U.S. patent application number 09/845374 was filed with the patent office on 2002-01-17 for electromagnetic device with embedded windings and method for its manufacture.
Invention is credited to Cho, Chahee Peter, Johnston, Ralph, Martin, Ronald A..
Application Number | 20020005675 09/845374 |
Document ID | / |
Family ID | 24127957 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005675 |
Kind Code |
A1 |
Johnston, Ralph ; et
al. |
January 17, 2002 |
Electromagnetic device with embedded windings and method for its
manufacture
Abstract
A powdered magnetic material stator core with embedded stator
windings and a method for its manufacture. Embedding the windings
within a radially compacted powdered magnetic material stator core
enables equivalent or better electromagnetic performance in a
significantly reduced size. Radial compaction of the powdered
magnetic material minimizes the distortion of the stator windings
during compaction.
Inventors: |
Johnston, Ralph; (Anderson,
IN) ; Cho, Chahee Peter; (Carmel, IN) ;
Martin, Ronald A.; (Pendleton, IN) |
Correspondence
Address: |
Thomas A. Walsh
ICE MILLER DONADIO & RYAN
One American Square
Box 82001
Indianapolis
IN
46282-0002
US
|
Family ID: |
24127957 |
Appl. No.: |
09/845374 |
Filed: |
April 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09845374 |
Apr 30, 2001 |
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09533915 |
Mar 23, 2000 |
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6232681 |
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Current U.S.
Class: |
310/44 ;
310/179 |
Current CPC
Class: |
H02K 1/12 20130101; Y10T
29/49009 20150115; H02K 15/02 20130101; H02K 3/04 20130101 |
Class at
Publication: |
310/216 ;
310/179 |
International
Class: |
H02K 001/00; H02K
003/00 |
Claims
We claim:
1. A method for fabricating a electromagnetic device comprising the
steps of: shaping an insulated electrical conductor into a
predetermined configuration of windings for an electromagnetic
device; placing the shaped windings into a compaction die cavity
dimensioned and shaped to form the exterior of the electromagnetic
device; filling the remaining volume of said compaction die cavity
with a quantity of powdered magnetic material capable of being
bound together to form a solid structure upon compaction so that
said windings are surrounded with said powdered magnetic material;
and radially compacting said powdered magnetic material within said
compaction die cavity with sufficient force to cause said powdered
magnetic material to bind together into a solid magnetic structure
with said shaped windings embedded therein.
2. The method of claim 1, wherein said electromagnetic device is a
stator.
3. The method of claim 1, wherein the step of radially compacting
said powdered magnetic material within said compaction die cavity
is accomplished by dynamic magnetic compaction.
4. The method of claim 1, wherein said powdered magnetic material
is composite iron.
5. The method of claim 4 wherein said composite iron comprises an
iron alloy material coated with a thermoplastic material.
6. The method of claim 4 wherein said composite iron comprises an
iron alloy material coated with a thermoset material.
7. The method of claim 4 wherein said composite iron comprises an
iron alloy comprising about 99.7% Fe, 0.003% C, 0.0005% N, 0.006%
S, and 0.004% P by weight.
8. The method of claim 1, further comprising the step of coating
said insulated electrical conductor with a binder coating to
promote bonding between said windings and said solid magnetic
structure.
9. The method of claim 1, further comprising the step of taping
said shaped windings prior to placement in said compaction die
cavity to retard movement of said shaped windings during the radial
compaction step.
10. A electromagnetic device comprising: one or more electrical
conductors shaped into windings of an electromagnetic device in a
substantially cylindrical configuration, wherein each adjacent
conductor of said windings is electrically insulated from each
other adjacent conductor; a cylindrical body comprising a plurality
of radially compacted powdered magnetic material particles bound
together due to compaction, surrounding and solidly enclosing said
one or more conductors, said cylindrical body being electrically
insulated from said one or more conductors, said cylindrical body
circumscribed by a continuous substantially cylindrical interior
surface and a continuous substantially cylindrical exterior surface
substantially concentric therewith, a first axial end surface
connected between one edge of said continuous cylindrical interior
surface and said continuous substantially cylindrical exterior
surface, and a second axial end surface connected between the other
edge of said continuous cylindrical interior surface and the other
edge of said continuous substantially cylindrical exterior surface,
said second axial end surface being opposite to and substantially
parallel to said first axial end surface; and a plurality of
electrical connector leads beginning outside said cylindrical body
and passing through at least one of the surfaces of said
cylindrical body, said plurality of electrical connector leads
electrically connected to said one or more conductors.
11. The electromagnetic device of claim 10 wherein said electrical
conductors are shaped into single phase stator windings.
12. The electromagnetic device of claim 10 wherein said electrical
conductors are shaped into multiple phase stator windings.
13. In an electromagnetic device of the type comprising a rotor and
a stator, an improved stator comprising: one or more electrical
conductors preformed into stator windings in a substantially
cylindrical configuration, wherein each adjacent conductor of said
windings is electrically insulated from each other adjacent
conductor of said windings; a cylindrical body comprising a
plurality of radially compacted powdered magnetic material
particles bound together due to compaction, surrounding and solidly
enclosing said one or more conductors, said cylindrical body being
electrically insulated from said one or more conductors; a
plurality of electrical connector leads beginning outside said
cylindrical body and passing through a surface of said body, said
electrical connector leads being electrically connected to said one
or more electrical conductors.
