In this paper, a novel scheme for the estimation of the equivalent series resistance (ESR) of the dc-link electrolytic capacitor in three-
phase ac/dc pulsewidth-modulation converters is proposed for condition monitoring. First, a controlled ac current component is
injected into the input. Then, it induces ac voltage ripples on the dc output. By manipulating these ac voltage and current
components with digital filters, the value of the ESR can be calculated, where the recursive least squares algorithm is used for reliable
estimation results. In addition, the value of the ESR is corrected by considering the temperature effect, for which a simple
temperature-sensing circuit has been designed. The simulation and experimental results show that the estimation error of the ESR
is within a reasonable range, thereby enabling the determination of the appropriate time for the replacement of the capacitor.
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