Speed estimation technique for flying start of medium-voltage induction machine under sensorless field-oriented control

This paper presents a new technique to estimate the initial speed of the medium voltage induction machine, typically driven by a Cascaded H Bridge converter, in case of the machine coasting while the converter modulation is off, often caused by a loss of the input voltage or the control command. Accurate identification of the machine’s freewheeling speed is indispensable for a smooth restart. Due to the characteristics of cascaded H bridge converter and medium voltage IM, conventional excitation-based techniques often only yield current signals with high signal-to-noise ratios, resulting in unacceptably poor identification accuracy. To address this issue, this paper proposes a normalized and observer based initial speed identification technique. The technique demonstrates good precision in full speed regions and therefore, it can be used as the first step in re-start protocol to reduce current impact. In the end, the feasibility and effectiveness of the proposed technique is verified by simulation results.