Chapter 18 - AC-AC converters

Abstract

This chapter introduces the power circuits, control methods and the operation of the ac voltage controllers, cycloconverters, and matrix converters. A power electronic ac–ac converter, in generic form, accepts electric power from one system and converts it for delivery to another ac system with waveforms of different amplitude, frequency, and phase. They may be single- or three-phase types depending on their power ratings. The ac–ac converters employed to vary the rms voltage across the load at constant frequency are known as ac voltage controllers or ac regulators. The voltage control is accomplished either by (i) phase control under natural commutation using pairs of silicon controlled rectifiers (SCRs) or triacs or (ii) by on/off control under forced commutation/ self-commutation using fully controlled self-commutated switches like gate turn-off thyristors (GTOs), power transistors, integrated gate bipolar transistor (IGBTs), MOS controlled thyristors (MCTs), integrated gate commutated thyristor (IGCTs), etc. The ac–ac power converters in which ac power at one frequency is directly converted to ac power at another frequency without any intermediate dc conversion link (as in the case of inverters) are known as cycloconverters, the majority of which use naturally commutated SCRs for their operation when the maximum output frequency is limited to a fraction of the input frequency.