WO2001028295A1

WO2001028295A1 - Method and apparatus for preparation of magnetic material

Info

Publication number: WO2001028295A1
Application number: PCT/US2000/028253
Authority: WO
Inventors: John F. Roesel, Jr.
Original assignee: Precise Power Corporation
Priority date: 1999-10-12
Filing date: 2000-10-12
Publication date: 2001-04-19
Also published as: AU8016600A

Abstract

An apparatus (10) for producing magnetic material from a slurry (13) containing magnetic particles and a fluid comprises a mold containing the slurry (13), a press (12), and a magnetic field generator (16). The press (12) presses the slurry (13) along a pressing line (PL), and the magnetic-field generator (16) produces a magnetic field that passes through the slurry (13) substantially perpendicular to the pressing line (PL) to orient the magnetic particles in the direction of the magnetic field. The magnetic-field generator (16) can be an electromagnet that includes two opposing poles (26, 28) that bound the mold. The press (12) can include a pair of opposing rams with the rams applying pressure to the slurry (13) and being aligned along the pressing line (PL). A method for using the apparatus to produce magnetic material and a method to use the material to produce a high speed controlled-pole rotor are also disclosed.

Description

METHOD AND APPARATUS FOR PREPARATION OF MAGNETIC MATERIAL

FIELD OF THE INVENTION This invention relates to magnetic material manufacturing. More particularly, this invention relates to a new and improved method and apparatus for manufacturing magnetic material.

BACKGROUND OF THE INVENTION Magnetic material is used in many applications including controlled-pole machines. Controlled-pole machines are a class of electric motors and generators in which the orientation of the rotor field poles can be varied during operation of the machine. These devices have the advantage of being able to continuously supply a current as a generator with a non-varying frequency even during variation in rotor speed (rpm) . Variation of rotor speed of typical generators causes the frequency of the current output to change accordingly. However, by using a stationary exciter pole in the controlled pole generator, the rotor field poles can be continuously induced while the device is operating to maintain the same frequency of current output. The controlled-pole magnetic material is the media upon which the rotor field poles are induced, and this magnetic material is located on the device's rotor surface.

Many types of materials are suitable for use as the controlled-pole magnetic material. However, ferrite hard magnetic material is typically used. This ferrite material includes the favorable characteristics of a low electrical conductivity, relatively low cost, and the ability to shape the hysteresis loop during production for optimum performance. Other examples of magnetic materials include high energy alnico or neodymium-iron-boron. Although these materials are typically more expensive, they have energy products of over 30 million Gauss-Oersted in contrast to an energy product of around 4 million Gauss-Oersted for the ferrite material.

To compensate for the lower energy product of ferrite, the thickness of a ferrite layer in the magnetic orientation can be increased. Typical controlled-pole devices use a single layer of ferrite that is approximately between 1 to 3 cm in thickness. In certain applications, thicknesses greater than 3 cm and as great as 10 cm or more are desired. However, manufacturing ferrite material greater than 3 cm is very difficult.

The difficulty in obtaining a ferrite layer with a greater thickness is due to current manufacturing processing. The current process of manufacturing ferrite layer involves pressing a mixture of ferrite powder and liquid. Concurrently with the pressing process, a magnetic field is applied to the mixture of the liquid and ferrite powder to align the individual ferrite particles in a particular orientation. The field lines of the magnetic field in the current process are oriented so that they are in the same direction of the pressing. Thus, the magnetic field aligns the magnetic particles along the direction of the pressing. Alignment of the magnetic particles is desired to maximize the energy product of the finished product. Once the pressing removes most of the liquid from the mixture, the ferrite "cake" or green part is fired to produce the ceramic ferrite magnet. The difficulty with producing thicker magnets is caused by the size of the particles in the powder. To obtain the desired properties of the magnet, these particles are often milled to less than one micron in thickness. With a thicker magnet, the small size of the particles results in an exponential increase in the time required to force the liquid out of the mixture. Thus, because of the small particle size and long pressing time needed, the cost of producing a thicker magnet becomes prohibitively expensive. Furthermore, proper drying of the "green" part before firing is much more difficult and yields of finished parts are greatly reduced due to excessive cracking during firing.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an apparatus and method for producing single-layer magnetic material having a length, in the magnetic orientation, of greater than about 3 cm. It is another object of this invention to provide an apparatus and method for producing single-layer magnetic material having a greater thickness while maintaining small particle size in the slurry, reduced time during pressing, and a greater production yield of acceptable parts.