14. An improved stator as claimed in claim 13, wherein said
cylindrical body is circumscribed by a continuous substantially
cylindrical interior surface and a continuous substantially
cylindrical exterior surface substantially concentric therewith, a
first axial end surface connected between one edge of said
continuous cylindrical interior surface and said continuous
substantially cylindrical exterior surface, and a second axial end
surface connected between the other edge of said continuous
cylindrical interior surface and the other edge of said continuous
substantially cylindrical exterior surface, said second axial end
surface being opposite to and substantially parallel to said first
axial end surface.
15. An improved stator as claimed in claim 13, wherein said
electrical conductors are shaped into single phase stator
windings.
16. An improved stator as claimed in claim 13, wherein said
electrical conductors are shaped into a multiple phase stator
windings.
17. An improved stator as claimed in claim 13, wherein said
powdered magnetic material is composite iron.
18. An improved stator as claimed in claim 17, wherein said
composite iron comprises an iron alloy material coated with a
thermoplastic material
19. An improved stator as claimed in claim 17, wherein said
composite iron comprises an iron alloy material coated with a
thermoset material
20. An improved stator as claimed in claim 17, wherein said
composite iron comprises an iron alloy comprising about 99.7% Fe,
0.003% C, 0.0005% N, 0.006% S, and 0.004% P by weight.
21. An improved stator as claimed in claim 13, wherein said
electrical conductors are coated with a binder coating to promote
bonding between the windings and said compacted magnetic
particles.
22. An improved stator as claimed in claim 13, wherein said
powdered magnetic material is radially compacted using dynamic
magnetic compaction.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to the field of electromagnetic
device design and manufacturing.
[0003] 2. Description of the Prior Art
[0004] An electromagnetic device, such as an electric motor or an
electric generator, contains two electromagnetic components: a
stationary component known as a "stator," and a rotating component
known as a "rotor." In the most common embodiment, the rotor and
the stator are cylindrical in shape. The cylindrical rotor is
installed inside the hollow, cylindrical stator in such a way that
when the rotor rotates, the outer surface of the rotor is proximate
to, but does not touch, the inner surface of the stator. The space
between the outer surface of the rotor and the inner surface of the
stator is known as the "air gap."
[0005] It is known in the art that a stator and a rotor each may be
manufactured from a core made from a magnetic material, around
which or within which insulated electrical conductors known as
"windings" are installed. The rotor core and stator core together
form the magnetic "flux path" for the electromagnetic device.
[0006] A typical stator of a design known in the art is comprised
of a hollow, cylindrical core, the inner surface of which contains
slots which extend the full length of the core parallel to the
axial direction of the core. The portions of the stator core
between the slots are known as the "teeth." The measurement made by
adding the width of one slot measured at the base of the slot to
the width of one adjacent tooth is known as the "slot pitch."
[0007] The prior art stator windings are inserted in the slots in
the core, usually by a manual means. After the stator windings are
installed into the stator core, the stator may be finished by
filling the remaining volume of the stator slots and coating the
external surface of the stator with a non-reactive, non-conducting
material such as, for example, varnish or epoxy. The non-reactive,
non-conducting material serves to protect the stator from
corrosion, and to prevent the stator windings from moving within
the stator slots during use. Such movement, if permitted, could
damage the electrical insulation on the stator windings, and/or
could alter the electromagnetic characteristics of the stator.
[0008] It is well known in the art to manufacture a stator core
from sheet steel. Steel laminations are punched from the sheet
steel. The punched steel laminations include slots, alignment
holes, and other assembly features. The punched steel laminations
are stacked so that the inner surface of the core, the outer
surface of the core, the slots, and the alignment holes are
aligned. The stacked steel laminations then are secured together by
methods known in the art including, for example, welding or
riveting.
[0009] The method of manufacturing a stator core from sheet steel
laminations possesses several disadvantages. The process of
punching the steel laminations from the sheet steel creates scrap
steel pieces, which often cannot be used productively by the
manufacturer. In addition to the cost of the wasted sheet steel
pieces, often the manufacturer must incur additional expense
involved with the disposal of the wasted sheet steel pieces.
Finally, the process of producing the finished stator core from the
raw sheet steel is a multiple step process requiring expensive
material handling to be performed during and/or between each
process step.
[0010] U.S. Pat. No. 4,947,065 to Ward et al. disclosed another
method for manufacturing a stator core using iron powder particles
coated with a thermoplastic material. The method disclosed in U.S.
