Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-RK524 FF. DRAWINGS: 2 SHEETS 2 1 6 7 0 1 8
FT FCTROMAGNFTIC TNDUCTION HFATTNG COIT
This invention relates to an electromagnetic induction coil, which may advantageously
be used in apparatus for inductive heating of objects, and to methods of inductive heating
using the coil.
It is well known that magnetic materials can be inductively heated by coupling them
with high-frequency alternating m~gn~tic fields generated by applying an alternating voltage
across a work coil formed by a number of`turns of electrical wire or other elongate
electrical conductor. During operation, the induction coil acts as the prirnary winding of a
transformer and the workpiece acts as the secondary winding. The material being heated is
not part of a closed electrical circuit and the generation of heat is due to the in~ ee~
electrical current which flows in the workpiece. Heating of the workpiece is the result of
internal energy losses, due either to resistance or hysteresis losses in the case of
ferrom~gn~tic material, which cause the temperature to rise. Conductive metal tubing may
be used as the coil conductor to enable cooling fluid to be passed through the coil in
operation. The actual voltage and frequency will depend on several factors, including the
size of the power unit, the workpiece and the type of coil. The success of the induction
heating process depends to a great degree on the proper design of the work coils which act
as inductors. Induction coils which completely surround the workpiece during the heating
process are inconvenient to use when the object to be heated is of a size or shape such that
it cannot be easily inserted and removed through the ends of the coil along the coil internal
axis. One such type of object is an electrical wiring bundle or harness, a short portion of
which is to be placed in the coil for inductive heating of materials intended to block
interstices in the harness for purposes hereinafter described.
This inconvenience may be alleviated by using an open-sided coil, into and out of
which the object to be heated can be moved in a direction lateral to the coil internal axis,
instead of along the axis as is unavoidable with a closed coil. Such an open-sided induction
coil may, for example, be made by coiling a suitable conductor into a sirnple flat "pancake"
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coil of rectangular shape and bending the pancake coil about a centre line running parallel
with the shorter rectangular sides, so that the longer sides of the rectangular coil arch
around to define a generally U-shaped or C-shaped internal space of the open-sided coil
thus formed. However, as the size of such open-sided "bent pancake" coils is increased to
accommodate larger inductively-heatable objects, for example the larger electrical wiring
harnesses of 20 mm or more diameter now becoming common in the automotive industry,
inconveniently large generators using undesirably high voltages and/or currents and/or
frequencies are required to generate a sufficiently powerful field in the coils. Very high
frequencies (eg 5MHz) have the further disadvantage that unacceptable power losses may
occur if the coil is ~ çh~d to the generator by extended (1 to 4 metre) flexible leads for
remote operation.
The present invention addresses this problem by providing an open-sided
electromagnetic induction coil comprising at least two coil sections (preferably made of
electrically-conductive tubing) electrically connected in parallel to one another.
By thus constructing the coil in electrically-parallel sections, the present invention
ingeniously enables the desired larger coils to be conveniently fabricated from small-bore
tubing or relatively narrow-gauge wire, while keeping the total coil inductance low enough
to be adequately driven by relatively low and safe voltages (e.g. below 250V), at high
frequencies (e.g. 0.5 - 1.5 MHz) obtainable from commercially-available generators, for
example the CEIA "Power Cube" (Trade Mark) 120V, lMHz, 2.5kW generators
sometimes used with small coils to heat small metal objects in the jewellery trade.
Preferably, in the coils according to the present invention, each of the said coil
sections comprises a proximal portion arched around the internal coil axis, a distal portion
arched around the internal coil axis, and a connecting portion extending substantially in the
same direction as the internal coil axis and connecting the proximal portion and the distal
portion together so that there is a space between them in a sense lying along the internal
coil axis.
