WO2011161321A2

WO2011161321A2 - Arrangement and method for protecting a permanent magnet

Info

Publication number: WO2011161321A2
Application number: PCT/FI2011/050603
Authority: WO
Inventors: Mikael Lamberg
Original assignee: Abb Oy
Priority date: 2010-06-23
Filing date: 2011-06-22
Publication date: 2011-12-29
Also published as: FI20105724A; WO2011161321A3; FI20105724A0

Abstract

An arrangement for protecting a permanent magnet according to the invention comprises a fixed permanent magnet (1). The permanent magnet (1) is surrounded by a gas-tight and airtight shell (2), with vacuum inside the shell (1).

Description

ARRANGEMENT AND METHOD FOR PROTECTING A PERMANENT MAGNET

Scope of the invention

The invention relates to an arrangement for protecting a permanent magnet according to the preamble part of Claim 1 , and a method for protecting a permanent magnet accord- ing to the preamble part of Claim 1 1.

Background art

A permanent magnet is a component manufactured from a magnetically hard material and is able to retain its magnetism permanently after magnetization. Permanent magnets are manufactured, for example, from an AINiCo mixture that includes aluminum, nickel, cobalt and steel, or, for example, from ceramic materials or rare earth metals.

Current powerful lanthanide-based permanent magnets are made from powdered materials by compaction and sintering. The finished items are fragile and extremely sensitive to corrosion, particularly in operational environments with varying humidity and temperature. A traditional way of protecting the permanent magnet pieces against corrosion is to coat them by painting or passivating. Mechanical protection is improved by encapsulating the pieces inside metal casings.

The industrial applications of permanent magnet motors have increased rapidly, particularly in the over-500 kW range. Permanent magnet motors have high efficiency, power density and reliability. Unlike a squirrel cage motor, for example, the properties of permanent magnet motors do not deteriorate when the rotation speed is low. Therefore, permanent magnet motors are well suited for the gearless drives used in, for example, windmills, electric trains and paper machines.

Description of invention

The purpose of the present invention is to create an arrangement and method for protect- ing a permanent magnet. In order to achieve this, the invention is characterized by the features specified in the characteristics sections of Claims 1 and 1 1. Some other preferred embodiments of the invention have the characteristics specified in the dependent claims.

An arrangement for protecting a permanent magnet according to the invention com- prises a fixed permanent magnet. The permanent magnet is surrounded by an airtight shell, with vacuum inside the shell.

A method for protecting a permanent magnet according to the invention comprises a fixed permanent magnet. The permanent magnet is placed inside a shell, vacuum is implemented inside the shell, and the shell is sealed airtight. The permanent magnet is protected against environmental conditions by packing it inside a gas- and airtight shell and closing the shell in such a way that vacuum remains inside the shell. A permanent magnet made of a powder by compacting and sintering, for example, is closed inside an airtight shell, and a pressure that is lower than atmospheric pressure is sucked inside the shell, for example by a vacuum packing machine. This will result in a lower proportion of chemically active gases, oxygen and water vapor in the air inside the shell than in the surrounding environment.

A permanent magnet packed in a vacuum shell is protected from the environment. The traditional protection, protecting a permanent magnet by painting its surface, seldom provides a faultless result. According to an embodiment of the invention, the pressure inside the shell is 10⁵ ... 10^"' Pa. The vacuum inside the shell provides additional rigidity for the shelled permanent magnet.

According to another embodiment of the invention, the shell is made of a flexible foil, such as aluminum foil. A ductile aluminum foil surface is more resistant to erosion and radiation than a painted or passivated surface. A permanent magnet piece is fragile and may splinter upon breaking. A paint surface is also fragile, and does not prevent splintering. Metal alloys are ductile. Ductile materials typically undergo considerable plastic deformation, absorb a lot of energy prior to breaking and therefore undergo a ductile fracture. A ductile shell decreases splintering, thereby also improving safety of han- dling. Existing handling phases of permanent magnets require the use of safety equipment, such as safety goggles.

Also, a foil comprising metal and plastic, for example, can be used as the material for the shell. Such a foil can, for example, be made of two different layers, an aluminum foil layer and a polypropylene foil layer, by lamination.

