how does ac motors spin in one direction if its powered by alternating current , and what is single phase and 3 phase alternating current ?
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Single phase alternating current is a current that rises to a maximum value in one direction, then falls to zero at which point it reverses, then rises to its maximum value in the other direction, then falls to zero again. This is called a cycle. In North America, the power grids are all 60 cycle.
Three phase current is three single phases arranged in a very particular manner. Call the individual phases A, B, and C, though most generation stations actually identify them by color. A phase is as described in the single phase description. B phase is the same, except it reaches its maximum current in the first direction 1/180 of a second after A phase does. C phase, again, is the same as A and B phase but it reaches its maximum current in the first direction 1/180 of a second after B phase does, and 2/180s of a second after A phase. And 3/180s of a second after A phase reaches its maximum, A phase does it again in its next cycle. This is what’s known as a phase sequence, A-B-C. As there is no beginning or end to a sequence, there are only two. The second one is commonly called B-A-C.
A three phase motor has a winding (a group of coils) for each phase so that when the current is applied, it creates a rotating magnetic field that drags the rotor around with it. Reversing the motors rotation is as simple as interchanging any two of the three power supply leads.
As for single phase motors, there are a great many different styles. I’m going to assume you’re interested in the squirrel cage types as they do the bulk of the work. Even among just the single phase squirrel cage motors, there are a handful of different types, but they all do the same thing, just in different ways.
A single phase motor with a single winding would not rotate, it would pulsate. That would be enough to keep the motor running, but it can’t start it. To start it they manufacture a second phase inside the motor. In smaller ones it’s done by installing a shading pole. It’s a rather inefficient, but cheap, way to do it, but it’s limited by its very low starting torque. Another is the permanent split capacitor motor. More efficient than the shaded pole, but more expensive, and you don’t get much more starting torque. Finally there are the split phase motors. They include capacitor start and capacitor start-capacitor run motors. They use capacitors to make a more efficient 2nd phase for both starting and running.
Excepting the shaded pole type, they all use the same configuration. There are two leads for the main winding and two for the starting winding. A main lead and a starting lead are connected to one power lead, and the other main and starting leads are connected to the other power lead. To reverse, just interchange the starting leads.
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