US2031975A - Electrical conductor - Google Patents

Description

Feb. 25, 1936. E. F. NORTHRUP 2,031,975

ELECTRICAL CONDUCTOR Filed May 14, 1930 Wow/mm Patented Feb. 25, 1936 ELECTRICAL CONDUCTOR Edwin Fitch Northrup, Princeton, N. 3., asaignor to Ajax Electrothermic Corporation,

AiaxPark,

Ewing Township, Mercer County, N. 1., a corporation of New Jersey Application May 14, 1930, Serial No. 452,181

My invention relates to electrical conductors, and particularly to conductors suited to carry heavy currents and currentsat high frequency.

A purpose of my invention is to decrease the losses in transmitting alternating current, especially with high amperage and at high frequency, by proper arrangement of the conductors and I suitable cooling.

A further purpose is to reduce the overhead of electrical installations by making it possible to employ conductors of smaller cross section.

A further purpose is to utilize the mutually inductive actions of adjacent conductors carrying opposite currents to distribute the currents uniformly in the conductors and reduce the stray field.

A further purpose is to dispose conductors carrying opposite currents in a square configuration so that the interaction of the opposing fields of force of the conductors will produce a more uniform current distribution throughout the cross section of each conductor.

A further purpose is to improve the conductors available for the oscillation circuits of high frequency induction furnaces.

Further purposes will appear in the specification and in the claims. 7

In the drawing I illustrate a few of the many forms in which my invention might be embodied, choosing them from the standpoint of convenience in operation and ready illustration of the principles involved.

Figure 1 is a broken perspective view, largely diagrammatic, of conductors to which my invention is applied.

Figure'2 is a section on the line 22 of Figure 1, omitting structure in the background.

Figure 2a is an enlarged diagrammatic view of the upper left hand conductor of Figure 2.

Figure 3 is an enlarged section on the line 3-4 of Figure 1.

Figure 4 corresponds to a section upon the line 4-4 of Figure 1, omitting the lower pair of tubes and the yokes.

Figure 4a is an enlarged diagrammatic view of the left hand conductor of Figure 4.

Figure 5 is a diagrammatic illustration of a circuit to which my invention may be applied.

Figure 6 corresponds generally to Figure 2, but shows six conductors.

Figure 7 is a different arrangement of the six conductors of Figure 6, with diagrammatic connections to the conductors.

In the drawing like numerals refer to like parts throughout.

In the past, numerous efforts have been made to decrease the losses in electrical conductors,

' and especially in conductors carrying large amounts of energy, by reducing the resistance to the lowest value possible.

In the alternating current circuit, resistance is a more complicated concept than in the direct current circuit. It is often desirable to make the inductance of the conductor as low as possible for any one of a great many possible reasons, as for example, to tune a circuit having high capacity. There is a distinct relation between the resistance of a pair of adjacent conductors in an alternating current circuit and the inductance of the conductors, and this serves to further complicate the question of resistance.

For convenience in discussing the question of resistance, I will separately consider a number of factors which influence it. I do not, however, wish to give the impression that my invention is dependent upon the theoretical accuracy of my discussion of the factors involved, or upon the completeness of the discussion in considering all possible pertinent matters.

The resistance due to the specific properties of the metallic conductor and to its gross cross section, which is found in the direct current circuit and calculated from the resistivity of the metallic conductor and its cross section, is referred to by me as gross cross sectional resistance. This resistance increases with the temperature, so that at first sight a great advantage will seem to accrue from maintaining a low temperature in the conductor.

