US 2738449 A
Description (OCR text may contain errors)
March 13, 1956 MASON ELECTROMAGNET CONSTRUCTION Filed Dec. 12, 1951 as H NVEMTOW Qj meJ Kfl.
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United States Patent ELECTROMAGNET CONSTRUCTION James A. Mason, Worcester, Mass., assignor to Warner Electric Brake & Clutch Company, South Beloit, llL, a corporation of Illinois Application December 12, 1951, Serial No. 261,234)
6 Claims. (Cl. 317-171) This invention relates to elec-tromagnets of the type in which the active magnetic force is produced by a permanent magnet but is rendered ineffectual by energization of an associated winding.
One object is to provide an electromagnet of the above character in which the effect of the permanent flux is dissipated by energization of the winding but without interrupting the threading of flux through the permanent magnet.
Another object is to provide such an electromagnet in which the permanent magnet and the winding are arranged in a novel manner within a hollow rigid ring which forms the magnet core.
A more detailed object is to create the permanent flux by a magnet spanning spaced legs of a magnetic ring of U-shaped cross section and constituting parts of two toroidal flux paths of different magnetic reluctances both extending through the permanent magnet.
Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which Figure 1 is a diametrical cross-sectional view of one form of the improved magnet and its associated armature.
Fig. 2 is a fragmentary face view of the magnet with parts broken away to show the internal construction of -1 the magnet.
Figs. 3, 4 and 5 are enlarged fragmentary views illustrating the direction of the flux in the magnet under different conditions.
While the improved electromagnet, indicated generally 4 by 10, is adapted for a wide variety of different uses, it is, for purposes of illustrating the invention, shown in the drawings in a form especially adapted for coaction .with anarmature 11 to produce a retarding or driving torque. In general, the working faces of the magnet and netic potential across the working faces of the magnet.
The armature 11 may as shown comprise a generally flat ring of magnetic material rotatable with a shaft 14 and mounted thereon for axial floating movement so as to remain at all times in close proximity to the working face of the magnet. This may be accomplished by splining the inner peripheral edge of the ring onto a collar 15 fixed to the shaft.
In the form shown, the magnet 10 has two laterally spaced pole faces 16 spanned by the armature ring and disposed flush with each other and also with the outer surface of wear resistant material 17. The faces 16 are at the outer ends of pole pieces 18 and 19 in the form of concentric generally cylindrical rings of magnetic iron joined by an end ring 20 to form a composite and rigid core 21 of U-shaped cross-section. Preferably, though not necessarily, the inner pole piece is formed as a separate part being fastened rigidly in abutment with the inner 2,738,449 Patented Mar. 13, 1956 edge of the end ring 20 by a thin flange 22 clenched around an under cut surface 23 on the ring 19. To impart a high reluctance to this part of the U, a thin layer 24 of nonmagnetic material, usually about .008 of an inch thick, is interposed between the opposed surfaces of the rings 19 and 20. If desired, the wear resistant material 17 may take the form of segments seated against shoulders 25 around the internal surfaces of the pole rings 18 and 19.
In accordance with the present invention, the permanent magnet 12 is associated magnetically with the core 21 and spaced from the pole faces 16 to provide a low reluctance flux path of toroidal shape through the pole faces, the armature and the permanent magnct and also a parallel toroidal flux path of substantially higher magnetic reluctance including the permanent magnet and that part of the core not included in the low reluctance path. Preferably, this is accomplished by disposing the permanent magnet within the hollow core or shell 21 and span ning the pole pieces 18 and 19 in a plane between the end faces 16 and the end ring 20. In the form shown, the permanent magnet 12 comprises a series of segmental bars closely spaced circumferentially around the interior of the core 21 each with its inner and outer arcuate end surfaces machined to fit in close abutment with opposed surfaces 26 machined on the pole pieces 18 and 19. The segments may be seated against shoulders 27 to locate them in the desired axial position.
The magnet pieces 12 are composed of permanent material such as an aluminum-nickel-cobalt alloy. After assembly in the core 21 as described above, the segments are magnetized in any preferred manner as by interposing each section of the core ring in a strong magnetic field threading radially through the segments.
