22 July 2017

Types of Cyclo Converter

There are following types of cycloconverter
( A ) According to output frequency
  • It is a cycloconverter in which output frequency is greater than the input frequency of the input supply.

Step down cycloconverter

  • It is a cycloconverter in which output frequency is less than the input frequency of the input supply.
( B ) According to supply voltage
     ( a ) Single phase to Single phase cycloconverter
  • Centre taped transformer configuration
  • Bridge configuration
     ( b ) Three phase to three phase cycloconverter
     ( c ) Three phase to single phase cycloconverter

Single phase to Single phase cyclo converter

  • It is a cycloconverter in which output frequency may be higher or lower than the input frequency. 
  • The input and output of this type of cycloconverter is single phase alternating. There are two configurations in this type of cyclo converter : centre taped configuration and bridge configuration

Single Phase to Single Phase centre taped configuration

  • 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 ). 
  • 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 cycloconverter may be 1/3 that of input frequency.  The load is connected between point C and D as shown in the figure A.

single-phase-to-single-phase-cyclo-converter.png

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. 
  • 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. 
  • 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 B ). 
  • 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.

Negative group

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

waveform-of-single-phase-to-single-phase-cyclo-converter.png

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

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.
  • 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. ) 
  • The voltage across load becomes zero when conduction of cycloconverter 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.

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. 
  • 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.

Single Phase to Single Phase bridge configuration

  • The centre tapping transformer is not required in this configuration. The power circuit of the single phase bridge configuration cycloconverter is shown in the figure A.
  • The SCR T1 to SCR T4 works as positive group ( P – converter ) and SCR T5 to SCR T8 works as negative group ( N – converter ). 
  • If the P – converter and N – converter conducts simultaneously, the supply is short circuited. 
  • It should be noted that when P – group SCRs conducts, the N group SCRs remains in off condition and vice versa. 
  • The output voltage becomes symmetrical if the firing angle of the both converter SCRs are kept same.
single-phase-bridge-cyclo-converter.png

Operation 

Positive Group

  • The SCR T1 and SCR T3 are turned on during positive half cycle of the alternating supply resulting load current flows through path A – SCR T1 – LOAD – SCR T3 – B. 
  • The SCR T1 and SCR T3 naturally turns off due to negative half cycle of the alternating supply. 
  • When SCR T2 and SCR T4 are turned on, the load current flows through path B – SCR T2 – LOAD – SCR T4 – A.  
  • The load current flows through supply to load during positive half cycle of the alternating supply. 
  • There are two positive half cycle at the output due to one positive and one negative half cycle of the alternating supply.

working-of-single-phase-bridge-cycloconverter.png


Negative supply

  • The SCR T5 and SCR T7 are turned on during positive half cycle of the alternating supply and load current flows through path A – SCR T7 – LOAD – SCR T5 – B. 
  • The direction of load current in the N – converter is opposite to that of current flow in the P – converter. 
  • The SCR T5 and SCR T7 naturally turns off due to negative half cycle of the alternating supply. 
  • When the SCR T6 and SCR T8 are turned on, the load current flows through path B – SCR T8 – LOAD – SCR T6 – A. 
  • The direction of load current reverses and there are two negative half cycle at the output due to one positive and one negative half cycle of the alternating supply at the input.

working-of-single-phase-bridge-cycloconverter.png
  • The input and output waveforms of the bridge configuration cycloconverter is shown in the figure D. 
  • When there are two complete cycles at the input sides, there is only one complete cycle at the output side resulting we can say that the output frequency is one half to that of input frequency. 
  • If the input frequency is 50 Hz, the output frequency will be 25 Hz. The waveform of the output voltage is adjusted by adjusting firing angle of the SCRs.
waveform-of-single-phase-bridge-cycloconverter.png


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