However, particularly at frequencies sufllciently high to cause appreciable eddy current losses in a solid conductor, regarded herein as high frequencies, with which frequencies I am most especially concerned, the current by no means distributes equally throughout the wall of the conductor. There is a distinct tendency for the current to concentrate near the outer surface of the conductor due to its effective depth of penetration at a given frequency, producing skin efiect". While not pronounced at low freqencies, skin effect increases with the frequency. An excellent discussion of surface or skin eflect appears in Steinmetzs Electrical Engineering Library, volume 8, chapters 6 and 7. On page 375 he discusses the depth of penetration of alternating current" and states that the arbitrary figure for this depth of penetration is such that the resistance of the whole conductor for the current at the frequency imposed is equal to the direct current resistance of the portion between 55 2' I the periphery and the depth of penetration. Formulas and equations are given which show how the depth of penetration can be calculated for difierent materials and at diiierent frequencies. In a conductor in which skin effect is present,

the current carrying cross section of the conductor is not the gross cross section as in the direct current circuit, but is some fraction of that amount. Current flows through a band around the edge of the conductor, of a thickness varying with the frequency, greatly increasing the effective resistance.

Skin eifect decreases with the temperature, but

reduction of the temperature of the conductor is not as pronounced as might at first be expected.

In order to reduce to a minimum the inductive drop between two conductors of a circuit carrying current in opposite directions, the conductors should be placed as close together as the voltage will permit. However, the arrangement of the conductors and the relationship of their size to their proximity has a distinct effect upon the resistance, as well as upon the inductance.

The circuit will tend to adjust itself so that the inductance is a minimum at all times. This condition will exist when opposite current paths are as close together as possible. For this reason, where two alternating current conductors in close proximity are carrying currents in opposite directions, the currents will tend to concentrate in the adjacent walls :of the conductors. There will be a reduction of the amount of the current carried along the remote walls as compared with that passing along the adjacent walls.

Where large conductors carrying opposite currents are separated by a distance less than their diameters, the inductance of the path including the adjacent walls of the conductors is smaller than that of the path including the outside walls, and therefore the current will concentrate along the adjacent walls. Then only'a small part of the gross cross section will carry current, and the resistance will be relatively large.

This is independent of the increase of the resistance due to skin effect, because, while the mutual inductance of the conductors causes the current to concentrate in the adjacent walls, skin effect causes the current to further concentrate aloxisg the adjacent surfaces of these adjacent wal As the conductors are moved farther apart, a limiting condition finally obtains in which the distance between the conductors is infinite with respect to the diameters of the conductors, and the resistances of the conductors will then be entirely those calculated from the gross cross section, modified by skin effect, without any resistance due to mutual inductive concentration.

Where two conductors only are used, the most desirable arrangement is as close together as the voltage will permit, in order to reduce the inductance to a minimum, while passing cooling fluid through them to minimize the resistance. If, however, four conductors be used instead of two, and they form a square configuration, with adjaoent ones carrying alternating current in opposite directions, the effective resistance of each conductor will bedecreased as compared with that of one of a pair. Due to the advantageous decrease in the mutual inductive concentration, the current will distribute more evenly along the wall of the conductor.

The resistance will decrease because the stray field about the conductors will be reduced and the concentration of the current flow in only part of the cross section of each conductor will be less pronounced. One external field will neutralize another, decreasing the stray field of the combination. This will be more fully understood by reference to the figures.

In Figure 1 I show four hollow tubes, 20, 2|, 22 and 23, preferably of copper or some similar material of high conductivity, arranged in a square configuration and connected so that diagonally opposite tubes are electrically in parallel. Electrical connection from one tube to the opposite one is made by forks or yokes 24 and 25, having collars 26 suitably attached to the tubes, as by brazing or welding.

Water connections to the tubes are made through hose 21, 21 and 28, 28', preferably of rubber or similar material attached to reduced ends of the tubes. Water may be supplied and drawn off at any desired point, but for convenience I illustrate the hose as arranged so that water may be supplied through 21 and 21' to one of the tubes conected electrically in parallel and drawn oil through 28 and 28', respectively, the other tube of the pair.

It is convenient to pass the water or other cooling medium through the two tubes of a pair in series so as to discharge the water near to the inlet, permitting use of the water over again. In that case water connections are made at 29 and 30 from each one of the tubes connected electrically in parallel to the other tube of the pair at the remote ends of the conductors.

