Asymmetry of inductance and torque ripple of multi-unit permanent magnet synchronous motor

Aiming at realizing the performance of high torque, low speed and high reliability for the Permanent Magnet Synchronous Machine (PMSM), a superpower multi-unit PMSM was designed, which had five units and each unit including an independent three-phase star-connection windings and an independent driver device. When the multi-unit PMSM was running under normal conditions, its operation was very similar to the general three-phase PMSM and even it can be regarded as general three-phase PMSM. It was different from the normal three-phase PMSM, when only part unit motors can be normally operated and the others were under the fault conditions. Due to the different magnetic conductivity of three-phase windings mutual inductances, the mutual inductance of the same unit was not symmetric in these unit motors. Furthermore, the electrical torque ripple would be increased due to the asymmetric mutual inductances, when part units were operating. However, there was a strong rule which can be estimated through the relationship of the running units' space position and quantity. In one word, the torque ripple is decreased using the correct rule when the whole unit motors are not normal working. In addition, the torque ripple of multi-unit PMSM is also restrained according to the analysis results.