Control Circuit & Synchronising Circuit for Cyclo-converters
Control Circuit for Cyclo-converters:
Fig: Control circuit block diagram for a cycloconverter with non-circulating current mode
- The function of the control circuit used in this case is to deliver correctly timed, 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 cyclo-converter.
- 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.
- The same control 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 are:
a. Synchronizing circuit
b. Reference voltage sources
c. Logic and triggering circuit
d. Converter group selection circuit
- The main function of the synchronizing circuit is to derive low voltage signals to the control circuit, which operates at low voltages.
- These low voltage signals must be synchronized 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 the firing 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.