Sensorless torque and rotor position control for interior permanent magnet machines at standstill and low speed

This thesis is focused on sensorless torque control of interior permanent magnet synchronous machines at standstill and low speed. It can be divided into two closely related sub-topics, i.e. sensorless rotor position estimation and torque control at standstill and low speed. For sensorless rotor position estimation, a fast initial rotor position estimation method is firstly proposed based on high frequency rotating voltage injection. Compared to conventional methods, the proposed method can detect the initial rotor position faster by comparing the three phase high frequency current amplitudes as a result of high frequency rotating voltage injection. Secondly, a novel incremental inductance measuring method is proposed based on both high frequency rotating and pulsating voltage injections. Different from conventional methods which use either high frequency rotating or pulsating voltage injection for measuring the incremental inductances, the proposed method is immune to the delay effects and does not require a complicated closed-loop position observer. Thirdly, the cross-coupling effect can result in position estimation error in high frequency signal injection based sensorless control and deteriorates the torque control performance. A novel method is proposed for offline measuring the cross-coupling effect induced position error by injecting high frequency pulsating voltages in the dq-axes. The measured errors are then used for online compensating the cross-coupling effect. Compared to conventional methods using the offline measured relationship between incremental inductances for cross-coupling effect compensation, the proposed method is simple to implement and has better dynamic performance. For improving the torque control performance, a simultaneous sensorless rotor position and torque estimation method is proposed for standstill and low speed operation. With the proposed method, the rotor position is estimated by injecting high frequency square wave voltages to the estimated dq-axes, which are simultaneously used for identifying the incremental inductances. The identified incremental inductances are subsequently used to calculate the electromagnetic torque by approximating the dq-axis flux linkages with curve-fitting technique. Besides, a whole speed range sensorless operation method is proposed using a single closed-loop position observer. Compared to conventional methods, unity-length position error signal can be obtained from the saliency model based on high frequency rotating voltage injection without using a scaling factor. Various simulations and experiments have been carried out to verify the effectiveness of the developed methods.