GB2165706A

GB2165706A - Alternating current sensor assembly and method of making same

Info

Publication number: GB2165706A
Application number: GB08524766A
Authority: GB
Inventors: Dayle Rigby Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-10-12
Filing date: 1985-10-08
Publication date: 1986-04-16
Also published as: IL76637A; AU4852485A; IL76637A0; AU589140B2; FR2571886B1; FR2571886A1; GB2165706B; DE3545095A1; GB8524766D0; JPS61180150A

Abstract

A current sensor has a secondary coil 14 disposed on a straight portion of a magnetic core member 12 whose ends are bent round and electrically and magnetically joined, for example by welding at 30. Alternatively the ends may be joined by a crimped-on magnetic sleeve, or bound together by magnetic wire. The arrangement may be disposed in a casing 32 filled with a potting compound. A single turn primary winding may link the core. <IMAGE>

Description

SPECIFICATION Alternating current sensor assembly and method of making same Due to the need for miniature inductor structures for use in micro-modules such as those used in the hearing aid industry there have been developed not only more sensitive and efficient devices but those that are more eco nominal This need has resulted in more precise and concentrated material refinement with the associated development technology. In the search for a tiny, reliable, sensitive telephone coil, the current sensor of the present invention was produced.

It is well known that a wound toroid coil provides a good current transformer. If a wire carrying a current to be measured is passed through the hole in the toroid a one-turn primary winding is formed and the turns ratio from the primary to the secondary is determined by the number of turns on the toroid core. Based on electrical characteristics, the theoretical ideal form for inductance devices is a toroid. Toroid core transformers are used extensively in electronics for power conversion and for signal transfer even at high frequencies; they serve a purpose which other devices cannot fill; however, toroid coils and transformers are difficult and expensive to construct, particularly because of the difficulty in applying multi-winding to completely closed toroid cores in mass production.

In an attempt to solve this problem, U.S.

Patent No.3,238,484 discloses a method of making an inductance device by applying a coil structure to an arched magnetic core member of substantially uniform cross sectional area and having two ends, by applying a magnetic bridge member directly across the ends of the arched member, by then adjusting the spacing between the bridge member and the arched member until the winding exhibits a predetermined value of inductance, and by finally fixing the relative position of arched and bridge members, thus making the adjusted Dshaped core juncture permanent. Such a device is still complex and difficult to produce.

The invention provides a current sensor assembly comprising: only one magnetic core member, having a straight central portion and two curved end portions magnetically and mechanically permanently connected together to form a single loop, one-turn coil; and a winding means on the straight central portion.

According to a further aspect, the invention provides a method of producing a current sensor assembly, comprising the steps of: mounting a single, straight magnetic core for rotation about its longitudinal axis; winding a coil of insulated, small-diameter, transformer wire onto the central portion of the straight core by rotating the core; bending the ends of the core outwardly of the coilsupporting straight central portion into a curved portion so that the ends are brought into contact with each other; and magnetically and permanently connecting the ends together.

The present invention produces a coil that simulates and compares favourably with a toroid wound coil, and is small in size. Such devices may be produced economically and with consistently high quality. In particular, they have a high sensitivity for an element of a given volume.

The straight magnetic core of the invention allows for ease in winding a multi-turn coil with a large number of turns in a simple winding chuck while protecting the coil against stress and strain in retaining the core straight under the coil by bending the ends of the straight core beyond the coil into a curved portion until the ends make magnetic contact and are permanently connected as by welding, crimping or wire wrapping to provide a single connection.

In the method of U.S. Patent 3,238,484, on the other hand, multiple connections are provided between multiple sections of curved core portions.

The transfer function of such a device is defined as the voltage at the current sensor output, which is taken across the multi-turn coil as a function of the current flowing in a conductor passing through the center hole of the single turn or D-shaped core. Transfer functions of thousands of micro-volts per milliampere have been obtained using the invention operated over a frequency range of 50 Hz to 20 MHz. Hysteresis and skin depth effects are evident at certain frequencies, but most important is the capability of transmitting the signal of the conducting wire under very light loading as an inductor rather than as an efficient power converter.

In order that the invention may be better understood, preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, wherein: Figure 1 is a plan view of a preferred form of the current sensor according to the invention mounted in a plastics case; Figure 2 is a sectional view along line 2-2 of Figure 1; Figure 3 is a plan view of the current sensor illustrating one method of connecting the ends of the D-shaped core; Figure 4 is a plan view of part of the Dshaped core illustrating another method of connecting its ends; Figure 5 is a plan view of part of the Dshaped core illustrating still another method of connecting its ends; Figure 6 is a sectional view along line 6-6 of Figure 1; Figure 7 illustrates another form of a coil end or spool using plastics strips cemented to the core;; Figure 8 is a graphical representation of the transfer function of a sensor type BG in volts output versus input current at 60 hertz; Figure 9 is a graphical representation of the transfer function of a sensor type BE in volts output versus input current at 60 hertz; Figure 10 is a graphical representation of the transfer function of a sensor type AO versus frequency at different test currents; and Figure 11 is a graphical representation of the transfer function of a sensor type BE versus frequency at different test currents.

