Three Phase to Single Phase Cycloconverter

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Three Phase to Single Phase Cycloconverter

Circuit Diagram of Three Phase to Single Phase Cycloconverter :

  • Figure A shows the circuit diagram of a three phase to single phase cycloconverter.
    • Three Phase to Single Phase Cycloconverter
    Figure A
  • This circuit is used for converting a three phase supply at one frequency to a single phase supply at a lower frequency.

Operation of Three Phase to Single Phase Cycloconverter :

  • As shown in Figure A, there are two three phase half wave circuits connected to the common load. These are called as the positive and negative groups.
  • The principle of operation is based upon progressively varying the firing angle a of SCRs connected in one of the groups.
  • If we want the load voltage to be positive then the SCRs of positive group should be fired whereas in order to produce a negative output voltage, the SCRs from negative group should be turned on.
  • For example refer to the load voltage waveform of Figure B. so as to produce the positive mean output voltage, the SCRs of the positive group are turned on.
  • But more importantly , the firing angle a is first reduced progressively from 90° to 0o so that the mean output voltage increases from OV to peak positive voltage sinusoidally. The firing angle is then increased progressively from 0o to 90° in order to reduce the mean output voltage sinusoidally from peak positive voltage to zero volt.
  • We can apply the same principle to produce the negative half cycle of the mean output voltage. To produce the negative half cycle of the mean output voltage, we have to turn on SCRs only from the negative group and vary their firing angle progressively from 90° to 0° and then from 0o to 90° as discussed for the positive group SCRs.

Fabricated and Mean Output Voltage Waveforms of a Three Phase to Single Phase Cycloconveter
Figure B

Output frequency of Three Phase to Single Phase Cycloconverter :

  • From the discussion till now it is clear that the output voltage is a single phase voltage and its frequency is less than the frequency of 3-phase input (150 Hz).
  • The output frequency depends on the number of pulses of input that correspond to one cycle of output. For example in Figure B, we have 12.5 input pulses per half cycle of output.

∴ Number of input pulses per cycle of output = 25

∴ Output frequency = fin / 24 = 150 / 25 = 6

Intergroup reactor (IG reactor) :

  • The intergroup (IG) reactor of Figur A is connected in order to limit the circulating current between the positive and negative group SCRs, when the cycloconverter is operated in the circulating current mode. SCRs from both positive and negative group conduct simultaneously in the circulating current mode.
  • In order to ensure that the average value of the two converters are equal in magnitude and opposite in sign, the sum of their firing angle should be 180°.

∴ αP + αN = 180°

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