GB2117979A - Electrical chokes - Google Patents

Abstract

An electrical choke is distinguished by the use in the core of a metallic glass (i.e. an amorphous or semi-amorphous metal) that has been uniformly subjected to a heat- treatment process until its maximum permeability (i.e. the permeability at the steepest point on the B/H [inductance/field] curve) is reduced at least to one thirtieth of its value measured in the substantially- amorphous "as cast" condition, but that a substantial degree of amorphous character remains. This material exhibits a permeability that is uniform over a wide range of polarising fields and little affected by D.C. biassing.

Description

SPECIFICATION Electrical chokes This invention relates to electrical chokes (inductors) and to magnetic cores for use in them.

Amorphous alloys, also known as metal glasses, have been developed to provide excellent high permeability and low loss magnetic properties. These properties are enhanced by heat-treatment, sometimes assistef by the application of a magnetic field, at temperatures below the crystallisation temperature. It has been discovered that the low-field permeability and loss can be improved by heat-treatment in the immediate vicinity of the crystallisation temperature to bring about a small degree of crystallisation. There is however a critical degree of crystallisation, and once this critical degree, reported to be in the range from 2 to 5 volume % in most cases, is exceeded the permeability falls dramatically, typically down to 1/100th of its maximum value.Metal glasses heat-treated beyond the critical degree of crystallisation (overtreated) have hitherto been considered of no value as magnetic materials and indeed to be avoided since it is not possible to reverse this degradation by any further heat-treatment.

We have now found that metal glasses in this overtreated low permeability condition do in fact have very useful magnetic characteristis which make them very suitable for use in a wide range of, chokes. Cores made from metal glasses heattreated to produce a substantial degree of crystallisation are found to have a relatively constant permeability over an extensive flux range and this property is maintained in the presence of a D.C. polarising field. Normally such a property can only be achieved, except with a special chemical formulation and processing as in the case of the conventional crystalline alloy once marketed under the designation "Isoperm", by the introduction of an air gap into the magnetic circuit.

In accordance with the invention therefore, an electrical choke is distinguished by the use in the core of a metal giass that has been uniformly subject to a heat-treatment process until its maximum permeability (i.e. the permeability at the steepest point on the B/H[inductance/field]curve) is reduced at least to one thirtieth of its value measured in the substantially-amorphous "as cast" condition, but that a substantial degree of amorphous character remains.

In most cases we prefer that the degree of crystallinity should not exceed 10 volume %, provided this is sufficient to achieve the specified reduction in maximum permeability, but a higher crystallinity, up to about 50%, may give useful results in some cases.

At the other end of the range, the degree of heat-treatment is in most cases preferably sufficient to reduce the maximum permeability down to or below one fiftieth of the as-cast value; this will mostly correspond to a degree of crystallinity of 2 to 5 volume %, depending on the composition and the heat-treatment conditions; exceptions may occur in the case of material of low quality, in the sense of having a relatively low maximum permeability in the as-cast condition.

The cores of chokes in accordance with the invention will usually be of the wound type, though stacked stamped laminations could be used; wound cores may be jointless (for optimum magnetic properties) or jointed in one or two (or more) places (for easier linking with the electrical coil of the choke); a single-joint arrangement is more practicable than with conventional crystalline magnetic materials, because the magnetic properties of metal glasses is less susceptible to deterioration from flexing.

Preferably any joint is made- without an airgap.

EXAMPLE 1 A toroidal core size 27 mm outside diameter, 19 mm inside diameter, width 6 mm was wound from 0.04 mm thick amorphous alloy tape of composition 3.7% B. 1.8% Si, 0.5% C, balance Fe (weight per cent). As cast, this tape had a maximum permeability of 50,000, which could be increased to 100,000 by heat-treatment at 3500C for two hours, or to 1 50,000 if a field of 1 kA/m was applied during heat-treatment. The core was heat-treated in argon at 3900C for two hours. The permeability was measured at frequencies up to 20 kHz and found to be 350 at low flux levels i.e.

below 0.001 Tesla and only to rise to 450 at a flux level of 0.5 Tesla. When a D.C. polarising field is applied the incremental permeability is maintained above 300 even with the polarising field of 1 kA/m.

A conventional choke winding was found on this core.

EXAMPLE 2 Toroidal cores size 28 mm outside diameter 19 mm inside diameter, width 10 m were wound from 0.04 mm thick amorphous alloy tape of composition 3% B, 5.6% Si, Balance Fe (weight %).

As cast, this tape had a maximum 50 Hz permeability of 40,000 which could be increased to 80,000 by heat treating at 4200 for one hour or to 120,000 if a 1 kA/m field was applied during the heat treatment.

One core was heat-treated for six hours at 4300C in argon. The permeability was measured at frequencies up to 20 kHZ and found to be 600 at low flux densities i.e. below 0.001 Tesla and rise only to 700 at a flux density of 0.5 Tesla.

When a DC polarising field is applied, the incremental permeability is maintained above 500 even with a polarising field of 1000 A/m.

A second core was heat-treated for three hours at 4500C in argon. The permeability was measured at frequencies up to 20 kHZ and found to be 380 at low flux densities i.e. below 0.001 Tesla and rise only to 460 at a flux density of 0.5 Tesla. When a DC polarising field is applied, the incremental permeability is maintained above 300 even with a polarising field of 2000 A/m.

A third core was heat-treated for six hours at 4450C in argon. The permeability was measured at frequencies up to 20 kHZ and found to be 200 at low flux densities, i.e. below 0.001 Tesla and rise only to 340 at a flux density of 0.5 Tesla.

When a DC polarising field is applied, the incremental permeability is maintained above 100 even with a polarising field of 5,000 A/m.

Conventional choke windings were found on these cores.

Claims (6)

1. An electrical choke distinguished by the use in the core of a metal glass that has been uniformly subject to a heat-treatment process until its maximum permeability is reduced to one thirtieth of its value measured in the substantially amorphous "as-cast" condition but that a substantial degree of amorphous character remains.

2. A choke as claimed in Claim 1 in which the degree of crystallinity does not exceed 10 volume %.

3. A choke as claimed in Claim 1 or Claim 2 in which the maximum permeability of the core has been reduced to below one fiftieth of the as-cast value.

4. A choke as claimed in any one of Claims 1 to 3 in which the core has no air-gap.

5. A choke substantially as described with reference to Example 1.

6. A choke substantially as described with reference to Example 2.