Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
CQRE FOR ELECTROMAGNETIC INDUCTION DEVICE
BAC~GROU~D OF THE INVENTION
Field of the Invention
. _ _
This invention rela~es to magnetic core
structures for use in electrical induction apparatus
such as transformers, motors, generators and the like.
Description of the Prior Art
Magnetic devicesl such as transformers,
motors, generators and -the like oftentimes include wound
core members composed of magnetically soft material.
The material, in the form of continuous strip is
typically wound on a suitable mandrel and annealed to
relieve winding stresses. The l~landrel is then removed
from the core, which is cut and treated for receiving
win~ings thereon.
One of the rla~or problems with toroidal core
members is the core loss produced by eddy currents
present in and between wound layers of the strip. This
lossl which varies as the square of strip width, is so
large that it has previously been necessary to form the
core from a number of laminated plates wound or stamped
from the strip, individually coated with insulating
material and wound or stacked one upon another on the
flat side thereof. As a result, magnetic cores for
electromagne~ic induction devices have low operating
efficiency and high construction and material costs.
SUMMARY OF THE INVENTIOM
_ _ _
Briefly stated the present invention provides
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a magnetic core for an electromagnetic induction device
that is economical to make and highly efficient in oper-
ation. The magnetic core comprises a plurali~y of mag-
netic core elements, each of which is formed by windiny
a plurality of layers of uninsulated strip of magnet
ically permeable material. The magnetic core elements
are juxtaposed together to form a core stack, the heiyht
of which is large relative to the strip width of each
element. The core elements are electrically isolated
from each other by insulating material interposed
between the elements at the region of juxtaposition.
BRIEF DESCRIPTION OF THE DRA:WINGS
The invention will be more fully unders~ood
and further advantages will become apparent when
reference is made to the following detailed description
of the preferred embodiment of the invention and the
accompanying drawings, in which:
Figure 1 is a perspective view of a trans-
former incorporating the composite core of this inven-
tion;
Figure 2 is a section taken through line 2-2
of Fi~. l; and
Figure 3 is an exploded perspective view
illustrating the composite core construction of the
transformer shown in Fig. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT~
Referring to Figures 1 and 2 of the drawings,
there is shown generally at 10 an electrorl~agnetic in-
duction device having the magnetic core 12 of this ln-
vention. The magnetic core 12 comprises a plurality ofmagnetic core elements 14. Each of the core elements
14 is formed by winding a plurality of layers 16 of
uninsulated strip 18 of magnetically permeable material~
The elements 14 are juxtaposed toyether to form a core
stack 20, the height~ h, of which is large rel~ative to
the strip width, w, of each element. Core elements 14
are electrically isolated from each other by insulating
material 22 interposed between the core elements 14 at
the region of juxtaposition 24. 1 1 5 8 3 2 5
The s~rip 18 used to wind the magnetic core
elements 14 is cornposed of rnagnetically soft material.
Such material desirably has the following combination of
properties: (al low hysteresis 105s; (b) low eddy
current loss; ~c) low coercive force; (d) high magnetic
permeability; (e~ high saturation value; and (f) mini~lum
change in permeability with tem~erature. Conventionally
- employed magnetically soft material in strip form, such
- 10 as high-purity iron, silicon steelsy iron/nickel alloys,
iron/cobalt alloys and the like, are all suitable for
use in the practice of the present invention Particu-
larly suitable, however, i5 strip 18 of amorphous
(glassy) magnetic alloys which have recently become
available Such alloys are at least about 50%
amorphous, as determined by x-ray diffraction Sush
alloys include those having the formula
M60_9o To_l5 X10_25, wherein M is at least one of the
elements iron, cobalt and nickel, wherein T is at least
one of the transition metal elements~ and X is at least
one of the metalloid ~lements of phosphorus, boron and
carbon. Up to 80 percent of the carbon, phosphorus
and/or boron in X may be replaced by aluminum, antimony,
beryllium, germanium, indium, silicon and tin. Used as
cores of magnetic devices, such amor~hous metal alloys
evidence generally superior properties as compared to
the conventional polycrystalline metal alloys commonly
utilized. Preferably~ strips of such amorphous alloys
are at least about 80% amorphous, more prefer~bly yet,
at least about 95% amorphous.
