US3521209A

US3521209A - Electromagnetic load lifting device

Info

Publication number: US3521209A
Application number: US706963A
Authority: US
Inventors: Lothar Fritz
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-02-21
Filing date: 1968-02-20
Publication date: 1970-07-21
Anticipated expiration: 1987-07-21

Description

July 21', 1970 FRITZ 3,521,209

I ELECTROMAGNETIC LOAD LIFTING DEVICE Filed Feb. 20, 1968 2 Sheeis-Sheet 1 FIG. 1. II

PRIOR ART F/GZ PRIOR ART I, FIG. 3.

' INVENTOR July 21 1970 L. FRITZ ELECTROMAGNETIC LOAD LIFTING DEVICE 2 Sheets-Sheet 2 .Filed Feb. 20, 1968 Pu. I F w 4 m 72 I2 2 F 2 m J 2 a 6 2 FIG] INVENTOR LOTHAR FR\T Z BY .2 Q/Wmfifi United States Patent Oihce 3,521,209 Patented July 21, 1970 3,521,209 ELECTROMAGNETIC LOAD LIFTING DEVICE Lothar Fritz, 3 Waldstn, 5159 Kerpen, Bezirk, Cologne, Germany Filed Feb. 20, 1968, Ser. No. 706,963 Claims priority, applieatiog 7Germany, Feb. 21, 1967, 25 Int. Cl. H01f 7/20 US. Cl. 335291 8 Claims ABSTRACT OF THE DISCLOSURE A load lifting electromagnet having a ferromagnetic housing and an excitation coil which is formed at least partially by a coil made of ferromagnetic material. The magnetically conductive coil is so arranged relative to the housing, that the magnetic cross section of the housing is increased, apart from the inherent excitation action of the magnetically conductive coil.

This invention relates to electromagnets, in particular to DC-energized electromagnets for lifting loads, es pecially of greater weight.

Conventional load lifting electromagnets generally comprise a ferromagnetic housing and a coil of excitation for generating a magnetic field associated with the housing. The housing includes pole pieces terminating in pole faces through which the lines of force of the magnetic field pass and by means of which the load is attracted. However, only those components of the lines of force, that extend in a normal direction with regard to the active pole faces, are capable of exerting a pulling force on the load. Tangential components remain without any effect upon the load. Therefore, in electromagnets for load lifting purposes, the ratio of normal components to tangential components of the lines of force should be as great as possible. In practice, in known electromagnets of the type here in question, the ratio of useful normal components to tangential and stray components is comparatively low.

Similarly, in prior art electromagnets, the ratio of active surface area basically available to the passage of flux to the total area of the magnet facing the load to 'be lifted is comparatively low, so that as a consequence, the efiiciency of prior art magnets is comparatively low, i.e. a relatively great amount of ampere turns as well as a ferromagnetic housing of relatively large size are necessary in order to provide a magnet capable of lifting a certain load. One of the main reasons for such an unsatisfactory efficiency is that a certain cross section of the ferromagnetic housing must be reserved for the coil of excitation, which cross section as a result is not available as a path for the lines of force.

Further details explaining the diificulties experienced with such prior art electromagnets will be set forth hereinbelow in connection with the description of preferred embodiments of this invention.

It is an object of this invention to provide an electromagnet having an efficiency materially greater than is true with prior art electromagnets of corresponding construction, that is an electromagnet having the same overall size and being provided with the same number of ampere turns as a prior art electromagnet, but yielding a substantially greater pulling force than such a prior art electromagnet.

It is a further object of this invention to provide an electromagnet having low magnetic losses in the magnetic path of its housing.

It is another object of this invention to provide an electromagnet 'being protected automatically against damages from overvoltages as produced, for instance, when the supply of electric energy to the magnet is suddenly interrupted.

Accordingly, the invention provides an electromagnet for lifting ferromagnetic loads, including a ferromagnetic housing and a coil of excitation associated therewith. The coil of excitation consists at least partially of a coil made of ferromagnetic material, which partial coil as a consequence may act as a coil of excitation generating a magnetic field and simultaneously may serve as a path for the lines of magnetic force so to augment the cross section available for the lines of magnetic force passing through the ferromagnetic housing. In view thereof, the ferromagnetic partial coil or coils are arranged in the immediate vicinity of those portions of the housing through which the lines of magnetic force extend. However, care is taken that in all circumstances bridging of pole pieces or pole faces, respectively, of opposite magnetic polarity is avoided. The partial coil may also, due to its intensive magnetic coupling with the remainder of the excitation coil, act as an attenuating coil capable of protecting the coil of excitation, if it is shorted before the disconnection of the remaining nonferromagnetic portion of the coil of excitation.

