Derivation of motor mean phase currents in PMSM drives operating with low switching-to-fundamental frequency ratio
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Gdansk University of Technology
Leszek Jarzebowicz   

Gdansk University of Technology
Pulse width modulation (PWM) of inverter output voltage causes that the waveforms of motor phase cur-rents consist of distinctive ripples. In order to provide suitable feedback for the motor current controllers, the mean value must be extracted from the currents’ waveforms in every PWM cycle. A common solution to deriving the mean phase currents is to sample their value at the mid-point of a symmetrical PWM cycle. Us-ing an assumption of linear current changes in steady PWM subintervals, this mid-point sample corre-sponds to the mean current in the PWM cycle. This way no hardware filtering nor high-rate current sam-pling is required. Nevertheless, the assumption of linear current changes has been recently reported as over simplistic in permanent magnet synchronous motor (PMSM) drives operating with low switching-to-fundamental frequency ratio. This, in turn, causes substantial errors in representation of the mean phase currents by the mid-point sample. This paper proposes a solution of deriving mean phase currents in low switching-to-fundamental frequency ratio PMSM drives, which does not rely on the linear current changes assumption. The simulation results show that the proposed method decreases the error of determining the mean phase currents approximately 10 times when compared to the classical mid-point sampling technique.
ISSN (print):2451-0262