Analysis of torque versus current capability of reluctance and interior PM machines under limited current and flux-linkage operation

Saturation strongly affects the performance of interior PM machines and thus the use of linear (constant parameter) equivalent-circuit models for such machines generally results in substantial errors. This paper investigates the use of a linear magnetic model which is combined with a flux-linkage limit, so that it corresponds to a flux density or saturation limit. The model is used to predict the low-speed torque versus current performance. It is verified for reluctance machines and interior PM machines, which show significant saturation, using finite-element and experimental results. The flux-linkage limit inherently defines the maximum ideal torque versus current characteristic for the machine which can be used as a performance benchmark. The selection of the magnet flux linkage to achieve the maximum torque at rated current is also discussed.