Thus it will be understood that the tubes 20 and 23 are electrically united to form one pair, and the tubes 2| and 22 are electrically united to form the other pair. To obtain the best use of my invention, one set, as for example 20 and 23, should be connected to one side of a piece of apparatus, and the other set, as for example 2| and 22, to the other side. I

My conductors are suited for use in any place where heavycurrents or currents at high frequency are to be handled. As one example, I illustrate in Figure 1 the application of my invention to the circuit between the inductor coil of a high frequency induction furnace and its capacity, represented diagrammatically by a condenser.

The inductor coil 3l is supplied with current from the source 32 through supply lines 33 and 34. At points 35 and 36 connections are made to the condenser 31.

In Figure 1 the connections from the points 35 and 36 are made to the ends of the respective pairs of conductors, and at the other extremity of the conductors connections are made to the condenser 31 through convenient yokes 38 and 39.

It will be understood that the suggestion as to the use of my conductor in connection with the oscillation circuit of an electric induction furnace is an example only of one application which might desirably be made, and that my conductor may be used in any one of a great variety of other circuits.

In the past it has been necessary to use a large amount of copper to conduct heavy currents. I have found by experiment that, of the copper previously used in the form of bars and rods, approximately one-third is needed to carry the current, and approximately two-thirds are required to provide the requisite cooling capacity so that the conductor will not become excessively heated during use.

Excessive heating of the conductor, besides being undesirable because of increase in the resistance, is dangerous because of the possibility that the conductor may melt and fail.

I believe that I am the first to water coolbus bars and other similar conductors.

By my invention I employ tubes instead of rods or bars for carrying the current, and I pass through the tubes 9, suitable cooling fluid, such as water or oil for example. This .permits me to save two-thirds of the amount of copper previously required (all that was used previously to provide'cooling capacity), without increasing the losses in the circuit. But in addition I have discovered that I can decrease the losses by specially arranging the conductors.

Figure 4 illustrates two tubes spaced a predetermined amount and carrying current in opposite directions as indicated by the plus and minus signs. Though this corresponds to a section of Figure 1, it will be understood that the ends of.

the conductors 20 and 2| are connected together, or to opposite sides of suitable apparatus, and that the lower conductors 22 and 23 seen in Figure 1 have been entirely removed.

Since the conductors of Figure 4 are relatively close together, and are carrying current in opposite directions at any instant, the current will be concentrated inthe adjacent portions of the conductors 20 and 2|.

In Figure 4a I show an enlarged view of the conductor 28, with the current concentrated in about half of the wall of the tube as indicated iagramrnatically by the shading. Due to skin efiect, the current will also be concentrated in the outer portion of this side, providing the frequency be sufiiciently high.

In any case, the portion 40 of the conductor, consisting of about half of the conductor, will carry very little current due to inductive action, while the inner part U will not carry current because of skin effect.

In Figure 2 I have the same spacing from center to center of the conductors as in Figure 4, and adjacent conductors are instantaneously carrying opposite currents. Here the current is again concentrated in the adjacent walls of the tube, but due to the fact that any conductor is acted upon by two other conductors carrying currents in opposite directions, instead of by one only, the current will be concentrated in a much larger portion of the tube wall.

Thus in Figure 2a the shading illustrates that about three-quarters of the wall of the conductor 20 is carrying current, instead of about onehalf as in Figure 4a. In Figure 2a, however, no appreciable amount of the current is carried in the portion of the wall at 42 because of inductive action, nor in the portion of the wall at 43 because of skin effect.

If gross cross section resistance alone were to be considered, it would be expected that the conductors of Figure 2 would have exactly one-half the resistance of the conductors of Figure 4, because in Figure 2 there is twice the cross section of conductor extending in each direction. On actual tests, however, I have found that the resistance of the form of Figure 2 is 44% of that of Figure 4 for a particular size and proximity.

While I do not wish to limit myself to any theory ofoperation, I believe that the abnormally lower resistance of the four conductor form is' due to the wider distribution of the current throughout the walls of each tube, as illustrated by the comparison of the shading in the diagrammatic views of Figures 2a and 4a.

