Insulated magnet wire, method of forming the same, and transformer windings formed therefrom

A magnet wire of modified cross-section is electrically insulated by adhering thereto an insulation tape which does not require a high temperature adherance step. The insulation tape has a pressure-sensitive adhesive coating which, prior to application to the magnet wire, is covered by a release strip. Just prior to application of the insulation tape to the magnet wire, the release strip is removed from the tape to uncover the adhesive coating and allow pressure-sensitive bonding of the insulation tape to the magnet wire.

FIELD OF THE INVENTION 
This invention relates to an improved insulated magnet wire, to a method of 
forming the same, and also to a coil formed therefrom. 
DESCRIPTION OF RELATED ART 
In commercial magnet wire applications, an electrical insulation material, 
in tape form is commonly employed as electrical insulation for the magnet 
wire. The insulating tape is coated with a heat-cured adhesive substance 
which substance is cured by heating after application of the tape to a 
wire, which wire is essentially square or rectangular in cross section. 
U.S. Pat. No. 4,159,920 to G. Andersson et al, granted Jul. 3, 1979, 
describes a typical prior art method for insulating a magnet wire with a 
wrapped insulation tape which is precoated with an epoxy adhesive resin. 
The epoxy must be thermally cured after application thereof to the magnet 
wire to insure adhesion between the tape and the wire. British Patent 
Specification 1,233,862 published Jun. 3, 1971 discloses a similar 
procedure for coating and forming magnet wire. 
One problem with the aforesaid magnet wire manufacturing procedure is the 
required heat curing of the adhesive to drive off volatile solvents that 
are employed when a binder material is used co-extensive with the 
adhesive. It would be economically advantageous to apply an adhesive 
coated electrical insulation tape to a magnet wire without incurring the 
extra process cost and time involved in heating and reheating the coated 
magnet wire to cure the adhesive. 
Other problems that occur with magnet wire manufactured in accordance with 
the aforesaid prior art include the use of rigid (or stiff) insulating 
tapes which result in splitting or cracking of such tapes as the magnet 
wire is wound around a square or rectangular mandrel to form an electrical 
winding. 
The splitting and cracking of the insulation tape is caused partly by the 
use of rigid insulation tape materials that do not stretch or flex to 
conform to the shape of the magnet wire when the latter is wound around 
the mandrel. Heating the insulation tape to cure the adhesive makes the 
insulation tape even more brittle and more susceptible to splitting or 
cracking. 
An additional contributor to rupture of insulation on a coiled magnet wire 
is the cross-sectional shape of the wire after it has been formed into a 
coil. When an insulated magnet wire of essentially square or rectangular 
shape, as described in the prior art, is wound around a mandrel to form a 
coil winding, plastic deformation of the wire results as the wire is taken 
through the ninety degree bends of the coil. The tensile forces on the 
side of the wire opposite the mandrel cause the width of that side of the 
wire to contract, while the compressive forces on the side of the wire 
facing the mandrel cause the width of that side of the wire to expand 
whereupon the resulting magnet wire cross section assumes a trapezoidal 
configuration. The resultant trapezoidal configuration of the wire 
increases the overall width of the magnet wire, so as to significantly 
increase the amount of space taken up by the wire in each adjacent turn in 
the coil. The trapezoidal cross-section of the wound magnet wire also 
creates sharp edges on the wire at the corners of the windings which can 
result in rupture of the insulation tape thereby causing electrical arcing 
between adjacent winding turns. 
Additionally, current prior art processes for applying the insulation tape 
are relatively slow and must be accomplished as separate, off-line 
operations since process speed is dependent on the time required to heat 
and reheat the wire. 
SUMMARY OF THE INVENTION 
This invention relates to a magnet wire which has a cross-sectional 
configuration that reduces abrasion of the insulation tape, and also 
occupies minimal space during the electrical magnet wire winding operation 
in that the cross sectional configuration of the wire of this invention 
produces a dimensionally stable wire that will not substantially deform 
when wound into a coil. 
The insulation tape is a fibrous soft, flexible material which has one side 
thereof coated with a pressure-sensitive adhesive to provide pressure 
sensitive bonding properties to the insulation tape without the need to 
heat-cure the wrapped wire. The adhesive is covered with a coated release 
strip that is stiffer or more rigid than the insulation tape to prevent 
stretching of the insulation tape prior to application thereof to the 
wire. The release strip also prevents the adhesive from being exposed to 
contaminants prior to application thereof to the wire. The coated release 
strip is removed from the insulation tape to uncover the adhesive 
immediately prior to application of the insulation tape to the magnet wire 
surface. 
