Designing High Power Density Induction Motors for Electric Propulsion

Designing high-power-density electric motors for propulsion has become an increasingly important issue in the past few decades. This is not only because electric vehicles are projected to become the main private transportation means in near future, but also because of the ever so important metro and railway transport requirements. Along with these application areas, electric aircraft propulsion is also coming into focus in recent years. Electric motors for traction are required to have high torque density, high efficiency over a wide speed range and are required to be robust. In recent years, permanent magnet (PM) motors became the favorite choice for such applications because of their higher efficiency than other types of motors. Increasing demand for permanent magnets is likely to cause supply problems. Therefore, permanent magnet-free alternative motor types are of much interest. In this paper, the authors present the design of a 125 kW induction motor for railway application. This design has 3-times the power density of a commercial induction motor. The designed motor is manufactured and its test results are used for establishing an accurate finite-element model for the prediction of its performance. This model is used to investigate the effect of magnetic loading choice, slot shape and magnetic material choice on the efficiency of the motor. It is shown that with the same basic dimensions the efficiency of the motor can be increased to 96% which is comparable with a similar size PM motor.