Keywords: back–emf, brushless machines, flux linkage, finite element method, FEM, cogging torque waveforms, permanent magnet motors, PM brushless motors, slot number, pole number, modelling
Improved torque in PM brushless motors with minimum difference in slot number and pole number
This paper presents design, modelling and prototyping of three–phase permanent–magnet brushless machines having a fractional slot to pole ratio. In particular, permanent magnet brushless machines which exhibits slot number and pole number differ by either ±2 or ±1 will be considered. Such small difference in slot and pole numbers will potentially improve motor characteristics and subsequently result in better motor performance. Having fractional slot–pole number motors may also facilitate the deployment of double–layer windings or single–layer windings in order to simplify the manufacturing process. Finite–element methods are first used to predict the flux–linkage, back–emf and cogging torque waveforms, and they are shown to be in good agreement with the measured results. Two prototypes are built, the first motor has 12–slot/10–pole and the other motor has 9–slot/10–pole for test and validations. It is observed that the unity difference in 9–slot/10–pole motor would produce more trapezoidal phase back–emf waveform, hence, slightly higher average torque output with smaller torque ripples, in comparison with that found in 12–slot/10–pole motor.