Assessment of harmonic distortion in a three-phase PWM chopper feeding a three-wire inductive wye load with an inaccessible star point

A three-phase symmetrical multipulse modulated (SMM) ac chopper containing six four-quadrant switch (4QSW) realizations is used. Each 4SQW encompasses a power MOSFET. The three 4QSW realizations intended for freewheeling are connected across the load line terminals while the other three are connected in series with the three-phase supply terminals. The source and load currents, and the load voltage are assessed for harmonic distortion when the chopper is connected to an inductive, three-wire, star-connected load with an inaccessible star point. The gate control logic, in conformity with SMM, pertaining to the star-connected, three-wire, inductive load is presented. The SMM method involves generation of pulses within a half cycle (M) that have the same duty cycle (8) and are symmetrically located. Variations in the rms magnitude of the fundamental load voltage, and the harmonic profiles of the load voltage and current, are achieved by manipulating 8 and M respectively. The SMM data suggest, considerable mitigation of low order harmonics and even selective elimination of specific harmonics are possible by choosing M prudently. Results include SMM data, simulation data and experimental waveforms.