Sensorless Direct Flux Vector Control of Synchronous Reluctance Motors Including Standstill, MTPA, and Flux Weakening

This paper proposes a sensorless direct flux vector control scheme for synchronous reluctance motor drives. Torque is controlled at constant switching frequency, via the closed loop regulation of the stator flux linkage vector and of the current component in quadrature with it, using the stator flux oriented reference frame. A hybrid flux and position observer combines back-electromotive force (EMF) integration with pulsating voltage injection around zero speed. Around zero speed, the position observer takes advantage of injected pulsating voltage. Instead of the commonly used current demodulation, the position error feedback is extracted here at the output of the observer's flux maps, thus resulting in immunity toward the cross-saturation position error. The maximum torque per ampere (MTPA) strategy is used. A detailed analysis puts in evidence the key advantages and disadvantages related to the use of the MTPA in the sensorless control of the synchronous reluctance machine, for both the saliency-based and the back-EMF based sensorless methods. Extensive experimental results are reported for a 2.2 kW synchronous reluctance motor prototype, showing the feasibility of the proposed method. These include speed response to step and sinusoidal load disturbances at standstill, up to 121% of rated torque, and speed response tests covering the flux weakening speed region.

I. Introduction

In recent years, synchronous reluctance motors (SyRMs) have been widely adopted due to their low manufacturing cost and simple structure, without windings nor magnets on their rotors, and for their good efficiency [1]–[4]. The application of SyRMs will presumably grow in the coming years, because of the price uncertainty of rare earth raw materials used in permanent magnets, and because of stricter regulations on motors’ efficiency [5]. A key condition toward a wider application of the SyRM in a large number of industrial applications is that the existing sensorless techniques are improved in terms of both performance and ease of calibration.