US3553618A - Phase splitting core for electromagnetic devices - Google Patents

Abstract

A MAGNETIC CORE CONSTRUCTION IN WHICH THE PHASE RETARDANT EFFECTS OF EDDY-CURRENTS AND HYSTERSIS IN A FLUX CARRYING CORE PORTION ARE EMPLOYED TO GENERATE AN ALTERNATING FLUX, LAGGING IN PHASE BEHIND THE FLUX WAVE FLOWING IN A NON-RETARDANT CORE PORTION.

Description

G. L.. LANG Jan. 5, 197B A PHASE SPLITTING CORE FOR ELECTROMAGNETIC DEVICES Filed Dec. 1i. 196s 2 Sheets-Sheet l PRIOR ART FQQ INVENTOIL GREGOR L. LANG Imst; SFM'ITING com Fon ELECTROMAGNETIC DEVICES 196B G. L. LANG Jan. 5, E71

2 Sheets-Sheet Filed Dec, 1 l.

Fig. 4

lNvrfNTmR. GREGOR L. LANG United States Patent O 3,553,618 PHASE SPLITTING CORE FOR ELECTRO- MAGNETIC DEVICES Gregor L. Lang, 295 East St., Suffield, Conn. 06078 Filed Dec. 11, 1968, Ser. No. 783,035 Int. Cl. Hlllf 7/13 US. Cl. 335-244 17 Claims ABSTRACT OF THE DISCLOSURE A magnetic core construction in which the phase retardant effects of eddy-currents and hysteresis in a flux carrying core portion are employed to generate an alternating flux, lagging in phase behind the flux wave flowing in a non-retardant core portion.

BACKGROUND This invention relates to alternating current electromagnetic devices of the type used to convert electrical power to mechanical motion or work. While useful in rotating devices such as motors of the split or retarded phase type, it is particularly applicable to the class of devices known as tractive electromagnets or solenoids, and is shown and described herein by way of example as applied thereto. Typical applications of such devices include relays, power contactors, valves, clutches, counters and linear and rotary actuators for many uses.

Long standing prior art design methods applied to such devices have in general followed transformer practice wherein a box-core magnetic shell is built up of thin laminations of low hysteresis magnetic iron, a portion of the core being surrounded by the magnetizing or exciting coil. By the above and other means, efforts are made to attain a high electrical efficiency through avoidance of the power loss and heating effects which otherwise result from the effects of hysteresis and circulating or eddy-currents in the flux carrying members of the magnetic core.

A further requirement in A.C. operated devices of this type, is the provision of shading or phase-splitting means for overcoming the buzz and weak pulling forces which otherwise result from the rise, fall, and reversals of the exciting alternating current wave. Such prior art shading means customarily take the form of a substantial copper short-circuited turn, or shading ring frequently of a D form, placed in a transverse slot in the magnetic pole-face in a manner such as to encircle somewhat more than half of the flux carrying area of the pole face abutting the attracted member or armature. Operating by transformer action, the shading ring establishes a heavy circulating current having an associated magnetic field which opposes and retards the flux changes due to the varying current in the exciting coil, thereby effecting phase retardation of the flux wave flowing in that portion of the pole-face enclosed by the shading ring.

The prior art shading ring does enable the 'attainment of a moderately effective sealing or holding pull value. It is however a relatively ineffective method of obtaining phase-splitting which adds significantly to the cost and bulk of the device. A further detriment stems from the fact that a portion of the otherwise available pole face area must be removed to provide the slot for insertion of the D ring. The coil and core structure must therefore be enlaged to offset the resulting loss of flux carrying area. A further loss in total flux stems from the fact that the magnetic flux due to the circulating current, is in opposition to that developed by the exciting coil. This results in a reduction in the flux density flowing in the portion of the pole face which is enclosed by the shading ring. This flux loss can be overcome only by increasing the value of ice input power or ampere-turns, thus adding to the size, cost, and power requirement.

THE PRESENT INVENTION The present invention makes use of the discovered fact that hysteresis and circulating currents in flux carrying members of the device, previously shunned in the prior art as detrimental, may be effectively used both singly and in combination, as `a low cost and highly efficient means of obtaining phase retardation in A.C. operated electromagnetic devices to thereby avoid the use of pole shading rings and the like.

It is accordingly an important object of this invention to provide a magnetic core construction in which the need for shading rings to secure phase shifting is obviated.

