Increase in operating range and efficiency for variable gap axial flux motors

For applications requiring a constant power source such as traction, efficient power electronic utilization requires an electric machine capable of flux weakening. Usually flux weakening is achieved by using an interior permanent magnet machine (IPM) and by applying current in the d-axis. Other recent work has used two inverters or pulses of current to demagnetize and magnetize the magnets depending on the operating point. This paper presents an axial flux machine with mechanical flux weakening capability. Experimental data shows that the torque constant can be reduced to five times its maximum value and the efficiency in the rated operating range can be increased by up to 15% points. To demonstrate the impact of the mechanical flux weakening mechanism on the efficiency of the system, the motor data was used to model the performance of an electric motorcycle over a World Harmonized Light Vehicles Test Procedures (WLTC)[1] drive cycle. If a self locking automated actuation mechanism is used, the peak power required by the actuation mechanism is 4% of the traction motor power rating and the energy used over the drive cycle is 0.05% of the traction motor energy use, suggesting negligible impact on the overall efficiency. If a non locking mechanism is used, the actuating power and energy will be substantially lower.