It is yet another object of this invention to provide an apparatus and method for producing low cost, low electrical conductivity magnetic material with superior magnetic properties that can be used with controlled-pole electrical machines. In accordance with the invention, these and other objects are accomplished by providing an apparatus that comprises a press, a magnetic field generator, and a mold for containing a slurry of magnetic particles. The press presses the slurry along a pressing line, and the magnetic-field generator produces a magnetic field that passes through the slurry substantially perpendicular to the pressing line to orient the magnetic particles in the direction of the magnetic field. The magnetic-field generator is preferably an electromagnet that includes two opposing poles that bound the mold. The press can include a pair of opposing rams. The rams applying pressure to the slurry and are aligned along the pressing line.

A method for use of the apparatus to produce magnetic material comprises the steps of pressing, orienting, and sintering the slurry. The pressing of the slurry is along a pressing line to produce a cake by removing the liquid from the slurry. Magnetic particles in the slurry are also subjected to a magnetic field that passes through the slurry substantially perpendicular to the pressing line to align the magnetic particles with the magnetic field. The orienting step is preferably conducted concurrent with the pressing step. Sintering of the cake removes additional fluid from the cake. To further enhance the magnetic alignment of the magnetic particles during pressing, the particles may be vibrated or agitated either mechanically or magnetically.

A further embodiment of the invention involves using the magnetic material to form a rotor for a controlled-pole electric machine by positioning the magnetic material about the surface of a rotor core. The magnetic length of the magnetic material is preferably oriented radially from the rotor core. In the case where the magnetic material is on the outside of the rotor, the magnetizable material can also be bound to the exterior surface using high strength wire or fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments of the invention that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein:

FIG. 1 is a front view of an apparatus for producing a magnet according to the invention.

FIG. 2 is an enlarged sectional view of a mold and press.

FIG. 3A is a diagrammatic view of a column subjected to a pressing force perpendicular to a magnetic field.

FIG. 3B is a diagrammatic view of a column subjected to a pressing force parallel to a magnetic field.

FIG. 4 is a longitudinal vertical cross-section of a high speed controlled- pole electric machine. FIG . 5 is a longitudinal vertical cross-section of a rotor.

FIG. 6 is an enlarged view of a portion of the rotor as illustrated in FIG . 5.

FIG. 7 is an enlarged transverse vertical partial cross-section of the high speed controlled-pole electric machine shown in FIG. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate an apparatus for producing a magnet from a slurry according to the invention. The magnet-production apparatus 10 comprises a press 1 2, a mold 1 4, and a magnetic-field generator 1 6. The press 1 2 presses the slurry 1 3 in a pressing direction. The mold 1 4 holds the slurry 1 3 during the pressing operation and allows any fluid pressed from the slurry 1 3 to escape, and the magnetic-field generator 1 6 produces a magnetic field that passes through the slurry 1 3 and is oriented substantially perpendicular to the pressing direction. The slurry 1 3 to be pressed into a magnet contains particles that are inherently magnetic and/or are capable of being magnetized. The particles are also susceptible to being oriented by a magnetic field. Although the particles can be composed of any material capable of being formed into a magnet, the presently preferred particles are composed of ferrite powder. An additional component of the slurry 1 3 is a fluid which acts as a binder for the particles.

Although this invention is not limited as to the type of fluid used within the slurry 1 3, the presently preferred fluid is water.

Particle size is a factor in determining the amount of time at a given pressure to which the slurry 1 3 must be exposed to remove the fluid from the slurry 1 3. As the size of the particles become smaller, the amount of time and pressure needed to remove the fluid from the slurry 1 3 exponentially increases. Although particle size is an important factor in the process of producing a magnet with the magnet production apparatus 1 0, the invention is not limited as to the size of particles within the slurry 1 3. The presently preferred size of individual particles are approximately a micron or less.

