Synchronization of Line-Start Permanent-Magnet AC Motors

The line-start PM (permanent magnet) motor is a very high efficiency synchronous motor designed to operate at fixed voltage and frequency from the same power supplies as induction motors. The excitation is provided partly by the magnets, which are mounted in the rotor, and partly by the line current. A squirrel cage is provided to accelerate the motor from standstill when started ``across the line.´´ This paper describes the synchronizing process and the characteristics of PM motors that lead to an inherently higher starting capability than that of comparable reluctance motors. The starting performance is described with the aid of computed transient torque/speed and related trajectories. The results have been verified with reference to detailed tests on a 25-hp motor that has been described in earlier papers. Two basic configurations of PM motor are shown in Fig. 1. On the left is a four-pole arrangement with radially magnetized magnets, and on the right a six-pole arrangement with circumferentially magnetized magnets.