Compact rotor power transfer in wound rotor synchronous motor (WR-SM) for high performance electric vehicles
The trend toward hybrid and electric vehicles pushes the developments of powertrain for high reliability, high efficient, high power density, easy mass production and high material recyclability.
Nowadays permanent magnet (PM) motors are commonly used for electric powertrains because of its compactness and efficiency. But high performance rare earth permanent magnets (NdFeB) are expensive and need special care during production. The resources of the material are limited. The recycling process is almost impossible. Alternatively induction motors with copper cages become popular for high performance vehicles like Tesla cars. But induction motors have low efficiency in general. The copper rotor manufacturing is complicated process.
Theoretically the perfect motor is a wound rotor synchronous motor (WR-SM), which is used for all large power generations. Because of the independent and controllable rotor excitation, the motor can be easily controlled to deliver the any desired torque at any speed with high efficiency. Speed range can be extended to more than 1:10 or as much as to the mechanical limitation. Its high power factor makes it possible to reduce the rating of the power electronics converters. The peak torque can be as high as 300% of the rated one. In addition, this motor technology is very suitable for mass production and conventional recycle process.
However, the brushes and slip rings for transferring power to the rotor windings need maintenance and may influence the operating reliability, especially in road vehicle applications. Therefore the WR-SM equipped with a rotating transformer is a promising solution. But the rotating transformer will significantly increase the overall size and weight of the motor.
The purpose of the project is to firstly find a compact concept to transfer power into the rotor windings and then verify the concept experimentally together with a WR-SM. Concept designs will be carried out for the target motors in power of 20kW for mild hybrid personal vehicles and 200kW for heavy-duty vehicles.
The project “Compact rotor power transfer in wound rotor synchronous motor (WR-SM) for high performance electric vehicles” is part of the Swedish Hybrid Vehicle Centre and is performed at Division of Electric Power Engineering, Chalmers University of Technology.