PERMANENT MAGNET DC MOTOR

A permanent magnet dc motor (PMDC) is a dc motor whose poles are made of permanent magnets. PMDC motor offer a number of benefits compared with shunt dc motors in some applications.

A PMDC motor is basically the same machine as a shunt dc motor, except that the flux of a PMDC motor is fixed. Therefore, it is not possible to control the speed of the PMDC motor by varying the field current or flux. The only methods of speed control available for a PMDC motor are armature voltage control and armature resistance control.

ADVANTAGE:

Since the motors do not require an external field circuit, they do not have the field circuit copper losses. Because no field windings are required, they can be smaller than corresponding shunt dc motors.

DISADVANTAGES:

Permanent magnets cannot produce as high flux density as an externally supplied shunt field. So a PMDC motor will have a lower induced torque per ampere of armature current than a shunt motor of the same size. Also, PMDC motors run the risk of demagnetization.