Flux-weakening performance comparison of conventional and E-core switched-flux permanent magnet machines

High torque density and efficiency are inherent merits of switched-flux permanent magnet machines due to flux concentration. The difference in the physical structure and winding configuration of the E-core and the conventional switched-flux machines influences their inductances, magnetic saturation and cross-coupling. In this paper the flux weakening characteristics of the machines is investigated with particular reference to output power capability. It is shown that both machines have the flux weakening characteristics of non-salient permanent magnet machines, i.e. with a saliency ratio approximately equals to 1. However, the E-core machine with single layer windings has up to twice larger d-axis inductance for similar open circuit permanent magnet flux-linkage compared to the conventional machine with double layer windings. As a consequence, the conventional switched-flux machine can achieve higher power over a wide speed range when excited with high currents compared to the E-core machine. It is further shown that for a fixed volt-ampere rating, both machines can achieve the same output power with the appropriate choice of the current-voltage ratio.