Another object is to provide an A.C. magnetic device in which novel, simple, and low cost means are employed to effect phase shifting of greater effectiveness than that obtainable by the use of the prior art shading rings.

Another object is to provide a magnetomotive device in which an innovation in magnetic actuation is employed whereby the value of mechanical work per unit of electrical power is greatly increased over that attainable by previous methods, and whereby the size, weight, and volume of the device per unit of mechanical work performed, are significantly reduced over the prior art values.

A further object is the provision of a magnetomotive device of improved electrical efficiency such that reduced watt-loss and temperature rise prevails over that attainable with prior art devices performing comparable mechanical work.

Another object is the provision of `a magnetomotve device of simplified nature, producible with relative economy in size, materials and number of parts, and incorporating in greater part commonly available and lowcost materials.

Another object is to provide for an electromagnetic device, a composite core including plural discrete portions having dissimilar electromagnetic properties, such that magnetic phase-shift occurs between flux waves flowing in the differing core portions.

A further object is to provide for an electromagnetic device, a method yielding high efficiency in operation, comprising a reduced number of parts, and wherefor such parts are economically producible by high volume production methods.

An important object is to provide for `an A.C. magnetomotive device, a magnetic core possessing inherent phaseshifting properties whereby said core is self-shading, and wherewith no external shading ring or other attachment is required, whereby said core may be installed in a restricted space comparable to that occupied by its D.C. operated counterpart.

Another object is to provide for a magnetomotive device, a phase shifting core of greater effectiveness than is obtained with shading rings, whereby increased work is obtained, thereby enabling reduced input power, and reduced cross-section of said core, with resultant reduction in the amount and weight of copper winding required for a given value of pull or work performed.

Another object is to provide in a magnetomotive device, a core assembly wherein major magnetic portions are constructed of commonly available materials such as coldrolled and hot-rolled mild steels, and wherein the magnetic property of hysteresis common to such materials is beneficially utilized to obtain magnetic phase retardation, whereby said core assembly is made effectively selfshading, with resultant benefits in cost, and in the values of tractive and sealing pull forces obtainable thereby.

Another object is to provide for an A.C. electromagnet, a self-shading core assembly readily manufacturable in a cylindrical form conveniently usable with a plunger type armature, in a pot type solenoid commonly used in D.C. electromagnet service, whereby the high magnetic efficiency and pull values comonly obtained in D.C. service by the use of said pot type solenoids, becomes essentially attainable in A.C. service.

A further object is to provide a simple and inexpensive means of constructing a polyphase magnetic core assembly, of relatively high electromagnetic efficiency, wherein multiple magnetic phases are generated by said core assembly, in response to single phase electromagnetic excitation.

The foregoing and other objects and advantages of the invention will become apparent from the following description and the accompanying drawings describing various embodiments thereof.

Generally speaking the present invention operates on the basis of the observed facts that a piece of hysteretic material subjected to an A.C. magnetizing force, will respond with a resultant flux, lagging or retarded in phase behind that of the magnetizing current by an amount varying with the degree of hysteretic remanence which is characteristic of that material; and further, if the above magnetic material be of suitable cross-sectional area, it will be subject to the flow of internal circulating currents which operate to oppose change in the flux value flowing therein, thus adding to the hysteretic phase retardation of the resultant flux wave. The flux phase lag is thus a composite effect.

It can be noted that the total phase retardation gained by the above means is the vector sum of that due to eddycurrent flow, and that due to hyseresis effects. It follows that the value of phase lag may be chosen or varied by choice of sectional dimensions of the retardant core member, and by the selection of a material which yields a desired value of hysteresis. These properties may be varied singly or in combination, to obtain an optimum value of phase lag angle varying with the application.

In applying the present invention, the above described phase retardant corepiece is used in company with a companion corepiece proportioned and designed to be relatively less susceptible to the phase retardant effects of hysteresis and eddy-current flow. The two or more pieces are arranged in a parallel, slightly spaced relationship, all being simultaneously magnetized by the single-phase current in the common exciting coil.

It will thus be seen that the phase or time relationship of the flux waves flowing in the various corepieces will differ according to the phase retardant properties of the individual flux path. The early or leading phase will flow in that corepiece having the least total retardant effect of hysteresis and eddy-currents. 'Ihe following phases will flow in time sequence in those corepieces having progressively greater phase retardant effectiveness. The small gap or spacing between corepieces above mentioned, serves to render the various flux paths substantially independent, by reducing interaction or magnetic shunting effects between phases.

