GB2260859A

GB2260859A - Electromagnetic lifting apparatus

Info

Publication number: GB2260859A
Application number: GB9220417A
Authority: GB
Inventors: Angel Manuel Quevedo
Original assignee: FELEMAMG S L
Current assignee: FELEMAMG S L
Priority date: 1991-09-26
Filing date: 1992-09-28
Publication date: 1993-04-28
Also published as: FR2683670A1; ES2042380A1; ES2042380B1; GB9220417D0

Abstract

An electromagnet such as a lifting electromagnet or an extraction electromagnet comprises at least one coil (2) and an internal pole (1) and an external pole (3) joined by a magnetically permeable yoke (5) having a lower surface which descends into the coil (2) as it extends from the external to the internal pole. The lower surface may be curved or stepped. <IMAGE>

Description

ELECTROMAGNET APPARATUS The invention relates to electromagnet apparatus such as a lifting or an extraction electromagnet. A lifting electromagnet is used, thanks to its magnetic properties, for lifting and supporting ferrous materials, especially scrap. An extraction electromagnet is used for extracting and separating ferrous materials from non-magnetic products.

Known electromagnets have an internal polar core which is enclosed by a coil that is circumscribed by one or more permeable magnetic parts. The surfaces of these pieces comprise the active part or attraction zone of the electromagnet and include the piece or pieces of non-magnetic material which support and protect the coil.

Such an electromagnet has the typical construction shown in Figure 1. There is an internal polar part 1 surrounded by a coil 2 which is also enclosed by one or more magnetic permeable pieces 3. The surfaces A of the pieces 1 and 3 are the active or attraction part of the electromagnet and include the piece or pieces 4 of non-magnetic material which support and protect the coil.

The other surfaces B of the pieces 1 and 3 are joined by another piece 5 generally called a housing. It is magnetically permeable so as to be suitable for transferring magnetic flux.

To obtain the greatest power advantage, the cross-sections of the pieces 1 and 3 are normally substantially simiLar. The piece 1 is surrounded by the coil and this fact makes its thickness bigger than that of the piece 3 which surrounds the coil and has a much bigger perimeter.

Because of this, the thickness of the piece 5 is not uniform since it needs to be much thicker where it contacts the central core 1 than where it touches the piece 3.

As an example of the traditional construction of this type of electromagnet, we show a typical section of an electromagnet, in this case circular, in Figure 2. Representative sizes for the dimensions M1, M2, M3, M, and Mj are 400mm, 1131mm, 1200mm, 35.4mm and lOOmm, respectively.

As the coil was without exception always manufactured with planar surfaces, the thickness increase of the piece 5 was always as indicated in Figure 2, i.e. above the coil.

According to the present invention, there is provided electromagnet apparatus such as lifting electromagnet or extraction electromagnet apparatus, comprising at least one coil and an internal pole and an external pole joined by a magnetically permeable connector having a lower surface which descends into the coil as it extends from the external to the internal pole.

By making the upper surface of the coil concave and the lower surface of the connector convex, particular advantages arise, as will be explained later.

Preferably, the winding is not uniform and has fewer ampere-turns per centimetre in an inner part of the coil(s) than in an external or peripheric part of the coil(s). Conveniently, the coil is made in a matrix which increases in width in the radially outward direction. Alternatively, the at least one coil comprises a plurality of coils of different diameters stacked inside one another and having thicknesses which increase in the outward direction.

Preferably, the or each coil has windings which vary in cross-section in the radial direction. By reducing the cross-section of the windings as the coil diameter increases, it is possible to magnify the increase in ampere-turns per centimetre in the outward direction. For the same reason, preferably the material of an external part of each coil has a bigger conductivity than that of an internal part.

The invention will now be described by way of non-limiting embodiments with reference to the following drawings, in which: Figures 1 and 2 are vertical cross-sections through a typical prior art electromagnet; Figure 3 is a vertical cross-section through a first embodiment of an electromagnet in accordance with the present invention; Figures 4 and 5 compare the magnetic characteristics of a traditional electromagnet and a second embodiment of an electromagnet in accordance with the present invention; Figure 6 shows the magnetic field characteristic of a third embodiment of an electromagnet in accordance with the present invention; Figure 7 is a vertical cross-section through a fourth embodiment of an electromagnet in accordance with the present invention; Figure 8 shows the magnetic field characteristic of a fifth embodiment in accordance with the present invention;; Figure 9 is a vertical cross-section through a sixth embodiment of an electromagnet in accordance with the present invention; Figure 10 shows a coil being formed in a matrix; and Figure 11 is a vertical cross-section through a seventh embodiment of the present invention.

