Methods for Speed Sensorless Control of AC Drives

The operation of speed controlled ac drives with- out mechanical speed or position sensors requires the estima- tion of internal state variables of the machine. The assessment is based exclusively on measured terminal voltages and cur- rents. Low cost, medium performance sensorless drives can be designed using simple algebraic speed estimators. High-per- formance systems rely on dynamic models for the estimation of the magnitude and spatial orientation of magnetic flux waves in the stator or in the rotor. Open loop estimators and closed loop observers differ with respect to accuracy, robust- ness, and limits of applicability. The overview in this paper uses signal flow graphs of complex space vector quantities to give an insightful description of the physical and mathemati- cal systems used in sensorless ac drive control. I. INTRODUCTION AC drives based on full digital control have reached the status of a maturing technology in a broad range of applica- tions ranging from low-cost to high-performance systems. Continuing research has concentrated on the elimination of the speed sensor at the machine shaft without deteriorating the dynamic performance of the drive control system. Speed estimation is an issue of particular interest with induction motor drives where the mechanical speed of the rotor is generally different from the speed of the revolving magnetic field. The advantages of speed sensorless induction motor drives are lower cost, reduced size of the drive machine, elimination of the sensor cable, and increased reliability. A variety of different solutions for sensorless ac drives have been proposed in the past few years. This paper reviews their merits and limits based on a survey of the available litera- ture.