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18
AC–AC Converters
A. K. Chattopadhyay, Ph.D.
Electrical Engg. Department, Bengal Engineering & Science University, Shibpur, Howrah, India

18.1 Introduction .......................................................................................... 483 18.2 Single-phase AC–AC Voltage Controller ..................................................... 484
18.2.1 Phase-controlled Single-phaseAC Voltage Controller • 18.2.2 Single-phase AC–AC Voltage Controller with On/Off Control

18.3 Three-phase AC–AC Voltage Controllers .................................................... 488
18.3.1 Phase-controlled Three-phase AC Voltage Controllers • 18.3.2 Fully Controlled Three-phase Three-wire AC Voltage Controller

18.4 Cycloconverters...................................................................................... 493
18.4.1 Single-phase to Single-phase Cycloconverter • 18.4.2 Three-phase Cycloconverters • 18.4.3 Cycloconverter Control Scheme • 18.4.4 Cycloconverter Harmonics and Input Current Waveform • 18.4.5 Cycloconverter Input Displacement/Power Factor • 18.4.6 Effect of Source Impedance • 18.4.7 Simulation Analysis of Cycloconverter Performance • 18.4.8 Power QualityIssues • 18.4.9 Forced Commutated Cycloconverter

18.5 Matrix Converter .................................................................................... 503
18.5.1 Operation and Control of the Matrix Converter • 18.5.2 Commutation and Protection Issues in a Matrix Converter

18.6 High Frequency Linked Single-phase to Three-phase Matrix Converters ........... 509
18.6.1 High FrequencyIntegral-pulse Cycloconverter [48] • 18.6.2 High Frequency Phase-controlled Cycloconverter [49]

18.7 Applications of AC–AC Converters ............................................................ 510
18.7.1 Applications of AC Voltage Controllers • 18.7.2 Applications of Cycloconverters • 18.7.3 Applications of Matrix Converters

References............................................................................................. 513

18.1 Introduction
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 voltageacross 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 selfcommutated switches like gate turn-off thyristors (GTOs), powertransistors, 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 naturallycommutated SCRs for their operation when the maximum output frequency is limited to a fraction of the input frequency. With rapid advancements of fast-acting fully controlled switches, forced commutated cycloconverters, or recently developed matrix converters with bi-directional on/off control switches provide independent control of the magnitude and the frequency of the generated output voltageas well as sinusoidal modulation of output voltage and current. While typical applications of ac voltage controllers include lighting and heating control, online transformer tap changing, soft-starting and speed control of pump and fan drives, the cycloconverters are mainly used for high power low speed large ac motor drives for application in cement kilns, rolling mills,

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