Active phase-converter for operation of three-phase induction motors on single-phase grid

Operation of a three-phase induction machine on single-phase supply has been an approach used for electromechanical energy conversion in rural communities with limited or no access to three-phase grid. Such single-phase to three-phase conversion can be achieved by passive and active means. In passive methods, fixed capacitors are used for starting and running the motor. Single-phase to three-phase conversion with reduced switch count is desirable, as this leads to a lower cost. However, it is a challenge to start an induction motor using such a power converter and to maintain balanced three-phase voltage under all loading conditions. In this work, unbalance in the motor terminal voltage with variable capacitor emulation method is analyzed quantitatively. This is used to show that perfect balance can not be achieved under all loading conditions. An active phase-converter configuration and its control is proposed that ensures balanced three-phase power supply at the motor terminals under all operating conditions. Also, this approach has the ability to soft start the motor. It is shown that, this can be done without drawing excessive currents from the power converter or from the single-phase grid. A design methodology based on the analytical results is presented for selection of the auto-transformer taps to limit startup transient currents to a desired level. Simulation and experimental results are presented to validate the proposed topology and control.