Patent GB-582367-A (GB)
Title: Improvements in methods of and apparatus for flux measurement
IPC: G01R33/028
Filed: 1944-08-31
Abstract: 582,367. Photo-electric apparatus. BRITISH THOMSON-HOUSTON CO., Ltd. Aug. 31, 1944, No. 16611. Convention date, Sept. 1, 1943. [Class 40 (iii)] [Also in Group XXXVI] Means for measuring a direct current comprises a flux measuring circuit containing a fluxmeter and means for causing the circuit to be linked by flux produced by the direct current flow in the conductor and by opposing flux produced by current flow in a metering circuit which is regulated in response to voltage drop across a section of the conductor, so that the fluxmeter responds to the difference of the flux linkages, and means controlled by the fluxmeter for causing the current in the metering circuit to be a predetermined fraction of the current to be measured. A bus-bar 10, shown as made of several sections, carries the direct current to be measured and has leads 11, 12 connected across a short section. These leads are in series with a variable resistance 13 and a voltage detector 14 having a mirror on its movable system which reflects light from a source 15 to a light-sensitive cell 16 which controls the bias of an amplifier 17. This controls the current in a circuit including a supply source 18, one coil 30 of a mutual inductance 19 in parallel with a variable resistance 20, an ammeter 21 and a section of the variable resistance 13. For a given setting of this resistance the current in the output circuit of the amplifier 17 is controlled so as to balance the voltage drop due to this current flowing in the resistance. 13 against the voltage drop produced therein by the current flowing in the bus-bar 10. Hence the current in the ammeter 21 is roughly proportional to the current to be measured. Also associated with the bus-bar 10 is a flux pick-up device consisting of coils 22 which may be wound on magnetic strips 23. These coils are mounted for rotation with a shaft 24 so that their axes may be turned from a position parallel to the bus-bar where they pick up no flux to the position shown at right angles thereto where they pick up a maximum flux due to current flowing in the bus-bar 10. The coils 22 are connected in series so as to produce, when turned, a flux linkage in a circuit 25 proportional to the current in the bus-bar 10. Included in the circuit 25 is a fluxmeter 26 and coils 29, 39 of mutual inductances 19, 27. In order to cause the flux linkage in the circuit 25 due to current flow in the coil 30 to vary with the turning of the coils 22, the resistance 20 is connected in shunt to the coil 30 and the arm 31 thereof is mounted to turn with the shaft 24. The coils 22, 30 induce opposing voltages in the circuit 25, the fluxmeter 26 being influenced by the differential voltage. The fluxmeter 26 has a mirror on its moving system reflecting light from a source 32 to a light beam divider 33 and a pair of photo-cell amplifiers 34, 35 to control relays 37, 36. These control a reversible motor 38 which adjusts the resistance 13 so as to regulate the current through the coil 30 and ammeter 21 to the correct measurement value. Rotation of the flux pick-up coils 22 is only necessary for occasional correction of minor errors in the reading of the ammeter 21 due, for example, to change in temperature of the bus-bar 10. To prevent undue current flowing in the fluxmeter 26 during rotation of the coils 22, the mutual inductance 27 is provided with coils 40, 41 in series with the relay coils 36; 37 in the output circuits of the photo-cell amplifiers 34, 35. Deflection of the fluxmeter 26 energizes one or other of the coils 40, 41 which induces a reverse E.M.F. in the coil 39, thereby tending to return the fluxmeter to zero. When the coils 22 reach their final position, the coils 40, 41 are short-circuited by switches 43, 44 carried by an arm 42 operated by the shaft 24. The fluxmeter 26 then responds to the net change in flux linkage due to the difference in linkage between the coils 22, 30 and operates the relays 36, 37 so as to control the motor 38 and adjust the resistance 13 in the direction required to reduce this difference. For measuring the flux linkage of the coils 22 only, the mutual inductance 19, switch 42 and relays 36, 37 are omitted. The mutual inductance 39 protects the fluxmeter as before.