GB1359048A - Cycloconvertors - Google Patents

Abstract

1359048 Frequency changing DEFENCE SECRETARY OF STATE FOR 12 July 1972 [3 Aug 1971] 36403/71 Heading H2F A forced-commutation cycloconverter includes control apparatus for generating a regular stream of switching pulses for controlling the conduction of the main converter elements, a feedback circuit for deriving a control signal from selected harmonic components of the cycloconverter waveform and causing said control signal to be amplified or attenuated by a factor varying substantially in accordance with the instantaneous value of the rate of change of the fundamental component of the output waveform and a control circuit for causing the timing of individual switching pulses to be perturbed by amounts depending on the instantaneous values of the control signal. The feedback circuit thus has the effect of minimizing harmonic content and reducing the size of output filter necessary. As shown in Fig. 1, a P-phase input at frequency fg is fed into a forced commutation cycloconverter 4 utilizing gate-turn-off switches connected in inverse parallel. The gateturn-off switches are controlled by firing circuits 2 in dependence on a triangular signal of frequency P (fg+fo) where fo is the required output frequency. Selected harmonics are derived by a filter 6 and divided at 3 by a cosine wave formed by integrating at 7 the fundamental frequency from the cycloconverter 4. The output from divider 3 is used to modify the signals from the firing control circuit 2 so as to minimize harmonics in the output of the cycloconverter 4. Fig. 2 relates to a block diagram of the divider 3; the harmonics from block 6 are fed into the in-phase input of a differential amplifier the output of which is fed into one input of an analogue multiplier, the other input of which is received from a Miller integrator 7. The output of the multiplier is fed to the anti-phase input of the differential amplifier. Fig. 3 (not shown) relates to a block diagram of the control signal generator 1 which comprises a mixer for producing (fg+fo) from which is filtered the Pth harmonic which is squared and integrated to provide the required triangular wave which, in turn, is fed into a comparator together with the output of divider 3. The comparator output is distributed by means of a ring counter to the various rectifiers of the cycloconverter. A phase-locked oscillator, arranged to oscillate in phase quadrature with the fundamental frequency of the output, may be used to replace the circuit 7.