GB2148751A - Manufacture of magnetic cores - Google Patents

Abstract

A magnetic core is made by winding a length of amorphous strip of ferromagnetic material onto a mandrel. Simultaneously heating current is passed through a portion of the strip that is approaching the mandrel in order to heat-treat it and so modify the magnetic properties obtained. In this way the advantages of a very rapid annealing step can be achieved without the complications of pulse current techniques, and the material reaching the winding point may be hot enough to ensure stress relief. Induction heating may be used, or electrode heating applied. Here two spaced adjustable sliding contact electrodes or one electrode and the mandrel are utilised.

Description

SPECIFICATION Manufacture of magnetic cores.

This invention relates to the manufacture of magnetic cores by winding into a coil strip of an amorphous ferromagnetic alloy.

It is known that the use of amorphous rather than crystalline strip makes it possible to obtain different and useful magnetic properties and that properties which are very attractive for a number of applications can be developed by a high temperature short-time heat-treatment. In the known technique, a pulse current of short duration is passed through the length of the strip (Paper by Jagielinski, presented at the conference entitled INTERMAG 1983 at Philadelphia, PA., USA, on 5 Aprii, 1983. Such a technique is difficult to apply under factory conditions because there are difficulties in providing connections, insulation and cooling if the core is processed in its final wound condition whereas undesirable stresses may be generated if it is wound or re-wound after processing.

In accordance with the present invention, strip of amorphous ferromagnetic material is wound onto a mandrel and is simultaneously heat-treated by heating current passed through a portion of the strip that is approaching the mandrel.

Induction heating is attractive in principle, but may be difficult to achieve as amorphous ferromagnetic strip can currently only be made in very small thicknesses (up to around 0.05 mm). Our present preference is therefore to introduce current into the strip by means of at least one electrode moveable along the strip by sliding or rolling. We prefer to use two such electrodes so as to provide a cooling zone between the downstream electrode and the winding mandrel, but in some cases, restricted to those in which the turns of the coil being wound are in electrical contact, it might be feasible for current to flow between a single moveable electrode and the mandrel itself acting as the second electrode.

Additional electrodes could be used to obtain a modified time-temperature profile.

When current is introduced by electrodes, direct current may be preferred, but alternating current is effective and acceptable with the proviso that if properties are required to be uniform along the length of the strip the cycle period of the current should be short compared with the time taken for a point in the strip to travel from electrode to electrode.

The extent of heating required will naturally vary with the composition and condition of the strip, but for the most readily available amorphous alloys in the as-cast condition an energy input of the order of 0.7 MJ/kg may be found to give useful results.

It may be desirable for the position of the electrode, or all the electrodes, to be adjustable as winding proceeds in order to keep constant, or at least control variation of, the spacing of the, or the last, electrode from the winding point as the size of the winding increases and the winding point moves outwards in consequence.

As it will be difficult to wind always at constant speed, without significant acceleration periods, it is desirable to provide control means responsive to winding speed to vary the magnitude of the heating current to obtain as nearly as possible consistent heat-treatment over a range of winding speeds.

After heating in the manner described, and either before or after reaching the winding point, the strip may be artificially cooled, for example by causing a stream of ambient air or other relatively cool fluid to impinge on it; by causing a suitable heat sink, e.g. a heavy brass roll, to contact it and preferably roll on it; or by force-cooling the mandrel on which it is being wound. The mandrel can be cooled, for example, by circulating fluid through ducts on it or by mounting it on a vertical axis with its lower end immersed in a open bath of water or other cooling fluid.

The invention, when adequately controlled, enables a high degree of annealing to be achieved with negligible incidence of crystallisation (because the temperature of the strip passes rapidly through the band in which the rate of nucleation is high) and at the same time the temperature at the winding point can be such that spontaneous stress-relieving takes place.

Alternatively, with a different set of operating conditions (e.g. reduced winding speed, increased current and/or increased spacing of electrodes) it is possible to induce a controlled degree of crystallisation and so obtain a further range of magnetic properties.

Prior to the heat-treatment in accordance with the invention the strip may be in "as-cast" condition, or it may have been given a preliminary heattreatment, mechanically worked and/or otherwise processed.

The winding formed by the process described may be used as such as a toroidal core, or it could be cut in one, or two (or more) places to facilitate assembly with one or more electrical winding; small degrees of shaping are acceptable, but if a shape substantially different from circular is required the use of a non-circular mandrel is recommended.

EXAMPLE 1 An amorphous ferromagnetic strip of composition Fe40 Vi40 P14 B6 and dimensions 1.8 x 0.04 mm is made by conventional techniques and is wound, without intermediate processing, at a linear speed of 90 mm/s onto a circular mandrel 35 mm in diameter to form atoroidal core. In accordance with the invention, heat-treatment is effected by passing a current of 10.5A between a pair of sliding electrodes of graphite, each extending the whole width of the strip and having a dimension of 1 mm in the direction of length of the strip; the downstream electrode of the pair is kept at a constant distance of 10 mm from the winding point and the gap between the two electrodes is kept constant at 10 mm.The temperature of the strip is estimated to reach a maximum temperature of 800"C at the downstream electrode and to cool to about 100'C at the winding point. Because of the short time at high temperature, a protective atmosphere is not essential.

The measured coercive force of the product was 0.5 A-1 in comparison with a minimum value of 0.78 A-' achieved by furnace annealing of this alloy (6 minutes at 293 C).

EXAMPLE 2 Amorphous strip of composition Fe40 Ni40 B20 (commercially available from Vacuumschmelze GmbH under the designation VAC 0040) with a width of 2 mm and thickness 0.025 mm was processed in the same way as in Example 1, except that the winding speed was 140 mm/s and the current 7.5 A. The coercive force achieved was 0.9 A -' compared with a minimum value of 1.28 A-' achieved by furnace annealing of this alloy (4 minutes at 377 C).

Claims (9)

1. A method of making a magnetic core comprising winding strip of amorphous ferromagnetic material onto a mandrel and simultaneously heat-treating the strip by heating current passed through a portion of the strip that is approaching the mandrel.

2. A method as claimed in Claim 1 in which the heating current is generated by induction.

3. A method as claimed in Claim 1 in which heating current is introduced into the strip by at least one electrode moveable along the strip by sliding or rolling.

4. A method as claimed in Claim 3 in which the heating current is a direct current.

5. A method as claimed in Claim 3 or Claim 4 in which the position of the electrode is, or the position of all the electrodes are, adjustable as winding proceeds.

6. A method as claimed in any one of the preceding claims in which the magnitude of the heating current is varied in response to changes in winding speed.

7. A method as claimed in any of the preceding claims in which the strip is artificially cooled after the heating step.

8. A method of making a magnetic core substantially as described with reference to either of the examples.

9. A magnetic core made by the method claimed in any one of the preceding claims.