Using a shell that is thin, such as a foil, can achieve even thinner surface thicknesses than painting, which is beneficial in terms of electromagnetic dimensioning. A nonmagnetic or thin magnetic shell has no impact on electromagnetic dimensioning. When a permanent magnet is installed inside the rotor pole, material between the permanent magnet and finger plates causes power loss, which makes small material thickness in the permanent magnet shell preferable. If the shell is made of a thick material, it is preferred that the material is non-magnetic.

A slippery shell surface is beneficial upon the installation of permanent magnets, because it prevents friction that wears the surfaces. Another embodiment of the invention comprises a shell of a single part wherein three seams are formed. The permanent magnet is typically a flat item with the shape of a rectangle or square when viewed from above or below. The length of a rectangular permanent magnet is bigger than its width. The length and width of a square permanent magnet are essentially the same. When the permanent magnet is packed in a shell comprised of a single part, it is preferred to be seamed with three seams, one longitudinal seam and two lateral seams.

In yet another embodiment of the invention, the shell is comprised of two parts attached to each other with four seams.

Placing the longitudinal seams of the shell on the longitudinal side surfaces of the per- manent magnet and placing the lateral seams on the lateral side surfaces of the permanent magnet leaves the top and bottom surfaces of the permanent magnet free of seams. When permanent magnets are installed in the rotor surface and fixed using, for example, pole pieces, the seamless top surface and bottom surface remain between the permanent magnet and the pole piece. The permanent magnet with shell is in such a case essen- tially of a uniform thickness. Standard thickness is mechanically preferred when the pole piece is tightened against the permanent magnets.

It is preferred to fold any skirts arising upon seaming to the level of the shell so that they will not cause obstacles to the flow of cooling agent passing the permanent magnet. A thin shell material offers various opportunities for implementing various kinds of graphics and markings on the packaging. The shell material can be provided to the pressing production machines in the form of rolls. The desired print pattern, graphics and markings are provided by the client in electronic format. The shell material passes as a ribbon in the production machine. The printed material roll produced by the production machine is but to the roll size specified by the client. Such a printing solution is considerably less costly than realizing graphics and markings in individual shells. For example, polarities can be marked ready on the shell for magnetizing the permanent magnet, and serial number and date for improving the traceability of the permanent magnet production chain up to the party implementing the protection of the permanent magnet. The seams and skirts of the shell can be utilized in order to eliminate any errors taking place in magnetization and the marking of polarity. When the seamed shell is formed so that it is asymmetric with regard to the longitudinal axis and/or lateral axis, the direction of the shell can be identified. An application of the arrangement according to the invention is the rotor of an electric machine. The rotor is equipped with permanent magnets. The rotor comprises a shaft and rotor core, and the rotor core is fitted to the shaft. In the rotor, a permanent magnet protected by a shell is installed on the outer surface of the rotor core or inside the rotor core. The permanent magnet is surrounded by a gastight and airtight shell, with vacuum inside the shell.

If the shelled permanent magnets are installed onto the surface of the rotor, they can be encapsulated, in which case the shell functions as a fixing body and ties the magnets to the surface of the rotor.

The shell protects the permanent magnet during handling and installation, helping to avoid scratching and damaging of the permanent magnet's surface, for example. Be- cause the shell of the permanent magnet unit is fully closed, raw, i.e. not surface-treated, magnets may be used as permanent magnets. However, commercial permanent magnets are usually phosphate coated as a basic surface treatment. A gastight and airtight enclosure protects the permanent magnet in demanding conditions. When vacuum is lost, damage in the shell of the permanent magnet can be seen as the shell material becoming loose and softer. It is easier to observe than damage in a paint surface or passivated surface.

The method for protecting a permanent magnet makes the production process of permanent magnets easier as a result of the simpler coating method. Vacuum packing meth- ods known in the prior art can be utilized in the manufacturing process and vacuum packing materials can be utilized as shell materials.

