Optimum speed ratio of induction motor drives for electrical vehicle propulsion

When sizing and subsequently designing an electric motor drive for any battery-powered electric vehicle (EV), the main objective is to get a large charge range while achieving low drive weight, volume and cost. The selection of the speed ratio (i.e., motor maximum speed divided by its rated speed) has a great impact on the drive efficiency, weight, volume and cost. To this end, the paper presents a methodology to determine the optimum speed ratio of an induction motor drive for EV applications using the EV America Technical Specifications as the basis. Factors like the peak power rating, continuous power rating, peak current, weight, volume and average loss of motor drives are functions of the speed ratio. The paper shows the need to consider several performance and design factors besides the peak power rating, and includes a set of criteria for selecting the optimum speed ratio of induction motors used in EV applications. The optimum speed ratio was found to be 3.0, where the motor weight, volume, cost, drive peak current and average losses (affecting the life of the battery pack) are low.