By the foregoing means a simple and economical phase transforming core is enabled of new and novel nature, and of such effectiveness as to satisfy all of the objects and advantages recited. It is in no sense limited to twophase operation, since three or more corepieces of differing phase retardant effectiveness may be used, thereby enabling the construction of a polyphase core. Further benefits in efficiency may be secured by the use of hysteretic core material in adjacent portions of the magnetic circuit, other than the parallel corepieces above described.

Referring to the drawings:

FIG. 1 illustrates a conventional box-core electromagnet having a pivoted clapper type armature, partially sectioned to show the use and placement of the prior art shading ring.

FIG. 2 is a sectional view of a plunger type solenoid embodying a preferred coaxial form of the present invention wherein the cylindrical phase splitting core is surrounded by the coil.

FIG. 3 is an enlarged sectional view of the central core of FIG. 2, showing details of the annular and peripheral gaps which contribute to the functioning of the present invention.

FIG. 4 depicts a box core electromagnet generally similar to FIG. 1, but employing a phase retardant core portion according to the present invention, in lieu of the prior art shading ring.

FIG. 5 is a plan view of a common type of bi-polar split-phase A.C. motor of the squirrel-cage rotor type, wherein c'ore sections according to the present invention are utilized to secure phase splitting, in lieu of the conventional or prior art shading rings. The resultant direction of rotation is indicated.

It will be understood that many changes may be made in details of construction and arrangement of parts without departing from the spirit of the invention as expressed in the appended claims. It will also be understood that the applications shown as applied to tractive electromagnets and induction motors are by way of illustration only, and that the phase transformation function of the present invention is also beneficially applicable to other magnetic devices such as polyphase motors, phase transformers, rotary actuators, and synchro rotary positioning devices. I therefore do not wish to be limited to the exact details of construction, or arrangement of parts as shown and described.

Referring more particularly to the drawings wherein similar reference characters designate corresponding parts throughout the several views, FIG. 1 depicts a conventional prior art solenoid with clapper armature embodying a laminated box-core structure l, built-up of U-shaped magnetic laminations, with a pivotally mounted armature 2, and a hinge pin therefor 3, passing loosely through clamp plates 4, which with rivets 5, serve to hold the lamination stack 1 in assembled condition.

Hinged armature 2, is also built up of a stack of laminations similarly held in assembled condtiion by clamp plates 6, and rivets 7. Exciting coil 8, is commonly installed on the leg of core stack 1, facing the free end of armature 2, being held in place by shading ring 9, which is pressed into transverse slot 10, thus separating the magnetic pole face into unequal areas 11, and 12. The magnetic flux lines in core stack 1, induced by current flowing in coil 8, link with armature 2, and fiowing across air gap 12A, through poleareas 11 and 12, cause the free end of armature 2, to be attracted theretoward. The flux waves flowing in Aareas 11 and 12, are caused to be out-of-phase, by the flux retardant effect of the circulating current flowing in shading ring 9 which, encircling pole area 12, causes the flux wave therein to lag in time behind the leading-phase wave flowing in pole area 11. Armature 2 is therefore acted upon by a two-phase flux wave.

The foregoing or an equivalent polyphase magnetic force is requisite in any A.C. tractive electromagnet, if the buzz, and intermittent tractive forces characteristic of a single-phase field are to be overcome. The present invention uses a broadly differing means of obtaining phase retardation. The use of a shading ring is avoided by the phase retardant effects of eddy-current flow, and hysteresis, which properties are intentionally incorporated in flux carrying portions of the core structure.

FIGS. 2 and 3 are sectional views of a preferred coaxial form of the present invention in which a two-piece annular central core 13 and 14 is surrounded by exciting coil 8, and in which cylindrical armature 2, is arranged for vertically slidable motion effected by magnetic attraction exerted between the lower face of armature 2, and the upper pole face formed by core members 13 and 14. The magnetic loop or circuit is completed by the magnetic shell 15, to which inner core assembly 13, and 14, is attached at the bottom 16, by spin-over or other means` Magnetic shell, has in its upper surface a centrally located bore or hole coaxial with core assembly 13-14, so dimensioned as to position cylindrical armature 2, for vertically slidable motion therethrough. A closed magnetic circuit is thus provided by outer shell 15, linking the lower end of core 13-14, with the upper peripheral area of armature 2, external to exciting coil 8.