Further details and advantages of this invention will become apparent from the following description of preferred embodiments of this invention in connection with the accompanying drawings, wherein:

FIG. 1 is a radial sectional view through a pot-shaped conventional electromagnet;

FIG. 2 is a radial sectional view through another potshaped prior art electromagnet somewhat similar to the electromagnet of FIG. 1, having, however, an improved arrangement of the coils of excitation and a somewhat more suitable configuration of its housing resulting in a better efficiency than this is possible with the magnet of FIG. 1;

FIG. 3 is a radial sectional view of a pot-shaped load lifting electromagnet basically having a structure similar to that of FIGS. 1 and 2, respectively, but which otherwise is constructed in compliance with this invention;

FIG. 4 shows a radial sectional view through one half of a pot-shaped electromagnet as illustrated in FIG. 3, but having a somewhat modified coil according toa further embodiment of this invention;

FIG. 5 is a schematical longitudinal sectional view of a U-shaped load lifting electromagnet having a coil of excitation designed in accordance with the principles of this invention;

FIG. 6 illustrates on an enlarged scale a fractional view of FIG. 4 representing a preferred practical embodiment of a partial coil as employed in accordance with this invention; and

FIG. 7 shows diagrammatically a preferred embodiment of this invention for the electrical connection of the coil of excitation and the partial coil, respectively, with each other and to the mains or a similar source of electric DC-energy.

Referring now to the drawings, FIGS. 1 and 2 represent embodiments of the prior art and will be briefly described in order to set forth more clearly the problems encountered in connection with load lifting electromagnets of the type here in question and the principle underlying this invention.

The electromagnet of FIG. 1 generally is of pot-like configuration. A circular disk-like bridge or yoke member 1 is provided with a substantially cylindrical center pole portion 2 and, at its periphery, terminates into an annular circumferential outer pole portion 3 substantially concentrical with the central pole portion 2. An annular coil of excitation 4 is located within the recess defined between pole pieces 2 and 3. When coil 4 is energized, a load 5 will be attracted by means of

pole faces

6, 7 of the pole piecesZ, 3. At its lower end, the center pole piece 2 may have a portion of enlarged diameter, as indicated in FIG. 1, so that a shoulder is formed protruding beyond the coil of excitation. The diameter of this shoulder may be of smaller size as illustrated in the' left half of FIG, 1, or of larger size as indicated with reference numeral 2a in the right portion of FIG. 1. i

A number of lines of force 1', 1" representative of the lines of force formed with this type of electromagnet are schematically illustrated in the left portion of FIG. 1. The lower portion of a line L1 indicates the area in which the normal components of magnetic induction are at a minimum and of zero amplitude. The intersection of line L1 and a further line L2 may be considered the center of the magnetic lines of force. The points, at which lines L1 and L2 intersect yoke member 1 and outer pole piece 3, respectively, indicate critical cross sections of the magnetic path susceptible to magnetic saturation. As apparent from the drawings, a considerable amount of the lines of force does not pass through the ferromagnetic housing, but through the recess between pole pieces 2 and 3, respectively, receiving coil 4. Accordingly, substantial magnetic losses take place within this nonferromagnetic area. Moreover, the normal components of the lines of magnetic force extending in the area between pole pieces 2 and 3 are of comparatively low amplitude, having a magnetic induction of between 0 and 1 kilogauss, and consequently may not exert a major effect on load 5. Only the magnetic force lines leaving or entering, respectively, the pole faces 6 and 7 comprise great normal components, the magnetic induction of these components being between 10 and 2.0 kilogauss.

In a well dimensioned circular load lifting magnet of the kind referred to in connection with FIG. 1, the percentage of surface area covered by the

active surfaces

6, 7 rarely amounts to more than 35% of the total area of the surface facing the load, the annular area included between the pole pieces 2 and 3 accordingly amounting to 65% of the total area. However, the pulling forces exerted on the load by this comparatively large area merely amounts to at most 10% of the total force by which load 5 is attracted.

Accordingly, if an appreciable increase in pulling force and consequently a higher efficiency of the electromagnet are to be accomplished, it will be necesary to amplify the percentage of normal components in the area between pole pieces 2 and 3.

