A new zero frequency flux position detection approach for direct field oriented control drives

The paper presents a novel approach to estimate the instantaneous position of the airgap flux in standard induction machines at low and zero speed. The proposed approach is based on the generation of a high frequency rotating field that, interacting with the main field, generates a high frequency homopolar flux component. By demodulating such a component, that is always present irrespectively of speed and load levels, it is possible to detect the airgap flux position. The proposed method allows some key drawbacks of sensorless approaches based on high frequency voltage injection to be overcome, and, compared to previous sensorless schemes exploiting the third harmonic voltage, improves the accuracy and extends the operating area up to zero speed. Experimental tests are shown in order to practically confirm the expected features of the proposed system.