Two-phase modulation technique for three-level inverter-fed AC drives

Two-phase dipolar and unipolar modulation techniques for three-level three-phase inverters are suggested and compared with conventional three-phase pulsewidth modulation (PWM) techniques. Two-phase PWMs with 60(degree) (0(degree) and +-30(degree) shift) and 120(degree) cycles are investigated from the point of view of harmonic losses, motor voltage spectra, and torque pulsations. It is shown that two-phase dipolar PWMs have no advantages in comparison with three-phase PWMs, while two-phase unipolar PWMs-in contrast with three-phase PWMs-considerably decrease the motor harmonic losses and torque pulsations in the whole motor voltage region. At the same time, the inverter neutral point control requires reversing to three-phase PWM technique for the duration of the control.