Note: Descriptions are shown in the official language in which they were submitted.
~2~358~
This invention relates to inductive windings for
electrical transformers, reactors and the like, and more parti-
cularly to low loss, flat or rectangular shaped cabled conductor
for use in such equipment and a method of making the same.
In any high current or high frequency induction
apparatus electrical losses occur in the windings due to
skin effects and proximity effects, and it is known
that such losses may be reduced by dividing the conductors of
which the windings are made into small sub-conductors which may
or may not be insulated from each other and which may be
transposed relative to each other. For maximum efficiency the
transposition of the subconductors should be such that all sub-
conductors are linked by the same quantity of magnetic flux so
as to ensure tha-t each subeonductor wilL have -the same effective
induetance and therefore eaeh will carry its proper share of
the total current.
One method for transposing subconductors for large
induction equipment is deseribed in Canadian Patent 768,775,
issued to Westinghouse Electric Corp. on October 3, 1967, and
employs an odd number of rectangular subconductors having a
width to thickness ratio of about 3:1, arranged in two columns.
Each subconductor in turn is diseretely transposed at intervals
alonc3 the length of the eonduetor. Sinee, however, the
transpositions are made at diserete points only, a complete
transposition of all subconductors in a conductor containing
a large number of subconduetors, takes a long lenclth of cable
to aehieve. In induetion apparatus where the mclgnetic FielP
ehanges rapidly, for example in the end ~c~iorl ot a sm~lL
diameter air core reaetor, it is difFicult iF not impcssib1e
~k
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3~B
.,
to achieve sufficient transpositions to ensure that thecurrents are shared equally by the subconductors. Eddy
current losses per unit length of subconductor vary as the
cube of the subconductor dimension normal to the incident
magnetic field, so that rectangular subconductors are not
the optimum shape for the construction of induction apparatus
in which the direction of the magnetic field is different in
different regions of the apparatus. For example, in large air
core inductors, the field near the center plane of the reactor
is axial whereas the field near the end plane of the reactor
is radial. If the -thin side of the subconductor is arranged to
be perpendicular to the axis of the reactor, this will ensure
that the eddy losses are small near the mid-plane of the reactor
but it will also ensure eddy losses will be very large in the
conductors near the end plane of the reactor. The optimum shape
of the conductor for such apparatus would be to have subconduc-
tors that are square or round.
A cable comprisiny rectangular subconductors is easy
to bend in a direction normal to the long side of the subconduc-
tors, but is very dificult to bend in the direction normal to
the thin side of the subconductors without buckling the cable.
The use of square or round subconductors facilitates bending of
the main conductor in either of its principal directions.
The problems surrounding the use of rectangular
subconductors are a-t least partially solved in a construction
known in the art as a "Litz" cable (such as that sold by New
England E]ectric Wire Corp. Lisbon N.H., which in its basic
orm, is essentially a standard 1 x 7 cable construction which
may be roll pressed to a rectangular final cross sectional
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shape. The disadvantage of this construction is that the
central subconductor, or even ropQ, which acts as a core and
around which the xemaining six subconductors are continuously
transposed, does not change its position, i.e. is not trans-
posed, and consequently it does not carry its proper share of
the current, and the cable therefore has a poor packing factor~
It is, therefore, an object of the present invention
to provide an improved, conductor cable in which all the sub-
conductors in the construction are continuously and equally
transposed and which has an improved packing factor.
Another object of the present invention is to
provide a method for producing the improved continuously
transposed cable of the present invention. According to a
principal aspect of this invention there is provided a method
for making a continuously and uniformly transposed electrical
cable, comprising:
simultaneously winding a plurality of
electrical conductors around an elongate mandrel so that the
conductors are in parallel side-by-side relation at a
selected acute angle to the axis of the mandrel; and simul-
taneously with winding the conductors on the mandrel,
continuously withdrawing the wound cable from the mandrel.
Also, there is provided in accordance with the
present invention, a cable manufactured in accordance with
the oregoing method wherein the conductors are uniformly
and continuously transposed throughout the length of the cable.
