New optimal current control methods for energy-efficient and wide speed-range operation of hybrid-field synchronous motor

This paper proposes new practical optimal current control methods for a newly emerging class of synchronous motors with hybrid rotor fields by both permanent-magnet and winding. In practical situation with limited voltage, the extensively used permanent-magnet synchronous motor hardly achieves an ideal performance that allows simultaneously both low-speed high-torque and wide speed-range operations, due to its constant magnet field. The hybrid field synchronous motors (HFSM) can achieve the ideal performance as practical motors with controllable hybrid rotor field. For HFSM, the same torque can be produced by a variety of currents due to nonlinearity between torque and currents. Consequently, appropriate determination of a set of stator and rotor current commands plays a key role to achieve possible energy-efficient and wide speed-range operation. Proposed methods determine the current commands corresponding to a given torque command such that total winding copper loss due to stator and rotor currents can be minimized if the exact solution exists, the best approximate torque can be produced if no exact solution exists. The determined current commands are the optimal in sense of energy-efficiency or degree of approximation in wide speed-range operation under voltage limitation. New real-time recursive algorithms searching the optimal current solution are also given. The proposed methods are analytical but practical, whose usefulness is verified through experiments