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Single Phase to Single Phase Center Taped Cycloconverter

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Introduction

  • The power circuit of single phase to single phase centre taped configuration is shown in the figure A.
  • There are two groups of SCRs.
  • The positive half cycle of the output voltage is generated by one group of SCRs ( SCR T1 and SCR T2 ).

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  • Similarly the negative half cycle of the output voltage is generated by second group of SCRs ( SCR T3 and SCR T4 ).
  • The output frequency generates by this type of cyclo converter may be 1/3 that of input frequency. The load is connected between point C and D as shown in the figure A.

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Operation : Resistive Load

Positive Group

  • The point A is made positive with respect to point C in the positive half cycle of the supply voltage.
  • The SCR T1 is turned on during this interval.

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  • The SCR T2 remains turns off when point B is negative with respect to point C.
  • The load current flows through path A – SCR T1 – D – Load – C – A during this interval.
  • The SCR T1 automatic turn off during negative cycle of the alternating supply because point A is made negative with respect to point C.

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  • As the SCR T2 is turned on, the load current flows through path point B – SCR T2 – D – Load – C – B.
  • The SCR T1 again turns on during positive half cycle and SCR T2 automatic turns off during that time.
  • The load current flows from supply to load during three half cycle of the alternating supply ( Figure D ).

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  • The voltage across load is equal to one half cycle which is made by combination of three half cycle of input supply.
  • The output voltage of the cyclo converter is adjusted by controlling firing angle of SCR T1 and SCR T2.

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Operation : Resistive Load

Negative Group

  • The SCR T3 and SCR T4 receive gate pulse and gate pulse of SCR T1 and SCR T2 remove in this mode.

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  • When point B is made positive with respect to point C, SCR T3 is switched on in spite of SCR T2 and load current flows through path C – LOAD – D – SCR T3 – A – C.
  • The direction of load current flows through path load to supply side.
  • The point A is made positive with respect to point C in the next half cycle and at that time SCR T3 automatic turns off and SCR T4 is turns on resulting load current flows through path C – LOAD – D – SCR T4 – B – C.

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  • The SCR T3 turns on and SCR T4 turns off in the next half cycle resulting voltage across load is equal to one negative half cycle equivalent to three half cycle of alternating supply.
  • The load current flows through load to supply side.
  • Finally we conclude that we receive only one complete cycle at the load side for three complete cycle at the input side therefore the output frequency is one – third that of input frequency.

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  • The output voltage of the cyclo converter is adjusted by controlling the firing angle of the SCRs.

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Operation : Inductive Load

  • The load current continues to flow in spite of complete half cycle due to energy stored in the inductor when load is inductive.
  • This will result in SCR does not turn off after completion of half cycle and load voltage becomes negative.

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  • When positive half cycle of the supply complete and negative half cycle starts, the SCR T3 turns on by applying gate pulse but it does not turns on because the conduction of SCR T1 of positive converter continue due to inductive load.
  • The SCR T1 reverse biases the SCR T3.
  • This will result in no voltage available at the load point during this half cycle. ( or we can say that the voltage across load becomes zero until SCR T1 does not turns off. )

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  • The voltage across load becomes zero when conduction of cyclo converter shift from positive half cycle to negative half cycle.
  • This will generate distortion in the output voltage waveform and its depend upon firing angle of SCRs and power factor of load.

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Effect of ratio of Input Frequency and Output Frequency

  • When the ratio of input frequency and output frequency does not integer, the conduction of positive group SCR continue in the last half cycle but at that time conduction of SCR T3 or SCR T4 of negative group starts.

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  • This will result in short circuit and load voltage becomes zero.
  • The time duration of short circuit is less than half cycle of the input supply.
  • The inductor is connected at the input side in order to limit the short circuit current.

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