Note: Descriptions are shown in the official language in which they were submitted.
I
PASSING CONDUCTOR WITH LAYER INCLUDING MAGNETICALLY
PERMEABLE PARTICLES THROUGH MAGNETIC FIELD
This invention relates to methods and apparatus for making
insulated electrical conductors.
In the telecommunications cable and power cable industries,
it is common practice to surround electrical conductors with at least
one layer of insulation which affects the electrical performance of the
conductor, e.g. by producing a desired dielectric effect and helping to
provide other design characteristics such as mutual capacitance. For
various reasons, continuous inductive loadings have been proposed and
used in dielectric layers of electrical conductors. These continuous
inductive loadings have comprised discrete particles of a magnetic
material such as -ferrite, which are dispersed throughout a continuous
dielectric carrier layer of polymeric substance such as rubber or other
plastic. Such a layer will be referred to in this specification as
"continuous loaded layer".
In order to provide the desired inductive characteristics
continuously along a conductor, it is necessary to have substantially
even dispersion of the particles throughout the layer. It is also
necessary that other parameters such as electrical and magnetic
influences of the particles should be controlled.
The present invention provides a method and apparatus for
making insulated conductor in which consistency in the magnetic
properties of magnetically permeable particles in a continuous loaded
layer is improved.
According to one aspect of -the invention, there is provided a
method of making an insulated electrical conductor comprising a solid
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conductor having an insulation layer formed from a dielectric carrier
provided with a plurality of magnetically permeable particles
homogeneously dispersed therein, wherein said method comprises applying
the layer to the solid conductor with the dielectric carrier in fluid
form and passing the solid conductor -through a magnetic field to cause
an increase in magnetization of magnetic domains of the particles
-towards a single direction while maintaining the particles
homogeneously dispersed.
The term "magnetic domains" is used in the sense of the use
of the term in magnetic domain theory as discussed in "College
Physics", 3rd edition, chapter 35, by Sears and Zemansky, 1961. As
explained therein during action of an applied Field to a ferromagnetic
substance, some aligning influence other than the applied field must
act upon molecular magnets forming the substance. This aligning
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influence is Noel understood to be small regions called "domains" which
exist in ferromagnetic materials. Domain size is typically between
10-6 and 10-2 cm3- -these domains the molecular magnetic moments
are all aligned parallel to one another as a result of molecular
interactions. While directions of magnetization in different domains
are not necessarily parallel to one another in an unmagnetized
specimen, when the specimen is subjected to a magnetic Field, the
resultant magnetization may increase either by growth of favorably
oriented domains or by domain rotation towards the direction of the
Field.
Each magnetically permeable particle referred to in -the
method of the invention is composed of such domains and in the method,
the increase in magnetization towards a single direction depends upon
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the strength of the field and is preferably obtained by domain rotation
although some growth of favorably oriented domains may take place.
The inventive method is applicable both to increase
magnetization towards a single direction in an insulation layer upon a
single conductor and upon a plurality of conductors forming the core
For a cable. In the case of the single conductor, it may be passed
through the magnetic field either before or after the dielectric
carrier is dried. In the event that the dielectric carrier is in a
liquid state, the conductor is coated with a mixture of fluid
carrier and magnetically permeable particles and is passed directly
through the field before entering or whilst inside a drying oven In
this particular method, increase in magnetization is also assisted by a
change in orientation of some at least of the particles and assists in
aligning the directions of magnetization of the domains of each of
these particles towards the single direction. Drying of the carrier
then holds the particles in their oriented positions.
According to another aspect of the invention there is
provided a method of making a core for an electric cable, the core
comprising a plurality of conductors each having a dry insulation layer
Formed from a dielectric carrier provided with a plurality of
magnetically permeable particles homogeneously dispersed therein,
wherein the method comprises taking the core with each conductor in
solid condition and surrounded with its dry insulation layer and
passing the core through a magnetic field to cause an increase in
magnetization of magnetic domains of -the particles towards a single
direction while maintaining the particles homogeneously dispersed.
