Patent GB-611107-A (GB)
Title: Device for measuring physical quantities by means of a direct current intensity proportional thereto
IPC: G01R17/08
Filed: 1945-09-21
Abstract: 611,107. Electric control systems. COMPAGNIE ELECTRO-MECANIQUE. Sept. 21, 1945, No. 24451. Convention date, April 2, 1943. [Class 40 (i)] A physical quantity is measured by a direct current automatically adjusted to balance the force produced on a mechanical element by the quantity, the D.C. being derived from at least one controlled rectifier fed from an A.C. which is a function of the quantity being measured, and movement of the mechanical element varying the self inductance of a circuit to detune it and produce a voltage proportional to the detuning, which is amplified, rectified, smoothed and then fed to the control grid of the rectifier for fine control of the D.C. As shown, applied to measuring the power in a three-phase line 4, 5, 6, wattmeter elements 2, 3 exert a torque M1 proportional to the power on a shaft 1 which is balanced by the torque M2 exerted on the shaft by the D.C. in a coil 7 rigid with the shaft and mounted in the field of a fixed permanent magnet 9. Coarse adjustment of the D.C. is effected by deriving an A.C. proportional to the current in lines 4, 5, 6 from the current transformers 29, 30 and rectifying this current in a triode 22 to produce the required D.C. which is fed through the coil 7 resistor 32 and an ammeter 31 shunted by a smoothing condenser 35. Fine adjustment of the D.C. is effected by grid control of the rectifier 22 as follows. A disc 10 fixed to the shaft 1 carries two abutments 17, 18, one of which is electrically conducting while the other is non-conducting, but bonded with magnetic powder. Between these abutments is disposed a fixed coil 16 so that movement of the shaft 1 in either direction varies the detuning of the oscillatory circuit 15, 16 relative to the stabilized frequency of an oscillator 11 feeding the amplifier 14. A voltage dependent on the magnitude and sense of the shaft movement is thereby applied through transformer 20, rectifier 21 and smoothing circuit 25-28, to the grid 23 to effect fine adjustment of the D.C. Modifications. In Fig. 2 the series-connected meter 31, coil 7 and rectifier 22 are fed from an independent A.C. source 55, and the control grid 23 biassed over condenser 25 for fine adjustment as above described, and over resistor 53 for coarse adjustment, the latter voltage being derived from the line current transformers 50, 51 after rectification at 52 and smoothing at 54. The two biassing voltages may be applied to separate rectifier valves in parallel or to separate grids of a multi-grid valve. The coarse biassing voltage may alternatively be applied to control the amplification of the tube 14, Fig. 1. To distinguish between the two directions of power flow along lines 4, 5, 6 a rectifier 33 is reversely connected across the rectifier 22 and adjusted by resistor 34 to pass only a fraction of a predetermined value of current flow through rectifier 22. The coil 7 and meter 31 respond to the smoothed difference of these currents, the meter 31 in this case being of the centre zero type to indicate positive or negative flow.