At the same time the stray field about the conductors in Figure 2 is less concentrated than that of Figure 4, due to the opposition of the fields of conductors carrying currents in one direction to those carrying currents in the opposite direction. When the two conductors only are used, the external field is greater than when four conductors are used. This' has been proved experimentally.

By placing my conductors as close together as possible I reduce the inductive drop to a minimum. The decrease of the alternating current resistance by the use of the form of Figure 2 instead of that of Figure 4 is accompanied by a further decrease in the effective inductance, so that in Figure 2, for a given spacing of conductors from center to center, the inductive drop is lower than in Figure 4. This I believe to be due also to the fact that in Figure 2 more of the walls of each tube are inductively opposed.

It will be understood that my invention is equally applicable to conductors in which any number of tubes are arranged alternately carrying opposite currents.

In Figure 6 I illustrate six tubes, 44, 45, 46,

41, 48 and 49, of which 44, 41 and 48 instantaneously carry current in one direction, while the others carry current in the opposite direction. In this form, as will be seen, the same general advantages exist as were explained in reference to Figure 2.

While I consider thearrangement of my conductors in the form of a square shown in Figure 2, or of a series of squares as shown in Figure 6, to be most advantageous, use may be made of my invention by employing other configurations. In Figure 7 I illustrate the conductors 50, 5|, 52, 53, 54 and 55 equally spaced about the arc of a circle and alternately connected in parallel so that adjacent conductors will instantaneously carry current in opposite directions.

Many variations in the arrangement of the conductors and in the method of applying or removing the cooling fluid will occur to designers, and doubtless some of them will be specially suited to particular uses.

From examination of Figures2a. and 4a, it will be evident that, except for convenience of manufacture and the desirability of water cooling, the conductor might advantageously be made in the form of an incomplete circle presenting those parts of the conductor only which would normally carry the current and, for example, omitting the portions 40 and 42. From electrical standpoints only, the thickness of the copper could also be reduced to that necessary to carry the current within the outer wall, omitting the portions 4 I and 43, for example.

However, such inductors would be unsatisfactory to manufacture, would add additional expense in the manufacture, would lack in strength and would heat so excessively as to require additional copper for cooling capacity. It is, therefore, very desirable that the complete tube be used in order to more conveniently permit effective cooling.

The discussion of shape indicates clearly that other types of tube than circular tubing could be used, and that other types than complete tubing can be used where a difierent form of cooling is available than that from water passage through the tube.

The rigidity of the tube is of great advantage in conductors of this character even if the cooling characteristics be not desired.

It will be evident that the reduction in temperature through water cooling and the inductive arrangement to reduce the inductive disturbance to a minimum and to increase the area about which the current flowing is distributed are interrelated and are capable of being carried out by a multiple of conductors greater than two.

It will be further evident that, though my invention has been illustrated in use for a special purpose, it is capable of broad application to conductors adapted for heavy currents such as bus bars and parts of oscillating circuits, etc.; and that, though it is useful for commercial frequency, it finds its highest utility with high frequencies.

I use the. term effective depth of penetration" in the accepted sense, as defined for example in the works of Charles P. Steinmetz.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain part or all of the benefits of my invention without copying the structure shown, and I, therefore, consider my invention to include all such in so far as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention, what I claim as new, and desire to secure by Letters Patent is:

1. A conductor system for supplying large amperages of alternating current comprising a pair of parallel hollow conductors of appreciable linear dimension side by side and close enough together for substantial mutual inductance, a source of water supply connected to pass water through the conductors for cooling purposes, in combination with high frequency electrical apparatus aside from the conductors themselves connected to the conductors, and a source of alternating current having frequency sufficiently high to cause appreciable eddy current losses in a solid conductor, connected to the conductors at points spaced along the conductors with respect to the points of connection of the high frequency electrical apparatus to pass the current from the source through the conductors in opposite directions to the high frequency electrical apparatus.