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The invention is illustrated by way of example
with reference to the accompanying drawings wherein:
Figure 1 is an isometric view of one form of
discretely transposed cable according to the prior art;
Figure 2 is an isometric view of another
transposed cable of the prior art;
Figure 3 is an isometric view of a continuously
transposed cable according to the present invention;
Figure ~ is an isometric view of another
embodiment of the cable according to the present invention;
Figure 5 is a diagrammatic sketch of an apparatus
arranged to p.roduce the cable of Figure 3; and
Figure 6 is a diagrammatic sketch of an alternative
apparatus arranged to produce the cable of Figure 3.
Figure 1 illustrates a transposed cable of
the prior art comprising an odd-numbered plurality of rectan-
gular subconductors or strands 1, each having a width to
thickness ratio of about 3:1, arranged in two columns or
layers wi.th a strand at the end of one of the layers
projecting past the adjacent layer and transposed about
the main axis of the cable at a specific transposition point,
by means of a first discrete bend which moves the strand to
the adjacent layer and a second discrete bend which moves
each of the strands in the newly vacated layer, one strand
position in the same layer to fill the layer. It will, of
course, be appreciated that this method of transposition is
not entirely uniform or continuous and, :Eurthermore, the odd
strand on the top of the layers creates a non-uniform appearance
and is relatively bulky.
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Some of the problems of the cable of Figure 1 are
eliminated wi-th the cable of Figure ~ which represents the
simplest and least complicated form of a "Litz" cable, in which
a plurality of subconductors or strands are continuously trans-
posed about a core conductor or strand 3. Core 3 may be an
electrical conductor or may simply be an inert core such as a
rope core. The strands 2 may be single conductors or may them-
selves consist of a number of sub-subconductors which are bunch
laid or otherwise transposed, and the result is a uniformly
shaped cable having a relatively poor space factor due to the
presence of the non-transposed and largely electrically useless
core 3. Litz cable may be roll formed to a rectangular shape or
Elattened and may have multilayers of unilaid conductors.
Figure 3 illustrates a cable 4 of the present
invention in its simplest form and which consists of a plurality
of circular insulated or uninsulated subconductors 5 cabled
together without a core conductor or the like, so that each
and every subconductor 5 is uniformly and continuously transposed
along the length of the cable. It will, of course, be appreci-
ated that each subconductor 5 may be a single strand or a number
of bunch laid or cabled sub-subconductors which may in turn be
cabled. The cable 4 may be roll formed to achieve compaction
and to form the conductor into a rectangular or sheet form.
The cable 4 of Figure 3 is manufactured using a
method and apparatus as illustrated schematically in Figure 5.
Referring now to Figure 5 there is illustrated a plurality of
cable spools 10, each carrying a supply of insulated or un-
insulated conductor wire strands, rotatably mounted adjacent
the periphery of a circular base member 11 which in turn is
mounted on an axle 12 driven for axial rotation by means of
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drive means 19. Wlres from spools 10 are drawn through
respective guide holes 13 in a feed strand guide 14 mounted
on axle 12 for rotation therewith. The wires, as they are
drawn from the spools are wound around a mandrel 15 which
extends axially from axle 12 and is stationary relative thereto.
The wires are drawn from the spools by means of any suitable
cable gripping and drawing device shown schematically at 16.
The cabled conductors are continuously drawn off mandrel 15
as they are wound therearound and downstream from the mandrel
the cable may be press rolled at 17 to compress and shape the
cable into a rectangular, square or sheet, i.e. thin strip
cross-sectional shape as required. The cable may also be
wrapped with insulation by a conventional cable wrapping device
18. A barrier strip 20, from spool 21., may be introduced between
the mandrel 15 and the conductors. The conductors and barrier
strip are pulled off the mandrel simultaneously so that, after
roll forming, the barrier stri.p lies between the two sides of the
conductor and prevents the subconductors from touching each
other. As indicated above, the subconductors may be insulated
or uninsulated depending on.the importance of eddy currents in
the apparatus in which the cable is to be used. Where the
magnetic field strength is large and/or the frequency is high,
the strands may require insulation so as to keep eddy currents
small. It will be observed that if a barrier strip is
introduced, as described above, during manufacture, it is only
necessary to insulate every second subconductor in order to
achieve full isolation between the conductors.
A slightly more complex embodiment of the invention
may be achieved by using subconductors that themselves are
formed from a number of sub-subconductors which are insulated
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. .
and then bunched, cabled or otherwise transposed to form a
subconductor all of the sub subconductors of which will share
current uniformly.
