Patent GB-681863-A (GB)
Title: Improvements in or relating to electrical apparatus for measuring resistance and temperature
IPC: G01R27/02
Filed: 1950-08-11
Abstract: 681,863. Temperature control s y s t e m s GENERAL ELECTRIC CO., Ltd., and MILNER, M. Aug. 9, 1951 [Aug. 11, 1950], No. 20030/50. Class 40 (i) [Also in Group XXXVI] Apparatus for indicating the resistance and hence the temperature of a resistive element, the resistance of which varies with temperature, comprises a magnetic amplifier having two saturating windings responsive to voltage across the resistive element and current there-through, the windings being arranged in opposition, and means responsive to the reactance of an A.C. winding of the amplifier for exerting an influence tending to maintain this reactance at a predetermined value, the extent of such influence being a measure of the resistance and hence of the temperature of the resistive element. An arrangement is shown for indicating the resistance of the rotor winding of a machine, for instance the field winding of an alternator. The field winding 10 is supplied with D:C. from an exciter 11 through brushes 12, 13 engaging slip-rings on the rotor carrying the winding 10. The exciting current is fed through two saturating windings 14, 15 of a magnetic amplifier 16, which has two further saturating windings 17, 18. These are connected in series and fed in accordance with the voltage across the field winding 10 with D.C. through variable resistances 20, 22 and a fixed resistance 21. The amplifier 16 has also two A.C. windings 23, 24 connected in parallel across the secondary of a constant voltage transformer 25, a bridge-connected rectifier 29 being included in the circuit. A relay winding 32 is connected in series with a resistance 33 and two feedback windings 34, 35 of the amplifier 16 across the D.C. terminals 30, 31 of the rectifier 29, the feedback increasing the sensitivity of the amplifier. The relay has an armature 40 which closes one or other of two pairs of contacts 41, 42, and 43, 44 controlling a motor 45 which drives the slider 46 of the variable resistance 20. The' amplifier 16 has two further saturating or polarizing windings 50, 51 connected in series and fed through a resistance 52 with D.C. from a rectifier 53, the A.C. terminals of which are connected across the secondary of the transformer'25 through a variable resistance 54, which enables the polarizing current to be adjusted. The saturation of the amplifier 16 and hence the reactance of the A.C. windings 23, 24 varies in dependence upon the difference between the ampere turns due to the current carrying windings 14, 15 and those due to the voltage responsive windings 17, 18, which are in opposition thereto.. At a certain degree of saturation the current through the feedback windings 34, 35 and the relay winding 32 has a value such that the relay armature 40 is in its central position and neither pair of contacts 41, 42 or 43, 44 is closed. If the resistance of the field winding 10 alters the motor 45 varies the resistance 20 to restore the balance, the temperature of the winding 10 being indicated on a scale by the slider 46. The variable resistance 22 is preset in accordance with the ambient temperature. To compensate for the voltage drop at the carbon brushes 12, 13 the D.C. current fed to the polarizing windings 50, 51 is adjusted by the resistance 54. Specification 654,918 is referred to.