Reliability Test Setup for Liquid Aluminum Electrolytic Capacitor Testing

Bus capacitors play a critical role in the operation of Variable Frequency Drives (VFDs). Failure of bus capacitors can cause a failure of the drive leading to unplanned downtime and substantial revenue loss for customers. In VFDs, liquid aluminum electrolytic capacitors are used as bus capacitors for smoothing of voltage and when line can't provide large bursts of energy (drive start-up) and during temporary power loss. These liquid aluminum electrolytic capacitors are critical for the successful functioning of the VFDs for various customer applications. The VFDs are used across many industries such as paper, tire and rubber, oil and gas, and automotive. In order to select robust liquid aluminum electrolytic capacitors for the drives, reliability testing is critical. Liquid aluminum electrolytic capacitors have long been considered as the weakest reliability component in power electronic systems. Primary failure mechanism observed is evaporation of liquid electrolyte leading to a decrease in capacitance and increase in equivalent series resistance (ESR) and eventually causing the failure of the capacitor. Ripple current increases the core temperature of these capacitors which leads to capacitance drop and ESR increase and reduces the useful life of these capacitors. As ESR increases, the heating effects due to ripple current worsens and this turn into a runaway failure situation. Therefore, it is paramount to perform the reliability/qualification testing on these capacitors with ripple current at elevated temperature and DC voltage bias. Testing multiple capacitors with just DC voltage is simple (connecting several capacitors in parallel) but testing multiple capacitors with combined DC voltage and ripple current is a challenge. A special test setup was prepared using two sub-systems. The first sub-system included an auto transformer, a rectifier, and liquid aluminum electrolytic capacitors. Second subsystem included a variable frequency drive (VFD), a sine wave filter and a 3-phase transformer. The first sub-system provided the adjustable DC bias voltage. The second sub-system provided adjustable magnitude/frequency of AC ripple current. The detailed construction of the test setup is included in this paper. To select robust liquid aluminum electrolytic capacitors, the capacitors were tested using this test setup at rated temperature, rated ripple current plus DC voltage bias. Electrical parameters for the capacitors like capacitance, ESR and leakage current were measured before starting the test. These electrical parameters were monitored by measurement at regular intervals during the test. The test was stopped and after capacitors cooled down to room temperature, the measurements were taken. Testing liquid aluminum electrolytic capacitors before using them in the final product can greatly reduce the probability of field failures.