Pat. No. 4,947,065 addresses the disadvantages present in the prior
art method of manufacturing a stator core from sheet steel
laminations. Scrap is eliminated by the use of a premeasured amount
of thermoplastic coated iron particles. The stator core is formed
by heating the premeasured amount of the thermoplastic coated iron
particles to a predetermined temperature, placing the heated
particles into a heated mold that is shaped to produce a stator
core of the desired shape, activating a means for compacting the
heated particles within the heated mold, thereby compacting the
heated particles within the heated mold for a predetermined time at
a predetermined pressure. Material handling is reduced because the
raw thermoplastic coated iron particle material is manufactured
into a finished stator in fewer process steps. The stator core of
Ward et al. does not overcome all disadvantages of a prior art
stator core made with steel laminations. To fabricate a finished
stator from a stator core according to the disclosure of Ward et
al., the stator windings must be installed into the slots by a
manual means after the stator core is formed, as was required in
the stator core made with steel lamination.
[0011] Stator windings are conventionally produced from an
insulated electrical conductor of types known in the art including,
for example, insulated single strand copper wire. The insulated
electrical conductor is conventionally formed by methods known in
the art into substantially cylindrical winding configurations which
will fit within the slots in the stator core, and which will
produce the desired electrical effect when the windings are placed
in a moving magnetic field, or the desired magnetic effect when the
windings are energized with an electric current. The windings are
inserted into slots in the stator to maximize the electromagnetic
coupling between the windings and the flux path, and to minimize
the air gap between the rotor and stator. The portion of the
windings which is aligned parallel to the axial direction of the
core is conventionally known as the "active portion" of the
windings. The portions of the windings which resides outside the
stator core at each axial end of the stator core, and which
function to conduct electricity from the active portion of the
windings which resides in a first slot to the active portion of the
windings which resides in a second slot, are conventionally known
as the "end turns."
[0012] Electric motors and generators operate on the principle of
magnetic flux cutting. Electric motors and generators have a source
of magnetic flux, such as an electromagnet or a permanent magnet,
and a set of windings that intercept the flux. The flux path is
always ferromagnetic. The flux is cut when rotation of the rotor
occurs. The desired torque and power set the rotor dimension, while
the stator dimensions are driven by both the rotor dimension and by
the flux return requirements. An important rotor dimension is the
"rotor active volume." If "r" is the rotor radius and "l" is the
rotor active length, then the rotor active volume "X" is calculated
as "X=(.pi.r.sup.2l)."
[0013] A vehicular alternator is an example of electromagnet based
electric generator. In a vehicular alternator, the magnetic flux is
generated with a multi-pole electromagnet in the rotor. It is
desired in the art to maximize the average magnetic flux density,
or the "magnetic loading," of the air gap. The magnetic loading may
be limited by magnetic saturation of the stator core. A
disadvantage present in prior art stator design using internally
slotted stator cores, is that the slots reduce the internal surface
area of the stator adjacent to the rotating rotor, thereby reducing
the ability for magnetic flux to flow between the stator and the
rotor. Due to the reduced internal surface area, the stator core
teeth reach magnetic saturation more readily than would a stator
core without internal slots. When the stator core teeth saturate,
the magnetic flux density in the air gap is limited to the ratio of
the tooth width to slot pitch multiplied by the saturation flux
density of the stator material. For a typical vehicular alternator
stator material the saturation flux density is about 1.5T, and the
tooth width to slot pitch ratio is about 1/2, making the magnetic
loading about 0.75T.
[0014] Reducing the slot width and increasing the tooth width
increases the magnetic loading by increasing the internal surface
area of the stator adjacent to the rotating rotor. However, because
the slot must carry a fixed total electric current to meet the
desired performance characteristics of the electromagnetic device,
decreasing slot width requires an increase in slot depth to enable
the slot to carry the same total electric current. Increasing the
slot depth while maintaining the same total electric current
requires increasing the radius of the stator. It is known in the
art of motor and generator design to balance magnetic loading and
stator dimension in an attempt to find the optimum solution for
each application of a particular electromagnetic device.
[0015] The magnetic loading of the stator core may be increased
without increasing the stator dimensions by fabricating the stator
core from a material known in the art to have a higher saturation
flux density than steel, such as, for example, an alloy of
neodymium iron boron. Such materials improve magnetic loading, but
at a substantially higher cost. It is desired to have a stator core
fabricated from a readily available, low cost, magnetic material,
wherein a higher degree of magnetic loading may be achieved without
a corresponding increase in stator size.
[0016] Another disadvantage of the prior art stator designs arises
from limitations on the amount of electrical current which can be
carried in the stator windings installed in a stator slot. The
total current carried in the slot is calculated from the current
carried in each conductor multiplied by the number of conductors
wound into the slot. A typical conductor packing factor for
vehicular alternator stators is limited to about 30%, which means
that only 30% of the slot volume is occupied by conductors.
[0017] It is known in the art that the total current carried in the
stator slots, and therefore the conductor packing factor, is
limited by the need to dissipate the heat generated by electrical
resistance in the conductors. The heat must be dissipated either
through the stator core, or through the conductors themselves to
the end turns of the conductors. The non-reactive, non-conducting
material used to fill the stator slots substantially thermally
isolates the conductors from the stator core. As a result, most of
the heat must be dissipated through the end turns. The total
current carried in the stator slots, and therefore the conductor
packing factor, can be significantly increased by providing a
direct cooling path through the stator core.
