In this paper we study the condition that the cogging torque occurs, that is caused by manufacturing errors in the stator of PM motors. Manufacturing errors can be classified into the distortion of the magnetic core, the degradation of magnetic properties etc. They are expressed as the asymmetric spatial distribution of permeance, that is, the component of the spatial distribution function whose number of spatial pulsations is not equal to the multiple of the stator slot number. The overviews of this paper are as follows. (1) We derive the relational expression between the spatial pulsation number of the asymmetric component of the permeance distribution function and the pulsation number of cogging torque per revolution. (2) Examples are shown for seven typical motor specifications, each of which has different pole/slot ratio. (3) We conduct finite element analysis for the seven typical motors. The stator cores were distorted and the spatial order of distortion was set to the parameter. The analyzed pulsation number of cogging torque agreed well with the theoretical solutions.
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