Spring biasing and stop means not shown, provide for upward motion of armature 2 to a pre-set stop position, leaving Ia working gap between the lower face of armature 2, and the juxtaposed upper face of core assembly 13-14, when coil 8 is deenergized. Upon passage of alternating current through coil 8, armature 2 is moved downward into contact with core assembly 13-14 by the two phase magnetic flux generated therein, being thereafter held in firm contact thereagainst by said flux. During the downward travel of armature 2, it overcomes the above mentioned spring bias, and performs such external mechanical work as may be assigned to it.

FIG. 3 is an enlarged sectional view of the two-phase core assembly of FIG. 2 showing details of the gap means which assist in securing optimum relative phase displacement by the core assembly. The outer cylindrical core member 13, is the leading-phase flux path, being designed to introduce a minimum of phase retardation of the flux flowing therein. To that end, it is constructed of low hysteresis material such as annealed magnetic iron, or magnetic grade silicon steel. Phase retardation by outer sleeve 13, is further inhibited by longitudinal slot 17, which extends the full length of sleeve 13, thus interrupting the circumferential circulating current which would otherwise flow therein. The flux flowing longitudinally in sleeve 13, is thus substantially in-phase with the alternating magnetomotive force established by the current flowing in exciting coil 8.

Inner core member 14 is the lagging-phase or retardant flux path, being designed to secure a substantial value of phase retardation, by the combined effects of hysteresis and internal circulating currents, or by either effect taken alone. As is known in the field of ferrous metallurgy, the property of magnetic hysteresis or remanence is a broadly variable function of iron and steel composition, varying with the content of carbon and alloying metals such as manganese and chrominum, and with the degree of hardness, whether obtained by heat treatment or by coldworking of the metal. It is thus apparent that a broad choice of steels Varying in hysteresis value is available for use in retardant core member 14, with the final value of remanence being further variable by heat treatment, as for instance by full or partial annealing. One material proven effective in this application is the commonly available cold-rolled steel identified as AISI C-1ll7, which has been used with good results. This steel contains 0.17% carbon, and 1.1% manganese. Many other steels and alloys are equally applicable.

The phase retardant effectiveness of inner member 14, is further enhanced by its design as an unbroken cylindrical part which is subject to the flow of circular induced eddy currents throughout its length. As described elsewhere, the effect of such eddy currents is to oppose changes in the flux value flowing therein, thus retarding the phase of the resultant magnetic flux wave, in addition to the hysteresis retardation described above.

The magnetic effectiveness of the present invention may be greatly enhanced by providing a gap or magnetic separation between the out-of-phase flux conducting members 13 and 14. Said gap serves to reduce inter-phase magnetic shunting which would otherwise bypass a significant portion of the fluxes, within the core assembly. An annular gap is indicated in FIG. 3, produced by forming member 14 with an annular shoulder or step at 18, whereby the upper portion of core 14 is slightly smaller in diameter than the inside diameter of sleeve 13. The annular space thus formed may be filled with nonmagnetic material such as plastic to secure adhesion, and maintain concentricity between the two members.

FIG. 2 discloses an additional design element serving to enhance the electromechanical efficiency of the present invention. Armature 2, will be seen to be significantly larger in diameter than the outside diameter of core assembly 13-14 which it faces. This disparity in dimensions enables armature 2, to capture and convert into useful mechanical pull, a significant amount of fringe or leakage flux which would otherwise pass around armature 2, directly between core assembly 13-14, and shell 15.

FIG. 4 discloses the means by which the benefits of the present invention may be obtained in a laminated box-core electromagnet, generally of the type shown in FIG. l. In the example of FIG. 4, the core area corresponding to area 12 of FIG. l, and the associated shading ring 9, are removed, a block of appropriately dimensioned hysteretic steel 19, being substituted therefor. Said hysteretic core block is cemented, or attached by other means to laminated core stack 1. Core block 19, is separated from the adjacent laminated pole area 1.1, by gap 20, which serves to separate the out-of-phase flux paths, in a manner similar to the annular gap separating members 13 and 14 in FIGS. 2 and 3. Core block 19 being phase retardant by virtue of its hysteretic composition, is also non-laminar, thus being subject to the flow of internal circulating currents, in a manner, and with results similar to those above described for the inner core member 14 of FIGS. 2 and 3.