According to another aspect of the invention, there is
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provided apparatus fur making an insulated electrical conductor wire
comprising means for moving a conductor wire in solid condition along a
feed path; means along the feed path for applying to the solid conductor
wire a surrounding insulation layer formed from a fluid dielectric
carrier provided with a plurality of magnetically permeable particles
dispersed therein; and means downstream along the feed path for
producing a magnetic field through a particular spatial region of the
feed path and of sufficient strength only to cause an increase in
magnetization o-f the magnetic domains of the particles in the
surrounding insulation layer towards a single direction.
In a practical arrangement, the feed path extends vertically
from the coating means with the coating and magnetic field producing
means disposed vertically one above the other. A drying oven may be
disposed downstream of the magnetic field producing means or may be
positioned to surround such means.
The magnetic field producing means may be operable to produce
flux lines either across or along the feed path.
The invention also includes apparatus for making an
electrical cable having a core comprising a plurality of conductors
each individually insulated with an insulation layer formed -from a
dielectric carrier provided with a plurality of magnetically permeable
particles dispersed therein, the apparatus comprising means to move the
cable along a feed path and through a particular spatial region, and
means to provide a magnetic field with flux lines extending through
said spatial region and of sufficient strength to cause an increase in
magnetization of the magnetic domains of the particles towards a single
d i fee it on .
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Embodiments of the invention will now be described, by way of
example, with reference to the accompanying drawings, in which:-
Figure 1 is an isometric view of a conductor having a
continuous loaded layer;
Figure 2 is a diagrammatic side elevation Al view, partly in
section of apparatus according to a first embodiment for applying the
loaded layer to conductor of Figure I
Figure 3 is a graph illustrating the magnetic flux strengths
required to produce desired forms of alignment of magnetization of
ferrite particles towards a desired direction;
Figure 4 is a side elevation Al view of apparatus according to
a second embodiment
Figure 5 is a greatly enlarged cross-sectional view through a
conductor after it has passed through the apparatus of the second
embodiment;
Figure 6 is a cross-sectional view through a cable comprising
a core formed from twisted pairs of conductors; and
Figures 7 and 8 are side-elevational views of apparatus
according to third and fourth embodiments for increasing magnetization
of ferrite particles in loaded layers of the cable of Figure 6 towards
a single direction.
Figure 1 shows an insulated conductor comprising a conductor
10 having two layers of insulation provided upon it, an inner layer 12
in the form of a continuous loaded layer and an outer layer 14 of other
insulating material. The continuous loaded layer is in the form of a
latex coating which forms a dielectric carrier for a plurality of
ferrite particles. These particles are of a size such that around
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99.5% of them Jill pass through a 325 mesh screen. The particles have
their magnetic domains oriented towards a single direction. This
direction is either axially of the conductor as provided by one of the
methods according to the invention or across the conductor at angles
substantially normal to its axis as provided by an alternative method.
To provide the ferrite particles with their general
orientation towards the axial direction, apparatus according to a first
embodiment and as shown in Figure 2 comprises a container 20 holding a
mixture of a fluid dielectric carrier which is any carrier suitable for
the purpose, e.g. a polymer or a latex material. It homogeneously
incorporates a plurality of magnetically permeable particles, which are
ferrite particles as discussed with regard to Figure 1. In this
embodiment, the carrier is a latex emulsion with a 45 solids content
by weight of the total emulsion and which is cross-linkable to enable
the finished coat to be made by curing. A suitable emulsion is an
acrylic emulsion sold by Rhoplex under their trade number NE 1612. The
base of the container is formed with an orifice 24 having a seal for
sealing engagement with conductor 10 as i-t is fed upwardly through the
bath to become coated with the mixture 22 and through a die means 28,
which is carried upon the surface of the mixture. The conductor then
proceeds upwardly away from the bath and through a vertical drying oven
30. The die means 28 is of any suitable construction and may be of
the construction described in Canadian Patent Application No. 450,801,
filed March 28, 1984 and entitled "Production of Insulated Electrical
Conductors" in the names of JO Walling, MA. Shannon and G.