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By "proximal" and "distal" portions is meant portions of a given coil section which
are respectively nearer to (proximal) and further from (distal) an observer looking along the
internal coil axis from one end of the coil. References to these proximal and distal portions
being "arched" around the internal coil axis are not intended to limit the arched portions to
any specific shape. Arches of angular shape, for example forming three sides of a
rectangle, would be conceivable, although more curved shapes may be preferable for
generating more uniform fields within the coil. Arches having "legsn on either side of the
internal coil axis, which legs extend in a relatively straight line from the side opening and
then curve approximately 180 around the coil axis towards the legs on the other side, may
be preferred for forming coils of convenient depth and side opening width, capable of
receiving relatively large objects wholly within the internal coil space. Substantially
circular arches, preferably subtending an angle of at least 180, preferably 225 or possibly
270, about their central coil axis, may also be useful.
It will be understood that the proximal (or distal) arched portion of a coil section may
be discontinuous if it is desired to form the arch from the two free end regions of the
continuous coil section. Preferably the free end regions will be positioned close enough
together so that any such discontinuity will have an acceptably small effect on the
uniformity of the field generated in the coil in use. Alternatively, the free ends of each coil
section may be located in the aforementioned connecting portion, allowing both arched
portions to be formed from continuous lengths of the coil section conductor. It is also
conceivable that the coil could be discontinuous both at the proximal portions and at the
distal portions of each section. Such a coil could be constructed of separate sections on
either side of the internal coil axis, each section having a proximal portion and a distal
portion, both of which portions arch around the axis toward the corresponding separate coil
section on the opposite side of the axis. However this structure would tend to complicate
the electrical and cooling fluid connections to an overlapping series of such "left-side and
right-side" coil sections. Continuous coil sections which extend along one side of the coil
axis and arch around it to return along the other side of the axis are therefore preferred.
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The indication that the said connecting portion of each coil section extends
substantially in the same direction as the internal coil axis is not intended to limit the
arrangement to strictly parallel alignments. The connecting portion need not be entirely
straight and may slope or deviate towards or away from the internal coil axis to some
extent, provided that it achieves the object of adequately spacing the distal portion from the
proximal portion along the axis. This longitudinal spacing is preferably sufficient to reduce
or nullify the degree of destructive interaction between the opposed fields generated in the
respective proximal and distal arched portions of each preferred coil section, which arched
portions may be im~gin~l as "travelling" in opposite directions around the internal coil axis
as one follows the coil section from one of its free ends to the other.
The longitll~1in~1 spacing is additionally advantageous when a first one of the said coil
sections has the proximal portion of at least one other said coil section aligned in the said
space between the proximal and distal portions of the first coil section with the distal
portion(s) of the said other coil section(s) aligned beyond the distal portion of the first coil
section.
This arrangement of the coil sections enables all the proximal arched portions
"travelling" in one direction (eg. clockwise) around the internal coil axis to be grouped
together separately from the corresponding group (further along the coil axis) of distal
arched portions "travelling" in the other direction (eg. anti-clockwise). Destructive field
interaction thus tends to be restricted to a small central area of the coil between the
respective proximal and distal groups. The spacing between immediately adjacent arched
portions will preferably be selected to maximise the axial length of the coil while
m~int~ining an acceptably uniform f1eld for the intended purposes in operation. Reference
to the proximal or distal portions being "aligned" is intended to convey the sense of the coil
sections being arranged to form a recognisable coil structure incorporating the four (or
more) arched portions provided by the two (or more) coil sections. Exact alignment is not
essential, and some deviation in ~lignm~nt and/or shape may be tolerable, provided that the
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field generated by the coil in use has a degree of uniformity suitable for the purpose in
question.
In an especially preferred form of the coils according to the present invention, two or
more, preferably not more than 5, said other coil sections are aligned with their respective
proximal portions in the said space of the first coil section and their respective distal
portions beyond that of the said first coil section. In this arrangement, the longitudinal
distance between the proximal and distal portion of the said first coil section will be
selected to suit the combined widths of the intervening proximal portions of the other coil
sections, together with the free space between adjacent proximal portions. It is furthermore
plefe~led that the distal portion of each successive other coil section is aligned beyond the
distal portion of the prece~ling coil section. This advantageously allows the respective coil
sections to be made fairly closely resembling one another in size and shape and proximal-
to-distal spacing, with only such minor variations as may be n~cess~ry for "nesting" the
overlapping coil sections within one another to form a recognisable, preferably substantially
uniformly aligned, open-sided coil.
