Parasitic rotor losses in a brushless permanent magnet traction machine

The paper reports on experimental and theoretical investigations into parasitic loss experienced in the rotor of a prototype 150 Nm, 45 kW at 3 krpm, brushless permanent magnet machine having inset, sintered NdFeB, rotor magnets and designed as the traction drive machine for a hybrid electric vehicle. The experimental investigation identifies and quantifies loss trends for the prototype machine as a function of machine stator current excitation and shaft speed, while the theoretical work aims to expand the understanding of the loss mechanisms to enable the formulation of design criteria for loss minimisation. The theoretical investigation employs simplifying assumptions that enable the calculation of loss via a number of 2-D finite element magneto-static field solutions however, such an approach is time consuming, hence the development of an analytic solution for the 2-D field distribution at the machine air gap has been implemented. Experimental and predicted results are presented for an inset rotor topology machine controlled via a PWM and overmodulated inverter operating a brushless DC (i.e. 120° discontinuous conduction) switching strategy, together with loss predictions for a surface magnet rotor design and operation of the prototype machine, with either rotor topology, via a vector oriented sinusoidal controller