Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Plural inputs
Patent
1997-10-28
2000-03-21
Nguyen, Vinh P.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Plural inputs
324117H, 324117R, G01R 1900
Patent
active
060406909
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an electricity measurement apparatus and a method of electricity measurement, in particular for current measurement, power measurement and watt hour metering.
When current flows through a conductor, a rotational magnetic field is set up. The magnitude of this magnetic field is proportional to the density of current flowing: conductor. This value I/A is the current density J.
It is known to use sensors to measure current flow through a conductor by detecting the magnetic field induced nearby. In the known art, magnetic field sensors for current measurement have been used individually such that the detected magnetic field is the sum of the rotational magnetic field due to current flow through the conductor and any external magnetic fields. To enable the accurate measurement of the rotational field a large amount of shielding is required to keep stray external magnetic fields away from the sensor as these would give rise to measurement inaccuracies, away from the sensor.
The present invention is defined in the claims to which reference should now be made. Preferred features are laid out in the sub-claims.
The present invention preferably provides an electrical measurement apparatus including two spaced-apart parallel conductors through which current flows in the same direction, a magnetic field being induced in the space between the conductors, and at least one magnetic field sensor disposed in that space.
The sensor or sensors are preferably disposed in the plane equidistant from the two conductors. The induced magnetic field in that plane is perpendicular to that plane, and sensors, in particular which detect field in that direction only, can operate with good accuracy.
These are preferably two sensors, one on either side of the central axis through the plane in which the conductors lie, the sensors providing signals representative of measured field strengths, and arithmetic processing means are provided operative to process the signals to provide a value representative of current flow. Components of the signals due to external magnetic fields cancel, giving improved accuracy.
It is preferred to position the two sensors in the plane equidistant from the two conductors or a plane parallel thereto. This has the advantage that the magnitudes of field components perpendicular to the plane vary linearly with sensor position within that plane. Errors in sensor misalignment can then be readily corrected for and there is less sensitivity to these errors than if a conventional single conductor were used.
There are several further advantages to the invention. In particular, no shielding is required around a sensor since the signal components due to external fields are removed. No magnetic concentrating cores are required, and both DC and AC external fields are automatically screened out. Another advantage is that the signal to noise ratio is increased and external magnetic field noise is efficiently removed.
Preferably, the two sensors are equidistant from the central axis. Errors due to an external magnetic field then cancel to zero.
It is preferred to position the two sensors in the plane equidistant from the two conductors. As there is a zero vertical field gradient in the direction between the conductors at this plane, misalignment of the sensors is least critical.
The two conductors are preferably of the same cross section, for example square, rectangular, circular or some other shape.
The two conductors are preferably arm parts of a single conductor in which a longitudinal slot is formed, the sensor or sensors being positioned in the slot.
The shape and configuration of the conductors is optimised to give good measurement accuracy, in particular, in view of the following advantages. A small conductor cross section gives a high current density and hence detected magnetic field intensity. A large conductor width gives a larger linear field region between the conductors. A large separation of conductors gives a larger space in which to place sensors, making small sensor size less c
REFERENCES:
patent: 3323057 (1967-05-01), Haley
patent: 4500838 (1985-02-01), Bloomer
patent: 4580095 (1986-04-01), De Vries
patent: 4926116 (1990-05-01), Talisa
patent: 5017804 (1991-05-01), Harnden et al.
patent: 5172052 (1992-12-01), Wells
patent: 5192877 (1993-03-01), Bitterbierre et al.
International Search Report, Int'l Appln. No. PCT/GB96/00040, Int'l Filing Date Jan. 10, 1996, 3 pp.
Horstmann Timers & Controls Limited
Nguyen Vinh P.
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