The Figure 3 embodiment looks similar to the prior art electromagnet shown in Figure 2, but has been manufactured in accordance with the invention.

The attraction force of an electromagnet depends, in closed circuit, on the magnetic induction on the pole surfaces and, in open circuit, it depends on the magnetic field intensity and its distribution adjacent to the poles.

The induction on the pole surfaces in closed circuit depends on the ampere-turns of the coil, the permeability of the circuit material, and the magnetic circuit length. The first and most immediate advantage of the invention is that, all parameters being equal, the circuit length L'm of the first embodiment is 9% less than the circuit length Lm of the prior art electromagnet of Figure 2. This results in an induction increase or it is possible to obtain the same induction with a smaller coil.

However, this is not the most important advantage of the invention. The most important ones occur in open circuit.

Most of the scrap handled by industry is of low apparent density, generally between 0.5 and 1.3 Tm/m3. When this scrap is lifted by an electromagnet, both the volume of the load and the airgaps in the circuit have such big values that the scrap does not really close the magnetic circuit and rather gives physical shape to the magnetic field generated by the electromagnet in open circuit. The ability to lift low density scrap depends on the magnetic field intensity and its distribution adjacent to the poles.

Figure 4 makes evident and clear the differences between the traditional electromagnet and a second embodiment in accordance with the invention.

This Figure represents graphically and by a continuous line the magnetic potential of a traditional electromagnet El. The same magnetic potential of an electromagnet E2 of the second embodiment of the invention is represented by a thin dashed line.

In both Figures the magnetic axis is represented by a thick dashed line.

The magnetic circuit sections, the number of turns of the coil and the electric intensity of both electromagnets are the same.

The differences between the two electromagnets El and E2 are very important and they can be summarized as follows: (A) The magnetic axis of the electromagnet has moved towards the electromagnet exterior. This is because the coil of the electromagnet of the invention has more turns towards its outside than towards its inside, whereas a traditional coil has a uniform distribution.

(B) After leaving the core or central pole, the potential of the traditional electromagnet El decreases in a straight line, while the potential of the electromagnet E2 decreases by means of a curve of minimum slope when leaving the central core, and maximum slope when reaching the external pole. As the magnetic potential is a direct function of the ampere-turns per centimetre of the coil, it is expected that the potential of a traditional electromagnet decreases linearly because the AV/cm of its coil is uniform across its width. On the contrary, the coil of the electromagnet of the invention has more Av/cm towards its radially external periphery. Consequently its potential decreases slowly adjacent to the centre pole (i.e. a few Av/cm for the inner part of the coil) and decreases more quickly adjacent to the external pole (i.e. more Av/cm for the outer part of the coil).The shape of the curve is dependent on the Av/cm increasing in the radially outward direction of the coil.

(C) As a consequence of the above two paragraphs A and B, and having taken into consideration the zero potential corresponding to the magnetic axis of each electromagnet, the potential of the electromagnet of the invention is moved towards the negative zone, i.e. the external pole.

These important differences in the magnetic potential distribution have their corresponding consequences in the magnetic field generated by an electromagnet of the invention.

Figure 5 represents graphically the magnetic field through the line which links the points where the electromagnet generates an attraction force of one gramme on a spheric ferrous mass of one gramme. The continuous line is for the traditional electromagnet El and the discontinuous line is for the electromagnetic E2 of the invention.

As it is to be expected from the graph of magnetic potential distribution (Figure 4), adjacent to the external pole the range of the magnetic field of the electromagnet E2 is much more than that of the traditional electromagnet El. However, adjacent to the internal pole, being near the same, the difference is a minimum and this is because the penetrating shape of the housing of electromagnet E2 reinforces the magnetic flux near to the central pole and increases the normal field component, all because the distance !'1 of electromagnet E2 is much shorter than the distance Al of the electromagnet El.