Further, it would be desirableto enlarge the cross sections of the ferromagnetic housing, particularly in the region corresponding to the intersection of line L1 with pole piece 3 and line L2 with yoke member 1, in which regions the ferromagnetic housing is subject to saturation increasing the magnetical losses 'which in turn cause a reduction of magnetic induction available in the surface area of the magnet. However, such an enlargement of the critical cross sections referred to is not possible without suffering from other disadvantages, as either the overall volume of the entire magnet would have to be increased or the cross section of the coil of excitation would have to be reduced.

As pointed out above, attempts have been made to improve the magnetic behavior of electromagnets by providing protrusions as the shoulder 2a of FIG. 1. The progress achieved by such a measure, however, is actually negligible. As indicated by the two lines of force 2 and 2", the flux in this area, in spite of the presence of shoulder 211, must still pass through a comparatively large nonmagnetic space causing high magnetic losses, and the normal components of the magnetic induction decrease very rapidly over the radial width of shoulder 2a, toward the outer edge thereof.

In order to overcome such disadvantages, and to increase the portion of normal components in the area between the

active pole faces

6, 7, a structure according to FIG. 2 has already been suggested, this structure having, in a manner similar as in FIG. 1, a yoke member 8, an outer annular pole piece 3 and a center pole piece 9. A part of the coil of excitation is arranged as an auxiliary coil 11 within the center pole portion 9, an annular auxiliary pole piece 10 extending between a main coil of excitation 4'? and the auxiliary coil 11, Both coils 4' and 11 are so related to each other, that the magnetic polarity of a pole face 12 of the auxiliary pole piece 10, on the one hand, and of a pole face- 14 of the center pole piece 9, on the other hand, are the same, so that both

pole pieces

9 and 10 may be bridged by a pole plate 9a, without causinga magnetic short circuit. The lines of magnetic force in this prior art embodiment extend in the manner illustrated in the left half of FIG. 2. ,As evident therefrom, the amount of normal components is enlarged and the distance between the center pole piece 9 and the outer pole piece 3 is reduced, but the fact remains that the magnetic force lines passing through the area filled by the auxiliary coil 11 must traverse a nonferromagnetic space. Consequently, the ratio of normal components of the magnetic induction to the total induction is improved in comparison with the structure of FIG. 1, but the total cross section of ferromagnetic material available for the magnetic flux lines is substantially the same as in FIG. 1, so that considerable magnetic losses are still suffered, the problem of saturation of the magnetic cross section likewise not being overcome. The normal components of the induction at the pole face 12 are substantially constant over the radial width of pole face 12in contrast to the normal components of the protrusion 2a which decrease toward the edge of protrusion 2abut their value is only 55% of that prevailing in pole face 14.

In connection with FIGS. 3 and 4, it will now be explained how the drawbacks of the before described prior art magnets of FIG. 1 or 2, respectively, largely may be over come. The ferromagnetic housing of the magnet of FIG. 3 has a center pole piece 15 terminating in a circular yoke member 16, which in turn terminates in an outer annular pole piece 17 corresponding to the

pole pieces

3 and 3 of FIGS. 1 and 2, respectively. In contrast to the embodiments of FIGS. 1 and 2, which both are provided with conventional coils, the coil of excitation provided between central pole piece 15 and peripheral pole piece 17 in this case is composed of a coil component constituted by first coil 19 of conventional structure and being made of nonferromagnetic material, such as aluminum or copper wire. However, the coil component 19 does not constitute the entire coil of excitation, but in addition, a second coil component 18 is provided, that is made of wire, tape or another suitable strip-like material having ferromagnetic properties, such as steel or iron, cobalt, nickel, iron due to its relatively good electric conductivity and due to its comparatively low price being a preferred material. In the embodiment of FIG. 3, the forromagnetic coil 18 concentrically surrounds center pole piece 15. A pole plate 15a is attached to the lower end of pole piece 15, that has about the same outer diameter as ferromagnetic coil 18 so that the lower surface of ferromagnetic coil 18 is covered by the plate 15a. The nonferromagnetic coil 19 concentrically surrounds the magnetic coil 18.

When coils 18 and 19 are energized so that a magnetic field is generated, load 20 may be attracted and within the area extending between center pole piece 15 and peripheral pole piece 17, the field lines extendin a manner as indicated schematically with

lines

16, 16" As evident, the distance between

pole pieces

15 and 17 substantially is the same as in FIG. 2, that is, it is considerably smaller than the corresponding distance of FIG. 1. However, in contrast to the embodiment of FIG. 2, the lines of magnetic force 16" do not have to travel through a nonmagnetic area as this is true with flux lines 8" of FIG. 2, but due to the location of coil 18 adjacent to pole piece 15 will find an additional ferromagnetic path constituted by the ferromagnetic coil 18.

