GB2183103A - Magnetic core - Google Patents

Abstract

A magnetic core 12 particularly for use in a residual current circuit breaker comprising a stack of laminations or rings 13 with staggered air gaps 14A, B, C, D, E. The core has a low remanence but retains a useful level of permeability. <IMAGE>

Description

SPECIFICATION Magnetic Core This invention relates to magnetic core structures, which may, for example, be used in residual current circuit breakers (RCCB's) complying with national and international specifications, requiring a minimum sensitivity to residual sinusoidal alternating currents and residual pulsating direct currents.

The latest requirement that an RCCB may be specified to be sensitive to pulsating direct current, in addition to the normal sinusoidal alternating fault current, imposes new demands on the characteristics of the sensing magnetic core structure. The flux change in a core due to pulsating direct current will depend on the magnitude of its remanence; the lower remanence the higher the flux change and vice versa. The typical RCCB high permeability nickel-iron sensing core has a remanence ratio BR/Bs of 0.5% or more and has a relatively poor sensitivity to pulsating DC. BR is flux density at remanence and Bs is flux density at saturation. One solution currently employed to achieve lower remanence is to employ an additional final heat treatment with a transverse magnetic field, in the manufacture of the sensing core. This heat treatment is relatively difficult and expensive to perform.

This invention is concerned with a magentic core structure which has low remanence and an additional advantage over the conventional core. It is well known that the introduction of an air gap into a core circuit will reduce the remanence, however this will also be accompanied by a substantial reduction in permeability. This reduction in permeability is normally so severe that the core would not be sufficiently sensitive to detect fault currents.

In accordance with the present invention there is a magnetic core fabricated from metallic laminations having magnetic properties, each lamination, or alternate laminations, containing an air gap, the laminations being arranged in such a manner that the gaps are distributed around the circumference. The preferred method of stacking the laminations is such that the gaps are progressively moved a distance of between one fiftieth and one third of the magnetic length of one lamination around the circumference relatively to the one below. The gaps are preferably spaced by equal increments but the increments may vary between 10 and 20% for example.

Magnetic length of a ring is (D1 +D2) n 2 where D1 is the external diameter and D2 is the internal diameter of the ring.

The gap is preferably of a length lying between 1/400th and 1/20th of the magnetic length of the ring.

This distributed form of lamination construction produces a magnetic core with a low remanence and a useful level of permeability.

The laminations may be, for example, ring shaped or rectangular or other shapes.

In the accompanying drawings: Figure 1 is an isometric view of a prior art magnetic core with an aligned air gap; and Figure 2 is a similar view of a core embodying the present invention.

The known core 10 shown in Figure 1, as mentioned above, has a single air gap 11 and suffers from the disadvantage described above.

The core 12 in Figure 2 is made, in accordance with the invention, from a number of rings 13 of equal diameter, each with a gap such as gap 14 in the uppermost ring 13, and other gaps such as 14A, B, C, D, E.

The following examples iilustrate the invention: EXAMPLE 1 Rings 12.5 mm outside diameter, 9.5 mm inside diameter were stamped from 0.35 mm thick nickel-iron alloy strip sold by the applicant under the name Mumetal (Registered Trade Mark). Air gaps of width either 0.46 mm, 0.87 mm or 1.6 mm were cut into the rings. The rings were heat treated to develop the soft magnetic properties. Sets of rings with each air gap were then assembled in such a manner that these gaps were uniformly distributed around the circumference as previously described. Tests were also carried out on similar ungapped rings and rings with gaps aligned for comparison. The 50 Hz permeability at B=0.4 AIM and BRIBE ratio for the three gaps and differing gap spacings and for the ungapped and aligned gap tests are given in Table 1.

EXAMPLE 2 Rings 70 mm outside diameter, 50 mm inside diameter were stamped from 0.35 mm Mumetal (Registered Trade Mark) strip and gaps of 0.46 mm. 0.87 mm and 1.6 mm introduced as before. The rings were heat treated to develop the soft magnetic properties, assembled into stacked with differing gap spacings and the magnetic performance measured. The magnetic performance results are given in Table 2, again with comparative figures for gapped, ungapped and aligned gap tests.

EXAMPLE 3 Sets of rings, as described in Examples 1 and 2, were stacked using gapped and ungapped rings alternatively. The magnetic performance results are given in Table 3.

