Torque improvement of single phase brushless PM motors by auxiliary magnets

The paper proposes a novel design arrangement to improve the torque performance of small rated single-phase brushless permanent magnet (PM) motors, especially as regards the starting performance. Unlike conventional techniques based on a suitable air gap shaping, such arrangement requires an auxiliary stator faced to one end of the main rotor PM, where some slots are made, generating an additional torque. A suitable design of the auxiliary PM and of the rotor PM slots enables higher starting torque than in asymmetric air-gap configurations, high stiffness around the standstill position and the mitigation of both the slot cogging torque and the load torque ripple. The additional cost due to the auxiliary PM is counterbalanced by the enhancements of both the starting performance and the torque ripple, prospecting a wider application of this class of motors to more demanding loads, such as pumps and portable devices.