Optimum efficiency control of interior permanent magnet synchronous motors in drive trains of electric and hybrid vehicles

In automotive traction applications the interior permanent magnet synchronous motor (IPMSM) is preferentially chosen as traction drive due to its high torque and power densities. In drive trains of electrical vehicles (EV) and most hybrid electrical vehicles (HEV) the traction motor is operated in torque controlled mode. In a field-oriented control scheme an operation point selection strategy is required to choose appropriate current setpoints to generate the requested torque with high precision and optimal efficiency. In this paper the potential to improve the efficiency by operating a motor with minimum overall losses instead of minimum current per torque is investigated. The interaction between the motors loss characteristic, the impact of the voltage limit and the relevance of operation points for vehicle operation is explored theoretically and based on loss measurements. It is shown, that the losses can potentially be reduced at medium to high motor speeds with moderate torque requirements. This operation range corresponds to traveling with constant medium to high speed and slight acceleration and deceleration in electric vehicles. As well most operation points of the New European Driving Cycle (NEDC) and the Worldwide harmonized Light vehicles Test Procedure (WLTP) are in that operation range due to the moderate accelerations and decelerations in both driving cycles.