TABLE 1 Permeability/Remanence Ratio Ratio of Distance between the Gap Spacing Gap to Magnetic Gap to Magnetic Gap to Magnetic in Successive Rings to the Length Ratio Length Ratio Length Ratio Magnetic Length of 1/22 of 1/40 of 1n5 1/2 18,500/0.27 18,000/0.26 19,000/0.23 1/3 19,000/0.2 18,000/0.2 19,000/0.2 1/4 17,000/0.15 18,000/0.17 18,500/0.19 1/8 9,500/0.05 13,000/0.08 15,500/0.11 1/11 9,000/0.04 10,500/0.05 15,000/0.06 Ungapped Rings 46,000/0.5 Gaps Aligned 80/ < 0.01 120/ < 0.01 180/0.02 TABLE 2 Permeability/Remanence Ratio Ratio of Distance between the Gap Spacing Gap to Magnetic Gap to Magnetic Gap to Magnetic in Successive Rings to the Length Ratio Length Ratio Length Ratio Magnetic Length of 1/118 of 1/217 of 1/410 1/2 35,500/0.51 37,500/0.48 36,000/0.5 1/3 37,000/0.43 37,000/0.4 36,000/0.41 1/4 35,000/0.41 37,000/0.34 35,000/0.39 1/8 32,500/0.32 35,000/0.33 33,000/0.32 1/11 31,500/0.27 32,000/0.27 30,000/0.28 1/15 26,500/0.19 28,500/0.19 26,500/0.16 1/45 - 15,000/0.06 Ungapped Rings 52,000/0.59 TABLE 3 (Alternate Gapped and Ungapped Rings) Permeability/Remanence Ratio Ratio of Distance between the Gap Spacing to the Gap to Magnetic Length Gap to Magnetic Length Magnetic Length Ratio of 1/410 Ratio of 1n5 1/4 39,000/0.37 32,000/0.33 Normally it is desirable that the magnetic core used in an RCCB should have as low a variation in permeability with temperature change as possible. This low temperature coefficient of permeability is achieved by suitable choice and control of alloy chemistry and final heat treatment conditions. In most instances the heat treatment conditions required to achieve the lowest remanence are different from those required to obtain the lowest temperature coefficient of permeability. It is therefore normal to use heat treatment conditions which are a compromise balance between remanence, temperature coefficient of permeability and permeability itself.

The constraints of this compromise mean that it is difficult to achieve a variation in permeability of less than 20% over the range -25" to +80"C. With this invention the heat treatment conditions do not have to be compromised to achieve low remanence and it is possible to achieve a change in permeability of less than 10% over the temperature range -30" to +80"C.

Claims (3)

1. A magnetic core fabricated from metallic laminations having magnetic properties, each lamination, or alternate laminations, containing an air gap, the laminations being arranged in such a manner that the gaps are distributed around the circumference.

2. A core according to claim 1 and in which the laminations are stacked such that the gaps are progressively moved a distance of between one fiftieth and one half of the magnetic length of one lamination around the circumference relatively to the one below.

3. A core according to claim 1 or claim 2 and in which the gaps are spaced by increments which vary up to 20%.

Claims 3, 6 and 7 above have been re-numbered as 2,4 and 5 and their appendancies corrected.

3. A core according to claim f or claim 2 and in which the gaps are spaced by equal increments.

4. A core according to claim 1 or claim 2 and in which the gaps are spaced by increments which vary between 10% and 20%.

5. A core according to any preceding claim and in which each gap is of a length lying between 1/200 th and 1/20 th of the magnetic length of the ruing.

6. A residual current circuit breaker incorporating a magnetic core in accordance with any preceding claim.

7. A magnetic core comprising gapped laminations substantially as hereinbefore particularly described with reference to Tables 1,2 and 3 of the specification and as illustrated in Figure 2 of the accompanying drawings.

Amendments to the Claims have been filed, and have the following effect: Claims 1,2,4 and 5 above have been deleted or textually amended.

New or textually amended claims have been filed as follows:-

1. A magnetic core fabricated from metallic laminations having magnetic properties, each lamination, or alternate laminations, containing an air gap, the laminations being arranged in such a manner that the gaps are distributed around the circumference, and in which either the laminations are stacked such that the gaps are progressively moved a distance of between one fiftieth and one half of the magnetic length of one lamination around the circumference relatively to the one below, or each gap is of a length lying between 1/200 th and 1/20 th of the magnetic length of the ring, or both.