Packing in vacuum is a preferred method for protecting a permanent magnet compared to painting, passivation or encapsulation. Vacuum packing means are not large investments, and they can be integrated into permanent magnet manufacturing lines. The pur- chase cost of shell material is considerably lower than the purchase cost of ready-made enclosures.

A vacuum packing line for permanent magnets is easier to realize than a painting or passivation line, also from the point of view of quality. As a working phase, vacuum packing is located in the same place of the manufacturing chain as painting, after the polishing of sintered pieces, prior to the magnetization of the permanent magnets.

Figures in the drawings

In the following, the invention will be described in more detail with the help of certain embodiments by referring to the enclosed drawings, where:

- Figure 1 illustrates an arrangement for protecting a permanent magnet; - Figure 2 illustrates an arrangement for protecting a permanent magnet, wherein graphics and markings are made on the shell; - Figure 2 is a partial illustration of an electrical machine's rotor equipped with permanent magnets.

Detailed description

Figure 1 illustrates an arrangement for protecting a permanent magnet. Permanent magnet 1 is made of powdered materials by compacting and sintering, and it is fixed. The permanent magnet is placed in shell 2, which is closed gastight and airtight. Shell 2 is of a gastight and airtight material. The shell is, for example, non-magnetic material, such as aluminum foil or a foil comprising metal and plastic.

Vacuum, pressure lower than the atmospheric pressure of the surrounding air, prevails inside the shell. This will result in a lower proportion of chemically active gases, oxygen and water vapor in the air inside the shell than in the surrounding conditions. The shell protects the permanent magnet from corrosion. A pressure of 10⁵ ... 10^"1 Pa pre- vails inside the shell, corresponding with a low vacuum ... moderate vacuum. In this vacuum range, active components and absorbed or dissolved gases are removed from air.

The shell comprises two parts 3a, b, which are seamed together with four seams 4. Placing the longitudinal seams 4 along the longitudinal side surfaces 5 of permanent magnet 1 , and placing the lateral seams 4 along the lateral side surfaces 6 of permanent magnet 1 leaves the top and bottom surfaces of the permanent magnet free of seams.

Seams 4 and skirts 7 of shell 2 of permanent magnet 1 in Figures 1 and 2 can be utilized in the haptic identification of permanent magnet 1. Skirts 7 emerged upon seaming are folded to the level of the shell, skirt 7 towards the bottom surface of permanent magnet 1. The seamed shell 2 is therefore asymmetric with regard to the longitudinal or lateral plane through permanent magnet 1.

If skirts 7 of shell 2 of permanent magnet 1 are of different shapes, or at least one skirt 7 is made to a different shape than other skirts 7, the skirts can be utilized for identifying that the permanent magnet is always in the orientation when the permanent magnet is magnetized with an excitation winding.

In Figure 2, polarity 8 has been marked ready for the magnetization of the permanent magnet on shell 2, with a warning of a powerful magnetic field 9. In addition, serial number 10 has been marked on shell 2, so that the production chain of the permanent magnet can be traced up to the party encapsulating permanent magnet 1. The markings on the shell and possible use of colors in the markings aid in visual identification.

Figure 3 is a partial illustration of an electrical machine's rotor 1 1 equipped with encap- sulated permanent magnets 1. Rotor 1 1 illustrated in Figure 3 is a motor's or generator's rotor.

Rotor 1 1 comprises a rotor core 12, fitted to the shaft. Rotor 1 1 rotates around its shaft. Rotor 11 comprises several magnetic poles 13. Permanent magnets 1 packed in shell 2 are installed on the outer surface of rotor core 12 at magnetic poles 13. The permanent magnets are according to Figure 1 or 2.

Pole piece 14 is installed on the air gap side of permanent magnets 1 in each magnetic pole 13. Each pole piece 14 attaches two permanent magnet arrays parallel to the shaft. In a rotor 11 with permanent magnets, it is useful to have much magnetic surface on the magnetic pole 13. Skirts 7 emerging upon seaming in shell 2 of permanent magnet 1 are folded to the level of the shell so that they will not cause obstacles to the flow of cooling agent when installed in the electric machine's rotor 1 1.

