US3668589A

US3668589A - Low frequency magnetic core inductor structure

Info

Publication number: US3668589A
Application number: US96005A
Authority: US
Inventors: Bruce L Wilkinson
Original assignee: Pioneer Magnetics Inc
Current assignee: Pioneer Magnetics Inc
Priority date: 1970-12-08
Filing date: 1970-12-08
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06

Abstract

A low frequency magnetic core inductor is provided which exhibits the desirable high quality factor (Q) and low external magnetic field of the prior art toroidal type inductor, but which is constructed to have a bobbin-like configuration for ease of assembly and winding the inductor winding on the core, and for compactness of size and efficient use of available space.

Description

United States Patent Wilkinson [54] LOW FREQUENCY MAGNETIC CORE INDUCTOR STRUCTURE [72] Inventor:

[73] Assignee: Pioneer Magnetics, Inc., Santa Monica,

Calif. v

[22] Filed: Dec. 8, 1970 [21] App1.No.: 96,005

I Bruce L. Wilkinson, Torrance, Calif.

52 u.s.c| ..336/2l2,336/233 [51] Int. Cl. ..n01r27/24 5s FieldofSearch ..336/2l2,22l,233,234;

[56] References Cited UNITED STATES PATENTS 2,200,263 5/ 1940 Kramolin ..340/ 174 28 51 June 6,1972

2,932,787 4/1960 Krabbe et al ..336/212 X 2,825,892 3/1958 Duinker ...336/22l X 2,059,393 11/1936 Polydoroff ..336/234 X 1,698,634 1/1929 Johannesen ..336/212 Primary Examiner-Thomas J Kozma AttorneyJessup & Beecher [57] ABSTRACT A low frequency magnetic core inductor is provided which exhibits the desirable high quality factor (Q) and low external magnetic field of the prior an toroidal type inductor, but

which is constructed to have a bobbin-like configuration for ease of assembly and winding the inductor winding on the core, and for compactness of size and efficient use of available space.

4 Claims, 6 Drawing Figures BACKGROUND OF THE INVENTION In the design of audio frequency and low radio frequency inductors, where high quality factor (Q) and low external magnetic field are required, it is usual for the inductor windings to be wound on a toroidal shaped powdered iron or low permeability ferrite core. It is well known to the art that inductors wound' on toroidal magnetic cores exhibit the properties of high Q, low external magnetic field, and good stability. The usual toroidal core is composed of, for example, a powdered ferromagnetic material such as molybdenum permalloy, carbonyl iron, or the like, or which may be composed of a ferrite.

Because of the uniformity in shape of the toroidal core, and because of the basic symmetry of its geometry, inductors wound on such a core produce a minimum external magnetic field. The toroidal geometry yields the best possible magnetic performance. However, the toroidal shape of the core does not lend itself to efficient and simple assembly of the inductor. For example, the inductor windings cannot be wound directly on the toroidal core from a spool of wire. Instead, amount of wire must first be wound intoa winding head from the spool, and the wire must then be wound off the head and around the toroid, with the winding head being passed through the center of the toroid so that the turns of the winding may be wound around the toroidal core.

In addition to the complexity of the aforesaid toroidal winding process, it is obvious that the windings of the finished coil cannot completely fill the central portion of the toroid, since the winding head must pass through the center throughout the entire winding operation. Therefore, toroidal inductors are of relatively large size, since the space available is not efficiently used. Also, the annular geometry of the core results in inefficient packing of the winding which also adds to the size of the unit.

An object of the present invention is to provide a magnetic core structure which will lend itself to normal layer or bobbin winding techniques, and yet which will preserve the desirable magnetic features of the toroidal inductor. It is clear that the use of layer or bobbin wound coils is advantageous from a fabrication standpoint since the windings may be wound directly from the wire spools. In addition, the windings may be wound on a multiple basis, and optimum packing may be realized, particularly when square or rectangular wire is used. In

addition, a transformer maybe assembled in the practice of the invention without the need for a residual opening in its center, as is the case with the prior art toroidal assemblies, as mentioned above, so that more compact and efficient units may be constructed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a section of a typical prior art toroidal inductor, which comprises a winding wound about a toroidal shaped magnetic core;

FIG. 2 is a section of an infinitely long magnetic rod of uniform permeability, and having a winding distributed evenly along its length, this representation being useful in the explanation of the invention;

FIG. 3 is a sectional view of a finite section of rod of uniform permeability, with a winding distributed evenly along its length, and with a high permeability C-shaped core piece contacting the ends of the rod, and illustrative of the practice of the present invention in one of its embodiments;

FIG. 4 is asectional view of a magnetic core structure coma measured 1 prising a high permeability core having a generally rectangular configuration, and with a winding evenly distributed along one leg of the core and across a non-magnetic gap in the leg, this representation being useful in describing the invention;

