U.S. patent application number 10/996562 was filed with the patent office on 2005-06-23 for method for manufacturing squirrel cage rotor.
Invention is credited to Sweo, Edwin A..
Application Number | 20050134137 10/996562 |
Document ID | / |
Family ID | 34681565 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050134137 |
Kind Code |
A1 |
Sweo, Edwin A. |
June 23, 2005 |
Method for manufacturing squirrel cage rotor
Abstract
Ceramic adhesive is applied inside the slots of an electric
machine squirrel cage rotor prior to casting or insertion of rotor
conductors. This heat-resistant insulation provides superior
electrical insulation that is also mechanically rugged, thus
enabling casting of squirrel cage rotors by die or cost-effective
consumable pattern casting. The consumable pattern cast copper
rotors are electrically well insulated, thus minimizing rotor
losses. The ceramic insulation in cast rotors enables longer
conductors to be cast by thermally insulating the molten metal from
the cooler laminations. The ceramic insulation in fabricated rotors
withstands the high temperatures of end ring attachment. This
method is applicable to both conventional induction machine rotors
and brushless doubly-fed induction machine rotors and provides the
degree of electrical isolation of the bars from the laminations
needed to avoid excessive parasitic torques in doubly-fed machines,
while ensuring adequate thermal conductivity to dissipate
conductive bar heat to the laminations.
Inventors: |
Sweo, Edwin A.; (Bellevue,
WA) |
Correspondence
Address: |
LAW OFFICES OF RONALD M ANDERSON
600 108TH AVE, NE
SUITE 507
BELLEVUE
WA
98004
US
|
Family ID: |
34681565 |
Appl. No.: |
10/996562 |
Filed: |
November 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60530218 |
Dec 17, 2003 |
|
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|
Current U.S.
Class: |
310/211 ;
29/598 |
Current CPC
Class: |
Y10T 29/49009 20150115;
H02K 15/0012 20130101; H02K 17/165 20130101; B22D 19/0054 20130101;
B22C 9/04 20130101; Y10T 29/49012 20150115 |
Class at
Publication: |
310/211 ;
029/598 |
International
Class: |
H02K 017/16; H02K
015/02 |
Claims
The invention in which an exclusive right is claimed is defined by
the following:
1. A squirrel cage rotor for use in an electrical machine,
comprising: (a) at least one stack of metallic laminations affixed
to one of a shaft and a mandrel, a plurality of spaced-apart
openings being formed in each metallic lamination around a
periphery of the lamination and aligned in the at least one stack,
so as to define slots that extend generally axially along the at
least one stack; (b) a ceramic insulating coating applied to at
least a portion of an internal surface of each of the slots, said
ceramic insulating coating having both a thermal insulating
characteristic and an electrical insulation characteristic that are
substantially greater than the thermal and electrical insulating
characteristics of the metallic laminations; (c) at least one
electrical conductor extending axially within each of at least a
majority of the slots; and (d) at least one electrically conductive
component at each axial end of each of the at least one stack, for
interconnecting electrical conductors that extend axially from the
slots in the at least one stack.
2. The squirrel cage rotor of claim 1, wherein the ceramic
insulating coating comprises a binder and a ceramic powder.
3. The squirrel cage rotor of claim 2, wherein the ceramic powder
includes at least one of an alumina, a berylia, a magnesia, a
thoria, a zirconia, a zircon, a spinel, and a mullite powder.
4. The squirrel cage rotor of claim 2, wherein the ceramic powder
most preferably includes an alumina powder.
5. The squirrel cage rotor of claim 2, wherein the binder of the
ceramic insulating coating comprises an inorganic binder.
6. The squirrel cage rotor of claim 2, wherein the binder of the
ceramic insulating coating comprises an organic-based material that
is processed after being applied, so as to substantially eliminate
an organic content.
7. The squirrel cage rotor of claim 1, wherein the at least one
conductor extending through each of the at least the majority of
the slots comprises a cast-in-place conductor that was formed in
the slot by casting a molten metal.
8. The squirrel cage rotor of claim 7, wherein the at least one
electrically conductive component at an axial end of the at least
one stack comprises a cast-in-place end cap that is formed using a
molten metal when the cast-in-place conductors are formed in the at
least the majority of the slots.
9. The squirrel cage rotor of claim 8, wherein the metal used for
the cast-in-place conductors and the cast-in-place end cap
comprises copper.
10. The squirrel cage rotor of claim 1, wherein the plurality of
slots are open at a peripheral edge of the at least one stack of
the metallic laminations.
11. The squirrel cage rotor of claim 1, wherein the at least one
conductor extending through each of the at least the majority of
slots comprises at least one metallic bar that has been inserted
axially through the slot and which extends generally from one end
of the stack to an opposite end of the stack.