[0018] Another disadvantage of the prior art stator designs arises
from the significant contribution to stator size made by the end
turns of the windings. It is known in the art of vehicular
alternator design that end turns add length to a stator while
serving no significant power producing function. The end turns are
extended to facilitate electric current conduction, and to serve as
the heat rejection surface for the windings. The end turns may
increase the overall length of a stator by a factor of about 2.5-3
times the active length. It is known in the art that the end turns
must be enclosed within the vehicular alternator housing, thus the
overall length of the vehicular alternator of this design is
increased by similar factor. A stator design wherein the end turns
are contained within the volume occupied by the stator core is
desirable.
[0019] U.S. Pat. No. 5,536,985 to Ward et al. disclosed a rotor
assembly wherein the rotor core is comprised of compacted soft
magnetic particles coated with a non-magnetic binder, and wherein
the rotor windings are embedded within the rotor core. Three
methods of manufacturing the rotor assembly are disclosed in U.S.
Pat. No. 5,536,985. In the first disclosed method, the rotor core
is manufactured by filling a die cavity with a predetermined amount
of soft magnetic particles coated with a nonmagnetic binder. The
soft magnetic particles within the die cavity are heated at a
predetermined temperature and axially compacted for a predetermined
time at a predetermined pressure to form the solid rotor core.
After the rotor core is removed from the die cavity, preformed
discrete rotor windings are embedded into the rotor core in a
circular pattern parallel to the axial direction of the core by
inserting the discrete rotor windings into preformed holes in the
core.
[0020] In the second method of manufacturing the rotor assembly
disclosed in U.S. Pat. No. 5,536,985, the discrete rotor windings
are preformed and inserted into holding devices within a die cavity
so that the rotor windings are oriented in a circular pattern
parallel to the axial direction of the core. The die cavity then is
filled with soft magnetic particles coated with a nonmagnetic
binder. The soft magnetic particles and the rotor windings within
the die cavity are heated at a predetermined temperature and
axially compacted for a predetermined time at a predetermined
pressure to form the solid rotor core with the rotor windings
embedded therein.
[0021] In the third method of manufacturing the rotor assembly
disclosed in U.S. Pat. No. 5,536,985, the discrete rotor windings
are preformed and inserted into holding devices within a two-part
mold comprised of an upper mold part and a lower mold part. The
holding devices are included in the lower mold part. The lower mold
part containing the rotor windings then is filled with soft
magnetic particles coated with a nonmagnetic binder, and the mold
is sealed by placing the upper mold part onto the lower mold part.
The sealed two-part mold containing the soft magnetic particles and
the rotor windings is placed inside a evacuated isostatic
compaction chamber. The isostatic compaction chamber is sealed, and
the sealed chamber is filled with hydraulic fluid heated to a
predetermined temperature, which exerts a predetermined pressure
uniformly on all surfaces of the mold. After a predetermined time,
the hydraulic fluid is drained from the compaction chamber. The
drained chamber is opened and the two-part mold is removed
therefrom. The mold is opened destructively to reveal a rotor core
with embedded rotor windings.
[0022] The performance of an electromagnetic device may be measured
by the torque density "D" and power density "Z." These measurements
can be derived from the magnetic loading and the electrical loading
of the device. The magnetic loading, "B," is the average magnetic
flux density of the air gap. The electrical loading "L" is
calculated by dividing the total current in the slots "I" by the
slot pitch "p." In equation form, it can be represented as
"L=(I/p)."
[0023] To arrive at the measurements of torque density and power
density, several intermediate calculations must be made. First, the
tangential force "F" acting on the rotor may be calculated by the
equation "F=(BL2.pi.rl)," where "B" is the magnetic loading, "L" is
the electrical loading, "r" is the rotor radius, and "l" is the
active length of the rotor.
[0024] The torque "Y" produced by the rotor may be calculated by
the equation "Y=(Fr)," where "F" is the tangential force acting on
the rotor, and "r" is the rotor radius. The power "P" produced by
the rotor may be calculated may be calculated by the equation
"P=(Fv)," where "F" is the tangential force acting on the rotor,
and "v" is the rotational speed of the rotor multiplied by the
radius of the rotor (known as the "tip speed" of the rotor).
[0025] We can normalize the power and torque to the rotor active
volume to obtain the torque density and the power density, which
are direct measures of performance of the motor or generator. The
torque density "D" therefore may be calculated by the equation
"D=(Y/(.pi.r.sup.2l))," where "Y" is torque, "r" is the rotor
radius, and "l" is the active length of the rotor. By algebraically
substituting for the variables and then algebraically reducing the
equation, an equivalent representation of the equation can be shown
as "D=(2BL)," where "B" is the magnetic loading, and "L" is the
electrical loading.
[0026] The power density "Z" may be calculated by the equation
"Z=(P/(.pi.r.sup.2l))," where "P" is power, "r" is the rotor
radius, and "l" is the active length of the rotor. By algebraically
substituting for the variables and then algebraically reducing the
equation, an equivalent representation of the equation can be shown
as "Z=((2BLv)/r)," where "B" is the magnetic loading, "L" is the
electrical loading, "v" is the tip speed of the rotor, and "r" is
the rotor radius.
