GB981787A - Improvements in or relating to the control of electrical power - Google Patents

Abstract

981,787. Automatic temperature control z systems. RESEARCH & ENGINEERING CONTROLS Ltd., and J. S. JOHNSTON. Sept. 4, 1963 [Oct. 29, 1962], No. 40841/62. Heading G3R. The current through an inductive heater 13 energized from a mains supply 11, 12 is controlled by means of a rectifier bridge 14 comprising rectifiers 15---18 and a silicon controlled rectifier 19, the arrangement being such that the average number of trigger pulses applied in a given time to the rectifier 19 is controlled in accordance with an error signal dependent on the temperature of an enclosure heated by the element 13, the temperature thus being maintained at any desired value. The control circuit for the trigger pulses is shown in Fig. 2 in which a pulse generator 22, energized from the same supply as the heater 13, provides a pulse every time the supply voltage crosses zero to the input of an "AND" gate 24 which, if open, passes the pulses through a mono-stable multivibrator 30 and power amplifier 26 to the control electrode of rectifier 19. The "AND" gate is controlled by a comparator 29 having two inputs, one from a controller 32 which produces a signal dependent on the temperature error, and the other from an averaging circuit 42 which provides a signal dependent on the number of trigger pulses passed to amplifier 26 in a given time. The controller 32 comprises an oscillator 33 supplying a transformer whose centre-tapped secondary 35 forms a bridge circuit with a temperature sensitive resistor 36 responsive to the enclosure temperature and a resistor 35 adjustable for temperature setting, the bridge output passing through a potentiometer 38 to an amplifier and detector 39 whose output is demodulated to be sensitive to the phase of the input signal, the phase reference signal to detector 39 coming from a further secondary coil 40 of the controller transformer. If the signal from the circuit 42 is less than that from controller 32, the comparator 29 produces a signal to open the "AND" gate 24, this gate being closed when the two signals are equal. Elements 24, 29, 30 and 42 form a closed control loop whose gain is controlled by potentiometer 38, the multivibrator 30 acting as a pulse shaper and also to cause the gate 24 to remain open until the pulse is over. The averaging circuit may be modified to incorporate the gain control, Fig. 3 (not shown), this modification also incorporating means to compensate for changes in the mains supply voltage. The averaging circuit may be further modified, Fig. 4 (not shown) and a mathematical analysis is given of the two modifications. The control system described may be applied to control variables other than temperature, and the signal applied to the comparator may be any desired function of the error voltage, e.g. integral and/or first and higher derivatives with respect to time. A divide-by-two circuit may be included in the input to the averaging circuit so that the rectifier 19 is switched to conduct integral multiples of whole cycles of the mains supply.