Discrete Rotor Flux and Speed Estimators for High-Speed Shaft-Sensorless IM Drives

In numerous motor drive applications, high rotor speed is the key factor for system cost, performance, and overall energy efficiency. As a result of energy crises and global market competition, the specified rotor speed and fundamental frequency of the induction motor (IM) in many drive applications noticeably go up. For the same cost and efficiency reasons, that increase of inverter fundamental output frequency cannot be followed with the increase of pulsewidth modulation (PWM) frequency. Therefore, a very low ratio between the PWM and motor fundamental frequencies is to be expected in the near future. In this paper, the shaft-sensorless drive performance is investigated at high speeds, with a very low sampling to fundamental frequency ratio. As a result, two main problems with rotor flux estimators were discovered: the integration problem in the current-based rotor flux model and the phase error in the voltage-based rotor flux model. Both problems were addressed, and a proper joint solution is suggested. The effectiveness of the proposed solution is tested in a model-reference-adaptive-system-based high-speed shaft-sensorless IM drive. The experimental results collected from the digitally controlled IM drive with a low frequency ratio validate the proposed solution.