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Principal of Cyclo converter


  • It is a apparatus or device which convert constant AC voltage, constant frequency input supply into variable voltage, variable frequency supply.

Principle of operation
  • The cyclo converter is made of two converters in which one converter works as a positive converter and second converter works as negative converter. 
  • The firing angle of both converters are set such that

       αPN  = π
      αP = Firing angle of positive converter and
      αN = Firing angle of negative converter 

equivalent circuit of cycloconverter

  • The firing angle of both converters is set such that it will change the output frequency. 
  • The P – converter and N – converter is made by series connection of variable alternating supply and diode
  • The output voltage of the both converter becomes equal in the P – type converter and N – type converter of the ideal cyclo converter. 
  • The output voltage at the terminal of the cyclo converter is either of the alternating voltage of the converter because each converter has equal alternating voltage amplitude, phase as well as frequency.
  • The power flows from either load to converter to converter to load by controlling the firing angle of the each converter. 
  • The unidirectional property of the P – converter and N – converter results in P – converter works in positive half cycle of alternating supply and N – converter works in negative half cycle of alternating supply. 
  • It does not depend upon phase between load voltage and load current.
  • The voltage and current waveform for the ideal cyclo converter for different load displacement is shown in the figure B. ( Displacement angle – It is angle between fundamental current and voltage of load current.)

Load displacement angle 0o
  • The current flows through converters when both converters are operated in the rectifier mode. 
  • Figure B shows waveform for each converter.

Load displacement angle 180o
  • Both converters operate in inverting mode when the load displacement angle is 180o
  • As the load is regenerative, the power flows from load terminal to output terminal of cyclo converter.

cycloconverter operation for various displacement angle

Load displacement angle 80o lagging

  • The converter operates in the rectifier mode during 100o of the positive half cycle from its first zero. 
  • The power flows from cyclo converter to load during this mode. 
  • The converter operates in the inverter mode during 80o of the remaining positive half cycle of the alternating supply.

Load displacement angle 80o leading
  • The converter operates in the inverter mode during 80o of the positive half cycle from its first zero. 
  • Similarly the converter operates in the rectifier mode in the 100o of the remaining positive half cycle.