Arbuthnot. The particular die means described in that application is
one which is free to Float across the mixture to assist in the forming
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of a concentric layer o-F the mixture 22 upon the conductor, the free
floating of the die means accompanying any lateral movement of the
conductor after it emerges from the bath to ensure this concentricity.
The drying oven 30 is of upwardly cylindrical construction
and is provided around its inside surface with a plurality of
circumferential and axially extending infrared heaters 32 of known
construction. Disposed within the oven is a means to produce a
magnetic field with flux lines extending through a central spatial
region of the oven, this region including the line of the feed path
through the oven. The magnetic field producing means comprise a
magnetic coil 34 which is concentrically disposed along the feed path
and extends upwardly through the oven. The coil 34 is connected at its
ends to a source of electric power (not shown). The coil is designed
to reduce the strength of the magnetic field towards the downstream end
of the feed path, i.e. towards the top of the coil, and for this purpose
the windings of the coil as they extend towards the top end, become
further spaced apart axially of the coil so that -the winding intensity
is reduced.
In use of the apparatus shown by Figure 2, the conductor is
passed upwardly through the container 20 and is provided with the
coating formed from the mixture 22.
After the conductor is coated with the particles by -the
apparatus and method described in the aforementioned cop ending
d pal cation en-titled "Production o-f Insulated Electrical Conductors",
then the conductor 10 is fed upwardly through -the coil 34 and through
-the drying oven. As the conductor passes through the coil, it is
subjected to the strength of the magnetic field, the flux lines of
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which extend generally in the direction of the feed path. Before
entering the coil, it is possible that the ferrite particles will
extend randomly in all directions within the coating on the conductor.
More exactly, the magnetization o-F the magnetic domains of the
particles extends randomly and haphazardly in various directions.
During passage of the conductor along the coil, the magnetic field
influences the magnetic domains so that the field increases the
magnetization of the domains of the particles towards the axial
direction of the conductor. The manner in which the domains are
orientated is dependent amongst other things upon the strength of the
field, the type of ferrite used and its geometry. For instance, for a
relatively weak field, the increase in magnetism will be changed by
means of domain boundary displacement so that favorably oriented
domains, i.e. those extending generally in the axial direction,
increase in size at the expense of other domains extending in other
directions. However, with this type of change in magnetization, there
is a possibility that a reversal in orientation will take place upon
-the magnetic field being removed. Hence a relatively weak field is
deemed to be undesirable if the object of this invention is to be
obtained. In stronger fields, the changes in magnetization are less
reversible and become irreversible as the Field increases in strength.
For the stronger fields change in orientation will take place, not only
by domain growth, but also by orientation caused by rotation of the
domains themselves towards alignment with the single direction, i.e.
the axis of the conductor. Should the magnetic field be of sufficient
strength, then the increase in magnetization may also be caused at
least partly by orientation of the particles themselves For the purpose
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of giving a general domain orientation which most favorably lies
towards the axial direction. This effect upon the increase in
magnetization is dependent upon the strength of the field as
illustrated by Figure 3, which shows a typical curve for magnetization
effect upon domains for different field strengths. The two axes of the
graph are the field strength and the percentage domains which are
oriented to the desired direction. As can be seen for weak field
strengths, there are small changes in the direction of magnetization of
the domains and these changes are by way of domain growth which is
reversible as has just been stated. At relatively stronger field
strengths, there is an irreversibility in the change in direction of
magnetization, and this change in magnetization takes place also
because of rotation of the domains themselves.
Hence with the use of the apparatus described with reference
to Figure 2, the degree of change in orientation to achieve the desired
effect and its degree of permanence is dependent upon the strength of
the field.
The use of a coil having its convolutions more widely spaced
apart towards the downstream end of the feed path ensures that the
strength of the magnetic field decreases towards the end of -the coil.
This decrease may be substantial and ensures that as the conductor
moves out of the coil, the influence of the magnetic field is
substantially negligible and will only have a negligible effect on the
direction of the magnetization of the domains. In contrast to this, if
the strength of the field at the end of the coil were substantial, then
any orientation into the desired axial direction of the domains would
be partly nullified by the sudden change in the direction of the -field
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existing at the end of the coil.
