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.
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.

waveform of step up cycloconverter