Electric Machine Torque Density Enhancement Using Spatial Harmonics

Torque-dense electric machines are beneficial across a wide range of applications. Unlike conventional three-phase stator windings that can only control a single spatial harmonic field in the airgap, multiphase, 'multi-harmonic' windings can independently control multiple spatial harmonics, each corresponding to a different number of pole-pairs. These spatial harmonics can be used to create additional torque, thereby improving the torque-density and the response time by increasing the torque-to-rotor-inertia ratio. The existing literature on torque enhancement is limited to radial flux designs and considers it as a motor drive / control problem of injecting 'time harmonic' currents, rather than machine design to control 'spatial harmonics'. This paper investigates torque enhancement as a machine design problem in radial flux (RFPM) and axial flux PM (AFPM) machines to i) quantify the torque enhancement limits in terms of the machine design parameters, and ii) identify optimal harmonic current amplitudes to maximize torque density. The paper develops a 7-phase multi-harmonic AFPM and shows that it can achieve up to 33% improvement in torque capability and 65% improvement in torque-to-rotor-inertia ratio relative to an equivalent 3-phase design. Experimental results are provided to validate these findings, making this the first work to demonstrate torque enhancement in AFPM designs.