Effect of power factor on rotor loss in high-speed PM alternators

Rotor electromagnetic loss in high-speed PM machines can be significant, causing overheating leading to failure. The paper calculates rotor loss in a 50 kW high speed alternator for different load power factors. A multi-layer current sheet model that was published previously by the authors is used to calculate the loss for each airgap flux harmonic travelling wave. The harmonics are evaluated from a number of magnetostatic FE solutions of the machine at different rotor positions. A close examination of the loss caused by each harmonic shows that MMF slot harmonics combine with no-load harmonics so as to increase rotor loss as the power factor changes from inductive to capacitive. The paper also discusses the accuracy of the current sheet model and presents results on the interaction between rotor eddy current flux and stator teeth which is not accounted for in the current sheet model. The results show that for the alternator under study this effect introduces less than 6% error in the calculation of rotor loss