Online electrolytic capacitor prognosis system for PWM drives

Electrolytic capacitors are ubiquitous in power electronic systems owing to their relatively large capacitance-to-volume ratio provided by virtue of their anodised electrode and electrolyte. As the capacitor ages, the electrolyte solution present in the capacitor is known to reduce and this leads to an increase in the equivalent series resistance (ESR) and a reduction in the capacitance. End-of-life for electrolytic capacitors is indicated by an increase in ESR by a factor of two. Here a prognostic system is proposed that can measure the ESR of a capacitor while it is operating in a PWM circuit. By using lock-in amplifiers synchronised to the PWM fundamental frequency, it is possible to extract precisely capacitor ripple voltage and ripple current. These signals are then combined to provide an estimate for the ESR.