A synchronous and synchronous electric motors are the two main categories of ac motors. The induction ac motor is a common form of asynchronous motor and is basically an ac transformer with a rotating secondary. The primary winding (stator) is connected to the power source and the shorted secondary (rotor) carries the induced secondary current. Torque is produced by the action of the rotor (secondary) currents on the air-gap flux. The synchronous motor differs greatly in design and operational characteristics, and is considered a separate class of ac motor.

Induction AC Motors: Induction ac motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction ac motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.

AC motors typically feature rotors, which consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This ac motor "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. The speed at which the ac motor magnetic field rotates is the synchronous speed of the ac motor and is determined by the number of poles in the stator and the frequency of the power supply: n_s = 120f/p, where n_s = synchronous speed, f = frequency, and p = the number of poles.

Synchronous speed is the absolute upper limit of ac motor speed. If the ac motor's rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. When ac motors are running, the rotor always rotates slower than the magnetic field. The ac motor's rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the ac motor's rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (n_s - n_a)/n_s, where s = slip, n_s = synchronous speed, and n_a = actual speed.