Influence of Manufacturing Tolerances on the Electromotive Force in Permanent-Magnet Motors

In permanent-magnet synchronous motors it is necessary that the induced electromotive force (EMF) is as sinusoidal as possible to achieve a low torque ripple. The techniques used for this purpose require great precision in the positioning and the magnetization of the magnets, which renders them highly susceptible to errors resulting from manufacturing tolerances. This paper analyzes the effect on the EMF of four types of common production errors: magnetization level, position and width of the magnets, and angular displacement between blocks of magnets. To this end, two totally analytical statistical models have been developed: one for the first harmonics and another for high frequencies, from which design rules for the minimization of the sensitivity to errors can be drawn. The techniques developed here allow the calculation of the deviations in the EMF spectra from the information on tolerances, and reciprocally estimate manufacturing errors from EMF spectra. The proposed techniques have been tested both numerically and experimentally showing good agreement with the developed models.