Time-stepping finite-element analysis of a dual three-phase salient-pole synchronous motor under voltage-source supply

In high-power electric drives the use of multiple three-phase stator windings, supplied by voltage-source PWM inverters, is a design strategy for power rating, performance and reliability enhancement. Possible drawbacks, highlighted in the literature, come from the high-frequency circulation currents that originate in motor phases due to PWM switching. When a synchronous machine is used as the motor, low-frequency current harmonics can arise too, as a consequence of the non-uniform air-gap permeance. This paper discusses the point focusing on a PWM-fed 22-MW salient-pole synchronous motor with dual three-phase stator arrangement conceived for electric ship propulsion. The machine behavior under voltage-source supply is studied through time-stepping Finite-Element Analysis and predicts high internally-generated circulating currents, with a dominant 5th harmonic component. Before its application to the 22-MW motor study, the time-stepping FE methodology for current harmonic prediction is assessed against measurements on a real synchronous machine and on a laboratory prototype.