Design Tradeoff Between Cogging Torque and Torque Ripple in Fractional Slot Surface-Mounted Permanent Magnet Machines

Owing to different magnetic saturation conditions in surface-mounted permanent magnet machines operating under different loads, the optimal design to minimize the cogging torque may not be the one to minimize torque ripple as well. This paper investigates the design tradeoff between the open-circuit cogging torque and the on-load torque ripple by taking the design of slot opening as an example. With the aid of the frozen permeability method, first, the torque components under different slot opening widths and load conditions are presented based on a 12-slot/8-pole machine. The result reveals that the on-load cogging increases significantly, especially when a small slot opening width is adopted, which is opposite to the condition for minimizing the open-circuit cogging torque. Then, two machines separately designed for minimum open-circuit cogging torque or minimum on-load torque ripple are compared to illustrate alternative design tradeoffs. Finally, a prototype machine is manufactured and tested to validate the analysis results.