The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the ac case. Three electrical phases are introduced through terminals, each phase energizing an individual field pole. When each phase reaches its maximum current, the magnetic field at that pole reaches a maximum value. As the current decreases, so does the magnetic field. Since each phase reaches its maximum at a different time within a cycle of the current, that field pole whose magnetic field is largest is constantly changing between the three poles, with the effect that the magnetic field seen by the rotor is rotating. The speed of rotation of the magnetic field, known as the synchronous speed, depends on the frequency of the power supply and the number of poles produced by the stator winding. For a standard 60 Hz supply, as used in the United States, the maximum synchronous speed is 3,600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to a power supply, but are essentially short circuits. The most common type of rotor winding, the squirrel cage winding, bears a strong resemblance to the running wheel used in cages for pet gerbils. When the motor is initially switched on and the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed. From Faraday's law, this situation results in the induction of currents round the rotor windings; the magnitude of this current depends on the impedance of the rotor windings. Since the conditions for motor action are now fulfilled, that is, current carrying conductors are found in a magnetic field, the rotor experiences a torque and starts to turn. The rotor can never rotate at the synchronous speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced. The induction motor has a high starting torque.