After such magnetization, the permanent magnet flux will thread the high and low reluctance paths above referred to and be distributed between the two paths in a proportion inverse to the reluctance values of the two paths. Thus, as illustrated in Fig. 3, the major part of the permanent magnetic flux will thread counter-clockwise radially through each magnet segment 12, outwardly along the outer pole piece 18 and into the armature 11, inwardly through the latter and finally through the inner pole face and piece 19 to the inner end of the magnet segment. At the same time, a minor part of the flux threads the high reluctance path passing outwardly through the magnet segment, then clockwise along the outer pole piece, inwardly through the end ring 21) and finally reversely through the inner pole piece 19 and the magnetic restriction 24 to the inner end of the magnet segment. Usually, it is desirable to proportion the reluctances of the two toroidal flux circuits so that about /2, of the total flux threads the low reluctance or working path. The magnets 12 are sized to produce a flux density in the working circuit corresponding to the frictional torque desired to be developed when the magnet and armature are drawn into gripping engagement.
The winding 13 is also disposed within the hollow magnet core 21 and comprises a multiplicity of turns each extending around the circumference of the core. Herein, the winding is disposed between the pole faces 16 and the permanent magnet segments 12 and is thus within the toroid formed by the low reluctance path above referred to. The direction of winding of the coil 13 and the flow of current therethrough are such that the magnetic flux that is produced by the coil tends to thread the pole faces 16 and the armature 11 in a direction opposite to that of the permanent magnet flux above described. Thus, if the permanent magnets 12 were omitted from the core as shown Fig. 4, the flux resulting from energization of the coil would thread the toroidal path through the core and armature as indicated by the arrows. It will be observed that the direction of this flux is radially outwardly through the armature 11 whereas the permanent magnet flux threads the armature inwardly as shown in Fig. 3.
The coil flux thus opposes the flow of the permanent magnet flux in the low reluctance flux circuit and reduces the magnetic potential drop across the opposite poles of the permanent magnet. The invention contemplates a design of the coil 33 such that when the coil is energized at a predetermined voltage, the magnetic potential across the ends 16 of the pole pieces 18 will be reduced to zero, thus cancelling the effect of the permanent magnet flux in attracting the armature to the magnet. This condition obtains when the coil is constructed to produce a total flux in the available path sushtantially equal to the total flux developed by the permanent magnets. Then, the magnet will be disabled whenever the winding 13 is en ergized to a predetermined degree but will be reactivated when the winding is again deenergized.
Such disabling of the magnet is not, however, accompanied by complete neutralization of the permanent mag net'fiux which would be conducive to demagnetization of the segments 12. This is due to the presence of the parallel and high reluctance flux path above described which extends through the permanent magnets and around the closed end of the U-shaped core section. Permanent magnet flux thus continues to thread this path but the density therein is doubled as illustrated in Fig. owing to the fact that reluctance of the working circuit has in effect been reduced to zero. As a result of this increased diversion of the permanent magnet fiux, the tendency toward demagnetization of the permanent magnets 12 is reduced appreciably.
In addition to maintaining the high degree of magnetization of the permanent magnets, the arrangement above described is highly advantageous for several reasons. Both the winding 13 and the permanent magnets 12 are disposed within the hollow shell 21 so that the core itself is utilized to support the permanent magnets which may be made of minimum size for a magnet of given capacity. The overall size of the magnet and its cost of construction are thus reduced to a minimum. At the same time, the core may be of the same construtcion as that used in ordinary electromagnetic friction brakes and clutches. in other words, a conventional friction brake or clutch magnet may be converted into the improved permanent magnet simply by adding the magnet segments 32 and properly designing the winding 13. The latter and the permanent magnets may be locked rigidly within the shell 21 in any suitable way as by a hardened nonmagnetic material 29 filling the spaces within the shell and forming a backing for the wear plates 17.
The magnet described above is particularly adapted for use in a so-called dead man brake. For such use, the magnet is mounted stationarily as through abracket 31) secured to the end ring Zl and supporting tne core 21 in axial alinement with the armature ring if. in such a brake, the armature would normally be attracted against the magnet face thus creating a friction force arresting or holding the shaft 14 against turning. The brake is released by energizing the winding 13 and remains released until-the current flow in the winding is again interrupted.
I claim as my invention:
1. A magnet comprising a ring of magnetic material of U-shaped cross section having parallel pole pieces termihating at annular end faces adapted to be spanned by an armature, a plurality of permanent magnets comprising permanent magnets and around the closed end of the U section, the other path extending through said permanent magnets and the armature spanning said pole faces, a
magnetic restriction interposed in said first mentioned magnetic circuit to direct the major portion of the magnetic flux around said second path, and a multiple turn annular Winding disposed within said ring between said pole pieces and on one side of said permanent magnets and adapted when energized to produce in said second fiux circuit magnetic flux opposing the permanent magnetic flux and of a magnitude such as to reduce the magnetic potential across said end faces whereby to divert a larger part of the permanent magnetic flux through said higher reluctance circuit.
