A Case Study: An Approach to Decrease Voltage Distortion and Enhance Field Weakening Capability in BMW i3 Traction Motor

Abstract

The transformation of modern vehicles to electric mobility is increasing day by day. This potential allows detailed studies of alternative motor topologies. Synchronous reluctance motor topologies are the main candidates among these alternatives. In particular, the permanent magnet assisted synchronous reluctance motor (PMaSynRM) topology is characterized by lower permanent magnet (PM) consumption and additional reluctance-based torque. However, PMaSynRM type EV motors suffer from torque ripple, voltage ripple, lower output power at higher speeds and lower power factor compared to the interior type permanent magnet (IPM) alternatives. In the scope of this study, the terminal voltage distortion level (TVDL) in PMaSynRM is investigated for the first time in the literature. Similarly, an approach to reduce voltage distortion and improve field weakening capability are proposed for the first time in literature. The proposed method is applied to a reference EV traction motor that is used in BMW i3. An improvement ratio of rotor pole shaping and rotor skew are presented separately. The proposed method is validated by comparison with technical datasheet of the reference motor and its Finite Element Analysis (FEA) results. According to the analysis results, the proposed design is reduced the terminal voltage distortion level to 1.08% from 45.4%, torque ripple to 15.5% from 45.9% and boost the output power to 135.3 kW from 94.9 kW at the maximum speed of 11400 rpm.

Keywords

• Electric vehicle
• traction motor
• permanent magnet assisted synchronous reluctance motor
• terminal voltage distortion
• field weakening
• maximum torque per ampere