US3161807A

US3161807A - Coil assembly for an electric magnet

Info

Publication number: US3161807A
Application number: US92537A
Authority: US
Inventors: Thomas R Brogan; John W Lothrop
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1961-03-01
Filing date: 1961-03-01
Publication date: 1964-12-15
Anticipated expiration: 1981-12-15

Description

Dec. 15, 1964 T, R. BROGAN ETAL 3,161,807

COIL. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1. 1961 5 Sheets-Sheet 1 THOMAS R. BROGAN JOHN W. LOTHROP INVENTORS Wm 17. PM

ATTORNEYS 1964 T. R. BROGAN ETAL 3,161,807

con. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1. 1961 3 Sheets-Sheet 2 Q THOMAS R. BROGAN JOHN W. LOTHROP INVENTORS AT TORNEYS Dec. 15, 1964 T. R. BROGAN ETAL COIL. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1. 1961 3 Sheets-Sheet 3 FIELD DENSITY (B) IRON CORE MAGNE T AIR CORE MAG NET MAGNETIZING FORCE (H) THOMAS R. BROGAN JOHN W. LOTHROP INVENTORS BYflMLwDF M W ATTORNEYS United States Patent O" 3,161,807 COIL ASSEMBLY FOR AN ELECTRIC MAGNET Thomas R. Brogan, Arlington, and John W. Lothrop, Essex, Mass, assignors to Avco Corporation, Cincirn nati, Ohio, a corporation of Delaware Filed Mar. 1, 1961, Ser. No. 92,537 8 Claims. (Cl. 317-158) The present invention relates to electric magnets and, more particularly, to electric magnets for providing substantially uniform high field strengths in a region perpendicular to the common axis of the magnet winding and extending through the magnet.

Some magnets, such as are used to furnish the mag netic field in a magnetohydrodynamic (hereinafter referred to as Ml-ID) generator, often operate at low voltages but require very large currents of the order of thousands of amperes. Such large currents make it necessary that the conductors forming the turns of these windings have such large cross sections that they cannot be bent into the required shape.

The general object of the present invention is to provide an electric magnet, the winding conductor being of large cross section and formed to provide a passage through the magnet perpendicular to the common axis of the winding to provide a substantially uniform and high strength field in the passage.

Another object of the present invention is the provision of a high field strength magnet comprised of individual turns formed from sheets of insulated conductive material laid on top of each other wherein oppositely disposed portions of the center turns are bent to the side to create a region extending through the magnet and parallel to the turns whereby a magnetic field is provided that is substantially constant in a direction transverse of the turns.

The novel features that are considered characteristic of the present invention are set forth in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when readlin conjunction with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic illustration of an MHD generator;

FIGURE 2 is a side view partly in section of a magnet constlucted in accordance with the present invention;

FIGURE 3 is a cross sectional view taken along line 3-3; t 7

FIGURE 4 and FIGURE 5 are perspective views of metal blanks from which the winding may be formed;

FIGURE6 is a fragmentary side view illustrating the mannerin which the turns are electrically connected one to another; and

FIGURE 7 is a graphic illustration of the BI-I curve from which it exhausts, as indicated by the arrow at 3.

The pressure at the exit of the duct is lower than at its inlet; and for this reason the ,plasma moves at high 3,161,807 Patented Dec. 15, 1964 velocity through the duct, as indicated by the arrow at 4. By properly choosing the pressure differential and shape of the duct, the plasma can be made to move through the duct at substantially constant velocity, which is desirable, although not necessary, to the operation of the generator. Surrounding the exterior of the duct is a continuous electrical conductor in the form of a coil 5 to which a unidirectional electrical current may be supplied from any conventional source or from the generator itself. Flow of electrical current through the coil establishes a magnetic flux through the duct perpendicular to the direction of plasma flow and the plane of the paper.

Within the duct are provided

opposed electrodes

6 and 7. These electrodes may extend along theinterior of the duct parallel to the direction of plasma movement and may be positioned opposite one another on an axis perpendicular to both the direction of plasma movement and the magnetic flux. High velocity movement of the electrically conductive plasma through the magnetic field induces a unidirectional electromotive force between the electrodes, as indicated by the arrows at 8.

The

electrodes

6 and 7 are connected by conductors 11 and 12 to a load 13 through which electrical current flows under the influence of the electromotive force induced between the electrodes.