The amorphous magnetic alloys of which strip
18 is preferably composed are formed by cooling a melt
at a rate of about 10 to 10 C/sec. A variety of well-
known techniques are available for fabricatiny rapid-
quenched continuous strip. When used in magnetic coresfor electromagnetic induction devices, the strip 18
typically has the form of wire or ribbon. The strip 18
is conveniently prepared by casting mol~en material
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direetly onto a ehill surface or into a quenehiny medium
of some sort. Such processing techniques considerably
reduce the cost of fabrication, since no intermediate
wire-drawing or ribbon-forminy procedures are required.
The amorphus metal alloys of which strip 18
is preferably composed evidenee high tensile strength,
typically about 200,000 to 600,000 psi (1.38-4.14 x
106 kPa), depending on the partieular composition. This
is to be compared with polycrystalline alloys, which are
used in the annealed eondition and whieh usually range
from about 40,000 to 80,000 psi (2.76~5.52 x 106 kPa).
A high tensile strength is an important consideration in
applications where high centrifugal forces are present,
such as experienced by cores in motors and generators,
sinee higher strength alloys allow higher rotational
speeds.
In addition, the amorphou~ metal al:Loys used
to form strip 18 evidence a high eleetrical resistivity,
ranging from about 160 to 180 microhm-cm at 25C, de-
pending on the particular composition. Typical priorart materials have resistivities of about 45 to 160
microhm-cm. The high resistivity possessed by the
amorphous metal alloys defined above is useful in AC
applieations for minimizing eddy current losses, whieh,
in turn, are a factor in reducing core loss.
A further advantage of using amorphous metal
alloys to form strip 18 is that lower eoereive forees
are obtained than with prior art COlllpOSi tions of sub-
stantially the same metallie eontent, thereby permitting
rnore iron, whieh is relatively inexpensive, to be
utilized in the strip 18~ as eompared with a greater
proportion of niekel, whieh is more expensive.
Referring to Figs. 2 and 3 of the drawings,
eaeh of the magnetic core elements 14 is formed by wind-
ing sueeessive turns of strip 18 on a mandrel (notshown). During winding of suecessive turns, strip 18 is
kept under tension to effect tight formation of the eore
element 14. The number of turns required for a given
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core element 14 can range from a few turns to several
thousand turns, depending upon the power capacity of the
electromagnetic device desired. When the required
number of turns are wound for a given core element 14,
the strip 18 is cut across the width, w, thereof, the
outer turn being held in wound relation to the preceding
turn. Typically, the cut end of the last turn of strip
18 is spot welded, clamped or otherwise secured to the
wound core element 14.
When sufficient turns have been wound to fori~l
a given magnetic core element 14 as above described, the
mandrel is removed therefrom to produce the core element
14 shown in Fig. 3. The core element 14 has a width
defined by the width of strip 18 and a build defined by
the number of turns of strip 18 times the strip thick-
ness, t. Amorphous metal strip is rela~ively thin as
compared to rolled crystalline strip. Moreover, the
composite core construction of magnetic core 12
eliminates the necessity for individually coating each
wound layer of strip 18 used to form core ele-~ent 14.
As a result, the core element 14 can be wound into a
smallerl lighter element at lower construction, process-
ing and material costs ~han magnetic cores having an
insulated interlaminar construction~ Generally, the
wi~th of strip 18 ranges froJn about .25 to 2.5 centi-
meters and the thickness of strip 18 ranges from about 1
to 2 mils. The build of each core element 14 can range
from as low as 4 mils to as great as 25 centimeters or
more depending upon the power requirements of the
electromagnetic device.
Magnetic core 12 is assembled by sandwiching
a layer of insulating material 22 between plural core
elements 14. The core elements 14 may be bonded
together by the insulating material 22. Alternatively,
core elements 14 and insulation layers 22 can be placed
successively on a spool composed of thermoplas~ic or
thermosetting material. The number of core elements 14
used to construct magnetic core 12, as well as the
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dimensions of the core elements 14 and overall hei~ht,
h, of the magnetic core 12 will vary depending on the
power capacity and operating frequency of the electro-
magnetic device. For electrornagnetic devices having an
operating frequency of 60f-1z and a power capacity rang-
ing from 100 to 20,000 watts, the maximum acceptable
strip width i9 about 1 inch (2.54 cm) the number of core
elements 14 used to construc-t magnetic core 12 is about
3 to 10, the height, h, of magnetic core 12 is about
2 to 10 inches ~5.08-25.4 cm) the inside diameter o:E each
core element 1~ is about 1 to 6 inches (2~54-1.52 cm)
and the outside diameter of each core element 14 is
about 2 to 20 inches (5.08-50~8 cm). For electro
maqnectic devices having an operating frequency of
about 10 K~z and a power capacity ranging from ]00 to
20,~00 watts, the maximum acceptable strip width is
about -~ inch (6.3 x 10~1 cm), the number of core ele-
ments 14 used to construct magnetic core 12 is about 3
to 10 inches, the height, h, of magnetic core 12 is
about 2 to 10 inches (5.08-25.4 cm), the inside
diameter of each core element 14 is about 1 to 3 inches
(2.54-7.62 cm) and the outside diameter of each core
element 14 is about 2 to 10 inches (5.08-25.4 cm).