[0027] The measurements of power density and torque density will
illustrate to those skilled in the art that magnetic loading and
electrical loading are fundamental to electromagnetic device
performance. An increase in either the magnetic loading or the
electrical loading, or both, relates directly to an increase in
power density and torque density. Increasing the tip speed and/or
decreasing rotor radius also will increase the power density.
[0028] It is desired to produce a stator which overcomes the
disadvantages present in the prior art. For example, it is desired
to produce a stator which may be manufactured economically from
readily available ferromagnetic materials with a minimum of waste
and a minimum of material handling, which possesses electromagnetic
capabilities required by a variety of applications, and which
embodies a smaller, more electrically efficient design.
SUMMARY OF THE INVENTION
[0029] The present invention is a novel powdered magnetic material
electromagnetic device with embedded windings, and a novel method
for its manufacture. In one embodiment, the electromagnetic device
is a stator. The windings comprise preformed insulated electrical
conductors, as are common in prior art electromagnetic device
designs. The windings are placed into a compaction die cavity, and
the remaining volume of the compaction die is filled with a
quantity of powdered magnetic material which is sufficient to fully
surround and encapsulate the windings after compaction. Radial
compaction is used to compact the powdered magnetic material into a
solid magnetic structure with the preformed windings embedded
therein. Radial compaction avoids or reduces distortion of the
windings during compaction. Radial compaction by the dynamic
magnetic compaction method has been found to produce a suitable
result. An electromagnetic device with embedded windings according
to the present invention will be of sufficient density and physical
strength to be a substitute for prior art devices in a variety of
applications including, for example, vehicular alternator
applications. By embedding the windings within the core, an
electromagnetic device according to the present invention may be
smaller and of a lesser mass than a prior art device offering
similar performance.
DESCRIPTION OF THE DRAWINGS
[0030] The features and advantages of this invention, and the
manner of attaining them, will be more apparent and better
understood by reference to the following descriptions of
embodiments of the invention taken in conjunction with the
accompanying drawings, wherein:
[0031] FIG. 1 is a cross-sectional view of one-quarter of a stator
according to the prior art.
[0032] FIG. 2 is a cross-sectional view of one-quarter of one
embodiment of a stator according to the present invention.
[0033] FIG. 3 is a cross-sectional view of a dynamic magnetic
compaction apparatus and concept.
[0034] FIG. 4 is a top view of the dynamic magnetic compaction
apparatus of FIG. 3.
[0035] FIG. 5 is a perspective partially fragmentary view of a
finished stator according to the present invention.
DESCRIPTION OF THE INVENTION
[0036] The present invention is a novel method for manufacturing a
stator core from powdered magnetic material, and embedding stator
windings within the stator core, and a novel powdered magnetic
material stator core with embedded stator windings produced by the
method.
[0037] To better illustrate the advantages of the present
invention, it is useful to examine a stator made according to the
prior art methods. FIG. 1 shows one-quarter of a cross-section of
such a prior art stator. In FIG. 1, stator 10 is shown with a
plurality of windings 11 installed in a plurality of slots 12.
Stator core 13 is fabricated from a plurality of stacked steel
laminations (only the top lamination being shown in FIG. 1). Stator
core 13 includes a plurality of teeth 14 which define the slots 12.
Windings 11 were shaped into the desired configuration prior to
installation into stator core 13. During installation into stator
core 13, windings 11 were inserted into slots 12 through slot
openings 15.
[0038] FIG. 2 shows one-quarter of a cross-section of one
embodiment of a stator according to the present invention. In FIG.
2, stator 20 is shown with a plurality of windings 21 embedded
therein. Stator core 22 of this embodiment is fabricated from
powdered magnetic material. As in the prior art, windings 21 are
shaped into the desired configuration prior to being surrounded
with the powdered magnetic material which comprises stator core 22.
Stator core 22 includes a plurality of slots 25 in which windings
21 reside. A plurality of teeth 23 separates the plurality of slots
25. Unlike the prior art, stator core 22 has a continuous inner
surface 24, with no slot openings.
[0039] In this embodiment of the present invention, the stator
windings 21 are produced from wire of a desired gauge, the wire
comprising a single strand copper conductor pre-coated with
insulation. The insulation must be rated to withstand 200.degree.
C.. In this embodiment, Phelps Dodge Industries GP/MR-200.RTM.
brand wire or equivalent may be used. In the preferred embodiment,
the wire gauge typically will be AWG-13 or smaller.
[0040] The wires are preshaped by methods known in the art into the
desired stator winding configuration. For example, the wires may be
shaped to form a complete multiple phase stator winding, or may be
shaped to form separate single phase stator windings which
subsequently may be combined into a multiple phase configuration,
if the desired application so requires. Because a stator produced
according to the present invention will have a smaller radius and
shorter axial length than a stator produced by prior art methods,
less wire is used in the manufacture of a stator according to the
present invention than in prior art methods. This reduces the cost
of the finished stator, and also reduces the mass of the finished
stator.
