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Module 4
AC to AC Voltage Converters
Version 2 EE IIT, Kharagpur 1

Lesson 32
Control Circuit for Threephase to Three-phase Cyclo-converters
Version 2 EE IIT, Kharagpur 2

Instructional Objectives
Study of the following: • • The control circuits used for the three-phase to three-phase cyclo-converters using two threephase converters, to generate the firing pulses for the thyristorsThe functional blocks, including the circuit and waveforms

Introduction
In the last lesson − third one in the second half of this module, firstly, the circuit along with the operation of the three-phase to three-phase cyclo-converter, are described in brief. Two threephase half-wave converters, with three thyristors as power switching devices in each converter, are needed, per phase, thus, usingsix such converters having a total of 18 thyristors. The mode of operation is non-circulating current one, in which only one converter is conducting at a time. Lastly, the analysis of the output waveform is presented. In this lesson − the fourth and final one in the second half, the complete control circuit for the three-phase to three-phase cyclo-converter, is presented in detail, showing howthe firing pulses are generated to trigger the thyristors. The function of the various blocks, with their respective functions, and also circuit diagrams as needed, is described. Keywords: The control circuit for the three-phase to three-phase cyclo-converter, functional blocks.

Control Circuit for Cyclo-converters
The function of the control circuit used in this case is to deliver correctlytimed, properly shaped, firing pulses to the gates of the thyristors in the power converter (rectifier/inverter) circuits, so as to generate a voltage of desired wave shape at the output terminals of a cycloconverter. The functional block diagram of the control circuit for the three-phase to three-phase cyclo-converter, in the non-circulating current mode of operation, is shown in Fig. 32.1. The samecontrol circuit is applicable to the cyclo-converter operating in circulating current mode, but the block designated as converter group selection will not be present in this case. There are four functional blocks in the circuit as given here. 1. 2. 3. 4. Synchronising circuit Reference voltage sources Logic and triggering circuit Converter group selection circuit

Version 2 EE IIT, Kharagpur 3 Synchronizing and modulating signals e er P-converter

e

Reference signals

Logic and trigger circuit

e0 Load i0 N-converter

er

Converter group selection i0

Fig. 32.1 Control circuit block diagram for a cycloconverter with non-circulating current mode

Synchronising Circuit
The main function of the synchronising circuit is to derive low voltage signals to the controlcircuit, which operates at low voltages. These low voltage signals must be synchronised to the voltages applied to the main power circuit. Step-down transformers may be used for this purpose with the filter circuit to avoid waveform distortion, if any. While deriving the modulating voltages at the supply frequency, the phase shifting network may also be required. To determine the instants at which thefiring signals are to be produced, to be fed to the gates of the thyristors in the two converter groups, the modulating signals are compared with the reference voltages.

Reference Voltage Sources
The reference signal is designed to control the output voltage in the sense that the output voltages tend to follow the reference signal. It means that, if the amplitude and frequency of thereference signal is varied, then the amplitude and frequency of the output voltage varies automatically. In the case of three-phase to three-phase cyclo-converter, the reference signal does additional function of shifting eOA , eOB & eOC , by phase shift of 120° . The three-phase variable frequency, variable voltage sine wave reference voltage can be designed in various ways. As the frequency of the...
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