FIG. 5 is an assemblysimilar to the assembly of FIG. 3, but in which the winding is not distributed over the'entire length of the aforesaid rod; and

of the rod 51,

FIG. 6 shows a structure representative of a further embodiment of the invention, and in which two rods composed of material of uniform permeability have windings distributed evenly therealong, and have their ends connected by high permeability core pieces, and representative of a further embodiment.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS The prior art toroidalinductor assembly illustrated in FIG. 1 comprises a toroidal magnetic core 11 of uniform permeability, and a winding which is evenly distributed around the core. An imaginary path defined by the

points

12, 13, 14 and 15 is shown to aid in estimating the external field of the assembly. A section of an infinitely long rod 21 is depicted in FIG. 2, the rod having uniform magnetic permeability. A winding is distributed evenly along the length of the rod. An imaginary path defined by the points 22, 23, 24 and 25 is shown to assist in estimating the external field of the structure of FIG. 2.

The embodiment of the invention shown in FIG. 3 is a sectional view of a rod 31 having finite length and of uniform permeability. A winding is distributed evenly along the length of the rod 31. A high permeability C-shaped core piece 32 is positioned in contact with the ends of the rod 31, as shown. The relative magnetic permeabilities of the core piece 32 and magnetic rod 31 are such that substantially all the magnetic reluctance around the resulting magnetic circuit formed by the rod 31 and core piece 32 resides in the rod 31.

The magnetic structure shown in section in FIG. 4 consists of a core of high magnetic permeability, and a winding 41 evenly distributed along one leg of the core 40. The leg has a non-magnetic gap 42 formed therein, and it is surrounded by the winding 41. The permeability of the magnetic core 40 is sufficiently high so that the reluctance of the magnetic path is substantially all due to the gap 42. A graph 43 is shown adjacent the core 40, and the graph illustrates that the magnetomotive force (MMF) at the outer surface of the winding 41 as a function of position along the length of the winding.

The representation of FIG. 5 is similar to that of FIG. 3 in that the magnetic core consists of a rod-like section 51 of uniform permeability, and a core piece section of high permeability. In the structure of FIG. 5, the total reluctance of the magnetic circuit is substantially all formed by the reluctance of the rod 51. The difference between the structure of FIG. 5 and the assembly of FIG. 3 is that in the structure of FIG. 5, the winding 52 is not distributed over the entire length as is the case with the-assembly of FIG. 3. A graph 53 adjacent the rod 51 illustrates the magnetomotive force (MMF) in the space just outside of the rod-51 as a function of position along the length of the rod 51.

The structure shown in FIG. 6 is one in which two rods 62 and 63 are provided, and each being composed of magnetic material of uniform permeability. The embodiment of FIG. 6 includes

windings

64 and 65 which are evenly distributed along the respective rods 62 and 63. The ends of the rods 62 and 63 are engaged by high permeability core pieces and 61. The reluctance of the magnetic path around the magnetic circuit of the structure of FIG. 6 is substantially totally due to the reluctance of the rods 62 and 63. Furthermore, the windings 64 and are excited with equal magnetomotive force (MMF) and in a direction to reinforce each other.

As mentioned above, the prior art toroidal inductor has an advantage in that a low external magnetic field is produced by the toroidal geometry, and as a result, the magnetic field is almost entirely confined to the core. This makes the properties of the inductor dependent upon the core material in a predictable manner. Several types of core materials are available which permit the design of high "Q" toroidal inductors, and units which are stable in their operation, and which have predictable characteristics that are not influenced significantly by external fields, and also which result in inductors which do not magnetic field may be explained by reference to the unit shown ,in FIG. 1. The flux in the core 11 is produced by current flowing in the winding 10. The magnitude of the flux is determined by the total magnetomotive force divided by the total reluctance of the magnetic path. Since the toroidal core 11 is made to be of uniform penneability, the magnetomotive force along a segment of the core is given by the ratio of the length of the segment to the entire path multiplied by the total magnetomotive force. Therefore, if the segment is, for example, of the path length, then the magnetomotive force along this segment is 10% of the total magnetomotive force. Moreover, if thewinding is'evenly distributed around the toroidal core, the percentage magnetomotive force developed )y a segment of the coilis proportional to the percentage of the entire coil that is within the segment.

