The current trend in production EVs/HEVs is to use interior permanent magnet synchronous motors (IPMSM) as the means of providing power to the vehicle drive train. The efficiency of these machines can be extremely high (>95%) although this typically occurs in a relatively narrow range in the middle of the machine speed-torque curve. This is a concern for automotive applications as vehicles operate across the full range of speeds and torques; typical urban drive cycles operate in the low speed/torque region where efficiencies can drop to below 80%. Clearly there is a mismatch between the region of high efficiency of the machine and the region of highest operation duty of EVs/HEVs. This paper presents a method of expanding the peak efficiency region of the machine by introducing a method for adjusting the permanent magnet flux linkage utilising a mechanism that short circuits the flux at the end caps of the rotor using solid steel plates. These plates are operated such that they can switch between open and closed positions depending on the demand on the machine allowing the flux linkage to be varied. Analysis shows that by applying the plates to short the flux in the rotor the flux linkage in low torque/high speed applications is lowered whilst reducing the need for high flux weakening currents in the d-axis. This reduces current consumption, improving the power factor and therefore increases the overall efficiency of the machine. The performance of the system is verified using a proof of concept IPMSM.
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