The electrically parallel connection of the separate coil sections enables them
advantageously to be made from small-bore metal (eg. copper) tubing, for example not
more than 5 mm, preferably up to 4 mm, more preferably up to 3.5 mm, especially 2.8 to
3.2 rnm, in outside diameter. This in turn enables each coil section to be conveniently
made by bending a separate continuous length of the tubing having substantially uniform
diameter, whereas tubing of more than 5mm diameter would tend to require cutting and
joining, being too wide for bending to the required complex shapes. Each coil section
preferably has a total length of the tubing arching around the internal coil axis which
exceeds the total length of the connecting sections extending substantially in the same
direction as the coil axis. Fluid cooling of the coil in use will usually be advantageous, in
which case it is preferred that first ends of the said coil sections are connected in parallel to
a shared cooling fluid inlet manifold, and the other ends of the said coil sections are
connected in parallel to a shared cooling fluid outlet manifold. The said manifolds may also
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conveniently connect the respective ends of the coil sections electrically in parallel to one
another, the respective manifolds being connected electrically to opposite sides of the high-
frequency energising circuit.
Useful coils according to this invention for automotive harness-blocking purposes may
preferably have a side opening of width at least 20 mm, preferably at least 25 mm, more
preferably at least 30 mm; and a depth of at least 20 mm, preferably at least 25 mm, more
preferably at least 30 mm. Axial coil length of at least 45 mm, preferably at least 50mm,
measured along the internal coil axis preferably from the first proximal portion to the last
distal portion of all the coil sections, may also be ~ler~llcd.
The coil may be more-or-less rigidly attached to a high-frequency generator
incorporating the other known components of a so-called "tank circuit" capable of
resonating at the desired frequency, in which case objects to be inductively heated will
normally be brought to the coil. However, it may often be more convenient, especially for
heating large objects such as wiring harnesses, for the coil to be arranged as part of a tank
circuit in an independently-moveable module capable of electrically-inductive coupling with
a remote high-frequency generator. Preferably the module is electrically-inductively
coupled to a remote high-frequency generator by flexible electrical lead means at least 1
metre, preferably at least 2 metres, more preferably 3 metres, especially 3.84.2 metres, in
length.
The induction coil is the inductor in this remote tank circuit and capacitance is added
to the coil. By matching the capacitance and inductance of this remote tank circuit, a tuned
resonant circuit is created, which can be fed with a voltage alternating at the resonant
frequency to keep losses in the leads very small. It is preferable to choose the remote tank
circuit capacitance and the coil inductance so that the resonant frequency is within the
operating range of a commercially-available generator, whose output frequency ispreferably self-tuning to match that of the remote tank circuit. Because the losses in this
system are low, the power and physical size of the generator may be kept conveniently
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small. To reduce electrical losses in the leads, the current passing through them is
preferably reduced to a minimum by the matching of the frequencies. To achieve this, the
coil is preferably connected directly to the tank circuit capacitors in a remote housing
attached to the ends of the flexible leads. However, to achieve sufficient current to create a
desirably high magnetic flux within the coil, the coil's reactive impedance is preferably as
low as possible, preferably less than 5 ohms, more preferably less than 2 ohms, especially
below 1 ohrn; and to supply desirable power levels, the capacitors in the remote housing are
preferably as large as possible, preferably greater than 200nF, more preferably greater than
350nF, especially greater than 500nF. With a frequency of lMHz, which is very efficient
for inductive heating of very small metallic particles, as hereinafter described, a capacitance
of 500 nF requires a matching in-luct~nre of 0.05 microhenry, which is preferably achieved
according to the present invention by increasing the number of parallel-connected coil
sections to reduce the coil in~ ct~nre to the desired value.