The path defined by the

points

12, 13, 14 and in FIG. 1, for example, includes a segment of the winding 10. The sum of allthe magnetomotive forces accumulated by tracing this path must be equal to the magnetomotive force developed by the segment of the winding 10 which is enclosed by the path. However, if the points 12 and 13 and the points 14 and 15 define two radial lines, the segment of core defined by the path 13 to 14 is the same percentage of the total magnetic path as the enclosed winding is of the total winding. Since the total magnetomotive force in the core must equal the total magnetomotive force developed by the windings, the magnetomotive force along the path 13-14 must be equal to the magnetomotive force developed by the enclosed segment of the coil. Therefore, the magnetomotive force along the other sections of the path must be zero. As a result, there is no net magnetomotive force and hence no appreciable magnetic field in the space outside of the coil 10. v

. As mentioned above, the structure of FIG. 2 represents a section of an infinitely long magnetic rod 12 with a winding evenly distributed along its length. Mathematically the rod 21 i maybe consideredto be a section of a toroid'of infinite radius.

Under these conditions, the aforesaid rationale for the magnetomotive force around the path defined by the points 22, 23, 24 and 25 is the same as for thepath in FIG. 1 defined by the points12, 13, 14 and 15. The reason a rod of finite length has a high external field is that the flux return path from one end to the other is through the surrounding'space.

In the structure of FIG. 3, however, the flux return path is provided by the high permeability C-shaped core section 32. For example, if the permeability of the core section 32 is of the order of several thousand, and the permeability of the rod 31 is in the range of 10-300, then the magnetomotive force developed by the winding is substantially all dropped along the length of the rod 31. As a result, the magnetomotive force between the ends of the rod 31 is very small, resulting in a low external magnetic field. Also, since the magnetomotive force along the rod 31 is over the same distance as the magnetomotive force produced by the winding 30, and furthermore since the rod 31 is of uniform permeability and the winding 30 is evenly distributed along the rod, there is no appreciable magnetic field produced in the space outside of the winding.

v The importance of assuring that the winding 30 is evenly distributed along the rod 31 in the embodiment of FIG. 3 is illustrated by the representations of FIGS. 4 and 5. In FIG. 4, for example, the magnetic structure is similar to that of FIG. 3, except that the entire core 40 is made up of high permeability 7 material, and the magnetomotive force is dropped across the reluctance of the non-magnetic gap 42. The winding 41 is evenly distributed along the leg of the core 40 containing the Starting at the top of the structure of FIG. 4 and going down, the magnetomotive force is observed to increase steadily as more of the coil is encountered. Since the core material drops very little magnetomotive force, all of the magnetomotive force developed by the coil appears in the space outside the coil. When the gap 42 is encountered, the total magnetomotive force of the magnetic circuit is dropped across the gap 42. This magnetomotive force is subtracted from the cumulated magnetomotive force of the coil 41 which causes the sign reversal, such as shown in the graph 43.

Continuing down the leg of the core 40 in FIG. 4, more winding is encountered which brings the magnetomotive force up from its negative value to a zero value at the lower end of the coil 41. The magnetomotive force between the ends of the coil 41 in FIG. 4 is substantially zero as is the case in the embodiment of FIG. 3. However, there is significant magnetomotive force in the space outside of the coil 41 in the unit of FIG. 4, and this external magnetomotive force results in substantial external fields,.particularly near the gap 42 where the gradient is the steepest.

The external magnetic field usually results in inferior performance of the unit. In addition, since it is difficult to predict the external magnetic field with any precision, inductors of this type must be designed by cut and try" methods in order to obtain desired tolerances. Moreover, any objects near the inductor of FIG. 4 which influence the external magnetic field also influence the parameters of the inductor. These influences take the form of changes in the inductance value of the flux magnitude of the external field is affected, and increased losses in the inductor or reduced 0" if the objects intercepting the external field dissipate energy in the field.

In the representation of FIG. 5, the magnetic structure is similar to that of FIG. 3 in that a low permeability magnetic rod 51 is provided with its ends intercoupled by a high permeability core section 50. The difference between the structures of FIGS. 3 and 5 is that the winding 52 in FIG. 5 is not distributed evenly along the low permeability rod 51, and this results in the establishment of an external magnetomotive force as shown by the graph 53. Starting from the top of the rod 51 in FIG. 5 and proceeding down from the rod, the flux in the low permeability rod results in a magnetomotive force which increases in strength as more rod length is included. The sign is shown as negative because the convention, as in the representation of FIG. 4, adopted was for the current in the winding to develop a positive magnetomotive force.

Therefore once the winding 52 is encountered, the magnetomotive force produced by the winding overcomes the magnetomotive force along the core which results in the net magnetomotive force moving in thepositive direction, as

shown. Proceeding to the lower end of the winding 52, all of the magnetomotive force of the winding has been developed but not all of the magnetomotive force from the core is present. As a result, the graph 53 indicates a net positive value. Proceeding down to the lower end of the rod, the magnetomotive force of the remaining part of the core comes into play reducing the net magnetomotive force gradually to zero at the bottom end of the rod as shown by the graph 53.

