Reduction of DC-bus capacitor ripple current with PAM/PWM converter

Electrolytic capacitors are used in nearly all adjustable-speed drives, and they are one of the components most prone to failure. The main failure mechanisms include loss of electrolyte through outgassing and chemical changes to the electrolyte and oxide layer. All the degradation mechanisms are exacerbated by ripple current heating. Since the equivalent series resistance of electrolytic capacitors is a very strong function of frequency it must be properly modeled to accurately calculate the power loss. In this paper, a method to reduce the ripple current in a constant Volts/Hertz pulse-amplitude-modulation (PAM)/pulsewidth-modulation (PWM) converter driving an induction motor is investigated. The dc-bus voltage amplitude is reduced in proportion to speed by a buck or current stiff rectifier and the PWM modulation index is maintained at a high level to achieve a reduced ripple current below base speed. By comparison with a diode-bridge-fed PWM voltage stiff inverter, it is shown that the PAM/PWM mode of operation can lead to a significant reduction in capacitor power loss leading to increased capacitor lifetime or decreased capacitor size. The capacitor heating is analyzed using numerical and analytical techniques. Experimental results are provided to verify the analytical results.