2. A magnet comprising a hollow ring of magnetic maerial of U-shaped cross section having annular pole pieces laterally spaced apart and terminating at annular pole faces adapted to cooperate with a magnetic armature, a plurality of an ularly spaced permanent magnets rigidly spanning and abutting at opposite ends against opposed surfaces of said ring at points spaced along said pole pieces away from said pole faces, said permanent magnets forming a common part of two magnetic flux circuits of toroidal shape one extending through the outer end portions of said pole pieces, said faces, and the armature, said other flux circuit extending around the closed end of the U-shaped ring section and having a magnetic reluctance substantially greater than said first circuit, and an annular multiple turn winding disposed between said pole pieces and adapted when energized to force through said pole faces and armature magnetic flux opposing and substantially cancelling the permanent magnetic flux therethrough whereby to divert a substantially larger part of the latter flux through said high reluctance path.
3. The combination of, a rigid shell of magnetic material and U-shaped radial cross section with the legs of the U laterally spaced apart and terminating in aunular faces at the ends of the-legs, an armature spanning said faces and adapted for axial engagement therewith, permanent magnetic means disposed within said shell and spanning said legs to cause magnetic flux to thread toroidal paths of relatively low and high reluctance, the high reluctance path extending from the poles of said magnetic means around the closed end of the U, and the low reluctance path extending through the end portions of said legs and through said armature between said faces, and an annular multiple turn magnetic we'll disposed within said ring between said legs and adapted when energized to cancel the permanent magnet flux threading through said faces while diverting part of such flux through said high reluctance path.
4. The combination of, a rigid shell of magnetic material and U-shaped radial cross section with the legs of the U laterally spaced apart and terminating in annular pole faces at the ends of the legs, an armature spanning said legs and adapted for axial gripping engagement therewith, a closed magnetic connection including permanent magnetic means spanning said legs at points spaced inwardly from said pole faces to cause magnetic flux to thread toroidal paths of relatively low and high reluctance, the high reluctance path extending from the poles of said magnetic means around the closed end of the U, and the low reluctance path extending through the end portions of said legs and through said armature between said pole faces, and an annular multiple turn magnetic coil disposed within said ring between said and adapted when energized to reduce the magnetic potential across said pole faces.
5. A magnet comprising a hollow rigid ring of magnetic material of U-shaped radial cross section having laterally spaced pole pieces terminating in annular faces at the ends of the legs of the U section, a closed ma netic connection between said legs intermediate their ends and away from said faces, said connection including a pen manent magnet encircling said ring within the latter and having end poles of opposite polarity at spaced points along said connection, :means providing a magnetic re- .striction in the toroidal flux circuit through :said connection and the closed end of said ring, and a multiple turn winding disposed within said ring and adapted when energized to produce a flux opposing the permanent magnet fiux through said faces.
6. The combination of, a hollow annular shell of magnetic material and generally U-shaped cross section having legs terminating in pole faces, an armature ring spanning said faces, a permanent magnet having end poles of opposite polarity, said shell, said armature, and said permanent magnet coacting to form two toroidal flux paths arranged in parallel and each extending through said permanent magnet and the poles thereof, one of said paths being of relatively low reluctance and threading back and forth through said shell faces and the opposed surface of said armature, said other path by-passing said 15 shell faces and including a magnetic restriction normally operable to divert the major portion of the permanent magnet flux around said first path whereby to cause magnetic attraction of said shell and armature, and an annular winding disposed within said shell and adapted when energized to create a magnetic potential opposing the flow of permanent magnet flux through said shell faces whereby to reduce the attraction between said magnet and armature substantially to zero.
References Cited in the file of this patent UNITED STATES PATENTS 2,188,803 Boehne Jan. 30, 1940 2,544,360 Schmidt Mar. 6, 1951 2,614,668 Waderlow Oct. 21, 1952 FOREIGN PATENTS 112,195 Germany Aug. 9, 1900 OTHER REFERENCES National Bureau of Standards: Technical News Bulletin, December 1950, pages 169 to 174.