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The steady-state performance of the surface magnet PM synchronous motor can be modeled to a good approximation by a simple equivalent circuit that includes the effect of the stator core loss. For operation at constant power with no restrictions placed on voltage or frequency, this model shows that there exists a unique operating point that yields the maximum efficiency and that the efficiency at this point is a function solely of the motor loss parameters, independent of the particular power level. Loss contour plots are introduced to show the existence of the optimum point and to show the nature of the losses as one departs from the optimum. The impact of these results on different operating modes for the machine is discussed. Test results are presented to verify that the optimal efficiency point exists and that the simple model predicts the machine performance reasonably well.