Configurations of high-power voltage source inverter drives

Synchronous machines fed by current source or cycloconverters have become standard solutions in high power applications. Mains side current shape and fundamental power factor are problems that arise using this type of drives. Therefore, recent developments tend to use induction machines fed by voltage source inverters (VSIs) also for high power drives. Inherent advantages are: a simple and less expensive induction motor which leads to an overall economic solution with a good power factor over the whole operation range. However, the switching and snubber losses of the gate turn-off thyristors (GTOs) limit the switching frequency to only a few hundred Hertz. This leads to severe drawbacks: high harmonics: low efficiency, and low maximum speed. In order to eliminate these drawbacks, the paper presents and compares improved solutions: combinations with series connected 2-level and 3-level VSIs and motors with 3 and 6 connections to the windings