The main application for permanent-magnet motors is in variable-speed drives where the stator is supplied from a variable-frequency, variable-voltage, electronically controlled source.
Such drives are capable of precise speed and position control. Because of the absence of power losses in the rotor, as compared with induction motor drives, they are also highly efficient.
Permanent-magnet motors can be designed to operate at synchronous speed from a supply of constant voltage and frequency.
The magnets are embedded in the rotor iron, and a damper winding is placed in slots in the rotor surface to provide starting capability. Such a motor does not, however, have means of controlling the stator power factor.
Permanent magnet motors exhibit an effect called cogging that results in torque ripple. It is caused by magnetic reluctance forces acting mainly in the teeth of the stator, and can be minimized by good motor design, but can still be a problem in sensitive applications.
