Cycloconverter (frequency changer)
Cycloconverter is a device which is convert ac to ac with variable frequency.
It also known as frequency changer.
 It is convert ac one power frequency to ac another power frequency.
 By changing a firing angle it is possible to control the frequency and amplitude of output voltage.
 Basically 2 types of converter.
 Single phase Cycloconverter.
 Three phase Cycloconverter.

Single phase Cycloconverter.
Its also known as Stepup cycloconverter. The load is assume to be Resistive for simplicity. stepup cycloconverter require forced commutation. The basic principle of stepup device is define as following.
 Mid point Cycloconverter
 Bridge type Cycloconverter
Mid point Cycloconverter.
 It consist of single phase transformer with mid point tapping at secondary side and four thyristor. there are two groups P1 and P2 are positive group and other two N1 and N2 are negative group.
 load is connected between secondary tapping and point Q.as shown in fig…
 during positive half cycle terminal X is positive respect to Y. hence in these positive half cycle the SCR P1 and N2 are forward bias from ωt= 0 to ωt=π.
 load voltage is positive with terminal Q positive and O negative. when SCR P1 is tern ON at ωt=0.
 At instant ωt1, P1 is forced commutated and N2 is turn ON so load voltage is negative with terminal O positive and Q negative. at instant ωt2, N2 is forced commutated and P1 is tern ON and load voltage is positive.
 after ωt=π, terminal Y is positive with respect to X. now SCR P2 and N1 are forward bias . from ωt=π to ωt=2π. at ωt=π N1 is forced commutated and forward bias SCR P2 is turn ON. now at ωt=2π, P2 is forced commutated and forward bias SCR N1 is tern ON. thyristor P1 , N2 for the first half cycle and P2 , N1 for the second half cycle.
 waveform are shown in fig,
bridge type Cycloconverter
 It consist of total of 8 thyristor. P1 to P4 for positive group and N1 to N4 negative group. When a is positive respect to X as shown in fig…
 Now during positive cycle thristor P1, P2 and N1 , N2 are ωt=0 to ωt=π. when forward bias SCR P1 and P2 turn ON togather at ωt=0, load voltage is positive with terminal A positive and O negative .
 At instant ωt1 , P1 and P2 are forced commutated and N1 and N2 are turn ON. therefore load voltage is negative with terminal O positive and A negative.
 At instant ωt2 ,N1 and N2 are forced commutated and P1 and P2 are turn ON the load voltage is positive .
 After ωt=π thyristor P3, P4 and N3, N4 are forward bias from ωt=π to ωt=2π in these manner high frequency turning ON and forced commutation of pair P1, P2, N1, N2 and P3, P4 ,N3 ,N4 give variable frquency modulated output voltage across the load resistance .
 waveform are shown in fig
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