A single phase motor may have as few as two magnetic poles, 3450 RPM. In the most common brushless variety the reversal of ac current reverses the magnetic field poles. Expansion, and collapse of magnetism induces current in the aluminum bars in the rotor. Current in these bars conducts through the rotor ends. This induces magnetic field at 90 mechanical degrees to the field poles. Magnetic attraction, and repulsion cause the rotor to move. Some means of starting is needed. Often, this is a second field winding with a switch to turn it off when it speeds up.

Three phase motors have at least 6 poles. Distance between field, and rotor magnetic poles is less. As the three phases are sequential the magnetic field rotates. These motors need no start winding, and direction of rotation can be reversed by switching any two leads. They offer more torque with a smaller package. Three phase motors are cheaper to build.

Not all buildings are provided with three phase power supply. For some, in rural areas, three phase installation can be very expensive.