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

This paper presents a novel approach to estimate the instantaneous position of the air-gap 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 zero-sequence flux component. By demodulating such a component, which is always present, irrespective of speed and load levels, it is possible to detect the air-gap flux position. The proposed method allows one to overcome some key drawbacks of sensorless approaches based on high-frequency voltage injection 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.