Separately excited synchronous motor with rotary transformer for hybrid vehicle application

The cost of rare earth (RE) permanent magnet along with the associated supply volatility have intensified the interests for machine topologies which eliminate or reduce the RE magnets usage. This paper presents one such design solution, the separately excited synchronous motor (SESM) which eliminates RE magnets, however, but does not sacrifice the peak torque and power of the motor. The major drawback of such motors is the necessity of brushes to supply the field current. This is especially a challenge for hybrid or electric vehicle applications where the machine is actively cooled with oil inside the transmission. Sealing the brushes from the oil is challenging and would limit the application of such motor inside a transmission. To overcome this problem, a contactless rotary transformer is designed and implemented for the rotor field excitation. The designed motor is built and tested. The test data show that the designed motor outperforms an equivalent interior permanent magnet (IPM) motor, which is optimized for a hybrid application, for both peak torque and power. Better drive system efficiency is measured at high speed compared to the IPM machine, while the later outperforms (for efficiency) the SESM at low and medium speed range.