Patent GB-1316675-A (GB)
Title: Circuit for regulating the power supplied from an alternating  current source to a load
IPC: G05F1/66
Filed: 1970-05-14
Abstract: 1316675 Automatic temperature or voltage control EUROPEAN ATOMIC ENERGY COMMUNITY 14 May 1970 [20 May 1969] 23440/70 Heading G3R [Also in Division H2] A circuit for regulating the power fed from an AC source to a load comprises an electrically controllable switching device connected in series or parallel with the load, a comparator producing a variable voltage by comparison of a set value and a measured value, and a control circuit, fed by the comparator, arranged to produce two phase-synchronised control voltages which act simultaneously and in parallel on the switching device, the first of the control voltages varying the duration of conductivity of the device within each period of possible conduction of it, whereas the second of the voltages produces a periodic conductivity composed one or more whole periods of possible conduction of the device, one of the periods occurring during each AC cycle. In Fig.1 the switching device comprises switches 3, 4 each of which may be a pair of anti-parallel connected thyratrons or thyristors and which control the heater 2 of an electrical furnace supplied from a stabilized AC source 1. A senser 5 detects the temperature of the furnace or the voltage across the heater 2 and supplies its signal to a comparator 6 fed with a reference signal. The output of the comparator is applied to a control unit 8, through a frequency filter 7 having an integral delayed action, and to a control unit 9, each unit 8, 9 controlling a respective switch 3, 4. The correcting signal B formed by the unit 8 closes the switch 3 for two cycles of the A.C. supply A and opens it for three cycles. The correcting signal C formed by the unit 9 determines the period of closure of the switch 4 during each A.C. cycle. Thus the resultant voltage across the heater 2 is that shown in the waveform D of Fig. 2. Switch 3 may be controlled to vary the ratio of on to off cycles if the duration of each combined on-off period is constant. Alternatively each on period may be of constant duration and the repetition frequency of the combined on-off periods may be varied. The two control loops are adjusted so that under conditions of stable dynamic equilibrium the proportional action loop 9 closes the switch 4 at the maximum and minimum points of the A.C. supply. Thus a balanced, constant, optimum regulating range is obtained and oscillations of the control system under disturbance conditions are prevented. Operation of the system is compared with a known system in which the proportional and integral action loops are combined in the usual way, so that both adjust the phase angle of the firing time. Switches 3, 4 may take the form of the single thyristor 10 shown in Fig.3, the control signals B, C being applied to the coils 11, 12 of a transformer whose secondary coil 13 supplies the resultant common control signal to the firing electrode of the thyristor. Combination of the control signals B, C may be carried out by superimposition in a disjunctive logic system or in resistors, depending on the working potential of the firing electrode. A resistor may be connected in parallel with the switches 3, 4 to reduce the proportion of high frequencies in the load current characteristic.