Patent GB-1226022-A (GB)
IPC: H02M5/27
Filed: 1968-08-16
Abstract: 1,226,022. Cycloconvertors. F. COVA. 16 Aug., 1968 [23 Aug., 1967], No. 39342/68. Heading H2F. A control system, for a cycloconvertor comprising positive and negative thyristor groups 11, 12 (Fig. 1) to supply a single phase load Z from 3 phase supply (alternatively there may be six groups to supply a 3 phase load Fig. 2, not shown) at a frequency f/6 where f is the supply frequency, includes a blocking circuit (Fig. 3) to prevent simultaneous operation of the two thyristor groups, a synchronizing or pilot network, and a logic control circuit (Fig. 7) to supply a train of firing pulses to the control electrode of each thyristor of a group which must be conductive throughout the desired conductive period of the group and to maintain blocked the thyristors of the other group. Thus group 11 will supply each positive halfcycle and group 12 each negative half cycle of current to the load Z. Blocking circuit (Fig. 3).-The blocking circuit comprises two ferrite ring detectors 301, 302 coupled to the load line 317, for detecting the magnitude and direction of current in the load. The rings 301, 302 sense the end of the positive and negative half cycles respectively of the load current and produce corresponding pulses from read out windings nL. Polarization windings nP saturate the cores in one direction, so that for example when a negative half cycle of load current flows the ring 301 is super-saturated and no output pulse is produced. The output pulses of the detectors are shaped and fed to change the state of a flip-flop circuit 307 alternatively in correspondance with the ends of the positive and negative half-cycles of load current. The flip-flop outputs 325, 326 are fed to OR gates 309, 310 and to a delay circuit 308 which makes pulses of a definite duration in dependence upon the magnitude of an input voltage from a line 327. These pulses are called "dead zone" DZ pulses (waveform 327, Fig. 5) since they correspond to and determine the period when neither thyristor group is conductive, and consequently short-circuiting of the supply network is prevented. The DZ pulses together with the flip-flop outputs are fed to the OR gates 309, 310 which produce outputs signifying the correct conduction period of each rectifier group. That is firing pulses cannot be produced when DZ pulses exist at the OR gate inputs. After inversion in amplifiers 311, 312 the gates outputs are mixed in AND gates 313, 314 with a pulse train 335 (Fig. 5) e.g. from a multivibrator connected to line 335 to produce interrupted pulse trains 315, 316 at terminals 339, 340. Synchronizing or "pilot" circuit (Fig. 6, not shown).-This comprises a 3 phase/6 phase transformer the delta connected primary of which is connected to the 3 phase supply and the six secondaries are connected to form three bi-phase pairs one end of each pair being connected to a respective integration amplifier. Three of the amplifiers have outputs RST each in-phase with a respective supply phase and the other three amplifiers outputs #R #S #T are respectively opposite in phase to the phases of the supply. The amplifiers outputs are fed respectively to six comparators in the logic control circuit. Firing control logic circuit (Fig. 7).-The other inputs to the said six comparators 501, 602, 703, 804, 905, 1006, comprise a common sinusoidal (f/6) reference signal. The outputs of the comparators are square waveforms (Fig. 8, not shown) with relatively phase displaced square pulses. In operation e.g. thyristor 101 is fired on the occurrance of the trailing edge of a pulse from comparator 501 and is turned OFF by natural commutation when 102 fires at the occurrance of the trailing edge of a signal from comparator 602. This is achieved by logic circuits such as 502. In addition an AND gate 510 therein receives signals from a blocking circuit (Fig. 3) and from the preceding OR gate 509 and delivers triggering signals (520), Fig. 8 (not shown), solely during the period thyristor 101 is to be conductive and via a pulse amplifier 503 and isolating transformer 505.