Sensorless control of Permanent Magnet Synchronous Motor at low speed

Abstract

Motor drives are essential in many industry applications such automotive, robotics, power and energy industry. The availability of cheap and powerful digital signal controller have increased the interest in its application in motor control. Sensorless control of motor is especially challenging because the rotor position and speed information need to be estimated without any shaft sensors. The use of machine model has been helpful in estimating the rotor speed using the phase currents of motor. However such control schemes have limitations at low speed because of absence of Back-EMF in the motor at low speed. Therefore these controlling schemes are useful only above a minimum speed above which there is substantial back-EMF in the motor. The aim of this thesis is to find a method to lower this speed limit as close to zero speed. Here in this thesis a brief study of the existing control scheme was carried out to understand the advantage and disadvantage of each control scheme. The current observer sensorless scheme was adopted for further study and improvements. The three improvements suggested were current injection, current averaging and voltage compensation. The current injection and current averaging was selected for implementation in this thesis. The current injection which was done on the rotating reference frames in the machine model and the current averaging was found to reduce the speed limit of an existing sensorless control. The stability and power efficiency of the changed control scheme was studied. The control strategy tested on the given motor was able to bring the minimum speed of control from 450rpm to 50rpm. The algorithm was also found to make the control unstable at speed ranges between 220-440 rpm. Besides that, the coupling effect of the injected current was found to reduce the efficiency of the motor at speeds less than 200rpm.