Direct frequency convertor with sinusoidal line currents for speed-variable AC motors

A novel concept for a static three-phase to three-phase power converter for an AC drive with a unity power factor and reduced harmonics on the line side is presented. The power circuit comprises two back-to-back connected six-pulse bridges having no energy storage elements in the DC link. This permits pulse-width modulation (PWM) control in both bridges while requiring active turn-off semiconductor switches in only one bridge. The line-side harmonics are suppressed by a three-phase second-order filter. The method of predictive optimization is used for the control of the power converter. The complex control structure of the system is based on an online prediction of space vector trajectories. The steady-state operation of the system is exemplified by simulation results