Another more complex embodiment of the invention
is illustrated in Figure 4. In this embodiment, a second layer
of subconductors 45 (which may be either simple conductors or
may consist of sub-subconductors) is wound in the same direc-
tion over the top of the first layer 46 after this layer has
been roll formed into a compact rectangular shape so as to form
a unidirectionally laid, or unilaid, cable. The second layer
is also roll formed in order to compact the cable and to make
its cross-section rectangular.
When a very large number of subconductors is to be
used to form a sheet which is very wide and thin, the use of
the rotating bobbin-stationary mandrel concept described with
reference to Figure 5 becomes difficult and the continuously
transposed cable may be manufactured as shown in Figure 6. In
this alternative process the subconductors are drawn from an
array of reels 61 rotatably mounted on a fixed frame through a
strand guide 62 onto a rotating mandrel 63 by a take up reel
67. A barrier strip 64 formed into a cylinder at 65 may be
introduced between the mandrel 63 and the conductor as
described above. The conductor and barrier strip are slid over
the rotating mandrel continuously and then roll formed at 66
to achieve compaction and to form the conductor into a
rectangular or sheet form. In this embodiment the roll forming
mechanism 66 must be rotated at the same speed as the mandrel
63 as must the taping machine (if provided) and the take-up
reel 67. An alternative method of achieving compaction is to
use subconductors which are already square in cross-section or
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to use round subconductors but to roll form them (68) into a
rectangular sheet prior to winding them on the rotating
mandrel (63).
Cables manufactured according to the present
invention offer several advantages over the transposed cables
of the prior art. For example:
(a) All subconductors of the new cable are identically,
continuously, and uniformly transposed. Conventional transposed
cables transpose strands discretely rather than continuously.
(b) Since all strands are transposed in an identical
manner, the cable has a very uniform appearance and has no
projections or bulges which would make it difficult to wind.
(c) The length along the cable which is required to
make a complete transposition of all subconductors can be
made very short by increasing the pitch when winding the sub-
conductors onto the mandrel. l`he length required for a complete
transposition can be made much shorter than is possible in
conventionally transposed cable.
(d) The continuously transposed cable of the present
invention can be made either from rectangular subconductors
or round subconductors. If round subconductors are used, the
cost of making the cable is considerably less than the cost of
making conventional transposed conductors which uses rectangular
subconductors.
(e) Since the subconductors even after roll forming have
a shape which is nearly square, the eddy loss in the sub-
conductors can be kept very small regardless of the orientation
of the subconductors with respect to the local field in the
piece of apparatus in which the cable is used. For example,
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:~L2~33588
the magnetic field of an air core reactor is mainly axial near
the mid~plane and nearly radial in the end-plane of the reactor.
Since the eddy 109s per unit length of a subconductor is pro-
portional to the cube of the dimension which is normal to the
incident field, it is difficult to achieve low eddy losses in
an air core reactor if rectangular subconductors are used. If
the subconductors are arranged so that their thin side is normal
to the axis of the reactor, then the eddy loss in the subconduc-
tors near the mid-plane of the air core reactor will be small but
the eddy loss in the subconductors near the end-plane of the
reactor will be very large since the long dimension will be
normal to the local field near the end-plane since this field
is radial. Since the subconductors in the present cable are
nearly square, their shape is nearly optimum in all regions of
the air core reactor.
(f) Since the present cable can be manufactured from
round conductors~ a much smaller inventory of subconductors is
required in order to achieve a very large variety of cable
cross-sections .
(g) The continuously transposed cable is caDable of
being wound with either side normal to the coil axis which
is not easily possible with the regular transposed conductor
which consists of rectangular shaped subconductors. The
shorter the pitch, the easier it is to wind the continually
transposed cable with its large side normal to the coil axis.
(h) The subconductors may themselves be composed of
bunched, cabled or otherwise transposed and insulated sub-
conductors.
(i) The con-tinuously transposed cable may be tapped at
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any point. This is not easily done with many other types of
transposed cable.
In order to illustrate the advantages of the
present invention in practice, in both low and high frequency
applications the following examples are provided.