2. A conductor system for supplying large amperages of alternating current comprising a pair of parallel hollow conductors of appreciable linear dimension and free from loops and all but simple bends side by side and close enough together for substantial mutual inductance, a source of water supply connected to pass water through the conductors for cooling purposes, in combination with high frequency electrical apparatus aside from the conductors themselves connected to the conductors, and a source of alternating current having frequency sufiiciently high to cause appreciable eddy current losses in a solid conductor, connected to the conductors at points spaced along the conductors with respect to the points of connection of the high frequency electrical apparatus to pass the current from the source through the conductors in opposite directions to the high frequency electrical apparatus, the walls of said conductors being so thin that the cross sectional area is only about one third of that required for similar conductors without the water cooling.

3. A conductor system for supplying large amperages of alternating current comprising a pair of parallel hollow conductors of appreciable linear dimension side by side and close enough together for substantial mutual inductance, a source of water supply connected to pass water through the conductors for cooling purposes, in combination with. high frequency electrical apparatus aside from the conductors themselves connected to the conductors, and a source of alternating current having frequency suiiiciently high to cause appre ciable eddy current losses in a solid conductor, connected to the conductors at points spaced along the conductors with respect to the points of connection of the high frequency electrical apparatus to pass the current from the source through the conductors in opposite directions to the high frequency electrical apparatus, the walls of said conductors having a cross sectional area which, with the effect of the water cooling, will allow the current at the frequency used to be transmitted without substantially heating the conductors.

4. A conductor system for supplying large amperages of alternating current comprising two pairs of hollow conductors of which the conductors of one pair are electrically and geometrically in parallel and the conductors of the second pair, also electrically and geometrically in parallel, are interspersed between the conductors of the first pair so that the four conductors are geometrically parallel, all of said conductors being of substantial linear dimension and free from all but simple bends, a source of water supply connected to pass water through the conductors for cooling purposes, in combination with high frequency electrical apparatus aside from the conductors themselves connected to the two pairs, and a source of alternating current at frequency sufiiciently high to cause appreciable eddy current losses in a solid conductor connected to the two pairs at points spaced along the conductors with respect to the points of connection to the high frequency electrical apparatus'so as to pass current in one pair through it in a direction opposite to the direction of the current in the second pair.

5. In a bus bar system for heavy alternating currents, a hollow metallic conductor carrying current instantaneously in one direction, a second hollow metallic conductor carrying current instantaneously in the opposite direction in close proximity with the first, said conductors being of substantial linear dimension free from loops and all but simple bends, connections to the hollow interior of the conductors for passing cooling fluid through the conductors, in combination with a high frequency electrical apparatus aside from the conductors themselves connected to said bus bar system at one point and an altemating electrical source of energy having a frequency sufficiently high to cause appreciable losses in a solid conductor due to skin and eddy current efiects, connected to said bus bar system at another point.

6. A bus bar carrying a heavy current at a frequency sufiiciently high to cause appreciable losses in a solid conductor, including a group of hollow metallic conductors electrically in parallel carrying current instantaneously in one direction, a second group of hollow metallic conductors electrically in parallel carrying current instantaneously in the opposite direction and interspersed among the conductors of the first group and connections for passing cooling fluid through the conductors.

7. A bus bar carrying a heavy current at a. frequency sufiiciently high to cause appreciable losses in a solid conductor, including a plurality of hollow metallic conductors arranged with their centers at the corners of a square, means for connecting the opposite corners of the square together, placing them electrically in parallel and connections for passing cooling fluid through the conductors.

8. An electrical current carrier for currents at frequencies higher than normal comprising a first pair of adjacent hollow metallic conductors geometrically parallel and electrically in parallel, a second pair of adjacent hollow metallic conductors geometrically parallel and electrically in parallel, interspersed between the conductors of the first pair so that adjacent conductors carry opposite currents, means for circulating a cooling medium through the conductors and connections for supplying high frequency current to the conductors, whereby an electrical current carrier of minimal inductance and resistance is produced.

EDWIN FI'ICH NORTHRUP.

Images