Permanent magnet machines torque considerations

An analytical model has been developed to predict accurately cogging and commutating torques of permanent-magnet brushless DC machines with buried and surface-mounted magnets. Computer programs have been written to evaluate the analytical models, and the results are compared with finite-element analysis and test results. A parametric study has been performed showing the effects of some of the most important machine variables. For both machine constructions the cogging torque is reduced to zero by skewing either the rotor or the stator by one slot pitch without a considerable loss of average torque. Slot opening has a marked effect over the magnitude of the cogging torque. The number of slots of the machine also produces a large effect over cogging torque. For a fractional number of slots per pole pitch the resulting cogging torque is considerably smaller than for an integral number of slots.<>