In the method for protecting permanent magnet 1 , fixed permanent magnet 1 is placed inside shell 2, vacuum is realized inside the shell, and shell 2 is sealed airtight. The method can be realized using, for example, vacuum packing equipment. Unmagnet- ized 100x100x20 mm permanent magnet piece 1 is ready for coating following polishing after sintering. In polishing, the surface of permanent magnets 1 is smoothed and its corners are rounded. Permanent magnet 1 is placed on the feeding bridge of the vacuum packing equipment, and the piece is automatically transferred to the vacuum packing table. The packing material, such as a metal foil, is rolled in a stand next to the vacuum packing equipment. The material is hard, ductile and smooth-surfaced. The customer company's identifica- tions, warnings of a powerful static magnetic field 9 and future polarities of the magnet 8 are printed ready on the material in bright blue and red symbols. The vacuum packing table closes and the material roll revolves forward. The equipment wraps permanent magnet 1 in the vacuum packing material, metal foil, removes air from between the material and permanent magnet 1 , and seams and glues the material to the edges of perma- nent magnet 1. The machine performs the run in a few seconds. The equipment opens up, and the unmagnetized permanent magnet 1 packed in vacuum shell 2 is transferred automatically to the ejection bridge. Permanent magnet 1 is ready for magnetization.

Package-specific markings, such as serial numbers 10 and dates, can be marked on shell 2 of permanent magnet 1 in connection with the protection of the permanent magnet. The means for implementing package-specific markings on shell 2 of permanent magnet 1 can be placed next to the vacuum packing equipment or integrated into the vacuum packing equipment. As a method stage, implementing the package-specific markings on shell 2 of permanent magnet 1 can be performed before or after the placement of the permanent magnet. Markings can be made by, for example, engraving with laser, heat treatment or with an additive on the shell surface.

Part list: 1 permanent magnet; 2 shell; 3a, b shell part; 4 seam; 5 longitudinal side surface; 6 lateral side surface; 7 skirt; 8 polarity marking; 9 warning marking; 10 serial number; 1 1 rotor; 12 rotor core; 13 magnetic pole; 14 pole piece.

Claims

1. An arrangement for protecting a permanent magnet, wherein the permanent magnet (1) is fixed, characterized in that the permanent magnet (1) is surrounded by an airtight shell (2), and vacuum prevails inside the shell (2).

2. An arrangement according to Claim 1, characterized in that a pressure of 10⁵ ... 10^"1 Pa prevails inside the shell (2).

3. An arrangement according to any of Claims 1-2, characterized in that the shell (2) is made of a flexible foil.

4. An arrangement according to Claim 3, characterized in that the shell (2) is made of aluminum foil.

5. An arrangement according to Claim 3, characterized in that the shell (2) is made of a foil comprising metal and plastic.

6. An arrangement according to any of Claims 1-5, characterized in that the shell (2) is comprised of a single part (3a, b), wherein a shell is formed from the said part with three seams (4).

7. An arrangement according to any of Claims 1-5, characterized in that the shell (2) is comprised of two parts (3a, b), wherein a shell is formed from the said parts (3a, b) with four seams (4).

8. An arrangement according to any of Claims 1-7, characterized in that the shell (2) is made of a non-magnetic material.

9. An electric machine's rotor, wherein the rotor ( 1 1) is equipped with permanent magnets (1), the said rotor comprising a shaft and a rotor core (12), the said rotor core (12) being fitted onto a shaft, and the said rotor comprising a permanent magnet (1) installed on the outer surface of the rotor core (12) or inside the rotor core (12), characterized in that the permanent magnet ( 1) is protected according to the arrangement of Claim 1.

10. A rotor according to Claim 9, characterized in that the permanent magnet (1) is installed on the surface of the rotor core (12), and a pole piece (14) is installed on the air gap side of the permanent magnet (1).

1 1. A method for protecting a permanent magnet, characterized in that a fixed permanent magnet ( 1) is placed inside the shell (2), vacuum is realized inside the shell (2), and the shell (2) is sealed airtight.

12. A method according to Claim 1 1, characterized in that a pressure of 10⁵ ... 10^"1 Pa is realized inside the shell (2).