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This paper aims to reveal an offset jitter-like waveform error, which is usually present in all three-phase current signals reconstructed from dc-link current samples. This paper also presents a method for the cancellation of that error, which could be useful in low-cost shaft-sensorless drives. A combination of two samples' time displacement and the line current pulse-width modulation (PWM) ripple produces a distinctive shape of the reconstructed current error, which is offset jitter like, and is always at the current ripple level. This unusual waveform distortion is often masked by other jitters and noise sources and therefore is typically not given in the relevant literature. However, the distortion is difficult to filter out, and it may be harmful, particularly in applications with low rated motor current. The proposed waveform correction scheme uses the readily available motor states in shaft-sensorless drives and does not require major computational effort. Experimental results show that the scheme recovers the α-β current waveforms and, as a result, prevent the increase in the sixth harmonic ripple in the d-q currents, which is inherent to the PWM inverter-controlled drives. Consequently, drives with the dc shunt will neither have current signal offset problems nor the need for d-q current signal prefilters with order significantly higher than usual.
Date of Publication: July 2010