Closed-Loop Rotor Flux Estimation in Vector Controlled Induction Motor Drives Operated at Low Switching Frequencies

Position sensorless vector control drives in the rotor-flux-oriented reference frame require precise estimation of rotor flux. A few open-loop rotor flux estimation methods are quite popular. However, the performance of these estimators is quite poor when the inverter switching frequency is quite low as in case of high power drives or when the ratio of switching frequency to maximum fundamental frequency is low as in case of high speed drives. This paper investigates an alternative approach of closed-loop rotor flux estimation for motor drives operated at low switching frequencies. This method is shown to be accurate in estimating the rotor flux, and thereby, ensuring good performance of the vector controlled drive at low as well as high speeds. Experimental results are presented on a 3.7-kW, 400-V, 50-Hz, vector controlled induction drive, switched at a frequency of 500 Hz.