It is therefore an object of this invention to provide an insulated magnet 
wire that does not require heat to cure the insulation materials or to 
drive off organic solvents from the adhesives used for bonding the 
insulation materials to the magnet wire. 
It is another object of this invention to provide an insulation tape and an 
application process that permits the use of insulation tape materials 
which allow the insulated magnet wire to be wound around square or 
rectangular mandrels without cracking or splitting the corners of the 
insulation tape. 
It is a further object of the invention is to provide a new conductor wire 
shape having a modified rectangular cross-section that minimizes the space 
required for each turn when the insulated magnet wire is formed, while 
decreasing damage to the insulation tape. 
It is an additional object of this invention to provide a high speed method 
of applying insulation tape to magnet wire.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 depicts the modified rectangular cross-sectional shape of a magnet 
wire conductor 10 prior to application of a layer of insulation tape where 
the radius of curvature R of the opposing ends 10C and 10D of the 
conductor 10 equals, or is greater than the wire thickness T defined 
between the opposing sides 10A and 10B of the conductor 10. It has been 
determined that this optimum side geometry, which is proportional to the 
thickness of the wire, beneficially reduces abrasion to the insulation 
tape when the latter is applied to the magnet wire and subjected to 
subsequent coil winding operations, as will be described in greater detail 
hereinafter. The optimum geometry also reduces the space that each turn of 
the magnetic wire requires within an electrical magnet wire winding. The 
magnet wire geometry depicted in FIG. 1 does not cause expansion of the 
width of the magnet wire during the winding operation because when the 
side radius R is equal to or larger than the wire thickness T the 
conductor wire 10 cannot be deformed to the trapezoidal configuration 
during the coil winding process. The formation of sharp edges on the 
covered wire is thus prevented. Since the conductor wire 10 will not 
physically deform during the coil winding operation, it will not, when 
wound, laterally expand at the corners of the winding. This allows the use 
of thicker insulation tapes. For example, a rectangular wire having a 
width dimension of 0.300 inch which is used to form a coil that can expand 
to a dimension of 0.312 inch wide when bent 90 degrees around a winding 
mandrel. When the modified wire of this invention is used, the width 
dimension of the wire will not expand when the insulated wire is wound 
into a coil, thus allowing the use of bulkier insulation in the same coil 
space. If the side radius of the wire is substantially less than the 
thickness of the wire, then the mass of the coil will be insufficient for 
optimum coil performance. 
Further conductor efficiency can be gained by forming the radius of 
curvature R only on the bottom half of the magnet wire that is closest to 
the mandrel as the magnet wire is being wound. The remainder of the sides, 
as indicated in 5E in FIG. 5 can be rectilinear. This results in a 
desirable increase in the overall cross sectional area of the magnet wire 
with no loss of space, and only a slight decrease in resistance to 
abrasion between adjacent windings, because the radiused side of the wire 
is the inner, normally expanded side when the wire is formed into a coil 
whereby formation of the trapezoidal cross section is avoided. 
It is noted that the improved abrasion resistance imparted to the magnet 
wire having the cross-section depicted in FIG. 1 allows the use of softer, 
more flexible insulation tapes that stretch in all directions to allow the 
insulation tape to elongate and conform to any changes in the magnet wire 
configuration that occur during the magnet wire winding operation. 
The fibrous highly stretchable insulation tape allows greater flexibility 
to the wrapped wire. The insulated magnet wire of the prior art must pass 
a flexibility qualification standard which requires that the wrapped wire 
must be wrappable on an arbor with a 4:1 diameter proportion relative to 
the major cross-sectional dimension of the wrapped wire, without 
exhibiting any insulation cracking or splitting. The insulated magnet wire 
of this invention can be wrapped on an arbor with a 1:1 arbor/wire 
diameter ratio without cracking or splitting the insulation tape. This 
quality is a highly desirable result of the invention, which cannot be met 
by the prior art heat-cured insulated magnet wire. Since the insulation 
tape of this invention does not require heat to bond it to the wire, it 
will retain its initial soft and flexible properties. 
The magnet wire conductor 10 (hereafter "conductor") is formed into an 
insulated covered magnet wire 11 by the application of a continuous web of 
electrical insulating material 12 which includes a coating of 
pressure-sensitive adhesive 13 as illustrated in FIG. 2. In some 
applications, the insulation covering may be omitted across the top and 
one end of the conductor, in order to conserve insulation. The electrical 
insulating material is a soft, flexible fibrous material, and can be 
formed, for example, of glass fibers aramid fibers, polymer fibers, and 
combinations thereof. Fibrous aramid materials that are manufactured 
utilizing spunlacing or hydraulic fiber entanglement techniques are 
especially beneficial since they normally possess multi-directional 
elongation properties. 
