Predicting Operating Temperature and Expected Lifetime of Aluminum-Electrolytic Bus Capacitors with Thermal Modeling

Large-can aluminum electrolytic capaci- tors are widely used as bus capacitors in variable-speed drives, UPS systems and inverter power systems. Accu- rate thermal modeling of the capacitor's internal tem- perature is needed to predict life, and this is a challenge because of the anisotropic nature of the capacitor wind- ing and the complexity of the thermal coupling between the winding and the capacitor case. This paper trans- lates analytical models for heat flow in bus capacitors into an equivalent three-loop, seven-resistor, lumped-pa- rameter thermal circuit model. This paper presents the results of a Finite-Element Analysis (FEA)-based par- tial differential equation solution and the results of the three-loop thermal circuit model. The latter model is the basis for an operating temperature and expected life- time Java applet which enables power-system designers to accurately predict capacitor operating temperature and expected life from operating conditions. Operating conditions permitted as inputs include applied voltage, ambient air temperature, air speed, thermal resistance of any heatsink attached, and capacitor characteristics like capacitance, ESR and case size.