A Real Time Method to Estimate Electrolytic Capacitor Condition in PWM Adjustable Speed Drives and Uninterruptible Power Supplies Eddy C. Aeloíza* Jang-Hwan Kim** Pedro Ruminot*** and Prasad N. Enjeti* * Department of Electrical Engineering Texas A&M University, College Station, TX. 77843-3128, USA. e-mail:enjeti@ee.tamu.edu ** School of Electrical Engineering Seoul National University, Kwanak-gu, Seoul 151-744, KOREA . e-mail:ghks95@eepel.snu.ac.kr *** Department of Electrical Engineering University of Concepcion Casilla 53-C, Concepcion, CHILE. e-mail: pruminot@udec.cl Abstract⎯ Electrolytic capacitors are widely used in various power electronic systems, such as adjustable speed drives (ASD) or uninterruptible power supplies (UPS). Their high energy density (J/cm 3 ) features make them an attractive candidate for smoothing voltage ripple and pulse discharge circuitry [1,2,3,4]. However, electrolytic capacitors have the shortest life span of components in power electronic circuits, usually due to their wear-out failure [1,2,5]. The main wear-out mechanisms in electrolytic capacitors are the loss of the electrolyte by vapor diffusion trough the seals and the deterioration of the electrolyte [1,6]. Both mechanisms can result in a fluctuation of the capacitor’s internal equivalent series resistance (ESR) [1,2]. In this paper, a real time diagnostic method of the ESR of the electrolytic capacitors in ASDs and UPSs is presented. This method is employed satisfactorily to estimate their deterioration condition. An on board implementation of this method is proposed, which can be very helpful for preventing down time and alerting plant operators to needed maintenance and/or replacement. The approach does not involve removing the capacitor bank from its device and relies on the fact that in steady state, the power in the capacitor is only due to the power losses in the ESR. An analog-DSP based diagnostic system has been implemented and experimental results from a 3-phase 6kVA/230V ASD are presented. I. INTRODUCTION Industrial applications, such as advanced motor drives, UPS, electric vehicles and dc power supplies are the most common examples of power electronics applications. These and other electronic devices commonly use electrolytic capacitors as an integral part of the device. The main functions of the electrolytic capacitors are smoothing voltage ripple and store electric energy [2,4]. Their large capacitance, large energy density (J/cm 3 ) and low price make them be the first choice to accomplish this objective [2,3,4]. Nevertheless, failures in electrolytic capacitors are the main cause of breakdowns in ac motor drives and UPS [1,2,5], mainly due wear-out. They have the shortest span of life out of all the other components, and then determine the whole system’s lifetime [1,2]. The load life specifications for aluminum electrolytic capacitors operating at maximum permitted core temperature are typically 1000 to 10,000 hours (6 weeks to 1 year), not long enough for most applications [7]. The literature states that the main causes of wear-out mechanism in electrolytic capacitors are the loss of the electrolyte by vapor diffusion trough the seals and/or the deterioration of the electrolyte [1,6]. Furthermore, they may have to withstand extreme and harsh temperature which accelerate the electrolyte evaporation process and reduces their life time [6]. Several methods to estimate the deterioration status of the electrolytic capacitor have been proposed; some methods are based on the fact that loss of electrolyte results in loss of capacitor weight [1,2] and others are based in the claim that the capacitor’s wear-out mechanisms can result in fluctuations of its internal equivalent series resistance (ESR) [1,2,8]. The ESR is a single resistance representing all of the ohmic losses of the capacitor and connected in series with the capacitance. For aluminum electrolytic capacitors, the ESR specified by the manufacturer is measured as the resistance of the equivalent series circuit at 25°C and 120 Hz/1V rms-max voltage source free of harmonic [7]. The methods based in capacitor’s weight are inherently inconvenient since the capacitors need to the removed to perform the test. Conventional methods to measure capacitor’s ESR utilize either frequency response analyzers or sophisticated RLC meters. Both methods also require removing the capacitors from the device, which make them impractical for real time applications. Nevertheless, methods based on ESR variations are fairly accepted to estimate the lifetime of electrolytic capacitors. In this paper, a real time diagnostic method of the ESR of electrolytic capacitor to estimate their deterioration condition is presented. The approach does not involve 2867 0-7803-9033-4/05/$20.00 ©2005 IEEE.