This effect is important for a lifting electromagnet and for an extraction electromagnet and it is preferable to increase even more the penetration of the housing into the coil, as is shown in Figure 6.

Figure 6 shows the field generated by a modified version E3 of the electromagnet E2 already shown in Figures 4 and 5. The modification involves lowering the lower face of the housing to as close to the lower face of the internal pole as is practially possible. The broken line indicates the field of electromagnet E2 and the continuous line indicates the field of the electromagnet E3.

Electromagnet E3 has a housing with a cross-section near to the internal pole that is much bigger than the cross-section near to the external pole. This does not in itself offer any advantage but adds to the weight of the electromagnet. The upper face of the housing may be lowered adjacent to the internal pole until the cross-sections adjacent to the internal and external poles are substantially the same. In fact, the cross-section adjacent to the internal pole may be made less than the crosssection adjacent to the external pole. Figure 7 shows an electromagnet having these characteristics.

It is known that for low density scrap the volume and weight that the electromagnet can lift are directly proportional to the magnetic field volume generated by the electromagnet and shown in Figures 5 and 6. Looking at these two Figures, it is easy to appreciate the big increase in the volume of scrap that can be lifted by an electromagnet of the invention compared with a traditional electromagnet.

From trials on a prototype electromagnet of the invention and a traditional electromagnet, for the same weight, sections of Av, power consumption etc..., it has been found that the prototype electromagnet can lift 38% more low density scrap (scrap capacity of 38%).

The beneficial effects resulting from the penetration by the housing and the coil shape can be increased even more by moving the coil magnetic axis further to the outside, for example by modifying the section or the type of coil conductor to increase even more the number of turns per centimetre at the radially outer part of the coil.

In a prototype, we wound with an aluminium conductor until the expected position of the magnetic axis and then with a copper conductor having a section that matches the resistivity of the aluminium conductor. This increased the weight of the electromagnet by 9.31%, but an improved magnetic field curve was obtained (see Figure 8). The curve of the Figure 8 embodiment is a thick line, the curve of the embodiment E2 of Figure 5 is a dashed line and the curve of the traditional embodiment El of Figure 5 is a dotted line.

The lower surface of the housing which penetrates the coil can be stepped as shown in Figure 9. With this construction, it is possible to use several coils with traditional rectangular cross-sections (as shown in Figure 9). Preferably, the lower surface of the housing is curved or conical and a single coil is used that has been manufactured in a matrix with a shape that is complementary to the housing7 using a wire or plate conductor, so as to have several turns in every coil layer.

Figure 10 shows a coil being formed in a matrix. The matrix determinate the number of turns in each layer. With impregnations of resin or varnish before the coil is out of the matrix, the coil has the exact shape of the hollow in the electromagnet housing.

Figure 11 shows a seventh embodiment of the present invention. It has an internal polar part 1, a field coil 2, a magnetically permeable outer part 3, a protection plate 4 of non-magnetic material, a housing 5 and an active or attraction zone 6.

The materials, shape, size and disposition of the elements may be varied by one skilled in the art from those of the non-limiting embodiments. The terms used herein must be considered in the broad sense.

Claims

1. Electromagnet apparatus such as lifting electromagnet or extraction electromagnet apparatus, comprising at least one coil and an internal pole and an external pole joined by a magnetically permeable connector having a lower surface which descends into the coil as it extends from the external to the internal pole.

2. Electromagnet apparatus according to claim 1, wherein the coil has fewer ampere-turns per centimetre in an inner part of the coil(s) than in an external or peripheral part of the coil(s).

3. Electromagnet apparatus according to claim 2, wherein the thickness of the coil increases in the outward direction.

4. Electromagnet apparatus according to claim 2, wherein the at least one coil comprises a plurality of coils of different diameters stacked inside one another and having thicknesses which increase in the outward direction.

5. Electromagnet apparatus according to any one of claims 1 to 4, wherein the or each coil has windings which vary in cross-section in the radial direction.

6. Electromagnet apparatus according to any one of claims 1 to 5, wherein the material of an external part of each coil has a bigger conductivity than that of an internal part.

7. Electromagnet apparatus substantially as herein described with reference to, or with reference to and as illustrated in, Figures 3 to 11 of the accompanying drawings.

8. All novel features and combinations thereof.