A modified direct torque control of surface permanent magnet synchronous motor drives without a speed sensor

Abstract

A modified speed sensorless direct torque control (DTC) scheme for surface permanent magnet synchronous machine (SPM) is investigated in this paper, which features in very low flux and torque ripple and almost fixed switching frequency. Instead of the switching table and hysteresis controllers in the basic DTC, a reference flux vector calculator (RFVC) is used to determine the reference stator flux linkage vector. The reference flux vector is produced according to the amplitude of the reference and estimated stator flux linkages, the position of the estimated stator flux linkage, the amplitude of the reference and estimated torques and the estimated rotor speed. Then the voltage vectors applied on the machine is calculated to compensate the flux and torque errors exactly, so the flux and torque are controlled in the deadbeat fashion. A speed estimation method is also introduced in this paper, in which the rotor flux linkage is calculated from the stator flux linkage and the stator current. Then the rotor speed is calculated from the rotor flux linkage. Simulation results show that the flux and torque ripples are greatly reduced when compared with those of the basic DTC. With the new scheme, the quick dynamic torque response of the basic DTC is still retained. Fixed switching frequency at different operating conditions becomes possible by means of space vector modulation.