FIG. 5 illustrates an application of phase retardant core blocks similar to the method of FIG. 4, to the laminated core stack of a common type of bi-polar squirrel-cage induction motor. Such motors are commonly produced with shading rings enclosing a portion of each of the two diametrically disposed magnetic poles which magnetize the rotor. In the motor application, the split, or lagging phase flux component is needed to establish an effective polyphase or rotating field which gives the motor a self-starting capability.

In FIG. 5, the laminated core stack 1, is of a generally U form, the lower horizontal portion of which is is enclosed by the exciting coil 8. The upward extending core legs 21, are shaped to form arcuate pole faces 22, adapted to receive the cylindrical rotating armature 23, which is carried by shaft 24, rotating in stationary shaft bearings, not shown. A small gap or clearance is provided between the outer periphery of armature 23, and the arcuate pole faces 22, to permit free rotation of said armature therebetween. In order to cause the motor to have a self-starting property, each of the arcuate pole faces 22, must be divided into two adjacent areas, with phase retarding means providing for the generation of a phase-retarded flux component by one of said areas. Phase retardant core blocks 25, and 26, are inserted in each of the opposed core legs, in a manner such as to form a pole extension, generally continguous with its associated non-retardant pole face 22, and similarly having arcuate pole face profiles coaxial with the armature. The hetardant core blocks will be seen to be separated from the adjacent non-retardant pole face areas 22, by gaps 20, to provide inter-phase separation as described above in connection with FIG. 4.

It will be noted that core blocks 2S and 26, are diametrically disposed about armature 23, and thus occupy different relative positions on the core legs 21, with core block 25 being located at the upper limit of its associated core leg, whereas block 26 is situated lower on the core leg, closer to exciting coil 8. This geometry is explained by the direction of rotation of the armature, indicated in FIG. 5, as counter-clockwise. Rotation is normally such that a given point on the periphery of the squirrel cage rotor, pases through a leading-phase pole area, moving toward the adjacent lagging-phase area, the geometry shown thus producing counter-clockwise rotation.

From the foregoing it will be seen that I have provided simple, efiicient, and economical means of attaining all of the objects and advantages of the invention in a variety of illustrative electromagnetic devices.

Having described my invention, I claim:

1. For electromagnetic devices having coil exciting means, a composite flux conducting element comprising a plurality of ferro-magnetic portions defining spaced magnetically parallel flux paths, said portions incorporating relative variations in at least one of the electromagnetic properties thereof including magnetic hysteresis and eddy-current susceptibility, said portions being magnetically separated over a substantial part of their common length by substantially non-magnetic gap means extending therebetween.

2. A composite flux conducting element as set forth in claim 1 wherein said plural ferro-magnetic portions are substantially cylindrical in form and are relatively disposed in an annular coaxial manner, said non-magnetic gap means being annular in form.

3. A composite flux conducting element as set forth in claim 1 wherein adjacent end surfaces of said plural portions are substantially co-planar, thereby jointly forming a planar polyphase magnetic pole face oriented substantially perpendicular to said flux paths, said pole face being intersected by said gap means.

4. A composite flux conducting element as set forth in claim 1 wherein at least one of said plural portions is constructed of a soft ferrous material chosen from the class known as mild mechanical steel of a composition containing at least 0.05% carbon, whereby a chosen value of hysteretc remanence is obtained in said portion.

5. A composite flux conducting element as set forth in claim 1 wherein said plural ferro-magnetic portions include an inner cylindrical portion, and an annular sleeve portion coaxially disposed about said inner cylindrical portion, said annular sleeve portion having its circumferential electrical continuity substantially interrupted for reducing the flow therein of circumferential induced eddy-currents.

6. A composite flux conducting element as set forth in claim 1 wherein at least one of said plural spaced portions is of laminar construction.

7. A magnetomotive device having a movable armature, a coil, a core magnetically linking said coil with said armature, said core including a phase-splitting flux conducting element comprising a plurality of ferro-magnetic portions defining spaced magnetically parallel flux paths, said portions incorporating relative variations in at least one of the electromagnetic properties thereof including magnetic hysteresis and eddy-current susceptibility, said portions being magnetically separated over a substantial portion of their common length by substantially nonmagnetic gap means extending therebetween.

