Comparison of two techniques for closed-loop drive of VR step motors without direct rotor position sensing

Closed-loop control of variable-reluctance (VR) step motors without using direct rotor-position sensing is investigated. Two nondirect rotor-position sensing techniques, namely the waveform detection (WD) and the pulse-width-modulation (PWM) techniques, were implemented and applied to a four-phase (15° step-angle) VR step motor. Test results show that a DC shunt motor and a DC series motor type of performance can be achieved from the step motor with PWM drive and WD drive, respectively. The stability problem encountered with the existing PWM controller due to motional EMF effects at higher operating speeds is investigated, and modifications are suggested to include speed as an additional control signal to make the system more stable. With the inclusion of the speed signal, the operating speed of the test motor is from 1000 to 1750 r/min. The performances for the two techniques are compared, and experimental results are presented