Molds 1 4 for containing a slurry 1 3 are well known in the art of magnet manufacturing and all are acceptable for use with this invention. The mold 14 preferably serves several functions besides containing the slurry 1 3. For instance, the mold 1 4 can provide a feature that allows the fluid to escape the slurry 1 3 during the pressing of the slurry 1 3, and any feature, for example a drain, capable of so doing is acceptable.

Preferably, the mold 1 4 is either adapted to allow the press 1 2 to apply pressure to the slurry 1 3 directly or adapted to transmit to the slurry 1 3 pressure applied by the press 1 2 to mold 14. Molds 14 so adapted are well- known in the art of magnet manufacturing and any of these molds 14 are acceptable for use with this invention. However, the preferred mold 1 4 is typically selected to be used in combination with a particular type of press 1 2. Presses 1 2 capable of pressing a slurry 1 3 are well known in the art of magnet manufacturing, and the invention is neither limited as to the type of press 1 2 used nor the manner in which the press 1 2 is used. Any press 1 2 capable of providing sufficient pressure to remove fluid from the slurry 1 3 is acceptable. The presently preferred press 1 2 includes opposing upper and lower rams 1 8, 20 driven by respective upper and lower hydraulic cylinders 22, 24.

As was previously discussed, depending upon the type of mold 14 is used, the rams 1 8, 20 can either directly apply pressure to the mold 1 4 which will then transmit that pressure to the slurry 1 3 or directly apply pressure to the slurry 1 3.

The press 1 2 applies force to the slurry 1 3 along a pressing line PL. This force can be applied along either direction of the pressing line PL or along both directions of the pressing line PL. Also, the press 1 2 is not limited as to a particular angular orientation of the pressing line PL. The pressing line PL of the presently preferred press 1 2 is oriented parallel to the direction of travel of the upper and lower rams 1 8, 20. Although rams 1 8, 20 typically travel in a vertical direction, the upper and lower rams 18, 20 are not limited in this manner.

Many factors determine the amount of time and pressure needed remove the liquid from the slurry 1 3 so as to produce a cake 1 3. As previously discussed, one factor in determining the amount of time and pressure is the size of the particles in the slurry 1 3. Another factor in determining the amount of time and pressure is the thickness to which to the cake 1 3 is to be pressed.

Thicker cakes 1 3 require more time and/or pressure to remove the fluid. Still another factor in determining the amount of time and pressure to produce a cake is the initial amount of fluid within the slurry 1 3. A slurry 1 3 having a higher initial fluid to particle concentration will require a greater amount of time/and or pressure to remove the fluid from the slurry 1 3.

The magnetic-field generator 1 6 produces a magnetic field having field lines that are substantially perpendicular to the pressing lines PL. Devices 1 6 capable of producing such a magnetic field are well known, and any are acceptable for use in this invention. The magnetic-field generator 1 6 also preferably generates magnetic field lines that are straight and parallel when passing through the cake. The presently preferred magnetic-field generator 1 6 is an electromagnet, and the preferred electromagnet 1 6 has two opposing poles 26, 28 that bound the mold 14 and that are substantially perpendicular to the pressing lines PL.

When the presently preferred electromagnet 1 6 is operated, the electromagnet generates a magnetic field that extends from the first pole 26 to the second pole 28 and passes through the slurry/cake 1 3. The magnetic field lines are substantially perpendicular to the pressing lines PL and are straight and parallel when passing through the slurry/cake 1 3. As the particles are susceptible to being oriented in a magnetic field, the magnetic field functions to orient the particles in the slurry 1 3 in the direction of the magnetic field lines. Also, because the magnetic field lines are oriented substantially perpendicular to the pressing lines PL, the particles themselves will also be magnetically oriented perpendicular to the pressing lines PL.

