Synchronization of Line-Start Permanent-Magnet AC Motors

The line-start permanent magnet ac motor has already been demonstrated to have significantly lower losses than comparable high-efficiency induction motors in low integral-horsepower sizes. Studies have shown that high power factors are also obtainable in PM motors, and that for many applications the magnet cost can be recovered in energy cost savings in a relatively short period, particularly when the duty factor is high. This paper complements earlier publications on the PM line-start motor, and describes the starting process in more detail. The synchronizing capability, which is measured by the critical combination of inertia and load torque which the motor will just synchronize, is inherently higher than that of the synchronous reluctance motor. The reasons for this are outlined in the paper.