[0041] During the compaction step of the method of present
invention (discussed in more detail hereinafter), windings 21 will
be exposed to compaction pressures above the pressure at which the
common wire insulation polymers will begin to flow. If the
insulation fails during the compaction process exposing the center
conductor, the electrical integrity of the stator will be
compromised and the stator must be scrapped. To reduce insulation
stresses during compaction, and to improve the insulation packing
factor, the plurality of wires comprising the active portion of
windings 21 should be substantially parallel to each other. In
addition, crossovers of individual wires in the end turn region of
stator 20 should be minimized. Windings 21 may be taped to reduce
the chance of movement during handling and compaction, but this is
not required.
[0042] The wires in the finished stator must bond to one another
and to the powdered iron. In one embodiment of the present
invention, this will occur due to changes to the surface of the
wire insulation polymer under the pressures and temperatures
achieved during compaction. In another embodiment, a binder coating
of a type known in the art, such as Phelps Dodge Industries
BONDEZE.RTM. M, may be applied to the insulation coating to promote
the bonding between the wires, and between the wires and the
powdered iron. Alternatively, a thermoset resin, such as a "B"
stage epoxy, may be applied. The thermoset resin must be of a type
which does not set before compaction.
[0043] After windings 21 are shaped into the desired configuration,
they are placed in a compaction die cavity. The compaction die
cavity is sized to receive windings 21 and the amount of powdered
magnetic material required to produce a finished stator of the
desired dimensions. After windings 21 are in place within the
compaction die cavity, the remaining volume of the compaction die
cavity is filled with powdered magnetic material.
[0044] The powdered magnetic material fill step of this embodiment
of the present will result in a powdered magnetic material fill
density of about 50% before compaction. To match the powdered
magnetic material "shrinkage" during compaction, windings 21 should
be produced with initial wire packing factor of about 50%. In
another embodiment of the present invention, a higher packing
factor, such as, for example, 75% may be used, provided the wire
insulation is of a type that can withstand powdered magnetic
material flow past it during compaction. In other embodiments of
the present invention, flow enhancers can be added to the powdered
magnetic material or the wires can be lubricated to reduce the
likelihood of insulation damage during compaction of the powdered
magnetic material.
[0045] In one embodiment of the present invention, the powdered
magnetic material is "composite iron." Composite iron is comprised
of iron alloy powder coated with a thermoplastic material or a
thermoset material. The particle sizes of the iron alloy powder
used in one embodiment of the present invention range from about 44
to about 250 microns. A small percentage of this iron alloy powder
may have a particle size as small as about 10 microns. The iron
alloy powder used in this embodiment is about 99.7% Fe, 0.003% C,
0.0005% N, 0.006% S, and 0.004% P by weight.
[0046] The iron alloy powder particles are either coated with a
thermoplastic material or a thermoset material, or the
thermoplastic material or a thermoset material is admixed with the
iron alloy powder to form the basic composite iron. The magnetic,
thermal, and mechanical properties of the composite iron are
interrelated through iron alloying, particle morphology, particle
size, thermoplastic material or thermoset material type, and the
ratio by weight of the iron alloy to the thermoplastic material or
to the thermoset material.
[0047] The thermoplastic material in one embodiment of the present
invention may be an amorphous thermoplastic polyethermide resin, an
example of which is a General Electric ULTEM.RTM. resin. The
thermoplastic material used in this embodiment may be replaced in
other embodiments of the present invention by a thermoset material,
such as polyethersulfone ("PES") and/or PPS. Other commercially
available materials which possess similar properties may be
used.
[0048] To prepare the composite iron, the particles of iron alloy
powder in this embodiment of the present invention are coated with
a thin layer of the thermoplastic material. If the thermoplastic
material is not liquid under ambient conditions, it must be
liquefied. In one embodiment, the thermoplastic material is mixed
with a solvent to provide a liquid material. In another embodiment,
the thermoplastic material is heated until liquefaction.
[0049] In one embodiment of the present invention, the iron alloy
powder is coated with the thermoplastic material when it is blown
by compressed air up through a vertical tube and, at the same time,
the liquid thermoplastic material is sprayed on the blown iron
alloy powder to coat the powder. The coated iron alloy powder falls
outside the tube and is directed back into an inlet of the tube
where it is blown up again and coated again. After a number of
passes through the tube, the particles of iron alloy powder are
coated to the extent desired. If a solvent has been used to liquefy
the thermoplastic material, the solvent evaporates or is recovered
during this process.
[0050] To provide output performance, power, and torque comparable
to a stator core structure formed from steel laminations, the
proportions of iron alloy powder and thermoplastic material in the
finished stator core should fall within certain ranges. In one
embodiment of the present invention, the stator core structure
should be approximately between 95 to 99.9% iron alloy powder by
weight, with the remaining weight comprised substantially by the
thermoplastic material. Since about 1% by weight of thermoplastic
material used in this embodiment is equal to about 4% by volume, a
stator core structure of this embodiment that is approximately 99%
iron alloy powder by weight and approximately 1% thermoplastic by
weight, would be in the range of approximately 96% iron alloy
powder by volume and approximately 4% thermoplastic material by
volume.
