Analysis of the Cyclo-converter Output Waveform
Analysis of the cyclo converter output waveform:
Fig: Output voltage waveform for m-phase converter with firing angle α
- An m-phase converter circuit is assumed in which each phase conducts for ((2.π)/m) electrical radians in one cycle of supply (input) voltage.
- For example, in a three-phase, half-wave (three-pulse) converter (m = 3), each phase conducts for ((2.π)/3 = 1200) radians in a cycle of 2π radians. Similarly, in a three-phase, full-wave (six-pulse) converter (m = 6), the conduction period of the periodic waveform is ((2.π)/3 =π/3 = 600) radians in one cycle.
- With the time origin, PP` taken at the peak value of the supply voltage, the instantaneous phase voltage is given by , where Eph = supply voltage per phase (rms). The conduction period is from (- π/m) to (π/m), if the firing angle is α = 00. For the firing angle α, the conduction period is from ((- π/m) α) to ((π/m) α). The total conduction period is ((2π)/m).
The average value of the output voltage is
- When the firing delay angle is α = 00, Edc has the maximum value of
- If the delay angle in the cyclo-converter is slowly varied as given earlier, the output phase voltage at any point of the low frequency cycle may be calculated as the average voltage for the appropriate delay angle. This ignores the rapid fluctuations superimposed on the average low frequency waveform. Assuming continuous conduction, the average voltage is Edc = Ed0 . cosα
- If E0r is the fundamental component of the output voltage (rms) per phase for the cyclo-converter, then the peak output voltage for firing angle of 00 is
- However, the firing angle of the positive group αp cannot be reduced to zero, for this value corresponds to a firing angle of (αn = 1800) in the negative group. The firing delay angles of the two (positive and negative) converters are related by (αp αn = 1800).
- Practically, inverter firing cannot be delayed by, because sufficient margin must be allowed for commutation overlap and thyristor turn-off time, as given in module 2. Consequently, the delay angle of the positive group cannot be reduced below a certain finite value,αmin. Therefore the maximum output voltage per phase is
Where, r = cosαmin , and is called the ‘voltage reduction factor’.
- Thus, the expression for the fundamental component of the phase voltage (rms) delivered by the cyclo-converter is,