Homeowner:

This post is going to be off topic because I believe your question has been answered.

All electric motors operate on the principle that a magnet will spin if you put it in a rotating magnetic field. It's producing a rotating magnetic field that can be a bit of a trick sometimes.

With three phase electric power, producing a rotating magnetic field is easy. If each phase of your power is 120 degrees apart, you just arrange the three windings 120 degrees apart around the stator and you have a near perfect rotating magnetic field.

With two phase power, you can do a similar thing and get good results.

It's when you get to single phase 120 VAC power that you have to get creative in using that single phase power to produce what appears to be a rotating magnetic field instead of just an oscillating magnetic field. ALL of the different kinds of 120 volt electric motors that you hear about (like split phase motors, shaded pole motors, capacitor start motors, etc.) are different only because they use a different way of making the rotor see what appears to be a rotating magnetic field.

The easiest of these to explain is the capacitor start motor.

If you imagine two metal plates in close proximity, if you apply a voltage to one plate, that applied voltage will repel the electrons in the second plate, and you'll get a small current flowing out of that second plate.

Now, if the voltage you apply to the first plate were in the form of a sinusoidal wave, just like the voltage in your wall outlets, the current coming out of the second plate would be at a maximum when the RATE OF CHANGE IN VOLTAGE in the first plate was at a maximum, and that actually occurs when the applied voltage is going from positive to negative or negative to positive, or when the applied voltage is actually ZERO for a very short period of time. That is, by putting a capacitor in a circuit, you completely change the relationship between the applied voltage and the resulting current through the circuit.

In a simple circuit with only a single resistor in the circuit, current through the resistor is maximum when the applied voltage is maximum. Similarily, current is theoretically zero when the voltage goes from positive to negative, or negative to positive, or when the applied voltage is temporarily zero.

If you replace that resistor with a capacitor, the current through the circuit is a maximum when the applied sinusoidal voltage is changing the fastest, and that occurs when the voltage goes from negative to positive, or positive to negative, or when it's actually temporarily zero volts.

So, one way to use single phase power to create an apparant rotating magnetic field is to build an electric motor for two phase power (with the windings 90 degrees apart) and apply the same 120 VAC to both windings. BUT, if you put a capacitor in series with one of those windings, the current through that winding will be 90 degrees out of phase with the current through the other winding.

Since a coil of wire develops it's magnetism as a result of the CURRENT flowing through the coil and not the voltage applied to it, the magnetic field of one winding will develop 1/4 of a AC voltage cycle before or after the other winding, thereby creating much the same thing as the rotor would see if it were in a two phase motor while the motor is starting.

In actuality, in a capacitor start motor the winding with the capacitor in series is cut out of the circuit by a centrifugal switch once the motor comes up to speed. After the "start" winding is shut off, the capacitor start motor continues to run on the other "run" winding only. It's been found that the motor will run smoother and more efficiently that way, and the explanation of "why" is something I just don't know.

In a "split phase" electric motor, you have very much the same thing happening as in a capacitor start motor, except that you don't have a capacitor in series with one of the windings.

Instead, in a split phase motor, one of the windings consists of a lot of turns of thin wire whereas the other winding consists of only a few turns of thick wire. This difference causes the two windings to have different "impedance", and that results in the winding with the thin wire developing it's magnetic field earlier in the AC voltage cycle than the winding with the thick wire.

And, that difference in the timing of the magnetic field from each winding creates the impression of a rotating magnetic field for the rotor to follow. And, just like in a capacitor start motor, once a split phase motor gets up to speed, a centrifugal switch cuts out the start winding and the motor will continue to turn on it's run winding only. (I can't remember now whether the start winding is the one with thin or thick wires.)

Now, both a capacitor start motor and a split phase motor will be happy to turn in the opposite direction if you want them to. All you have to do is reverse the polarity of ONE of the windings. It can be the start winding or the run winding; doesn't matter which. If you reverse the polarity of one of the windings, the motor will turn happily in the opposite direction. Washing machines use that feature to advantage by having the timer (or something called a "motor reversing relay") reverse the polarity of one of the windings on the motor so that the motor turns in one direction while the washer is in the agitate cycle, and in the opposite direction when the washer is in the spin cycle. This is important because during the spin cycle, the motor turns the washer's pump one way to pump water OUT of the washer, but during the agitate cycle, the pump turns so as to pump any water that leaks into it back into the wash basket. Similarily, Maytag top loading washing machines have a pulley which turns on a threaded shaft. The pulley screws UP the shaft when the motor turns one way, and down the shaft when the motor reverses direction, and it's which direction that pulley is pushing that determines what the transmission does; agitate or spin.

I've been told not to reverse the polarity of BOTH windings on a capacitor start or split phase motor. Apparantly, doing that will create a dislocation in the space-time continuum causing you to complete the reversal procedure before you began, with the necessary result that the motor will turn in it's original direction. (smirk)

A shaded pole motor is a different kettle of fish. A shaded pole motor has only two poles in it's stator. But, there will be a thick loop of copper wire going through the middle of both of those two poles, thereby separating each pole in half. A current flows in that thick wire as a result of the magnetism produced at the two poles, and the magnetic field of the thick wire opposes the magnetic field on one side of the pole and adds to the magnetic field on the other side of the pole. So, what the rotor sees is a weak magnetic field on one side of the pole becoming a strong magnetic field on the other side of the pole, and that gives the rotor the illusion of the magnetism "sweeping across" the pole, or something similar to a rotating magnetic field.

The thing to remember here is that there is only ONE kind of three phase motor and only ONE kind of two phase motor, but several different kinds of single phase electric motors. That's because each different kind of 120 VAC electric motor uses a different method of getting the stator to create the appearance of a rotating magnetic field using only single phase electric power. Two and three phase electric motors don't need to do that because their stators actually produce a rotating magnetic field for the rotor to follow.

There, now you know more about electric motors than 99 percent of people named "Homeowner".