## 22 July 2017

### Step up Cycloconverter

• 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.
• The output frequency of the cycloconverter is greater than the input frequency therefore it is called as step up cycloconverter.

### Operation

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 cyclo converter fis always greater than the supply or input frequency.