Permanent magnet

[57] ABSTRACT

A permanent magnet is described, which is particularly adapted for use in stators of electric motors or in attraction devices which should present an involved attracting surface, together with high magnetic performance. A plurality of small ferrites pieces are stuck, closely juxtaposed on a flexible sheet of magnetic or non magnetic material. Preferably, the supporting sheet is non magnetic, and the ferrite pieces have a magnetization pattern which alternates the polarities according to a predetermined pitch.

Claims, 3 Drawing Figures

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PATENTED jam 91973 + 710.291

Rcj+1

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3,710,291

PERMANENT MAGNET

The present indention relates to permanent magnets and, more particularly, to magnets made of a material having a high coercive force, from which the ferrites are a preferred example.

In a number of applications, conventional magnets obtained by pressing a ferrite powder in a forming die, in the presence of an orientating magnetic field, and then by sintering the resulting piece at a high temperature, have serious disadvantages. For instance, in a motor stator, the conventional magnets, shaped as comparatively thin ring portions with radial magnetization, usually termed “tiles” are difficult to build with a large angular span, or with a thickness which is comparatively small with respect to the stator diameter. The radial orientation of the field is not always perfect. The “tile” is deformed by sintering and a subsequent machining is necessary. Special pressing tools should be provided for each stator size.

In the above application and also in the manufacture of magnets, the attractive force of which will be used for various purposes, for instance insulating closing devices or fixing devices, use has been made in prior art of a permanent magnet material usually termed “magnetic rubber.”

This material has been disclosed in French Pat. Specification No. 1,135,734 filed on Nov. 24, 1955 by the firm “Tesla Narodin Podnik.” It consists of an elastic binder mixed with ferrite powder. It has the property of adhering, due to its elasticity, to plane or profiled surfaces. It is adapted, moreover, to be subsected to various types of magnetization.

However, due to the binder, its magnetic characteristics are lower than those of a conventional ferrite magnet. The (BH) max product there of is for instance between 1.3 and 1.8, as compared to between 2.5 and 3.5 for the “tiles” and consequently, the use of such a material for manufacturing certain types of stators is prohibited.

It is an object of the present invention to provide a permanent magnet which has substantially the same magnetic properties as the conventional ferrite magnets, while possessing at the same time a good flexibility of pliancy which affords the manufacture of stator of various sizes and of attraction devices the surfaces of which is not necessarily plane. Such a magnet will combine the advantages of both the conventional magnets and the magnetic rubber, without having their drawbacks.

Permanent magnets are priorly known which combine a plurality of piece of an Alnico type alloy fixed to a support. However, the support is a rigid yoke sintered at the same time as the pieces. When it is desired to deform the support, for instance for shaping the magnet into a ring, the rigidity should be lowered by opering out notches in the support. This does not provide a magnet having the same flexibility as a magnetic rubber and adapted for the same kind of applications.

In accordance with the present invention, a permanent magnet comprising a plurality of pieces made of hard magnetic material fastened to a supporting member and magnetized in a direction substantially at right angles with said supporting member, has the supporting member made of at least one thin flexible sheet on which the said pieces are stuck.

In a first embodiment, the said sheet is made of a non magnetic material, the pieces being made of ferrite and having a magnetization which alternates with a given pitch.

In a second embodiment, all the pieces are magnetized so as to present the same polarity along their contact surface with the supporting member and the latter may then be indifferently made from a magnetic or non magnetic material.

These and other objects of this invention will become apparent from the following description.

In the accompanying drawings :

FIG. 1 is a perspective view of a permanent magnet according to the invention ;

FIG. 2 is a plan view showing two such magnets mounted in a stator arrangement and,

FIG. 3 shows a portion of a magnet with alternate poles.

There is shown on FIG. 1 a comparatively pliant supporting member 1, for instance made of a steel ribbon or sheet of other metal, such as aluminum or brass, or even paper or other material, either magnetic or not. A layer of glue 2 affords fastening of magnetic pieces, advantageously made of ferrite, cut out of a massive block.

In the illustrated embodiment, a plurality of small juxtaposed bars, 3,4,5 having for instance a length of 2 cm, a width of 7 mm and a thickness of 4 mm will be adapted for manufacturing a stator “tile.” It will be understood that the illustrated shape and arrangement of the magnetic pieces are by no way limiting, and will depend upon the desired application. For example, to provide an attraction magnetic device designed for adhering to an involved surface, the bars will be subjected to a further cutting operation through directions at right angle with those of the first cutting operation. This will render the arrangement deformable along two mutually perpendicular directions.

The cutting operation(s) may be effected either before or after the lay out and fastening of the magnetic pieces on their supporting member, while the magnetization in effected last of all, in a direction substantially perpendicular to the surface of the support. FIG. 2 illustrates a particularly advantageous application. Reference numeral 6 is the yoke of a motor stator, 7 and 8 two tiles made of ferrite bars stuck on two steelribbons (7α and 8α respectively), in accordance with the technique illustrated in FIG. 1. All the bars of a same tile are magnetized radially with the same polarity and the magnetic sheets 7α and 8α act as pole pieces. The “tiles thus obtained are adapted to the design of stators of various diameters. They may have as small a thickness as desired, with respect to the stator diameter. No machining tool is necessary and the manufacture is quite simple and may be automatized. For example, for use in a D.C. motor having a 40 mm diameter rotor and with a strontium ferrite cut out into bars having the above mentioned dimensions, tiles have been manufacture in accordance with the above disclosed technique, having a span angle of 140° and providing a flux of 28,500 Maxwell in the rotor with a conventional massive tile made of strontium ferrite, the maximum span angle which may be obtained is 120°, with a flux of 29,500 Maxwell through the rotor. With a tile made of magnetic rubber, a span angle of 140° may be obtained,

3,710,291 but the flux through the rotor then does not exceed 19,500 Maxwell.

For a 55 mm diameter rotor, a tile made of bars of 6,5 X U X 26 mm made as disclosed hereinabove may have a span angle as high as 130° and generate a flux of 43,000 Maxwell through the rotor. Such a flux is of the same order of magnitude as that which should be generated by a conventional massive tile, but the latter could not be designed with a span angle exceeding 120°. However, a tile made of magnetic rubber could have a span angle as high as 140°, but with a flux not exceeding 30,000 Maxwell.

It is finally seen that the composite tile of the invention substantially combines the advantages of the two prior art types of tiles, yet without having these drawbacks.

For motors of larger size it would be possible to manufacture the tiles by stiching the ferrite bars on a paper support, shaping the resulting assembly, placing it in a mould and pouring a resin, such as “Araldite,” about the assembly. The resin will penetrate into the interstices between the bars and, when hardening, will fix the shape of the assembly. The paper support will then be removed and a pole piece thicker than that the said support will be glued in place of it.

On FIG. 3, there is shown a portion of a permanent magnet made of small ferrite bars, such as 9 to 17, bonded with a glue 18 on a sheet of non magnetic material 19, for instance paper. Bars 9-11 have for instance their North pole engaging the support 18-19, and also the bars 15-17, while the bars 12 - 14 have their South pole engaging the support 18 - 19. It results that the magnetization pattern alternates with a pitch of for instance three bars, such a permanent magnet may be used in the design of a magnetic attraction device or a magnetic coupling.