The invention includes a method of electromagnetic induction heating, wherein a coil
according to any aspect(s) of the present invention is energised by a suitable high-frequency
generator and an object capable of being heated by electromagnetic induction is placed
within the field generated within the coil and is thereby inductively heated. In one
embodiment of this method, the inductively-heatable object is associated with insulated
electrical wires, preferably part of a wiring harness, placed so that the wires extend in a
direction substantially parallel to the internal coil axis, thereby minimi~ing inductive heating
of the wires. The inductive heating of the wires is approximately doubled if the lines of
flux within the coil intersect the wires at right angles rather than rurming substantially along
the wires as in this preferred arrangement. On the other hand, when the inductively-
heatable object comprises a heat shrinkable tubular sleeve carrying or incorporating
inductively-heatable magnetic particles, this is preferably placed so that the tubular axis of
the sleeve lies substantially parallel to the coil internal axis (which naturally occurs when
the sleeve surrounds the aforementioned part of a wiring harness). This time, the alignment
maximises the inductive heating of the sleeve (and preferably causes the sleeve to shrink),
since the m~gnPtic flux lines flowing along the direction of the sleeve wall will have a
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greater chance of interaction with the inductively-heatable particles than would flux lines
passing through the sleeve wall at right angles to its surface. Thus, the sleeve can be
shrunk by inductive heating while serendipitously minimi.~ing the risk of thermal damage to
the wire insulation.
In one especially preferred method according to this aspect of the invention, the
inductively-heatable object comprises the said part of a wiring harness surrounded by a heat
shrinkable sleeve, preferably an inductively-heatable heat-shrinkable sleeve (that is
preferably, a sleeve carrying or incorporating inductively-heatable magnetic particles), and
the sleeve also encloses a separate body of heat-activatable sealant material, preferably
inductively-heatable heat-activatable sealant material (that is preferably incorporating the
aforementioned inductively-heatable m~gn~tic particles), which melts and flows to block the
interstices within the said part of the harness when the sleeve and/or sealant material and/or
the wires of the harness is or are inductively heated by the field within the coil.
Inductively-heatable materials and induction heating methods for blocking electrical cables
or harnesses are described in US-A-5378879 (MP1474), the disclosure of which is
incorporated herein by reference.
The invention will now be further illustrated by way of example with reference to the
accompanying drawings, wherein :-
Figure 1 shows for comparison purposes in schematic perspective an example of theaforementioned known "bent pancake" coils;
Figure 2 shows in schematic perspective a coil according to the present invention
composed of three parallel-connected coil sections;
Figures 3A and 3B are schematic views from the end and looking into the side
opening of a coil similar to that shown in Figure 2;
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g
Figures 4A, 4B and 4C are schematic views of a similar coil according to the present
invention having five coil sections instead of the three illustrated in Figures 2 and 3; and
Figure 5 shows schematically a possible form of module or housing for the coil and
other components of the tank circuit for use with a remotely-coupled generator.
The known open-sided coil shown in Figure 1 is made from a single length of copper
pipe 10 extending from an inlet end 11 to an outlet end 12, the originally flat pancake coil
of roughly rectangular shape having been bent as shown around the inner coil axis inrlicated
by the line A to form proximal and distal arched portions "travelling" in the directions
indicated by the arrows on the arches, with conn~cting portions 13 extending approximately
parallel with the internal axis A. Even using the largest bendable ~ m~ter (about S mm) of
copper tubing to minimi~e the inductance, the maximum tubing length for a coil of this type
in a system with a remote tank circuit is about 300 mm, which limits the practical coil
dimensions to an internal diameter and depth of at most about 20 mm each and coil length
along the internal axis of at most about 40 mm. This limits the diameter of wire bundles
which can be placed inside the coil to about 15 mm, which represents a bundle of only up
to about 30 wires, which is small for the kinds of wires usually used in modern automotive
wiring harnesses.