In a second embodiment as shown in Figure 4, an application
container 20 and associated eke pent for the mixture 22 is of the same
construction as described in the first embodiment. The second
embodiment has means for creating a magnetic field which is different
from that described in the first embodiment. As shown by Figure 4,
this means for creating a magnetic field is disposed along the feed path
between the applicator bath and a drying oven 37. Thus the insulation
is acted on by the magnetic means only with the layer in a fluid
state. In this particular embodiment, the means for creating a
magnetic field comprises two solenoids 38 each having a soft iron core
40 disposed within a coil 42 of the solenoid. Solenoids are disposed
with north and south poles facing each other across the feed path
whereby the solenoids operate to create intensity in the magnetic field
across the feed path. In use of the apparatus of Figure 4, after the
conductor 10 has passed through the applicator bath, it moves through
the magnetic field and the field strength increases the magnetization
of the domains transversely of -the length of the conductor with the
domains tending to be orientated towards a single line of direction
passing through the conductor. This direction which follows the flux
lines of the field is shown by Figure 5 which represents the domains
diagrammatically and substantially enlarged. As can be seen in this
particular structure through the conductor, the domains 41 lying to the
sides of the conductor in the figure lie in a radial direction whereas
those lying -towards the top and bottom of the conductor at 43 extend
tangentially of the conductor. After domain orientation, the conductor
is then fed through -the oven to dry the coating and help to stabilize
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the magnetization direction.
Change in direction if magnetization may also be effected for
conductors in a finished cable. For instance, the telecommunications
cable 50 of Figure 6 comprises a core 52 formed from a plurality of
twisted conductor pairs 54 and having a polymer jacket 55. Each
conductor has continuous loaded layer including ferrite particles.
This is a typical structure for a telecommunications cable. This
cable, however, should no-t be provided with a metallic sheath as this
will detract from any influence that the magnetizing effect will have
upon the domains of the conductors.
The cable 50 is fed along a feed path (Figure 7) to a take-up
reel (not shown) during which time it passes through a magnetic field
produced by a means 52 which, in a similar manner to the second
embodiment, is provided by two solenoid 54. The soft iron cores 56 of
the solenoids cause an intensity in the magnetic field across the
feed path, and this intensity in the strength of the field increases
the magnetization of the domains of the ferrite particles in the
conductors towards a direction which lies normal to the axial length of
the cable. As will be appreciated, this direction is not with regard
to the axis of each of the conductors, because the conductors follow
tortuous paths along the cable. Thus the direction of the general
magnetization of the domains of each conductor changes along the
conductor length in a continuous fashion and this change is dependent
upon the angular positional change of the conductor in the cable axis.
Hence, each conductor has its domains oriented at a specific angle
relative to the cable axis and the general direction of orientation is
substantially the same throughout the length of the cable for all
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conductors. As a result of this, the loaded layers upon the conductors
average out the inductive effect from one conductor to another.
In a fourth embodiment as shown by Figure 8, the cable 50 is
fed through a means for creating a magnetic field provided by two
solenoid coils 58 and 60 disposed along the length of the cable.
The downstream end of the solenoid coil 60 has its windings 62
increasing in axial length, whereby there are fewer windings per unit
length of the coil. the purpose for this is to reduce the strength of
the magnetic -Field at the downstream end so as to not suddenly effect
undesirable directional change upon the magnetization of the domains as
the cable emerges from the magnetic field. The magnetic field created
by the two coils increases the magnetization of the domains of the
ferrite particles on the conductors towards the axial direction of -the
cable. As in the third embodiment, this direction is not consistent
along each conductor because of the angular change in position of each
conductor along the cable. However, the effect of this directional
change is substantially the same in all conductors, whereby there is an
averaging effect in inductance of all of the conductors along the
cable.
As may be seen from the third and fourth embodiments, the
change in direction of magnetization of the domains may be effected
after manufacture of the cable even though the orientation of the
particles themselves cannot be effected because of the solidified
nature of the insulation upon the conductors.