8. A magnetomotive device as set forth in claim 7 in which said plural ferro-magnetic portions are substantially cylindrical in form and are relatively disposed in an annular coaxial manner, said non-magnetic gap means being annular in form, adjacent ends of said plural portions jointly forming a polyphase magnetic pole intersected by said gap means.

9. A magnetomotive device as set forth in claim`7 wherein adjacent contiguous surfaces of sald plural v`portions cooperatively form a polyphase magnetic pole intersected by said gap means, said pole and said armature being adapted for magnetic cooperation therebetween, said armature being of magnetically homogeneous construction.

10. A magnetomotive device as set forth in claim 7 wherein adjacent end surfaces of said plural portions are substantially co-planar thereby jointly forming a planar polyphase magnetic pole-face oriented substantially perpendicular to said flux paths, said pole-face being 8 intersected by said gap means, said armature being of magnetically homogeneous construction and arranged with a planar surface juxtaposed said pole-face.

11. A magnetomotive device as set forth in claim 7 wherein said plural ferro-magnetic portions include an inner cylindrical portion comprised of mild steel of a composition containing at least 0.05% carbon, and an annular sleeve portion comprised of a ferrous material of selected composition coaxially disposed about said inner cylindrical portion, said annular sleeve portion having its circumferential electrical continuity substantially interrupted for reducing the fiow therein of circumferential induced eddy-currents.

12. For electromagnetic devices having coil exciting means, a flux carrying core comprising a first portion formed of a magnetic material having a finite hysteresis characteristic and a second portion having a relatively lower hysteresis characteristic disposed adjacent said first portion and forming therewith a composite core, said first and second portions being spaced apart over a substantial portion of their length thereby providing a gap therebetween which is substantially non-magnetic in relation to said first and second portions for maintaining phase displacement of flux flowing in said portions.

13. A flux carrying core as set forth in claim 12 in which said first portion is a central member and in which said second portion is in the form'of a sleeve disposed coaxiallyl about said central member and spaced from the outer surface thereof over a substantial portion of the length of said core, said sleeve being formed of a material having a lower hysteresis characteristic than said central member and forming therewith generally parallel flux paths.

14. For electromagnetic devices having coil exciting means, a flux carrying core comprising a first portion of a magnetic material and configuration having a relatively high eddy-current susceptibility and a second portion of a magnetic material and configuration having a relatively lower eddy-current susceptibility for phase displacement of flux waves flowing in said portions, said second portion disposed adjacent the first portion and forming therewith a composite core, said first and second portions being spaced apart over a substantial portion of their length thereby providing a gap therebetween which is substantially non-magnetic in relation to said first and second portions for maintaining said phase displacement of flux flowing in said portions.

15. For electromagnetic devices having coil exciting means, a flux carrying core as set forth in claim 14 in which said first portion is a central member and in which said second portion is in the form of a sleeve disposed coaxially about said central member and spaced from the outer surface thereof over a substantial portion of the length of said core, said sleeve portion having a longitudinally extending discontinuity for preventing the flow therein of circumferential induced eddy-currents.

16. For electromagnetic devices having coil exciting means, a flux carrying core comprising a first portion formed of a magnetic material of a finite hysteresis characteristic and configuration to provide a relatively high eddy-current susceptibility and a second portion of relatively lower hysteresis characteristic and configuration t0 provide lower eddy-current susceptibility for phase displacement of flux flowing in said first and second portions, said portions being spaced apart over a substantial portion of their length thereby providing a gap therebetween which is substantially non-magnetic with respect to said first and second portions for maintaining said phase displacement of flux flowing in said portions.

17. For electromagnetic devices having coil exciting means, a flux carrying core as set forth in claim 16 in which said first portion is a central member having a hysteresis characteristic related to its composition, carbon 9 l0 content, and degree of hardness, and in which said second References Cited portion is a sleeve coaxially disposed about said first mem- UNITED STATES PATENTS ber and formed of a ferrous material having a lower hysteresis characteristic than said rst portion, said gap of substantially non-magnetic material being dis osed between said sleeve and said central member, snid sleeve 5 GEORGE HARRIS Pnmary Examiner having a longitudinally extending discontinuity for U S C1 XR preventing the circumferential ilow of induced eddycurrents in said sleeve. 335-249, 251

989,018 4/1911 Lindstrom 335-244

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