Stator Harmonic Currents in VSI-Fed Synchronous Motors With Multiple Three-Phase Armature Windings

Electric motors equipped with multiple three-phase windings are often used for the advantages they offer in terms of reliability, performance, and inverter power segmentation. When the windings are fed by independent voltage-source inverters (VSIs), circulation harmonic currents can occur in stator phases. If the motor is an induction machine, circulation currents are known to originate form transient imbalances in the applied voltages due to inverter switching. This paper investigates the problem when a synchronous machine is used and shows how additional (and possibly major) sources for circulation currents can arise in this case (even with a round-rotor design) due to the nonsinusoidal air-gap flux distribution. The phenomenon is illustrated for a 45-MW quadruple three-phase synchronous motor supplied by four medium-voltage (MV) multilevel VSIs. Its circulation currents are predicted with two alternative methods, i.e., analytically and from time-stepping finite-element simulation. The results obtained in both ways are shown to well match measurement results collected on the actual motor during full-load system testing.