12. The squirrel cage rotor of claim 1, wherein the squirrel cage
rotor is adapted to be energized by one of: (a) a singly-fed
stator; (b) a doubly-fed stator having two sets of windings with a
different number of poles on a common lamination stack; and (c) a
doubly-fed stator having one set of windings on each of two
lamination stacks.
13. The squirrel cage rotor of claim 1, wherein the ceramic
insulating coating is applied over substantially an entire internal
surface of each slot.
14. The squirrel cage rotor of claim 13, wherein the ceramic
insulating coating is formed as a plurality of generally axially
extending ridges that are spaced apart around the internal surface
of the plurality of slots and are substantially thicker than the
ceramic insulating coating covering the internal surface of the
plurality of slots between said ridges.
15. A method for fabricating a squirrel cage rotor for use in an
electrical machine, comprising the steps of: (a) forming at least
one stack of metallic laminations affixed to one of a shaft and a
mandrel, each metallic lamination including a plurality of
spaced-apart openings disposed around a periphery of the lamination
and generally aligned in the at least one stack, so as to define
slots that extend generally axially along the at least one stack;
(b) applying a ceramic insulating coating to at least a portion of
an internal surface of each of the slots, said ceramic insulating
coating having both a thermal insulating characteristic and an
electrical insulation characteristic that are substantially greater
than thermal and electrical insulating characteristics of the
metallic laminations; (c) providing at least one electrical
conductor extending axially within each of at least a majority of
the slots; and (d) electrically interconnecting the electrical
conductors extending axially within the slots in the at least one
stack.
16. The method of claim 15, wherein the step of providing at least
one electrical conductor comprises the steps of: (a) heating an
electrically conductive metal to a molten state; (b) casting a
conductor within each of the at least the majority of the slots
using the electrically conductive metal that has been heated to the
molten state and a consumable pattern; and (c) enabling the
electrically conductive metal cast as the conductor within each of
the at least the majority of the slots to cool to an ambient
temperature.
17. The method of claim 16, wherein the step of casting comprises
the step of employing one of: (a) a lost foam casting process in
which a plastic foam comprises the consumable pattern; and (b) a
lost wax casting process in which a wax material comprises the
consumable pattern.
18. The method of claim 16, wherein the electrically conductive
metal comprises copper.
19. The method of claim 15, wherein the step of applying the
ceramic insulating coating comprises the steps of: (a) cleaning the
interior surface of the slots; (b) filling any gaps between the
metallic laminations comprising the stack; (c) coating the interior
surface of the slots with a ceramic adhesive; and (d) heat treating
the stack at a substantially elevated temperature, to substantially
increase a coating strength for the ceramic adhesive.
20. The method of claim 19, wherein the step of applying the
ceramic insulating coating further comprises the steps of: (a) air
drying the ceramic adhesive coating; (b) applying another coat of
the ceramic adhesive over the ceramic adhesive already coating the
interior surface of the slots; and (c) air drying the other coat of
the ceramic adhesive before the step of heat treating.
21. The method of claim 19, wherein the step of coating the
interior surface of the slots comprises the step of applying a
relatively thicker coating of the ceramic adhesive to form ridges
that extend axially through the slots and are spaced apart on the
internal surface of the slots, with a relatively thinner coating of
the ceramic adhesive on the internal surface of the slots between
the ridges.
22. The method of claim 15, wherein the ceramic insulating coating
comprises a binder and a ceramic powder.
23. The method of claim 22, wherein the ceramic powder includes at
least one of an alumina, a berylia, a magnesia, a thoria, a
zirconia, a zircon, a spinel, and a mullite powder.
24. The method of claim 22, wherein the ceramic powder most
preferably comprises an alumina powder.
25. The method of claim 22, wherein the binder of the ceramic
insulating coating comprises an inorganic binder.
26. The method of claim 22, wherein the binder of the ceramic
insulating coating comprises an organic-based material that is
processed after being applied to the interior surface to
substantially eliminate an organic content.
27. The method of claim 15, wherein the step of providing at least
one electrical conductor extending axially within each of the at
least the majority of the slots comprises the step of inserting a
conductive metal bar into each such slot so that the conductive
metal bar extends axially through the slot from opposite ends
thereof, the conductive metal bar comprising the at least one
electrical conductor.
28. The method of claim 15, wherein the step of electrically
interconnecting the electrical conductors comprises the steps of:
(a) providing a consumable pattern defining a shape of at least one
end cap adapted to fit an end of the squirrel cage rotor; (b)
heating an electrically conductive metal to form a molten metal;
(c) using the consumable pattern, casting at least one end cap from
the molten metal; and (d) enabling the at least one end cap to cool
substantially to an ambient temperature, said at least one end cap
electrically interconnecting the electrical conductors extending
through the slots.