The insulating layers 22 disposed between core
elements 14 can be composed of any sui-table insulating
rnaterial such as thermosetting or thermplastic
material, glass, cloth, fiberglass, polycarbonates,
mica, CAPSTAN, (a registered trademark of ~upont
Corporation), LEXA2~1 (a registered trademark of General
Electric Corporation) fish paper and the like, having
the required flexibility dielectric strength, toughness
and stability at the design operating temperature of
the magnetic core 12, normally in the vicinity of
130C. As shown in Fi~. 3, insulating layers 22 are in
the form of a flexible film having a thickness of about
~2 mil and inside and outside diameters substantially
equivalent to those of core elements 14. Electrical
~,
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isloation of core elemen-ts 14 can alternatively be
accomplished by disposing insulating material over part
of the buil.d portions between ad~oining core elements
14. Thus, the insulating
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layer 22 disposed between adjoinlng core elelnents 14 can
have the form of a spider or other suitable configura-
tion adapted to physically separate and electrically
isolate the adjacent core elements 14. In such a case,
electrical isolation of core elements 14 is effected by
an insulatiny layer 22 comprised in part of air. Still
further, the insulating layer 22 can be painted, sprayed
or otherwise applied to one or both of the adjoining
surfaces of core elements 14.
Construction of a transformer 11 incorporating
magnetic core 12 can be readily effected by toroidal
winding of primary and secondary ~urns 30, 32 of copper
or aluminum wire or ribbon about the magnetic core 12,
or by hand threading the copper or aluminum wire turns
about the magnetic core 12 in a conventional Inanner-
The elimination of interlaminar insulation afforded by
the sectionalized construction of magnetic core 1~
substantially reduces -the length of the copper turn
required, and decreases the copper loss of the
electromagnetic device 10.
The reduction in core loss resulting from the
composite core construction of magnetic core 12 has been
demonstrated by a 15 KVA transformer wound with seven
core elements 14 of 1 inch (2,54 cm) wide uncoated s~rip
18.
If the transformer core had been wound with
uncoated strip 18 in the form of wide ribbon in a single
section, a current, I, would have flowed between the
ribbon layers. This current would be R where E is the
voltage induced in the magnetic core and R the effective
interlayer resistance. As per Faraday's Law of Induc-
tion, E is proportional to the frequency, the flux
density and the core area. R, the effective inter-
layer resistance, is proportional to the interlayer
resistivity and inversely proportional to the area of
contact between the layersO The interlayer core loss
would then be: P = I R = E
If, on the other hand, the ~nagnetic core 12 is sectioned
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into "nl' number of core elements 14 insulated from each
other, in accordance with the invention, there will now
be a current i flowing in each core element 14 due to an
induced voltage e and an interlayer resistance r.
Since the core area of the core element 14 is now n
times smaller, the induced voltage e :is:
e = En (l)
The contact area determining the inter.layer resistance
is now n times smaller, therefore the effective inter-
0 layer resistance of each element is:r = nR (2)
The interlayer core loss, p, of each element will be:
p = i2r = e2
Substituting for e and i from equations l and 2:
p - E
R n3
Since the core is composed of n sections/ the total
interlayer core loss, P, will be:
P = np = ~
Or, in other words, n2 times less than w~en the core is0 wound as a single element. In the 15 KVA transformer
consisting of seven elements~ the .interlayer core loss is
49 times lower than it would be if it had been wound as
a single section with 7 inch (17.78 cm) wide uncoated
strip.
Having thus described the invention in rather
full detail it will be understood that these details
need not be strictly adhered to but that various changes
and modifications may suggest themselves to one skilled
in the art, all falling within the scope oE the inven-0 tion as defined by the subjoined claims.
