Life-Cycle Monitoring and Voltage-Managing Unit for DC-Link Electrolytic Capacitors in PWM Converters

A novel life-cycle monitoring and voltage-managing device for dc-link electrolytic capacitors in pulsewidth modulation converters is presented. The system performs online identification of the capacitor's equivalent series resistance (ESR) in order to detect the life-cycle status and permit preventive maintenance. The ESR detection is based on the capacitor's ac losses calculated from voltage/current measurements using a simple low-cost microcontroller. The unit is designed as small printed circuit board located directly at the capacitor's screw terminals in order to simplify the required temperature sensing and to minimize wiring effort. The minimized energy consumption allows a power supply taken out of the capacitor to be tested. Besides life-cycle monitoring, the unit further facilitates energy-efficient voltage balancing for capacitors in series arrangements avoiding any power resistors typically used in balancing circuits. Instead, the unit controls the capacitor voltage by influencing its power consumption. Each individual monitoring unit (one per each power capacitor of the converter) transfers the ESR results to the converter's main controller and receives control commands via a common optoisolated fieldbus. Alternatively, this data transfer is performed using wireless near-field communication leading to a completely autonomous monitoring unit without any wiring for power supply and data transmission.