Multi-star induction motors fed by voltage source inverters

This paper deals with the problems of multi-star induction motors fed by voltage source inverters. After an introduction, the structure of this drive system is described using the example of a 3-star motor with three independent 3-phase windings in the stator which are phase shifted mechanically against each other. Based on the transient equations derived, a control structure is proposed. Special attention is given to the harmonics of the inverter output currents. They can reach a high level due to the interactions of the inverters allocated to the individual 3-phase windings, which are coupled to each other magnetically. The current harmonics determine the dimensioning of the turn-off capability of the inverters. The modes of operation with sinusoidal PWM and square wave mode with fundamental switching frequency are illustrated and compared to normal single-star drives. It is shown how the pulse patterns can be optimized in order to get the lowest possible current harmonics at a given inverter switching frequency and a given fundamental frequency. The paper concentrates on the motor side of the drive system.