In a synchronous motor, the rotors rotate at a speed similar to the speed of a magnetic field, or stator field, while asynchronous motors have the rotor running at a slightly slower speed than that of the induced magnetic field. Synchronous motors can be found in dryers, fans and the axles of off-road vehicles.
Synchronous motors run more efficiently than asynchronous motors. Their output is more easily regulated and they offer wide support for load power variations. Synchronous motors are however more complicated than asynchronous motors and are more expensive to build. This makes them a less attractive option, especially in situations that call for extended use.
All AC induction motors are asynchronous. In order to produce the required torque, the stator magnetic field must interact with the induced current in the squirrel cage. Because the rotor speed is reliant on the magnetic field, the speed is always slower. The difference between speeds is referred to as the slip and is usually calculated as a percentage, which typically ranges from 2 to 4 percent. Different factors can affect the amount of a slip measured in an asynchronous motor. These include the load, the strength of the stator field and the resistance of the wires.