Optimal pulse patterns of a nine‐phase voltage source PWM inverter for use with a triple three‐phase wound ac motor

The authors have already described the nine‐phase inverter driving system providing both the triple three‐phase voltage source inverter with 180‐deg conducting period and the ac motor windings with triple three‐phase construction. The nine‐pulse inverter driving system used three small‐capacity three‐phase coupling reactors having special windings for current balance and reduction of higher harmonics. In addition, for voltage control, current balance, and waveform improvement, optimal PWM pulse patterns were applied to the six‐phase inverter driving system using a three‐phase coupling reactor to provide double three‐phase construction. In the system based on the mode in the title, the PWM control was applied to the nine‐phase inverter driving system described in the foregoing. The coupling reactors in the system balance the fundamental currents of three sets of three‐phase inverters and also absorb higher harmonic voltages other than 18p±1 (p = 1, 2, …) orders. The optimal pulse patterns found by the approach to make the performance index minimum using the Lagrange multiplier method can be applied to the PWM to reduce the higher harmonic currents greater than those calculated by the modulation method comparing the sinusoidal signal wave with triangular carrier signal. This system can have a larger capacity than the six‐phase PWM inverter driving system in which coupling reactors absorb higher harmonic voltages other than the 12p±1 orders and also improve output current waveforms. In addition, the system includes the capacity of coupling reactors slightly larger than the six‐phase PWM inverter driving system but enables operation with smaller torque ripples and electromagnetic noises in low‐ to high‐frequency ranges.