Guidelines for Selecting Minimum Capacitance for a Floating Bridge Dual Inverter Drive

A method for selecting the floating bridge capacitance of a dual inverter drive with an open ended winding induction machine is presented. It is shown that the floating bridge capacitor voltage can be kept constant even during large step transients in motor speed reference and load torque. Hence, the capacitor can be made very small and can be chosen based on the steady state voltage ripple, which is a result of the inverter switching action. The ripple voltage is a function of the motor speed, torque and inverter switching frequency. Equations are provided that can predict the ripple voltage over the whole operating range of the motor. A guideline equation for selecting the capacitor based on the predicted worst case ripple and drive parameters is also presented.