Example 1
Comparison of Continuously Transposed Cable and Rectangular
Discrete Transposed Cable in 42 MVA, VAR Compensation Reactor
The overall dimensions of this coil will be the
same whether it i5 made with traditional rectangular trans-
posed cable or with continuously transposed cable. Howeverthere is a significant difference in the conductor eddy losses.
Rectangular transposed cable consisted of sub-
conductors of rectangular section. The width to thickness
ratios of the subconductors were in the range of 2:1 or 3:1.
Eddy loss is proportional to the cube of the dimension which
is normal to the incident field. In the case of rectangular
subconductors it is only possible to orientate them so that
their smallest dimension sees the incident field in one part
of the winding (normally the middle portion) but the larger
dimension will see the largest portion of the field in another
part of the winding (the end portion) and the eddy loss will
be significantly higher. In the case of the continuously
transposed cable the subconductors can be round or square
and hence the eddy loss will be the same in all parts of the
winding.
Now consider a typical rectangular subconductor
of .05 by .10 with a cross-sectional area of .005 in.2. The
equivalent area of round subconductors from continuously
transposed sheet would be .08 in. in diameter. The net saving
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in eddy loss is calculated to be 10~ e~ the round subconduc-
tor would have 10~ lower eddy loss. However i~ must be emphasized
the rectangular subconductors of .05 x .1 in. are about the
smallest that existing transposing machines can handle. In
the case of continuously transposed cable, subconductor sizes
on the order of .05 in. in diameter were utilized with no
difficulty. The eddy loss was, therefore, 39% of that for
.08 in. diameter round and 29~ of the rectangular subconductor
(.05 x .10). The continuously transposed cable will have more
subconductors.
RECTANGUI.AR TRANSPOSEDCONTINUOUSLY TRANSPOSED
CABLE DESIGN CABLE DESIGN
.
outar diameter120" 120"
overall height80" 80"
weight18,000 lbs. 18,000 lbs.
I2R loss 84 KW 84 KW
Conductor eddy loss 14 KW 4 KW
Support Structure
(spider,etc) eddy loss 26 KW26 KW
TOTAL LOSSES 124 KW 114 KW
Example 2
Comparison of Continuously Transposed Cable and Rectangular
Discrete Transposed Cable ln an Air Core Commutation Reactor
rated 10-~uH, 300 amperes r.m.s.
The ringing frequency is 4 kHz and the Q at the
frequency must be approximately 250. In order to meet the Q-
requirements and the rms current requirements, it can be shown
that approximately 2800 strands of #30 AWG insulated copper in
parallel are required. Assuming that the basic sub-conductor
will comprise 80 #30 conductors in bunch lay, then 34 subconductors
~ z(~3588
will be required.
Using the cable construction described herein, 34
subconductors each comprising 80 #30 bunched copper strands
may be used to produce a flat cable about two inches wide and
0.2 inches thick. This cable will be perfectly transposed and
have a packing factor of about 0.6. In the alternative it is
possible to use 17 subconductors each comprising 80 bunched
#30 copper strands to produce a 1 inch by 0.2 inch flat cable
having a packing factor of 0.6. Two of these cables can be
used in parallel providing that they are properly transposed
themselves to carry equal currents.
If regular "Iitz"-wire is to be made, there are
several options. If only one cabling operation is permitted,
then in order to guarantee perfect transposition, a type 4 Litz
construction must be used which consists of 34 subconductors
cabled around a central non-conducting core. The diameter of
this cable will be approximately 1.2 inchesand the packing
factor approximately 0.19, only 1/3 of that for the continuously
transposed sheet. A 17 subconductor cable would have a packing
factor of only 0.26, less than 1/2 that of the continuously
transposed cable.
The coil made from the continuously transposed
sheet is approximately 20% lighter and 25% smaller in diameter
and height for the same Q factor.
When making cable for high-frequency use, it is
possible with the cable construction method described herein
to perfectly transpose very large numbers of strands with only
two opera-tions (bunching and cabling) and to provide at the
same time a very good packing factor (greater than 0.5). With
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conventiona Litz, the only way to perfec-tly transpose a large
number of subconductors in one pass is to use a non-conducting
central cylinder about which to cable the subconductors. This
provides a very poor packing factor, the larger the numb~r of
subconductors the poorer the packing factor.