An insulating tape composite 15 consisting of an insulation tape component 
14 and release strip component 16 can best be seen by referring now to 
FIG. 3C. The insulation tape component 14 is prepared by coating the 
insulation material 12 with the pressure sensitive adhesive 13, as 
depicted in FIG. 3A. The release strip component 16 is prepared by coating 
a release paper material 17 with a release agent 18. Immediately after the 
application of the adhesive 13 to the insulation material 12 to form the 
insulation tape component 14 the release strip component 16 is affixed to 
the insulation tape component 14 by covering the adhesive 13 with the 
release agent 18. The resulting insulating tape composite 15 can be rolled 
into a continuous reel for easy shipment and handling and can be later 
applied to the wire conductor 10 as best seen by referring to FIG. 4. 
The wire coating assembly 19 is arranged next to a continuous source of the 
conductor 10 as it is being formed or extruded, or can be arranged 
independently as an off-line operation, if desired. 
The insulating tape composite 15 described earlier is drawn from a supply 
reel 20 by a pair of driven rollers 21. The driven rollers are 
synchronized with the speed of the continuous source of the conductor 10 
as it is drawn through the wire covering assembly so that slack as 
indicated at 15' is created thereby eliminating any tension on the 
insulating tape composite 15 between the drive rollers 21 and the point of 
application to the conductor, so as to prevent any premature stretching of 
the insulating tape composite 15. The insulating tape composite 15 is 
guided to the conductor 10 by passing through the guide block 22. 
Immediately prior to making contact with the conductor at the leading edge 
of base plate 23, the release strip component 16 is separated from the 
insulating tape component by means of a stripper block 24 and taken up by 
red 29. The adhesive 13 is thus exposed so that pressure sensitive bonding 
of the insulation tape component 14 to wire side 10A is accomplished, 
while folding the remaining unbonded portions of the insulation tape 
component 14 to facilitate bonding thereof to the ends 10C and 10D of the 
conductor 10. A set of opposing elastomeric rollers indicated generally by 
the numeral 25 apply pressure to the ends 10C, 10D and deform to press and 
bond the insulation tape component 14 to the conductor ends 10C and 10D, 
while folding the remainder of the insulation tape component 14 around 
each conductor end 10C and 10D in position to faciltate bonding to 
conductor side 10A or 10B. The final bonding step is completed as roller 
27 applies pressure to side 10A or 10B to complete the insulation covered 
magnet wire 11 as illustrated in FIG. 2. The completed insulated magnet 
wire is then collected on a reel 28 as shown for later use, or as 
mentioned previously can be fed directly into a coil winding station. 
Still referring to FIG. 4 it is noted that when the insulating tape 
composite 15 is drawn from the supply reel 20 by the driven rollers 21, 
the release strip component 16, due to the stiffness properties of the 
paper material, functions to prevent any premature stretching of the 
insulation tape component 14, thus preserving the elongation properties of 
the insulation tape 14. The release strip component 16 functions to also 
protect the adhesive from contaminants until the moment that the 
adhesive-coated insulation tape 14 is applied to the conductor. 
An insulated covered magnet wire is produced for use in transformers, 
motors, and the like according to this invention by using a stretchable 
insulation covering, which is applied to a magnet wire of modified 
rectangular cross-section. By using a pressure-sensitive adhesive, without 
the use of supplemental solvents or the application of heat, the invention 
results in a cost-effective environmentally favorable magnet wire-forming 
process. 
The fibrous, flexible, soft insulating tape, and its insulation 
tape/release strip composite is the invention of Martin Weinberg, and is 
disclosed and claimed in a copending U.S. patent application Ser. No. 
07/801,745 filed Dec. 3, 1991, entitled Magnet Wire Insulation. The 
pressure sensitive adhesive which is preferred for use with the spunlaced 
aramid insulation is a thermosetting polymethymethacrylate crosslinkable 
pressure sensitive adhesive of high molecular weight which is saturated 
and resistant to oxidation. The preferred release agent coated onto the 
paper release strip is polydimethylsiloxane which is thermoset with a 
crosslinker and catalyst, and which forms a surface on the paper release 
strip which resists penetration by the acrylic adhesive which it covers. 
The release strip thus peels readily away of the adhesive coated surface 
of the insulation when the insulation is laid onto the conductor wire. 
It will be appreciated that this invention involves the use of a wire with 
a modified cross section, which allows the application of a soft, flexible 
insulation tape to the wire. No lateral expansion of the wire will occur 
when the covered wire is formed into a coil. The result is a faster 
insulating process and a more flexible insulated wire. 
Since many modifications and variations of the above described embodiment 
of the invention will be readily apparent to those skilled in the art, it 
is not intended to limit the invention otherwise than as required by the 
appended claims.