Resolving the startability-efficiency trade-off in line-start permanent magnet synchronous motors

Line-Start Permanent Magnet Synchronous Motors (LSPMSMs) have emerged as a promising solution for high-efficiency fixed-speed applications by employing a squirrel-cage and permanent magnets on the same rotor. This paper investigates the inherent trade-off between line-start capability and steady-state performance in LSPMSMs through finite-element analysis of machines with varying permanent magnet excitation. The results show that high back-EMF configurations exhibit superior steady-state efficiency and power factor but suffer from significantly reduced direct-on-line startability. To address this limitation, a temporary auxiliary starting device with ramped voltage and frequency is proposed. Once synchronous operation is achieved, the motor is directly connected to the grid, avoiding any continuous converter losses.