Therefore, in this preferred embodiment of the invention, the percentage of normal components of the magnetic induction in pole plate 15a is much greater than in pole plate 9a and pole face 12 and is essentially constant over the radial width of pole plate 15a, so that the efficiency of the electromagnet of this invention is ma- ,terially increased. I

The outer surface of ferromagnetic coil 18 adjacent another preferred embodiment of this invention generally similar to the embodiment of FIG. 3, the ferromagnetic housing of FIG. 4 comprising a cylindrical center pole piece 21 and an annular outer pole piece 23, both pole pieces being bridged by the common preferably circular yoke member 22.

Pole pieces

21 and 23 at their lower ends in this case both terminate in

pole plates

21a and 23a, protruding toward each otherbeyond the outer diameter of pole piece 21 and the inner diameter of pole piece 23, respectively. In this embodiment, three different

ferromagnetic coils

24, 25, and 26 are provided, each of these ferromagnetic coils extending adjacent to and parallel with a ferromagnetic flux conducting portion of the ferromagnetic housing thereby supplementing and augmenting the cross section of the ferromagnetic housing without, however, appreciably reducing the ampere turns and, likewise, without bridging the

magnetic pole pieces

21, 23 which would of course result in a magnetic short circuit. The outer diameter of ferromagnetic coil 24 corresponds substantially to the outer diameter of pole plate 21a and, similarly, the inner diameter of ferromagnetic coil 26 is only slightly less than the inner diameter of pole plate 23a. The upper ferromagnetic coil 26 has an inner diameter corresponding to the outer diameter of pole piece 21 and an outer diameter corresponding approximately to the inner diameter of pole plate 23a. Coil 26 extends over the entire height of the recess receiving the coil of excitation while the height of coil 24 and 25 together corresponds to the entire height of the recess. The remainder of the coil of excitation, that is coil 27, in a conventional manner is made of aluminum or copper wire or the like. The elfect achieved by this arrangement of the coil of excitation is principally the same as in FIG. 3, that is, active magnetic pole faces of comparatively large size are obtained and the normal components emanating from pole faces 21b or 231; are considerably increased in amplitude and density. Consequently, the embodiment of FIG. 4 provides for a very high ratio of pulling force to ampere turns.

FIG. 5 illustrates another embodiment of this invention having a U-shaped core 28 whose yoke portion is first surrounded by a ferromagnetic coil 29 and, in addition thereto, with a nonferromagnetic coil 30 enclosing the ferromagnetic coil 29, so that the magnetic cross section of the yoke section is increased and that stray flux, which as is known in electromagnets having U-shaped cores is comparatively pronounced in the yoke region, may be reduced. In spite of this enlargement of the magnetic cross section, no reduction takes place with regard to the space between the two lateral legs of core 28 for receiving the coil of excitation.

FIG. 6 shows a particularly advantageous possibility of forming the ferromagnetic coil as used for example in connection with the embodiment of FIG. 4. A strip or tape of electrically conductive, ferromagnetic material is wound in layers 31 about an axis corresponding with the center line 21' of FIG. 4. Together with the strip of ferromagnetic material, there is wound a

second layer

32, 33 for electrically insulating adjacent layers 31 of the ferromagnetic coil from each other. The insulating layer may be purely electrically insulating material. In lieu thereof, the insulating layer may likewise consist of two

different layers

32, 33. In this instance, layer 32 may be made of a nonferromagnetic, but electrically conductive material whereas the other layer 33 surrounding layer 32 may possess the required electrically insulating properties. For example, layer 32 may be constituted by aluminum foil having a thickness of about 20 1, whereas the isolating layer 33 may be an electrolytically applied oxide-insulation having a thickness of about 5,1,0. This kind of oxideinsulation is very heat resistant and likewise is resistant to electric voltages to a degree suflicient to insulate adjacent layers 31 as well as

layers

31 and 32 from each other, respectively. If layer 32 is made of an electrically conductive material, its windings may be connected in parallel with the windings of layer 31 so to enhance the total conductivity of the ferromagnetic coil 26.

The ferromagnetic coil separated from the coil 27 may yield the additional advantage of protecting the coil of excitation from destruction due to overvoltages. Such overvoltages may take place when the electric current fed through the excitation coil is suddenly interrupted so that voltage peaks of considerable amplitude are produced. Specific devices may be associated with the coils of excitation of electromagnets for protecting the excitation coils from the deteriorating effect of overvoltages, but these protecting devices remain ineffective as soon as the feeding cable will break. With the invention, the ferromagnetic coil may be bridged without the interconnection of the feeding cable by a device whose resistivity is a function of the voltage across the ferromagnetic coil, this resistance dropping in response to a rise in voltage across the coil.