[0051] In the final molded state, the thermoplastic material is
bonded to the outer surface of each iron alloy powder particle so
that the particles within the finished stator core are insulated
from each other by thin layers of thermoplastic material. In
addition, the thermoplastic material surrounding each iron alloy
particle will bond to the thermoplastic material surrounding
adjacent iron alloy particles, creating the stator core structure.
Those skilled in the art will appreciate that, since the
thermoplastic material separates the particles, there are gaps
formed between the particles. These gaps act like air gaps, since
the thermoplastic material separating the particles has about the
same magnetic permeability as air. This gap effect increases
resistivity and, consequently, reduces eddy current loses.
[0052] To produce the finished stator core 20, the powdered
magnetic material surrounding windings 21 must be compacted within
the compaction die cavity to form the powdered magnetic material
into the solid stator core 22. A stator core produced with powdered
magnetic material compacted by conventional means, such as the
axial means or the isostatic means used to produce the rotor
disclosed in U.S. Pat. No. 5,536,985, can achieve densities of only
approximately 95% of full density under optimal circumstances. The
permeability and saturation flux density of a stator core produced
from conventionally compacted powdered magnetic material of this
density both are significantly lower than that of steel
laminations. A stator core produced from conventionally compacted
powdered magnetic material also is significantly weaker than a
stator core produced from steel laminations, often resulting in
damage during handling. For a stator core produced from compacted
powdered magnetic material to be a practical alternative to a
stator core produced from steel laminations, higher densities must
be achieved. The magnetic properties of a stator core fabricated
from compacted powdered magnetic material improve with density. The
mechanical and thermal performance of compacted powdered magnetic
material increases with density.
[0053] To achieve an optimal finished stator, the powdered magnetic
material surrounding windings 21 must be compacted in a way which
does not result in buckling and distorted geometry in the active
portion of windings 21. If the geometry of the preformed windings
21 is altered, the electromagnetic characteristics of stator 20
will be changed. Compacting the powdered magnetic material by to
the axial compaction method or the isostatic method disclosed in
U.S. Pat. No. 5,536,985 will cause the active portion of windings
21 to buckle and distort. To prevent buckling and distortion in the
active portion of windings 21, the powdered magnetic material
surrounding windings 21 must be compacted radially.
[0054] In one embodiment of the present invention, the necessary
stator core density and undisturbed windings geometry may be
achieved by radially compacting the powdered magnetic material
using the dynamic magnetic compaction ("DMC") process disclosed by
Chelluri et al. in U.S. Pat. No. 5,405,574, and in U.S. Pat. No.
5,689,797. DMC provides two advantages over prior art compaction
techniques. First, powdered metal structures compacted by DMC are
more dense than powdered metal structures compacted by other
compaction techniques known in the art. DMC can produce a nearly
fully dense powdered magnetic material stator core. High density is
required to withstand the mechanical and thermal stresses, and to
provide the magnetic performance necessary to make powdered
magnetic material a viable substitute for steel laminations in
stator fabrication. The second advantage provided by DMC is that
DMC enables radial compaction of the powder magnetic material.
Prior art devices employing embedded windings used axial compaction
or isostatic compaction, neither of which is suitable for the
present invention. Both would result in an unacceptable distortion
of the stator windings during compaction of the powdered magnetic
material. An alternate means of radial compaction may be used if
the means is capable of producing a powdered magnetic material
stator core of the required density.
[0055] The most critical areas of magnetic performance are the
teeth 23 of stator 20. Teeth 23 also are high mechanical and
thermal stress areas, so mechanical and thermal stress resistance
must be maximized in teeth 23. The radial compaction inherent in
DMC promotes powdered magnetic material powder flow into the teeth
during compaction to maximize densification, thereby improving
magnetic performance, and mechanical and thermal stress
resistance.
[0056] FIG. 3 shows a cross-sectional view of a typical DMC
apparatus. FIG. 4 shows a top view of the DMC apparatus of FIG. 3,
after compaction is complete. In FIG. 3 and FIG. 4, a compaction
die cavity is formed by an electrically conductive outer compaction
container 30, an inner die core 31, a die cavity floor 35, and a
removable die cavity lid 36. A coil 32 surrounds outer compaction
container 30. Inner die core 31 is solid, cylindrical and has an
outer diameter equal to the desired inner diameter of the stator.
The compaction die cavity is sized to receive windings 21 and a
predetermined amount of powdered magnetic material particles 33.
The DMC apparatus also possesses a means for permitting the release
of air from the compaction die cavity during compaction (not
illustrated in FIG. 3 or FIG. 4). This air release means is sized
to prevent the release of powdered magnetic material during
compaction.
