Low Harmonic Loss PWM for a Dual Inverter Drive using a Floating Capacitor Inverter

Abstract

Two carrier-based PWM schemes are 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. Since the output voltages of the two inverters have a nominal 90° phase shift relative to each other, low quality pwm motor voltages, resulting in high pwm frequency harmonic losses, can be 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 also 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. Experimentally measured high frequency motor harmonic losses illustrate that the 1R2C scheme produces lower losses at low modulation depths, while 2R2C produces lower losses at high modulation depths. Results are experimentally validated using a DSP digital controller.