Sensorless Direct Torque Control for Electrically Excited Synchronous Motor Based on Injecting High-Frequency Ripple Current Into Rotor Winding

A novel rotor position angle estimator is proposed to achieve zero and very low operation speed for sensorless direct torque-controlled electrically excited synchronous motor (EESM) drive. First, high-frequency ripple current is injected into rotor winding, and second, using closed-loop torque control, high-frequency component is injected into electromagnetic torque to produce high-frequency stator current only in d-axis direction. At last, the rotor position angle is estimated with phase-locked loop principle. The stator flux and electromagnetic torque are calculated with stator flux current model and estimated rotor position angle, and then a sensorless direct torque-controlled EESM drive is constructed with estimated flux and torque. The experimental results show that the proposed sensorless EESM drive system can operate at zero and very low speed.