Patent GB-1399920-A (GB)
Title: Systems for firing grid-controlled rectifiers
IPC: H02M1/08
Filed: 1972-07-15
Abstract: 1399920 Automatic control of speed FERRANTI Ltd 9 Oct 1973 [15 July 1972] 33216/72 Headings G3N and G3R [Also in Division H2]  In a firing system for a controlled rectifier 11, an input counter 14 is arranged to receive pulses at a rate depending on the frequency of the A.C. source 13, a transfer stage 22 is arranged to be operated at the end of each half cycle of like sense to transfer to an output counter 21 the count then held in the input counter 14, means operable from the start of each half cycle of opposite sense so as to restore the count in 21 to a predetermined value (such as zero) before the end of that half cycle and means to derive a pulse for firing the controlled rectifier 11 on completion of such restoration. The Specification describes a system to provide controlled acceleration and retroactive speed control of a D.C. motor 12. The A.C. supply 13 is squared at 15 and positive half cycles fed to a divider 16 so as to divide the frequency by 8. The ninth pulse thus sets a 1 into the input counter 14. The next half cycle from supply 13 transfers the 1 to the output counter 21. At the beginning of the next positive half cycle, gate 26 is opened and 25kHz pulses are fed into counter 21 which is arranged to reach zero slightly before the end of. the positive half cycle. This is detected at 31 and triggers thyristor 11 which passes only a small pulse of current since it is shut off almost immediately by the next zero crossing of the A.C. supply 13. The motor thus tends to run at a low speed for the next 8 cycles after which, two will be stored in input counter 14 and hence, after transference to output counter 21, a zero will be detected slightly earlier so that thyristor 11 will pass a slightly larger current for the next 8 cycles. The motor thus accelerates in steps until the input counter 14 is full when switch 17 is closed, shortcircuiting the frequency divider 16 so that the speed increases progressively during each cycle. Constant speed operation may be achieved by interrupting the supply of pulses to input counter 14. One or more initial pulses may be fed into counter 14 to produce instantaneous starting. Detector 31 may alternatively respond to maximum value of counter 21.  Retroactive speed controls. A tacho alternator 41 is driven by motor 12 and provides a sine wave, half wave pulses of which are derived at 42 and used to fill counter 46. A zero detector 51 now generates the leading edge of a pulse the duration of which is a function of desired speed. A gate 3 is opened by control signal 52 and passes pulses from a binary rate multiplier 44 at a frequency proportional to the product of the 100 kHz clock at 23 and the contents of input counter 14 so as to reduce the contents of counter 46 to zero. At this time, the zero detector 51 terminates the desired speed pulse. If the speed is too low, the tacho half pulse is longer than the desired speed pulse provided by 51 so that the comparator 43 opens a gate 53 to add pulses to counter 21, so as to reduce the firing angle and increase the speed. If the speed is too high, pulses are subtracted from counter 21. Modifications. Multiplier 44 may be replaced by a Miller run down stage which derives the quantity which is reciprocal of speed as represented by the number of pulses in counter 14. In Fig. 4 (not shown), separate up and down counters are respectively synchronized to tacho 41 and to the supply 13.