Patent GB-697950-A (GB)
Abstract: 697,950. Electric control systems. BRITISH THOMSON-HOUSTON CO., Ltd. June 17, 1952 [June 21, 1951], No. 15230/52. Class 40 (i) [Also in Groups XXXV, XXXVI and XXXIX] Small direct currents are measured by converting to A.C. amplifying, rectifying, employing the amplified and rectified current to vary the saturation of a saturable reactor, and opposing this variation by means of a permanent magnet. As shown in Fig. 1, a temperature measuring thermocouple 10 is connected in series with a feed-back resistor 11 and a Hall plate converter 12 for converting the D.C. voltage of the thermocouple into an A.C. voltage, which is fed through a transformer 13 and amplifier 14 to a polarity sensitive rectifier 15, the output of which is a D.C. at a higher level than is obtainable from the thermocouple directly. An A.C. bridge 16 has resistance arms, 17, 18 and reactance arms 19, 20. D.C. from the rectifier 15 flows through windings on the core of the saturable reactor 20, and a proportion is also fed back through the feed-back resistor 11 in opposition to the thermocouple current. The A.C. output of the bridge 16 is amplified at 21 and controls a servo-motor 22 through thyratrons 36. The motor 22 drives a permanent magnet rotor 24 of the saturable reactor 20, the action being such as to rebalance the bridge and stop the motor. The motor 22 also operates an indicating and recording device 25 calibrated in terms of the temperature of the thermocouple 10, and may operate a resistance 26 to control this temperature. The reactance arm 19 comprises a saturable reactor having no D.C. winding. Its reactance is adjusted by adjusting the permanent magnet rotor 24a by hand and is normally fixed during operation. The range for a given temperature change may be adjusted by varying a resistance 11a shunting the feedback resistance 11. The rotor 24 is driven from the motor 22 through a cam 40a and cam follower 40b, which is made from temperature sensitive bimetal to compensate for changes in ambient temperature. Change in temperature of the cold junction of the thermocouple circuit is also compensated for by placing the cold junction 10a in thermal contact with the cam follower 40b. In a modification, Fig. 2, for maintaining a temperature constant, the rotor 24 of the variable reactance arm of the bridge is manually adjustable by a knob 51 having a pointer indicating on a temperature scale 52. The output of the bridge controls a motor operated valve 53 in a conduit 54 which supplies gas to heat a furnace 55, the temperature of which is measured by the thermocouple 10. In the arrangement shown in Fig. 3 the second harmonic current induced in the D.C. circuit of the saturable reactor is employed for control purposes, no bridge being used. Across the D.C. winding 35a, 35b is connected a condenser 56 in series with a parallel connected condenser 56' and inductance 57 tuned to the second harmonic. The A.C. input to the variable reactor 20 is adjusted so that with the rotor 24 in a neutral position and no D.C. flowing in the winding 35a, 35b, the A.C. voltages induced in the two equal coils 35a, 35b are equal and opposite and there is no resultant A.C. voltage across the D.C. winding. When a unidirectional flux is introduced into the reactor by either the D.C. winding 35a, 35b or the rotor 24 the tops of the plus and minus voltage waves induced in the D.C. coils are shifted relative to each other and no longer cancel, so that there remains a resultant of double frequency which is passed through a transformer 58 and amplifier 21 and controls the motor 22 through thyratrons 36 and windings 38, 39. A second saturable reactor 59 is supplied from the A.C. source 23 for producing a reference voltage of double frequency. the phase of which does not change relative to the fundamental. This voltage is amplified and supplied to the winding 37 of the motor 22. Fig. 3a shows another means for obtaining a double frequency reference voltage. A full-wave rectifier has a transformer 61 in the D.C. output circuit to pick up the double frequency ripple and convert it into an A.C. voltage at terminal 62 of twice the frequency of the supply source 23 and having a fixed phase relation. Specification 665,841 is referred to.