PWM Control of a Dual Inverter Drive using a Floating Capacitor Inverter

A carrier-based pwm scheme is presented for a dual inverter drive operating an open winding induction machine. One inverter is supplied from a dc power source while the other uses a floating capacitor dc voltage source. The output voltages of the two inverters have a nominal 90° phase shift relative to each other and low quality pwm motor voltages is obtained when using "standard" pwm techniques. Two pwm controllers are compared: 1R2C uses a single reference and a dual carrier to control a switch; 2R2C uses both a dual carrier and a dual reference signal (phase-difference and phase-average). The latter scheme allows for independent control of the two inverter output voltages while simultaneously coordinating their pwm switching patterns to obtain high quality motor pwm voltages. Independent inverter voltage control allows a field-oriented controller to be used for a fast motor speed response, whilst allowing decoupled control of the floating inverter dc voltage. The 2R2C pwm scheme is shown to produce a 5-level "effective line voltage" and is compared with several alternatives using either a single or dual inverter drive. The quality of the pwm waveforms is demonstrated using simulations, load current THDF, and the line voltage harmonic volt-seconds. Results are experimentally validated using a DSP digital controller.