor. Split-phase motors are designed to use inductance, capacitance, or resistance to develop a starting torque. The principles are those that you learned in your study of alternating current.
We will cover types of split phase induction motors in the next tutorial.
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