Synchronous 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 and commutators.

AC Motors can be broadly classified into two categories:

• Asynchronous / Induction Motors
• Synchronous Motors

The rotor in synchronous AC motors rotates in sync with the excitation field (i.e. there is no "slip"). The magnetization of the rotor is produced by a permanent magnet in brushless designs or by windings with an AC current supplied through slip rings or brushes (large, high-power motors).

These motors maintain a constant speed at all loads. When the load exceeds the rated load, the motor 'pulls out' of synchronism and ceases to operate. As these motors operate at a fixed speed (that can be adjusted by varying the supply frequency), they are suitable for precision drives where accurate speed control is required.

Synchronous motors can be made to operate at different power factors based on rotor excitation. It is common to use a synchronous motor as a synchronous condenser by over exciting the rotor, causing it to operate on a leading power factor. By using such a synchronous condenser, the overall power factor, of a manufacturing plant for example, can be improved.

Since these motors only operate at synchronous speed, a separate starting arrangement is required. In most cases a 3-phase synchronous motor will be started as an induction motor (by shorting the rotor winding) and then switched into synchronous mode.

Manufacturers

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] AC Motor, Wikipedia.

[2] AC Motors, CoolMagnetMan.com.

[3] Induction Motor Action, tutorial on Georgia State University HyperPhysics web site.

[4] Electric Motor Assembly, YouTube, Jan. 15, 2009.

[5] Most Popular Electric Car Motors, electric-cars-are-for-girls.com.