[0057] In operation, die cavity lid 36 is removed and windings 21
are placed in the desired position within the compaction die
cavity. The remaining volume of the compaction die cavity is filled
with powdered magnetic material particles 33. Die cavity lid 36 is
replaced. A short pulse (typically less than 1 ms) of a very large
electrical current (approximately 40,000-60,000 amps) is conducted
through coil 32, producing a high magnetic field (typically
30-40T), which in turn produces strong magnetic forces (represented
by arrows 34) in the radial direction of the coil 32 and inward on
the electrically conductive outer compaction container 30. Outer
compaction container 30 is collapsed radially inwardly, compacting
the powdered magnetic material particles 33 into a solid powdered
magnetic material structure, and embedding windings 21 within the
solid powdered magnetic material structure.
[0058] Inner die core 31, die cavity floor 35, and removable die
cavity lid 36 must be able to withstand the compaction forces
generated by coil 32 without distorting or changing position
relative to each other. The magnetic forces produced by coil 32
force the powdered magnetic material particles 33 together within
the compaction die cavity, forming a very dense solid powdered
magnetic material structure. The windings 21 may move slightly
inward toward the inner die core 31 because of the magnetic forces
34, but this movement may be anticipated and compensated for in the
design of the DMC apparatus and windings 21, and in the placement
of the windings 21 within the compaction die cavity. After
compaction, outer compaction container 30 is removed destructively,
revealing the finished stator 20.
[0059] Those skilled in the art will be appreciate that the above
described methods of manufacture are embodied by the process
comprising the steps of (a) shaping an insulated conductor of a
desired gauge into stator windings 21 of a desired configuration;
(b) placing stator windings 21 into a compaction die cavity; (c)
filling the remaining volume of the compaction die cavity with a
predetermined quantity of powdered magnetic material particles 33;
and (d) radially compacting the powdered magnetic material
particles 33 to cause the powdered magnetic material particles 33
to bind together into a solid powdered magnetic material structure
with windings 21 embedded therein.
[0060] FIG. 5 shows a perspective partially fragmentary view of a
stator according to the present invention. In FIG. 5, stator 50 is
shown having a first flat axial end surface 51, a second flat axial
end surface 52, an outer annular surface 53, an inner annular
surface 54, and a plurality of electrical connector leads 55. The
missing fragment reveals a conceptual representation of stator
windings 56. The stator windings, including the end turns, are
fully enclosed within the core of stator 50. The only exposed
electrical conductors are electrical connector leads 55, which pass
through the surface of stator 50 and electrically connect to the
embedded stator windings 56. In preferred embodiments, the
electrical connector leads 55 may pass through either the first
axial end surface 51, the second axial end surface 52, or the outer
annular surface 53.
[0061] For a stator slot carrying a given total current, the
present invention enables the conductor packing factor to be
increased from about 30%, which was typical of the prior art, to
greater than 90%. The conductor packing factor is increased by
reducing the size of the stator slot. The air is virtually
eliminated from the stator slot. Several advantages of the present
invention accrue from the reduced stator slot size.
[0062] First, the reduced stator slot size enables the diameter of
the inner surface 54 and the diameter of the outer surface 53 to be
reduced without affecting the magnetic loading. Reducing the
diameter of the inner surface 54 of the stator enables the rotor
radius to be reduced correspondingly. For a given total current and
a given magnetic loading, a smaller rotor radius results in an
increase in power density. It may be necessary to increase the
rotor's rotational speed to maintain a rotor tip speed adequate to
sustain the increase in power density.
[0063] Second, the reduced stator slot size results in a reduced
stator slot pitch. For a given total current, a smaller slot pitch
results in an increase in electrical loading. For a given magnetic
loading, an increase in electrical loading results in an increase
in torque density and in power density.
[0064] In prior art stators the heat generated by electrical
resistance in the windings is primarily conducted to the end turns
and rejected to the air from the end turns via either natural or
forced convection. The low conductor packing factor and use of
non-reactive, non-conductive material to fill the stator slots
essentially prevents thermal conduction to the stator core. In a
stator according to the present invention, the conductor packing
factor is higher and there is no material insulating the conductors
from the stator core. The complete windings, including end turns,
are embedded in the stator core. All the resistive losses are
conducted through the core and rejected from the surfaces of the
stator. In an alternate embodiment, the thermal conductivity of the
stator core may be enhanced by the addition of high thermal
conductivity carbon fibers to the composite iron before
compaction.
[0065] Those skilled in the art will be appreciate that the
foregoing method will produce a stator containing embedded windings
which is suitable for use in many applications, such as, for
example, in vehicular alternator applications. It will be further
appreciated by those skilled in the art that a stator according to
the present invention will produce output performance, power, and
torque equivalent to that produced by a stator fabricated from
slotted steel laminations, but with smaller overall dimensions and
lower mass. Production waste is reduced in the present invention by
fabricating the stator core from powdered magnetic material instead
of steel laminations. Production cost also is reduced in the
present invention by reducing production waste, and by reducing the
material handling required during stator fabrication.
[0066] While this invention has been described as having a
preferred design, the present invention can be further modified
within the scope and spirit of this disclosure. This application is
therefore intended to cover any variations, uses, or adaptations of
the invention using its general principles. Further, this
application is intended to cover such departures from the present
disclosure as come within known or customary practice in the art to
which this invention pertains and which fall within the limits of
the appended claims.
* * * * *