Comparison of Direct and Axle Drives with electrically excited Synchronous Machines for Traction Applications

While permanent magnet synchronous machines (PSM) dominate the market of electrical machines for traction application, electrically excited synchronous machines (ESM) are still under-representated, although they can be designed to feature the highest mechanical output power to total inverter output power ratio. This paper addresses the advantages of ESM especially for axle drives and evaluates if they are also valid for ESM designs for wheel hub drives. The main characteristics of three optimized ESM drive concepts for the use in a fullsize car with 220 kW peak output power and almost identical acceleration characteristics are presented and compared. Based on the available installation space, the influence of the required torque density and of the total leakage factor on the machines performance is identified and important design rules are derived.