Automotive Electronics

Asynchronous AC Motors

Basic Description

AC Motors are electrical machines that convert electrical energy (supplied in the form of sinusoidally time varying or "alternating" current) to rotational mechanical energy by the interaction of magnetic fields and conductors. Unlike motors that run directly on a DC current, AC motors generally do not require brushes or commutators. One type of AC motor is the asynchronous or induction AC motor.

Asynchronous or induction motors consist of a stator with a winding capable of producing a rotating magnetic field, and a rotor with a shorted conductor winding in which a current is induced by the rotating magnetic field. The fields generated by the current induced in the rotor generate a restoring torque responsible for the rotation of the rotor. The rotating magnetic field produced by the stator is easily set up using a multi-phase AC supply.

The term "asynchronous" refers to the fact that the rotation of the rotor is always slower than the rotational speed of the magnetic field. The difference in speed of the field and the rotor is called 'slip' and the torque of the motor is proportional to this slip. Thus, the rotational speed of the motors depends on both the excitation frequency and the load.

The synchronous speed, or the theoretical maximum speed of an induction motor is a function of the supply frequency (e.g. often 60 Hz in the US) and the number of poles. Induction motors are often called 'squirrel cage' motors due to the rotor winding design.

An induction motor starts with maximum slip and has a tendency to draw a very high current initially, especially if starting at a high load. This results in the need to have a separate starting mechanism. In case of single phase AC motors, the rotor needs to be set in motion initially to start the motor. This is accomplished by using a mechanical starting force or employing a separate starter winding.

Although most electric and hybrid-electric cars employ synchronous AC motors for the main drive, the Tesla Roadster, Tesla Model S, Mercedes B-Class electric drive, and a few others use an asynchronous AC motor.

Baldor, Bircraft, Century, Circor, Emerson, Empire Magnetics, Fasco, Groschopp, Kinetek, Leeson, Met Motors, Motion Control Group, North American Electric, Pittman, Powertec, Remy, Siemens, Sterling Electric, Teco, Toshiba, WEG, Zhongda
For More Information
[1] Induction Motor, Wikipedia.
[2] AC Motors,
[3] Induction Motor Action, tutorial on Georgia State University HyperPhysics web site.
[4] Electric Motor Assembly, YouTube, Jan. 15, 2009.
[5] 3-Phase AC Induction Motor,
[6] AC Motors,YouTube, May 19, 2010.
[7] Squirrel Cage Motors,YouTube, July 18, 2010.