A current sensor 10 comprises a straight core 12 of magnetic material onto which a wire coil 14 is wound of small diameter transformer wire having an insulated coating thereon. The straight portion 12a of core 12 is provided with flanges, i.e. stops or dams 16, at either end of that portion of the core where the coil is to be wound, to limit the width of the coil. The stops 16 may be made up of one or more pieces 1 8a and 1 8b of shrink tubing shrunk onto the core. The core 12 is placed in a suitable rotating winding chuck and rotated about its longitudinal axis with the core serving as a straight wire mandrel. The wire coil 14 or winding means is wound onto the straight mandrel and secured at the ends by pigtails 20 tied around the stops 16.The ends 22 and 24 of the straight wire core 12 are then bent to form a D-shaped core while the portion of the core supporting the coil is maintained straight. The ends 22 and 24 of the core 12 are magnetically connected together by suitable means. In Figure 3, a preferred method of magnetically connecting the bent ends 22 and 24 is shown as by placing a magnetic sleeve 26 over the ends 22 and 24 abutting one another and then crimping the sleeve to physically secure and hold the ends together in a magnetic connection. Figure 4 illustrates still another suitable method of connecting the bent ends 22 and 24 together.

Here, the ends 22 and 24 are overlapped and magnetic wire 28 is wound tightly over the lapped core ends. The wire may be tied, cemented or soldered at low heat to hold the wire in place. Figure 5 illustrates still another method of magnetically connecting the bent ends of the core 12. Here, the bent ends 22 and 24 of the core are overlapped and secured together by resistance compression welding as at 30.

As seen in Figure 6, the stops 16 are preferably formed of short pieces of shrink tubing 18a and 18b slid over the straight wire core and firstly shrunk onto the core and then shrunk togehter. Strips of plastics 40 cemented to the core 12 may also be used as illustrated in Figure 7.

As shown in Figure 1, the current sensor 10 is mounted in a plastics case 32 which is filled with a suitable potting compound. It may be desirable to dip the entire coil and core assembly into a polyurethane cushion so that shrinking action of the potting compound will not break coil wires or pigtails. The case 32 is shaped to receive the D-shaped structure of the sensor and is provided with two solder lugs 34 and 36 to which the pigtails 20 are soldered with low temperature solder. The case is further provided with an opening 38 through which a current carrying conductor (not shown) is threaded, to position same as near the single-turn primary 12 of the sensor 10 as possible for maximum current sensitivity.

One type of sensor, type BG, the characteristics of which are illustrated in Figure 8, has a high mu (magnetic permeability) core so that at very low current levels the transfer function is high compared with the transfer function characteristics of sensor type BE, illustrated in Figure 9, both at a frequency of 60 hertz. The type BE, however, can be used to 240 amps (maximum amperage tested) and even up to probably 1000 amps before saturation begins, since the core 12 of the type BE sensor is of a much lower permeability than the type BG sensor. Type BG sensor starts to saturate before 10 amps and is usable at from 100 to 240 amps, but the output decreases sharply at the higher value. The type BE sensor output, at this higher value of current, is still increasing with increasing current passing through the conductor extending through opening 38.

The graphs of transfer function versus frequency do not lend themselves to direct comparison, since the cores 12 in the different cases are of different diameters. If the type AO sensor, Figure 10, were tested to 0.1 microamp, its transfer function would be higher than sensor type BE, Figure 9. The curves of Figures 8, 9 and 10 relate to sensors having the same number of turns in coil 14. The sensor type AO, Figure 10, is designed for use on low level instrumentation applications whereas the sensor type BE is a high alternating current 60 hertz sensor which nonetheless works very well across a wide frequency band at somewhat lower output, Figure 11.

While there have been described what at present are considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from scope of the invention.

Claims

1. A current sensor assembly comprising: only one magnetic core member, having a straight central portion and two curved end portions magnetically and mechanically permanently connected together to form a single loop, one-turn coil; and a winding means on the straight central portion.

2. A current sensor assembly according to claim 1, wherein the straight central portion of the core member is provided with a spool end means positioned on said straight portion at each end of the winding means.

3. A current sensor assembly according to claim 1 or 2, wherein the winding means is comprised of insulated transformer wire.

4. A current sensor assembly according to any preceding. claim, wherein the end portions are connected together at a point diametrically opposite the straight central portion.

5. A method of producing a current sensor assembly, comprising the steps of mounting a single, straight magnetic core for rotation about its longitudinal axis; winding a coil of insulated, small-diameter, transformer wire onto the central portion of the straight core by rotating the core; bending the ends of the core outwardly of the coil-supporting straight central portion into a curved portion so that the ends are brought into contact with each other; and magnetically and permanently connecting the ends together.

6. A method according to claim 5, wherein the ends of the straight core are bent while maintaining the central straight portion of the core straight.

7. A method according to claim 5 or 6, wherein a pair of flanges are positioned on the central portion of the straight core between which the coil is wound and outwardly of which the core ends are bent.

8. A current sensor assembly, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. A method of producing a current sensor assembly, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.