Patent GB-1059288-A (GB)
Title: Apparatus for measuring current
IPC: G01R33/07
Filed: 1964-06-30
Abstract: 1,059,288. Measuring direct current. HALMAR ELECTRONICS Inc. June 30, 1964 [July 5, 1963], No. 27005/64. Heading G1U. [Also in Division H3] An apparatus for measuring direct current passing through a conductor 10 comprises a core 23 of high permeability which surrounds the conductor and has a number of air gaps, Hall generators 49 being disposed in two or more air gaps and conductive coils 27, 28 wound on the core being provided for each generator. A D.C. source is connected to the coils to create a flux in the core in opposition to the flux created by the current through the conductor and a control arrangement varies the magnitude of the current applied to the coils in direct relation to the amplitude of the outputs of the Hall generators, a meter 85 measuring this direct current. In an embodiment (Fig. 7) two Hall plates are situated in gaps on opposite sides of the core, which takes the form of a head in two halves, clamped about a bus-bar carrying a heavy D. C. current to be measured. The Hall plates 49 are supplied with A. C. from the supply 55 via the transformer 53. The Hall plate output is used to control the current to the windings 27, 28 on the core which produce a field in opposition to that produced by the bus-bar current. By balancing out the flux in the core errors due to saturation effects are avoided. To achieve the necessary control, the outputs from the Hall plates 49 are supplied via the transformer 62 and thence to an amplifier 63, a chopper 66, which converts the amplified Hall plate output to D. C., and a differential D. C. amplifier 73 which passes a smoothed D. C. to the control block 88. This control block is arranged to provide a pulse whose angle (i.e. the instant during the supply half cycle when it occurs) depends on the voltage output from the differential D. C. amplifier (see Division H3). This pulse is used to control a silicon control rectifier 82 in the path of the supply to the windings 27 and 28, and thus determines the length of the part of the supply cycle that is supplied to those windings and thus the average current supplied to the windings. This current is measured on the meter 85, the reading giving an indication of the bus-bar D. C. current. A compensating arrangement for adjusting the system so that correct measurements are obtained whatever the position of the bus-bar 10 relative to the core incorporates a potentiometer 105 in the supply to the windings 27 and 28. After clamping the core around the bar, this potentiometer is adjusted until the outputs from the two Hall plates, as shown on the meters 107, are identical. An alternative embodiment Fig. 8 (not shown), incorporates four Hall plates in suitable gaps in the core each associated with its own negative feedback system. The Hall plate outputs are fed to amplifiers (124), the amplifier outputs then being fed back via the resistors (125) to the core windings (123). The sum of the voltages across the resistors (125) is shown in the Specification to be proportional to the current in the bus-bar (10) and is indicated on the meter (127). This arrangement eliminates errors due to local saturation of the core 123. Core Head. The head which clamps about the busbar consists of four silicon steel L-shaped segments (24) Fig. 2 (not shown), mounted at their ends to two split centre core plates (19). These core plates are mounted within split aluminium casings (18), the inside wall (21) of which is of non conductive plastic to permit the casing to rest on the bus-bar. On each of the L-shaped segments (24) are mounted a pair of coils (27 or 28). A dowel pin (29) fits into a block (30) at one side of the core head to ensure correct alignment when mounted about the conductor (10) while at the other side an electrical connection (34, 35) is established. All electrical elements connect to a jack (36) fitted on the lower half of the core casing.