Computation of the voltage-driven flux-MMF diagram for saturated PM brushless motors

The flux-MMF diagram or "energy conversion loop" is a powerful tool for computing the average torque of saturated permanent-magnet brushless motors, especially when the assumptions underlying classical dq theory are violated, as they often are in modern practice. Efficient finite-element computation of the flux-MMF diagram is possible when the motor current is known a priori; but in high-speed operation the current-regulator can lose control of the current waveform and the computation becomes "voltage-driven" rather than "current-driven". This paper describes an efficient method for solving the voltage-driven problem together with experimental validation on a small 2-pole brushless interior-magnet motor. The paper also discusses the general conditions when this method is appropriate, and compares it with alternative approaches.