Identifying manufacturing induced rotor and stator misalignment in brushless permanent magnet motors

Cogging torque in fractional pitch PMSM is minimized when manufacturing induced errors are also minimized. This paper investigates additional cogging torque harmonics resulting from misalignment of the rotor and stator caused by assembly and manufacturing tolerances in an axial flux, fractional pitch PMSM. A hybrid analytical / FEA method utilizing superposition of a pole transition over a single stator slot is developed to predict the effect on cogging torque of angular misalignment in combination with pole and slot placement inaccuracies. This work extends on that previously published by inducing varying degrees of assembly misalignment of the stator and rotor to determine the effect on cogging torque waveforms. Experimental data is presented supporting the analytical data indicating that static angular misalignment produces sidebands about the rotor affected harmonics and dynamic angular misalignment creates sidebands around the stator affected harmonic. There is good agreement between the analytical method, FEA and measured experimental data.