ABSTRACT
Three-phase systems are used at higher power levels. They can provide energy to loads connected with a star or delta connection. Three-phase voltage systems can also be created with electronic converters. A three-phase inverter can be built with three inverter branches, controlled at the same modulation index, and a phase shift of 1200. This configuration is not very used since the large majority of practical loads do not offer access to the neutral point. The same converter topology with star or delta loads is observed within this Chapter. Low harmonics from the output voltage are observed – a fundamental component and the harmonic components around the multiplies of the carrier frequency. Supply current has an important DC component, and harmonics around the multiplies of the carrier frequency. Fundamental component of the output voltage can be modified with a variation of the modulation index. A linear dependency is achieved up to certain modulation index, when this is defined up to a half of the supply voltage. Fundamental component of the output voltage can be increased through a nonlinear dependency, up to the un-modulated (six step) operation. Multilevel converters can be used at large voltages (400–10,000 V dc) where they can be built with low voltage rated transistors. Three-phase inverters are discussed herein as an alternative for AC voltage generation and this will be used further in Chapters 13–14 towards uninterruptible power supplies.