As in the structures of FIGS. 3 and 4, the magnetomotive force between the ends of the rod 51 of the structure of FIG. 5 is essentially zero. However, as in the structure of FIG. 4, the unit of FIG. 5 develops a significant amount of magnetomotive force in the space surrounding the low permeability rod 51 on which the winding 52 is' wound. This magnetomotive force produces external magnetic field with the same disadvantages as explained above.

From the foregoing discussion, it is apparent that the low external field of a toroid may be metin a straight core segment by meeting certain criteria. That is, the core segment must bev of uniform permeability throughout its length; the winding must be uniformly distributed over the core segment; and the ends of the core segment must be connected by a low reluctance and high permeability path. It might be noted, however, that although the windingmust be uniformly distributed over the core section, it need not be a single layer winding, since the only restriction is that the current for unit length does not change from one end of the core section to the other.

Another feature of the toroid is the rejection of induced voltages from external stray magnetic fields. This capability exists as a result of the basic symmetryof the toroidal core. The structure of FIG. 6 shows a symmetrical embodiment of the unit of FIG. 3, the unit of FIG. 6 providing the same stray field induced voltage rejection as the toroid.

In the unit of FIG. 6, the flux return path for the first rod is through a second rod of low permeability on which a second winding is mounted. That is, the flux return path for the rod 62 and winding 64 is through the rod 63 and winding 65, the ends of the low permeability rods 62 and 63'being intercoupled by the high permeability magnetic core pieces 60 and 61. Since there is no net magnetomotive force appearing at the ends of each low permeability rod 62 or 63 when the magnetomotive force developed by the corresponding winding 64 or 65 matches the magnetomotive force dropped along the length of the rod, each rod and winding functions satisfactorily as a flux return path. However, if two or more such assemblies of windings and low permeability rods are to be in series in a magnetic path, the winding magnetomotive forces and the rod magnetomotive forces must all be in balance at the same flux level.

It is not necessary for each of the aforesaid rods to have either the same reluctance or permeability, so long as the ampere turns of the respective windings are adjusted to match the magnetomotive forces of the respective rods at the operating flux level. In normal operation, the windings are usually connected together either in series or parallel or both, depending upon the application, and the turns are adjusted to provide the correct magnetomotive force in each winding. Such an assembly provides the proper match of magnetomotive forces over a range of flux levels. The only limitation is that the permeability of the core material must not be significantly affected by the flux density in the core material.

The invention provides, therefore, an improved structure for an inductor which exhibits the desired magnetic characteristics of a toroid type inductor, and yet which lends itself to simple and economical fabrication. As represented by the embodiments of FIGS. 3 and 6, various embodiments of the invention are feasible, so long as the underlying principle is followed. Also, additional windings may be incorporated into the inductors so that it may be used as a transformer. It is also clear that the structures themselves may have different configurations, so long as the concepts of the invention are incorporated.

Therefore, although particular embodiments of the invention have been shown and described, modifications may be made. It is intended that all such modifications which fall within the spirit and scope of the invention be covered by the following claims.

What is claimed is;

1. A magnetic inductor unit which exhibits no appreciable external magnetic field, comprising: a first straight magnetic core section with a rod-like configuration and of a particular length and of a relatively low and uniform permeability throughout its length; a second C-shaped magnetic core section of relatively high permeability in contact with the ends of said first core section and completing a magnetic circuit with said first magnetic core section, said second and first magnetic core sections exhibiting relative permeabilities in a ratio range of the order of 10-300 such that substantially all of the magnetomotive force developed around the aforesaid magnetic structure resides in said first magnetic core section; and a winding positioned on saif rist magnetic core section and evenly distributed along the entire length of said first core section.

2. The magnetic inductor unit defined in claim 1, in which said first core section is composed of a magnetic material of a permeability in a range of the order of 10-300.

3. The magnetic inductor defined in claim 1, in which said second core section is composed of a magnetic material of a penneability of the order of 3,000.

4. A magnetic inductor unit which exhibits no appreciable external magnetic field, comprising: first and second straight core sections spaced and parallel to one another, each with a rod-like configuration and each of a particular length and of relatively low and uniform permeability throughout its length; third and fourth magnetic core sections engaging the ends of said first and second core sections and completing a magnetic circuit with said first and second magnetic core sections and of relatively high permeability, said first and second and third and fourth core sections exhibiting relative permeabilities in a ratio range of the order of 10-300, such that substantially all of the magnetomotive force developed around the aforesaid magnetic circuit resides in said first and second magnetic core sections; and windings mounted on said first and second core sections and evenly distributed along the entire lengths thereof.

Claims

3. The magnetic inductor defined in claim 1, in which said second core section is composed of a magnetic material of a permeability of the order of 3,000.