The magnetic length of the cake 1 3 is defined as the length of the cake 1 3 along the magnetic orientation. As illustrated in FIG. 2, the magnetic length of the cake 1 3 will be the distance from a first inside surface 30 to a second inside surface 32. Thus, a cake 1 3 can be produced that has a greater magnetic length than the magnetic length of a cake produced by an apparatus in which the magnetic field is oriented parallel to a pressing line. Besides providing a cake having a longer magnetic length, the magnet- production apparatus 1 0 also provides a cake 1 3 for producing magnetic material having improved material and magnetic properties. An explanation for the improved properties can be illustrated by considering each magnetic particle within the slurry 1 3 as a column 36 having north N and south S poles.

Application of a magnetic field to the slurry aligns these columns 36 in the direction of the magnetic field. However, whether the columns 36 remain aligned depend upon the direction PL of the pressing force relative to the orientation of the columns 36. A column 36 can only bear a certain amount of load before the column 36 buckles. The amount of load at which the column 36 buckles is also known as the critical load. The length of the column 36 is defined as the length of the column in the direction of the load being applied to the column 36. One factor in determining the amount of critical load for a particular column 36 is the material of the column 36. Also, the critical load is inversely proportional to the squared length of the column 36 and directly proportional to the moment of inertia of a cross section transverse to the length. As such, a short and squat column 36 is much more stable than a long thin column 36 of the same material. When a column 36 is instable, the slightest misalignment or disturbance will cause the column 36 to buckle.

When force is applied to the columns/particles in the slurry 1 3 that is perpendicular to the magnetic field lines, these columns/particles 36 have greater stability than if the force was applied parallel to the magnetic field lines. When force is being applied perpendicular to the magnetic field lines, the columns/particles 36 can be analogized as short and squat, as illustrated in FIG. 3A. When the force is being applied parallel to the magnetic field lines, the column/particles 36 can be analogized as long and thin, as illustrated in FIG. 3B. As such, the columns/particles 36 are more stable when force is applied in a direction PL perpendicular to the magnetic field lines.

Particles that are more stable are less likely to become misaligned. Misalignment of the particles results in lower magnetic properties for the magnet because all the poles of the particles do not point in the same direction. Also, the particles have lesser strength when they are misaligned. Thus, by pressing the slurry 1 3 in a direction PL perpendicular to the magnetic field, the particles within the slurry are better aligned and the magnetic material produced from the cake 1 3 has better material and magnetic properties.

The preferred process of using the magnetic-production apparatus 10 is as follows. A slurry 1 3 of particles is introduced into the mold 14. The slurry

1 3 will then be subjected to pressure from a press 1 2 along pressing lines PL and concurrently subjected to a magnetic field passing through the slurry 1 3 with the magnetic field being oriented substantially perpendicular to the direction of the pressing lines PL. The magnetic field is produced by a magnetic-field generator 1 6, and the magnetic filed acts to orient the particles in the direction of the magnetic field. The pressing of slurry 1 3 will continue until sufficient fluid is removed from the slurry 1 3 so as to produce a cake 1 3, and the subjecting of the slurry 1 3 to the magnetic field will continue until the particles have been aligned and will not become non-aligned. Once the cake 1 3 is removed from the mold 1 4, the cake 1 3 can then be sintered to remove additional fluid from the cake 1 3 to produce the magnetic material.

A rotor for use in a high speed controlled-pole electric machine can further be produced using the above identified process. FIG. 4 illustrates a rotor 38 employed in a controlled-pole electric machine 39. The controlled-pole electric machine 39 includes a stator core 50, a winding 52 and a rotor 38. The rotor 38 preferably has a rotor shaft 54, a laminated steel rotor core 44 surrounding the rotor shaft 54, and a layer 46 of remagnetizable magnetic material 40 enveloping the laminated steel rotor core 44. The magnetic length of the magnetic material 40 is preferably oriented radially with respect to the rotor core 44 for improved magnetic properties. In addition, two rotor end rings 56 preferably are disposed at each end of the laminated steel rotor core 44.

