Table of Contents.Introduction to Cyclo-ConvertersThe cyclo-converter is a device that converts AC power of certain frequency to AC power of another frequency (usually lower frequency).It converts the frequency without help of any intermediate DC link. The output voltage and frequency of a cyclo-converter can be varied continuously and independently using a control circuit.
Therefore, unlike other converters, it is a single stage frequency converter.Cyclo-converters are constructed using naturally commutated thyristors with inherent capability of bidirectional power flow. These can be single phase to single phase, single phase to three- phase and three-phase to three phase converters.So the control circuit implementation is not simple because large number of SCRs, typically 4 or 8 SCRs for single phase and 36 for three- phase supply. For such controller, a microcontroller or microprocessor or DSP is used to trigger SCRs.Basically, these are divided into two main types, and are given below. Step-down cyclo-converterIt acts like a step-down transformer that provides the output frequency less than that of input, fo fi.In case of step-down cyclo-converter, the output frequency is limited to a fraction of input frequency, typically it is below 20Hz in case 50Hz supply frequency. In this case, no separate commutation circuits are needed as SCRs are line commutated devices.But in case of step-up cyclo-converter, forced commutation circuits are needed to turn OFF SCRs at desired frequency. Such circuits are relatively very complex. Therefore, majority of cyclo-converters are of step-down type that lowers the frequency than input frequency.Step-down cyclo-converter circuits can be further classified into following types.
Single-phase to single-phase cyclo-converters. Three-phase to single-phase cyclo-converters. Three-phase to three-phase cyclo-convertersBesides the frequency control, cyclo-converter output voltage can be varied by applying phase control technique. These can be used to provide either fixed frequency output from variable frequency input value or variable frequency output from fixed frequency input.These are mainly used in very high power, low speed AC motors and traction systems, especially low frequency three-phase to single phase systems. Basic Principle of Operation of Cyclo-converterThe equivalent circuit of a cyclo-converter is shown in figure below. Here each two quadrant phase controlled converter is represented by a voltage source of desired frequency and consider that the output power is generated by the alternating current and voltage at desired frequency.The diodes connected in series with each voltage source represent the unidirectional conduction of each two quadrant converter.
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If the output voltage ripples of each converter are neglected, then it becomes ideal and represents the desired output voltage. DIY Project Kit: »If the firing angles of individual converters are modulated continuously, each converter produces same sinusoidal voltages at its output terminals.So the voltages produced by these two converters have same phase, voltage and frequency.
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