# Power Circuit of Step up Cycloconverter

• It is Cycloconverter in which the output frequency is greater than the input frequency of the supply.
• It is only Cycloconverter in which commutation of SCRs is done by forced commutation.
• The power circuit of the step up Cycloconverter is shown in the figure A.
• There are two group of SCRs in which one group ( SCR T1 and SCR T2 ) generates positive half cycle and other group ( SCR T3 and SCR T4 ) generates negative half cycle.

## Operation of Step up Cycloconverter

The operation of the Cycloconverter is explained as below.

### Positive half cycle

• The SCR T1 is turned on during positive half cycle of the alternating supply during time 0 to t1 therefore the load current flows through path A – SCR T1 – D – LOAD – C.
• The output voltage becomes positive. The SCR T1 is turned off by forced commutation at time t1 and SCR T4 is turned on.
• The load current flows through path C – LOAD – D – SCR T4 – B. The output voltage becomes negative during this time.

### Negative half cycle

• The SCR T2 is turned on during negative half cycle ( at π time ) of alternating cycle therefore the load current flows through path B – SCR T2 – D – LOAD – C .
• The output voltage becomes positive. The SCR T4 is tuned off at time t4 by forced commutation and SCR T2 is turned on.
• The output voltage becomes negative during this time interval.
• The SCR T3 is turned off at time t5 by forced commutation and SCR T2 is again turned on.
• This will result in output voltage becomes positive. The cycle completes in this way. The output frequency ( from figure B )

f0 = 1 / ( t2 – t1 ) Hz

• The output frequency of the Cycloconverter fis always greater than the supply or input frequency.

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