Three-phase to Three-phase Cyclo-converter
Three – phase to three phase cycloconverter:
Fig: 3 phase to 3 phase cycloconverter
- The circuit of a three-phase to three-phase cyclo-converter is two three-phase half-wave (three-pulse) converters connected back to back for each phase, with three thyristors for each bridge.
- The total number of thyristors used is 18, thus reducing the cost of power components, and also of control circuits needed to generate the firing pulses for the thyristors.
- This may be compared to the case with 6 (six) three-phase full-wave (6-pulse) bridge converters, having six thyristors for each converter, with total devices used being 36.
- Though this will reduce the harmonic content in both output voltage and current waveforms, but is more costly.
- This may be used, where the total cost may be justified, along with the merit stated.
- The ripple frequency is 150 Hz, three times the input frequency of 50 Hz.
- The circulating current mode of operation is used, in which both (positive and negative) converters in each phase, conduct at the same time.
- Inter-group reactor in each phase is needed.
- But, if non-circulating current mode of operation is used, where only one converter (positive or negative) in each phase, conducts at a time, the reactors are not needed.
- The circuit in each of the three phases is similar to the cyclo-converter circuit.
- The firing sequence of the thyristors for the phase groups, B & C are same as that for phase group A, but lag by the angle,1200 and2400 , respectively. Thus, a balanced three-phase voltage is obtained at the output terminals, to be fed to the three-phase load.
- The average value of the output voltage is changed by varying the firing angles (α) of the thyristors, whereas its frequency is varied by changing the time interval (T/3 = 1/(3.f0 ), after which the next (incoming) thyristor is triggered.
- With a balanced load, the neutral connection is not necessary, and may be omitted, thereby suppressing all triplen harmonics.
- Normally, the output frequency of the cyclo-converter is lower than the supply (input) frequency (step-down region), limited to about one-third of it ( f0 = fi/3 ).
- This is necessary for obtaining reasonable power output, efficiency and harmonic content. If the output frequency is to be increased, the harmonic distortion in the output voltage increases, because its waveform is composed of fewer segments of the supply voltage.
- Thus, the losses in cyclo-converter and also in ac motor become excessive. By using more complex converter circuits with higher pulse numbers, the output voltage waveform is improved, with the maximum useful ratio of output to input frequency is increased to about one-half.