In FIG. 7, such a circuit is shown. Coil 34 is representative of the nonferromagnetic component of the coil of excitation, and coil 35 is representative of the ferromagnetic coil. In operation, both coils are connected to a suitable source of direct current through electrical switches 37 and 38, respectively. Coil 35 is bridged by a device 36 having a resistance which is great if the potential difference between the two terminals of coil 35 is at its nominal value, but drops considerably in response to an increase of the aforementioned potential difference. This voltage response device may be constituted by a varistor or any other arrangement having similar characteristics.

In this arrangement, switch 37 may be actuated and overvoltages arising at the terminals of coil 35 as a result of the interrupted current flow will then be attenuated by the voltage responsive device. If subsequently switch 38 is opened, current flow through coil 34 will be interrupted which normally likewise may lead to overvoltages. Ferromagnetic coil 35, which remains bridged by device 36 and which because of its good magnetic conductivity is very closely magnetically coupled with coil 34, will then act as an attenuating coil with regard to coil 34.

It will be understood, that various modifications of the embodiments described will be possible without departing from the scope of this invention. In particular, it should be noted in this conjunction, that the concept of this invention may be applied in an analogous manner to other kinds of electromagnets not especially designed for the purpose of lifting loads.

What is claimed is:

1. A DC-electromagnet, particularly for lifting loads or the like, having a ferromagntic housing with pole pieces of opposite polarity and at least one coil of excitation associated with said housing for generating lines of magnetic force, said coil of excitation being composed of one coil-section made of electrically conductive and ferromagnetic material and a remaining coil-section made of velectrically conductive, but non-magnetic materiaLrsaid one coil-section extending contiguously along portions of said housing conducting said lines of magnetic force and confining an intermediate space betweensaid pole pieces of opposite polarity preventing bridging of saidpole I pieces, thereby increasing the efiectivemagnetic crosssections of said portions conducting said lines .of magnetic force, and said remaining coil-section being located within said intermediate space. q f 7 s I 2. An electromagnet accordingtoclaim 1, wherein said portions of said housingconducting lines. of magnetic force and contiguous to said one coil-section terminate in pole plates extending across the. lateral width of said one coil-section.

3. An electromagnet according to vclaim l wherein said .one coil-section at least partially ismade ofvstrip offer-romagnetic material Wound into the form of said coil, the

.said outer layer enclosing said inner layer at least at opposite sides adjacent said ferromagnetic material.

4. An electromagnet according to claim 3, wherein said inner layer is made of aluminum foil, whereas said outer insulating layer is constituted by an electrolytically applied oxide-insulation.

5. An electromagnet according to claim 4, wherein said inner layer of said additional layer forms an additional electrical coil electrically connected in parallel-with said one coil-section.

6. An electromagnet according to claim 1, wherein said one coil-section. is bridged by a voltage responsive device.

7. An electromagnet according to claim 1, wherein said ferromagnetic housing has a center pole and a peripheral outer pole piece substantially concentrical withsaid central pole piece said central pole piece terminating in a pole plate having an outer diameter enlargedwith regard'to the outer diameter of .the center pole piece, saidannular peripheral pole piece terminating ,atits'lovrer end inra pale, plate having .a smaller inn er. diameter than saidouter peripheral pole ,pieee, saido co'il of. excitation comprising va first ferromagnetic coil surrounding said center pole piece and avling an outer diameterfsligh tly less than thatoffthefpole plate associatedwith said center pole p iece a second ferromagnetic coil extending along the inner's'urfac e on the vouter annular pole piece andhaving an inner diameter slightly less than the pole, plate associated with said outer polep'iece and a third ferromagnetic coil extending along theiniler surface or the yoke member arid having an inner diameter substantially equalft o the outer diameter of the ,center pol piece and an outer diametersubstantially equal to' the innerdiameter of 'said second ferromagnetic coil 1 8". An electromagnet according to claim '1, wherein said ferromagnetic housing is' U-sli'aped and has two' lateral legs bridged by a yoke member, said yoke 'member being surrounded by said ferromagneticcoil and said ferromagneti'c coil being surrounded by said coil 'ofnonferromagnetic material. we

References Cited UNITED STATES PATENTS 2,548,179 4/1951 Underwood '336--177'XR 2,716,736 8/1955 Rex 3se 177 XR 2,761,094- 8/1956 Frampton 335-291 GEORGE HARRIS, Primary Examiner