Accuracy investigation of closed-form predictions for the operating envelope performance characteristics of interior PM synchronous machines

Closed-form expressions are available to predict the torque-speed performance limits of interior permanent magnet (IPM) synchronous machines under various simplifying assumptions. This paper compares the performance that is predicted by using these closed-form expressions and that directly calculated by finite element analysis (FEA) considering maximum limits in both the constant-torque and flux-weakening operating regimes. It confirms that the closed-form expressions cause substantial errors in the constant-torque regime where magnetic saturation effects are large. However, the accuracy of the closed-form predictions improves significantly at elevated speeds in the flux-weakening regime due to the reduced impact of magnetic saturation as flux levels are decreased.