In contrast with the coil of Figure 1, the coil according to the present invention
illustrated in Figure 2 is formed from three separate coil sections respectively having
proximal portions 20, 21, 22 and distal portions 23, 24, 25 arching around the internal axis
A and "travelling" in the direction of the arrows on the arches, with respective connecting
portions 26, 27, 28 extending roughly parallel to the internal axis A. It may be seen that
the arches formed by proximal portions 21 and 22, in the space between proximal portion
20 and its corresponding distal portion 23, are slightly wider than the arches formed by the
first proximal portion 20 and the three distal portions 23, 24, 25. This is done in order to
accommodate the preferred overlapping arrangement of the three generally similar coil
sections with the second distal portion 24 positioned beyond the first distal portion 23 and
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the third distal portion 25 beyond the second distal portion 24. If desired, the tubing could
be bent to allow proximal portions 21 and 22 over most of their length to align more closely
with the other arched portions, with the widening taking place only in the region where
proximal portions 21 and 22 approach closely to the connecting portions 27 and 28.
The free ends of the coil sections are connected respectively to cooling fluid inlet
manifold 29 and outlet manifold 30 made of copper, which effects the necessary electrical
connection of the three coil sections in parallel to one another. With this structure, the coil
may be made of 3 mm diameter copper tubing with each of the three sections 300 mm long,
making a total of 900 mm of easily-bendable copper tubing in the coil, while still achieving
a total in~ ct~n~e well within the operating range (for example less than 0.5 micro henries,
preferably less than 0.3 micro henries, more preferably 0.08 to 0.12 micro henries) suitable
for use in a system with a remote tank circuit. This enables the coil to be made with a
much more useful working volume, having an internal width of about 30 mm, a depth of
about 32 mm, and a length along the internal axis A of about 50 mm, capable of receiving
the increasingly common automotive wire bundles of up to 25 mm diameter incorporating
about 60 wires in an average automotive harness.
Figures 3A and 3B respectively show schematic views from an end and looking intothe side opening of coils approximately corresponding to that illustrated in Figure 2, with
the various parts numbered correspondingly. The aforementioned wider arches of proximal
portions 21 and 22 are illustrated in Figure 3A, and the preferred bending of connecting
portions 27 and 28 to bring their respective distal arched portions 24 and 25 into line with
the first distal portion 23 is indicated in Figure 3B.
Figure 4A is a view generally similar to that of Figure 3B of a coil comprising five
overlapping coil sections with the proximal portions 41, distal portions 42 and connecting
portions 43 arranged in a manner generally similar to that of Figure 3B. The inlet and
outlet manifolds have been omiKed in this view.
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P~K524 FF
Figure 4B shows an end view generally similar to that of Figure 3A with a coil 45
attached to a module 46 housing other components of the aforementioned remote tank
circuit. This view also illustrates the aforementioned alternative arrangement where the
proximal and distal arched portions are all more-or-less in ~lignment with widening only in
regions 47 near the connecting portion 48 to accommodate the overlapping structure. In all
versions, the coil may be protectively potted using appropriate known resin materials and
methods, as indicated by the broken lines 49.
Figure 4C shows an i(le~li.ced side view of the five-section coil having proximal
arched sections 50 conn~ct~cl to inlet and outlet manifolds 51, 52, distal arched portions
53, and conn~cting portions 54, in overlapping arrangement as before.
Figure 5 illustrates schem~tir~l~y a preferred construction for robust industrial use
such as on automotive harness production lines, wherein a coil 51 according to this
invention is enclosed within a housing 52 having a projecting lip 53 which protects the coil
from direct contact contact with an object such as an automotive wiring harness about
which the coil is to be positioned in use. This housing 52 may conveniently contain the
other components of the remote tank circuit to form an independently-moveable module,
which in use will be conn~cted to the required high-frequency generator by the
aforementioned flexible electrical leads, preferably incorporating fluid coolant conduits.
