Design and control of axial-flux brushless DC wheel motors for electric Vehicles-part II: optimal current waveforms and performance test

We present dedicated designs of optimal current waveforms for disk-type axial-flux wheel motors. The four-phase dedicated wheel motor has been designed and installed directly inside the wheel of electrical vehicles without mechanical differentials and reduction gears. We performed a torque-oriented optimization to obtain the optimal current waveform subject to various constraints for the independent winding structure. We found that the best optimal waveform with maximized torque and confined ohmic loss is proportional to the magnetic flux variation in the air gap between the stator and the rotor and has the same shape as the back-electromotive force (EMF). This finding is confirmed by both theoretical and numerical analyses. As expected, the current control waveform of the back-EMF extracted by experiments renders the best performance in terms of maximum torque and motor efficiency.