From the foregoing description, it will be immediately recognized that an MHD generator of the type described employs. a stationary magnetic field and unidirectional gas flow. As a result, such a generator is inherently a source of direct current. If alternating currentis desired, specially designed generators or auxiliary equipment must be provided to invert the direct current to alternating current. v

The electric magnet shown in FIGURE 2 and FIG- URE 3 includes a winding 21 comprised of end turns indicated generally by the

numerals

22 and 23 and center turns indicated by the numeral 24. The end turns 22 and 23 are essentially alike as are the center turns 24 except as will be hereinafter pointed out. The end turns 22 and 23 surround

respectivelyiron pole pieces

25 and 26. Each

pole piece

25 and 26 is attached as by

bolts

27 and 28 to respectively

end pieces

29 and 30 which extend to about oppositely disposed edges of the turns as best shown in FIGURE 3. The center portion of the winding intermediate its ends rests upon and is supported by the bottom end piece 30 and the top end piece 29, in turn, rests upon the center portion of the winding. Insulation 31 is provided between the turns of the winding and the wind-ing and the end pieces to prevent shorting of the winding. End piece 30 is mounted on a suitable base 32, such as, for example, I beams as shown. I The bolts 28 serve the additional function of attaching the bottom pole piece 26 to the base 32, and the bolts 27 similarly serve the additional function of attaching the top pole piece 25 to crossmembers 33, such as, for-example, I beams as shown. The cross members 33 and the base 32 which compress the winding 21, are held in fixed relation one with another by rods 34. Rods 34 and

bolts

27 and 28 function to provide a rigid structure to prevent displacement of the various parts of the magnet due to the'force exerted thereon when current is first supplied to the magnet and when. the magnet is shut down. Suitable means such as, for example, I beams 41 and members 42, prevent displacement of the bent portions 44 (more fully described hereinafter) of the center turns 24- that are bent aside.

Members 42 are T-shaped and maybe formed of a nonconductive material or insulated from the turns as by insulation 31. By reason of their effective abutment with the end pieces 29 and 3t and the innermost surface 43 of the bent portions 44 of the center turns that are bent aside, members 42 in addition to supporting the overhang of the winding 21 prevent displacement of the said bent portions 44 inwardly toward the common axis of the turns. I beams 41 connected by stress members 45 are essential to prevent displacement of the bent portions 44 of the center turns in the opposite direction (away from the common axis) which would otherwise occur during operation of the magnet if I beams 41 were not present. It is essential that the bent portions 44 be maintained in a fixed position for the reason that operation of the magnet will tend to cause displacement thereof and consequent failure of one or more of the connections between the turns due to the stresses on these connections which will result from displacement of the bent portions of the center turns.

The magnet having now been generally described, the winding and its construction will now be described in detail.

FIGURE 4 and FIGURE illustrate respectively by way of example the general configuration of the end turns and the center turns. As shown in FIGURE 4 and FIG- URE 5, the turns of the winding are made from

blanks

51 and 52 in the form of a heavy fiat metal plate, preferably copper, with respectively

openings

53 and 54. Each blank 51 and 52 is thus a continuous turn surrounding its respective opening. These blanks may be .formed in any desired way as by stamping, shearing, cutting, or punching them from solid fiat metal sheets.

Each end turn in the finished winding is fiat and extends around the space formed by openings 53 and in which are located the pole pieces. These turns are electrically connected one to another, such as, for example, in the manner hereinafter to be described. At one point on each blank a slot 55 (FIGURE 6) is cut through the blank to change it from a closed blank or turn to an open turn with two

ends

56 and 57. The adjacent turns are also slotted but in a slightly different displaced position so that each turn may be connected to the opposite end of an adjacent turn. With reference now to FIGURE 4, the first or bottom turn may, for example, be cut at

lines

58 and 59. The next succeeding turn is cut at

lines

60 and 61. The cuts in the succeeding turns are similarly progressively displaced such that when the turns are assembled a portion indicated by way of example in FIG- URF. 4 by the numeral 62 will comprise the left-hand end 56 of the second turn and will extend over this same portion which comprises the right-hand end 57 of the first turn as shown in FIGURE 6. Insulation 31 such as Mylar having a thickness of about .010 inch is provided to insulate the turns from the pole pieces or base and from each other with the exception of the area 63 where the left-hand end 56 of each turn projects over the righthand end 57 of the preceding turn as shown in FIG- URE 6.

Inspection of FIGURE 6 will show that the left end 56 of each turn extends across the slot 55 resulting from conversion from the blank from a closed to an open turn and over a small portion of the opposite or right end 57 of the preceding turn. An electrically conductive spacer 64 equal in thickness to the thickness of the insulation 31 is inserted between the

overlapped end portions

56 and 57 of the turns and is fused as by welding or soldering to the aforementioned overlapping portions 55 and 57 to provide a strong and low resistance connection. In this manner, and as illustrated in FIGURE 6, the turns are insulated one from another and serially connected to form the winding. A power source (not shown) may be connected to the free ends of the outermost end turns.

