The authors have previously presented a control scheme for synchronous reluctance motors where the motor current is controlled to keep the MMF phase angle constant, and made it clear that a simpler control scheme is possible by using a N-speed synchro for a 2N pole motor. However, conventional brushless synchros are expensive and bulky due to the intricate construction where the rotary transformer supplies current to the exciting winding on the rotor core. The authors have therefore developed a novel VR (variable reluctance) synchro without a rotary transformer, consisting of a stator core with both the exciting and output windings and a uniquely shaped rotor core with no winding. This paper describes the theory of the novel VR synchro and presents measured waveforms of the output voltages of a prototype VR 4-speed synchro. In addition, a variable speed drive system for 8-pole reluctance motors which uses the new control scheme is introduced. In this system, the VR synchro is directly connected to the motor shaft, whose output voltages are demodulated and directly used as the current phase command for the inverter current supplied to the motor. Experimental results show a satisfactory speed response for 4 quadrant drive.
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