An open-loop operation strategy for induction motors considering iron losses and saturation effects in automotive applications

Induction motors (IM) are suitable traction drives for electric vehicles (EV). In comparison to permanent magnet synchronous motors (PMSM) the lack of power and torque density can be compensated by inferior production costs and a greater level of robustness. However, IM have to be operated at maximum efficiency since the amount of stored energy is still very limited in automotive applications and the driving energy demand is directly related to costs. To generate a certain torque with minimum losses a precise motor model considering the impact of saturation effects as well as iron losses is required. To consider these nonlinear effects a lookup table (LUT) based open-loop operation strategy (OS) in the rotor flux-oriented coordinate system is proposed. The presented approach allows a smooth transition between the constant torque and the flux weakening area as well as a high level of voltage utilisation above base speed. The LUTs can be generated offline using a maximum efficiency (ME) strategy based on finite element analysis or measured motor data. Efficiency enhancements in the range of 0.1-0.2% for a 60 kW IM in contrast to the classical minimal copper losses (MCL) strategy can be achieved.