As illustrated in FIG. 5, the laminated steel rotor core 44 can include a steel rotor core 58 having rods 60 inserted therethrough. Each rod 60 is preferably welded into each end ring 56 and holds the rotor core 58 intact. Encasing the laminated steel rotor core 44, a layer 46 of remagnetizable magnetic material 40 can be positioned on the exterior surface 42 of the laminated steel rotor core 44. FIG. 6 illustrates an enlarged view of the layer 46 of remagnetizable magnetic material 40 as illustrated in FIG. 5. A layer of magnetic high strength wire 48 is preferably wrapped about the layer 46 of remagnetizable magnetic material 40. The layer of magnetic high strength wire 48 is preferably attached at each end to groove 62 etched in rotor end ring 56, and preferably binds the layer 46 of remagnetizable magnetic material 40 to the laminated steel rotor core 44.

FIG. 7 illustrates a cross-sectional view of the rotor 38. Individual magnetic blocks 72 of the remagnetizable magnetic material 40 form a layer 46 that is disposed on the exterior surface 42 of the rotor core 44. Each magnetic block 72 is positioned on the rotor core 44 so that the magnetic length ML of each magnetic block 72 is oriented radially with respect to the rotor core 44. The direction of the magnetic length ML for each magnetic block 72 corresponds to the direction of the magnetic field through the cake 1 3 during the pressing of the cake 1 3. The cake 1 3 thereafter being sintered to form the magnetic block 72.

The separation gap 70 between each block 72 of magnetic material 40 widens from the narrowest gap 74 near the base of each magnetic block 72 to the broadest gap 76 near the surface of each magnetic block 72. The amount of the separation 70 is preferably calculated to allow room for the independent thermal expansion of each magnetic block 72 relative to the laminated steel rotor core 44 and neighboring magnetic blocks 72 at the narrowest portion of the separation 70. Typically, the expansion of a single magnetic block 72 is less than .002 inches per inch of magnetic block 72 length. Thus, each magnet block 72 should require less than .002 inches of space between each of its sides and adjacent magnetic blocks 72.

Claims

What is claimed is: 1 . A method for producing magnetic material from a slurry containing magnetic particles and a fluid, comprising the steps of: pressing the slurry along a pressing line to produce a cake by removing the liquid from the slurry; and, orienting a magnetic field through the slurry substantially perpendicular to said pressing line to align the magnetic particles with said magnetic field, said orienting concurrent with said pressing step.

2. The method for producing magnetic material according to claim 1 , further comprising the step of sintering the cake.

3. The method for producing magnetic material according to claim 1 , wherein the magnetic particles are sized less than one micron.

4. The method for producing magnetic material according to claim 1 , wherein said pressing step is initiated before said orienting step.

5. The method for producing magnetic material according to claim 1 , wherein said orienting step is initiated before said pressing step.

6. An apparatus for producing magnetic material from a slurry containing magnetic particles and a fluid, comprising: a mold for containing the slurry; a press for pressing the slurry along a pressing line; and, a magnetic-field generator for producing a magnetic field passing through the slurry oriented substantially perpendicular to said pressing line.

7. The apparatus for producing magnetic material according to claim 6, wherein said magnetic-field generator is an electromagnet.

8. The apparatus for producing magnetic material according to claim 6, wherein said electromagnet includes two opposing poles bounding said mold.

9. The apparatus for producing magnetic material according to claim 6, wherein said press includes a pair of opposing rams, said rams are aligned along said pressing line and apply pressure to the slurry.

1 0. A method for producing rotor for use in a high speed controlled- pole electric machine from a rotor core and a slurry containing magnetic particles and a fluid, comprising the steps of: pressing the slurry along a pressing line to produce a cake by removing the liquid from the slurry; orienting a magnetic field in a field direction through the slurry substantially perpendicular to said pressing line to align the magnetic particles with said magnetic field to produce a cake having a magnetic length in said field direction, said orienting concurrent with said pressing step; sintering the cake to produce magnetic material; and, forming the rotor by positioning a plurality of the magnetic material about the exterior surface of the rotor core with the magnetic length being oriented radially from the rotor core, the plurality of magnetic material forming a first layer of remagnetizable magnetic material.

1 1 . A method for producing a rotor according to claim 10, further comprising the step of binding the first layer to the exterior surface using high strength wire.