Stator shifting in dual m-phase SPM machines with single-layer windings for space harmonic cancellation

This paper investigates the ability for elimination of unwanted magneto-motive force (MMF) harmonics in fractional slot dual m-phase single-layer PM machines. By doubling the number of slots and applying stator shifting, the 1st order and one unwanted higher order winding MMF harmonic can be suppressed. Then, through the addition of a second converter operating at a specified phase shift one higher order harmonic can be eliminated. This work identifies the required shift angle of the second winding set as well as converter phase shift in any dual m-phase machine to achieve elimination of an unwanted winding MMF harmonic. This methodology is validated by studying 3- and 5-phase machines through analytical modelling. Then two 3-phase machines, 12-slot/10-pole (12s/10p) and 12s/14p, are investigated using FEA. It is found that the 24s/10p dual 3-phase machine can achieve higher torque than the conventional 3-phase 12s/10p but the 24s/14p dual 3-phase performs worse. However, in both cases the significant suppression of unwanted harmonics reduces the rotor iron losses and PM eddy current loss. As a result, the efficiency of each machine is increased by about 3% over its initial 12-slot counterpart. In particular, the PM eddy current losses are reduced by over 95% leading to a substantial reduction in thermal demagnetization risk. The dual 3-phase single-layer machine is also found to achieve higher torque than two similar topology dual 3-phase double-layer machines while having equivalent PM eddy current losses. Both analytical and numerical results have been validated by a series of tests. A prototype machine has been manufactured to validate EMF, static torque, and efficiency measurements of the proposed machine.