The invention relates to inductive current measuring devices and more particularly a current sensing device for use in a power monitoring device or watthour meter.
Two well-known approaches for non-intrusive sensing of current flowing in a conductor are: (1) the transformer approach wherein inductive coupling between a primary coil (the conductor in which current is to be measured) and a secondary coil generates a voltage in the secondary coil proportional to the current flowing in the primary coil; and (2) using a current sensor, such as a Hall-effect device to sense the magnetic field associated with current flow in the conductor. With the first approach, accurate detection of the current flowing through the primary coil is dependent upon a variety of factors. In addition to the typical turns ratio calculation, interference attributable to capacitive coupling between the surface of the secondary coil and nearby metallic structures interferes with the accuracy of the voltage signal induced in the secondary coil.
Additional considerations in the design of a current detecting sensor are presented when a watthour meter design must measure current from low amperage up to 200 amps flowing in a conductor or primary winding. In addition, the current detecting device must be unaffected by current flowing in conductors situated nearby.
Various prior devices are known for detecting current flowing in a conductor. U.S. Pat. No. 4,473,810 to Souques et al. discloses an AC current sensor for measuring the current flowing in a conductor. Souques discusses an insulating frame having a central compartment open at its upper end and encompassed by a U-shaped receptacle for a bent portion of the conductor is disclosed. A coil is housed in the central compartment. The frame is slidingly mounted within a sleeve having two assembled half-shells made of a soft magnetic material. The voltage generated is proportional to the intensity of current flowing in the main or primary conductor.
Another example of a current detection device is disclosed in U.S. Pat. No. 4,506,214 to Lienhard et al. Lienhard et al. discloses a measuring transformer including a measuring conductor carrying a current I.sub.m to be measured, a premagnetizing winding which carries a premagnetizing current I.sub.v, and a magnetic field comparing means which is exposed to the magnetic field produced by the current I.sub.m and the magnetic field produced by the premagnetizing current I.sub.v. This comparing means is alternately controlled in both directions of saturation. The magnetic field comparing means in one embodiment is a magnetic film (which may be anisotropic) operated in the magnetic preferential direction, and has a thickness of at most a few microns. It may be secured to pole shoes of a magnetic core or arranged between a flat measuring conductor and a premagnetizing coil of flat cross-section. The measuring transformer can be used as an input transformer in a static electricity meter. In another embodiment, output pulses are obtained directly from a magnetic field comparing means.
Another example of a current sensing device is disclosed in U.S. Pat. No. 4,362,990 to Schneider et al. The Schneider et al. device includes a transducer for measuring a current, which is comprised of a magnetic core having a gap substantially impeding passage of any magnetic flux, a coil arranged to pass a premagnetizing current for producing a first magnetic field, a loop for carrying the current to be measured, so as to produce a second magnetic field, and a magnetic field comparison device exposed to the magnetic fields. The magnetic field comparison device includes a magnetic film bridging the flux gap. The film is alternately controllable in respective opposite directions of saturation substantially by the first magnetic field, and evaluates the measuring current in dependence of the magnetic fields. The magnetic core, the coil, the loop, and the magnetic film are substantially concentric.
None of the known devices include a means for preventing coupling between the secondary pick-up coil and surrounding metallic structures to thereby enhance the accuracy of the signal in the secondary coil. Thus a need remains for an improved current sensing device for sensing current having high accuracy coupled with high linearity is needed for use in conjunction with watthour meter systems known in the art.