A computationally efficient PM power loss derivation for surface-mounted brushless AC PM machines

This paper presents a simple and computationally efficient algorithm for mapping magnet loss within field-orientated controlled surface-mounted brushless AC PM machines over a wide range of operating conditions. Induced eddy current loss in rotor magnets can be a significant proportion of the overall loss and should not be overlooked when analysing thermal behaviour and efficiency over the required torque-speed envelope. Employing finite element analysis (FEA) to determine magnet loss at every load point would be computationally intensive. Here, a technique is proposed, where from a limited FEA the magnet loss over the entire torque-speed envelope is derived, catering for both rated flux and field weakened operation. The method requires four discrete time-step FEA solutions accounting for open-circuit, rated current in the quadrature axis, rated current in the direct axis and reduced current in the direct axis. The loss predictions from the FE analyses are then used to define a functional representation of the magnet loss. The proposed method has been validated on two surface-mounted brushless AC PM machine designs showing good agreement with direct FE predictions of the PM power loss.