Control of Permanent Magnet Machine (PM)

Introduction

While induction motors are the most commonly used motors for most of the simple applications, much attention is currently being given to permanent magnet motors, specially as servo drives. Permanent magnet AC (PMAC) machines use magnets instead of windings to produce the air gap magnetic field. The replacement of rotor winding by magnets results in simplification in construction, reduction in losses, and improvement in efficiency. With the availability of Samarium –Cobalt and neodymium-iron –boron materials for permanent magnets the replacement of dc motors and induction motors with PM motors is on the increase. The PM motors are basically synchronous machines and can be operated at unity power factor, thus making them more efficient than induction motors. The cost of PM motor is more than the induction motor due to cost of the magnet, but due to its high efficiency the running cost is smaller.