To provide a region extending through the winding perpendicular tothe common axis thereof, or stating it another way. to provide access through the sides of the winding to the space 53 of an assembled winding the length of the blanks 52 from which the center turns 24 are {formed (see FIGURE 5) is increased over that of the blanks 51 from which the end turns 22 and 23 are formed. When oppositely disposed

portions

66 and 67 of the two innermost center turns in particular, and the remaining center turns in general, are bent aside preferably at right angles to the plane of the turns, such as, for example, at or

adjacent lines

68 and 69,

passages

70 and 71 are provided through the turns at opposite ends of the winding which communicate with the interior of the winding.

If each pole piece extends into the winding through openings 53 a distance approximately equal to the total thickness of the end turns as shown in FIGURE 2, a

region comprising passages

70 and 71 and openings 54 will extend through the winding 21 and will be determined by the total thickness of the center turns and the size of the openings in the center turns. Thus, this region is defined by the openings 54 in the center turns 24, and the inner surfaces 72 and 73 of the

pole pieces

25 and 26.

Obviously, the

pole pieces

25 and 26 and/or

end pieces

29 and 30 may be omitted if desired. However, use of pole pieces as described herein provides the optimum design and substantial reduction of the amount of power required vfor operation of the magnet, particularly at very high field strengths. Since iron saturates at about 20,000 gauss, a pure iron core design is not desirable for very high field strength magnets and a pure air core magnet will require considerably more power than an iron core magnet. Thus, for a field strength of about 32,000 gauss a magnet incorporating pole pieces as shown and described herein will permit a reduction in the power required by about 50% over that required for a pure air core magnet. This permits a reduction in the power requirements or conductor weight for a given field and hence a substantial reduction of the cost of conductor material at the expense of a small increase in the cost of a power source of greater capacity than would otherwise be required. As a result, a total system of minimum cost is provided.

The effect of the pole pieces is illustrated in FIGURE 7. Curve 74 is the B-H curve for an air core magnet, while curve 75 is the B-H curve for the same magnet with an iron core. Up to iron saturation, the iron core magnet produces a given field for a much smaller expenditure of power than the air core magnet. Above saturation, the slope of the two curves as shown in FIG- URI-I '7 is the same, the difference in field strengths thus remaining the same. In effect, above saturation, the pole pieces contribute a bonus field above the air core value. The value of this bonus field may be determined by extrapolating the iron core B-H curve back to zero current with the air core BH curve slope.

For a construction of the Winding in accordance with that described hereinabove, the packing factor (ratio of conductor volume to total volume) can approach unity. In fact, a packing factor of 0.96 has been achieved. Since, for a given field, the power dissipation varies as the reciprocal of the packing factor this is a most efficient design from the point of view of utilization of power.

At the expense of reduction of the packing factor the turns may be maintained in spaced relationship one with another to permit cooling thereof if desired.

When a search coil consisting of many turns of fine wire on a small bobbin is placed in the magnet, a voltage will be induced in the search coil which is proportional to dB/dt. By observing dB/a't on an oscilloscope when current is supplied to the magnet, a voltage vs. time curve may be obtained which can be integrated to obtain the total field strength. Moving the search coil to different locations inside the magnet will give data for field distribution curves. The search coil may be calibrated by inserting it in a long A.C. solenoid in which the field can be calculated and'measuring the coil output voltage.

A power source of 3300 -kw. was required for a magnet constructed in accordance with the embodiment described herein and successfully reduced to practice for MHD applications. This magnet had a field with a maximum strength of about 32,000 gauss, a field length of about 5 feet, and a hollow cross section of 14 inches by 24 inches to receive an MHD generator duct. At maximum power conditions the current in the magnet was 22,000 amperes and the voltage drop was 150 volts. The winding was comprised of 144 copper plates or turns, 52 plates of which were utilized as the center turns. Each end turn weighed 280 pounds and each center turn, all of which were of identical length before bending, weighed 430 pounds for a total copper weight of 48,200 pounds. At high field strengths the pole pieces contributed about 10,000 gauss.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims:

We claim:

1. An electric magnet comprising: a conducting winding having a plurality of center turns disposed between a plurality of end turns, oppositely disposed portions comprising the entire cross section of said center turns extending past the outer periphery of said end turns and being bent aside out of the plane of said turns to provide openings in said winding, said end turns being coplanar with the unbent portion of said center turns and located inwardly of and spaced away from the said oppositely disposed portions of said center turns; and means to prevent displacement of said turns when said winding is connected to and disconnected from a source of current.

