Superposition techniques for analyzing cogging torque sensitivity to manufacturing errors in fractional pitch PMSM

Theoretically, the layout of fractional pitch permanent magnet motors should ensure the cancellation of most cogging torque harmonics. In reality, manufacturing errors lead to incomplete cancellation and higher than expected cogging torque. Two potential manufacturing errors are the variation in angle between successive magnets and the strength of the magnets. To determine the sensitivity of cogging torque to magnet strength and placement, a superposition technique is used. This technique is validated by first demonstrating that the total cogging torque can be reconstructed by summing offset versions of the torque from experimentally determined single-magnet/single- slot interactions. Following this validation, Monte Carlo simulations are conducted to statistically determine the sensitivity of cogging torque to magnet placement and strength errors. It is demonstrated that the mean RMS cogging torque increases by 4 % for every degree increase in magnet placement standard deviation and by 0.06% for every 1% increase in magnet strength standard deviation.