Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With coupling means
Reexamination Certificate
2000-01-04
2001-11-27
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With coupling means
C324S11700H, C324S11700H
Reexamination Certificate
active
06323636
ABSTRACT:
The present invention relates to an electrical current sensor for measuring current flowing in a primary conductor, comprising a magnetic circuit coupled to a secondary coil and arranged such that it may be coupled with the primary conductor, and further comprising means for measuring the magnetic flux in this magnetic circuit, control means for producing a compensation current in the secondary coil in order to cancel said magnetic flux and means for measuring the compensation current.
Such sensors are often used in power supply circuits in which the current to be measured is switched at high frequency. A typical example is the supply of an asynchronous motor from a three phase inverter bridge, the sensors being placed on the three lines connecting the motor phases to the inverter bridge.
The switching speeds can attain very high values that lead, for example, to voltage variations in the order of 10 kV/&mgr;s and more.
Since these voltage variations are found on the primary conductors of sensors, leak capacitances between these conductors and the corresponding secondary coils may lead to very large parasitic currents. These leak capacitances are particularly high in sensors of small dimensions as they result from miniaturisation developments in this field. Moreover, insulation and sealing are often ensured by the introduction of a moulding resin inside the sensor. Leak capacitances in the order of 5 to 10 pF may thus exist between the primary conductor and the secondary coil, resulting in the appearance of leak currents reaching 50 to 100 mA during a transition corresponding to a voltage variation of 10 kV/&mgr;s. Such values lead to extremely high measurement errors.
An existing solution consists in providing a shield in the form of copper foil connected to a fixed potential and arranged around the primary conductor. This solution has the disadvantage of providing insufficient protection because the shielding does not completely surround the primary conductor nor the secondary coil and thus allows non-negligible residual leak capacitances to subsist. Moreover, the provision and particularly the connect ion to a fixed potential of this shielding is neither simple nor cheap.
Another solution consists in placing, a round the secondary coil, a copper foil insulated on both of its faces and bordered in such a manner as to avoid shortcircuiting the windings. This method is however very costly, particularly in the case where the secondary coil is a tore and especially if the diameter of the tore is small.
According to a first aspect of the invention, an object is to overcome these disadvantages and to provide a current sensor in which the effects of leak capacitances between the primary conductor and the secondary coil are practically totally eliminated by very simple and economic means.
To this effect, the current sensor according to the invention is characterized in that the secondary coil comprises, at the surface of a winding through which the compensation current flows, at least one layer of insulated wire spires forming an open protective winding connected as a whole to a point of fixed electrical potential.
Preferably, the spires of the protective winding are interconnected.
According to another aspect of the invention, a sensor of the above mentioned type comprises, on the one hand, an opening through the inside of the magnetic circuit in order to allow insertion of at least a first primary conductor and, on the other hand, two or more separate portions of primary conductor partially surrounding respective sections of the magnetic circuit, these conductor portions being arranged such that they may be connected, at the exterior of the sensor, individually, in series, in parallel or in series-parallel to at least a second primary conductor.
The separate portions of primary conductor are preferably portions in the form of a U, whereby the extremities are arranged such that they enable assembly of said portions on a circuit board exterior to the sensor for their electrical connection and/or for supporting the sensor.
Such an arrangement aims particularly to provide a sensor that may be used for the differential measure of two primary currents of opposite direction, flowing respectively in a conductor traversing said opening and in at least one of said separate portions of primary conductor.
This aspect of the invention results, in particular, in very good insulation between the primary conductors coupled to the sensor, by passing through said sensor opening and by means of said separate portions of primary conductor, respectively.
The above described arrangements relating to protecting the secondary winding and insulating the separate primary conductors may advantageously be combined in the same sensor.
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Cattaneo Frederic
Cattaneo Pierre
Vuillermoz Guy
Brown Glenn W.
Browning Clifford W.
Hamdan Wasseem H.
Liaisons Electroniques-Mecaniques Lem S.A.
Woodard Emhardt Naughton Moriarty & McNett
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