2. An electric magnet comprising: a conducting winding having a plurality of center turns disposed between a plurality of end turns, oppositely disposed portions comprising the entire cross section of said center turns extending past the outer periphery of said end turns and being bent aside out of the plane of said turns to provide oppositely disposed openings in said winding, said end turns being coplanar with the unbent portion of said center turns and located inwardly of and spaced away from the said oppositely disposed portions of said center turns; a pole piece disposed in each end of said winding; and means to prevent displacement of said turns when said winding is connected to and disconnected from a source of current.

3. An electric magnet comprising: a conducting winding having a plurality of center turns disposed between a plurality of end turns, oppositely disposed portions comprising the entire cross section of said center turns extending past the outer periphery of said end turns and being bent at substantially right angles to the plane of said turns to provide oppositely disposed openings in said winding, said end turns being coplanar with the unbent portion of said center turns and located inwardly of and spaced away from the said oppositely disposed portions of said center turns; and means to prevent displacement of said turns when said winding is connected to and disconnected from a source of current.

4. An electric magnet comprising: a conducting winding having a plurality of center turns disposed between a plurality of end turns, oppositely disposed portions of said center turns extending outwardly from said winding and being bent at substantially right angles to the plane of said turns to provide oppositely disposed openings in said winding, said end turns being coplanar with the unbent portion of said center turns and located inwardly 6. of and spaced away from the said oppositely disposed portions of said center turns; and means to prevent displacement of said turns in a direction normal to the plane of said turns and displacement of said oppositely disposed portions of said center turns in a direction parallel to the plane of said turns when said winding is connected to and disconnected from a source of current.

5. An electric magnet comprising: a winding having a plurality of turns disposed about a common axis said turns comprising a plurality of center turns disposed between a plurality of end turns, each turn comprising a single piece of flat metal having one end overlapping and connected to an end of an adjoining turn, oppositely disposed portions of said center turns extending outwardly past said end turns and being bent aside out of the plane of said turns to provide oppositely disposed openings in said winding; means to prevent displacement of said turns in the direction of said common axis; and means to prevent displacement of the end portions of said center turns in a direction substantially normal to said common axis.

6. An electric magnet comprising: a winding having a plurality of turns disposed about a common axis, said turns comprising a plurality of center turns disposed between a plurality of end turns, each turn comprising a single piece of fiat metal having one end overlapping and connected to an end of an adjoining turn, oppositely disposed portions of said center turns extending outwardly past said end turns and being bent aside out of the plane of said turns to provide oppositely disposed openings in said winding; means for insulating said turns; a pole piece disposed in each end of said winding; means to prevent displacement of said turns in the direction of said common axis; and means to prevent displacement of the end portions of said center turns in a direction substantially normal to said common axis.

7. In an electric magnet the combination comprising: a winding having a plurality of turns disposed about a common axis, said turns comprising a plurality of center turns disposed between a plurality of end turns, each turn comprising a single piece of flat metal having one end overlapping and connected to an end of an adjoining turn, oppositely disposed portions of said center turns extending outwardly past said end turns and being bent at substantially right angles to the plane of said turns to provide oppositely disposed openings in said winding, said end portions being located close to the outer surface of said winding; a pole piece disposed in each end of said winding; first means to prevent displacement of said turns in the direction of said common axis; second means to prevent displacement of the end portions of said center turns in a direction substantially normal to said common axis; and means for insulating said turns one from another and from said first and second means.

8. In an electromagnet for supplying very high field strengths, the combination comprising: a winding having a plurality of serially connected turns disposed about a common axis, said winding comprising a first and second group of end turns and a third group of center turns disposed between said end turns, each turn comprising a single piece of flat conductive metal severed at one point to form the ends of said turn, each end except one end of the outermost turns overlapping and being connected to the opposite end of an adjoining turn to form said serially connected turns, said connections between turns being progressively displaced in successive turns about said common axis, said center turns having oppositely disposed portions extendin'g outwardly past said end turns, one half of said oppositely disposed portions of said center turns being bent upwardly and one half be ing bent downwardly at substantially right angles to the plane of said turns to provide oppositely disposed openings in said winding normal to said common axis, said bent portions of said center turns being located close to the outer surface of said end turns; means for insulating said turns one from another and from ground; an iron pole piece disposed in each end of said winding; first means for preventing displacement of said turns in either direction parallel to said common axis; and second means for preventing displacement of said bent portions of said center turns in either direction perpendicular to said common axis.

References Cited in the file of this patent UNITED STATES PATENTS Behlmer Aug. 8, Woodfolk Nov. 18, Sommerville Apr. 10, Freas May 1,