29. The method of claim 15, wherein the step of electrically
interconnecting the electrical conductors comprises the steps of:
(a) forming a consumable pattern defining an end cap for each end
of the stack, said consumable pattern also defining fan blades,
vents extending into the slots, and balancing posts for each end
cap; (b) assembling the consumable patterns to at least one end of
the stack; and (c) casting the end cap for at least one end of the
at least one stack and the electrical conductor extending axially
through the at least the majority of the slots, in place, with a
molten metal.
30. The method of claim 28, wherein the step of casting comprises
the step of casting the at least one end cap in place on the at
least one stack by employing one of: (a) a lost foam casting
process, in which a plastic foam comprises the consumable pattern;
and (b) a lost wax casting process in which a wax material
comprises the consumable pattern.
31. The method of claim 28, further comprising the step of
vaporizing the consumable pattern with one of the steps of: (a)
vaporizing the consumable pattern with the molten metal; and (b)
preheating an assembly that includes the consumable pattern before
the step of casting with the molten metal, so that heat thus
applied vaporizes the consumable pattern, yielding a void into
which the molten metal subsequently flows during the step of
casting.
32. The method of claim 31, wherein before the step of preheating
the assembly, further comprising the step of coating an assembly
that includes the at least one stack and the consumable pattern
with a ceramic slurry, said ceramic slurry drying to form a shell
that defines the void after the step of preheating has vaporized
the consumable pattern, and before the step of casting that fills
the void with the molten metal.
33. The method of claim 28, wherein the electrical conductor
comprises copper.
34. The method of claim 28, wherein before the step of casting,
further comprising the steps of: (a) creating an assembly of
components that includes a sprue, at least one foam gate, at least
one vent, the at least one stack in which the at least the portion
of the internal surface of the slots is coated with the ceramic
insulating coating, and the consumable pattern for at least one end
cap; (b) coating the assembly with a gas permeable layer, producing
a coated assembly; (c) placing the coated assembly into a
container; and (d) adding particulate material that is heat
resistant and conforms around the coated assembly within the
container to provide support during the step of casting.
Description
RELATED APPLICATIONS
[0001] This application is based on a prior copending provisional
application, Ser. No. 60/530,218, filed on Dec. 17, 2003, the
benefit of the filing date of which is hereby claimed under 35
U.S.C. .sctn. 119(e).
FIELD OF THE INVENTION
[0002] The present invention generally pertains to squirrel cage
rotors of electrical induction machines, including both
conventional singly-fed induction machine rotors and brushless
doubly-fed rotors, and more specifically, pertains to the use of a
ceramic adhesive insulation that is applied on the interior surface
of the slots during the manufacture of such rotors, prior to either
the casting or insertion of rotor conductors within the slots.
BACKGROUND OF THE INVENTION
[0003] The rotor slot conductors of squirrel cage rotor electric
machines must be electrically insulated from the steel lamination
stack of the rotor to prevent stray electrical currents from
leaking into the lamination stack. Such currents cause loss of
efficiency and an unbalanced magnetic pull in conventional
induction machines. The insulation system that is employed for such
machines must withstand subsequent manufacturing operations, such
as the die casting of cast rotors or the insertion of fabricated
bars within the slots, and the welding or brazing of end rings on
the fabricated rotors. In addition, the insulation system that is
employed must also withstand the thermal, electrical, and
centrifugal stresses that occur during operation of the electrical
machine.
[0004] In brushless doubly-fed induction machines, cage rotor bar
voltages during normal operation can be much higher than in
conventional induction machines, and these machines therefore
require better insulation than conventional induction machines for
successful operation. Other effects must also be taken into
consideration in doubly-fed electrical induction machines. For
example, the parasitic torque effects of doubly-fed rotor leakage
are described by Koch and Spee in their paper entitled, "A
Comparison of Stack Preparation Methods for Bar Insulation in Die
Cast Rotors," pp. 182-187, presented at the IEEE Industry
Applications Society Annual Meeting, Oct. 5-9, 1997.
[0005] Prior Art Die-Cast Rotors and Insulation Systems
[0006] FIG. 1 shows a typical (prior art) die-cast squirrel cage
rotor 100 for a conventional induction machine employing a stack of
laminations 101 with individual laminations 110, which are shown in
FIG. 2 (prior art). Laminations 110 are stacked with slots 111
either aligned axially or aligned with a small skew relative to the
axial direction. End rings 102 on rotor 100 typically include fan
blades, as shown, and often include other elements, such as
balancing posts (not shown). The end rings and bars 103 are
integrally die cast of aluminum to form a complete rotor conductor
structure. Slot insulation 104, which as shown in FIG. 1, is
disposed between each cast bar 103 and lamination stack 101, may or
may not be included in standard induction machines. Laminations 110
are each typically oxide coated to insulate the laminations from
each other. Insulation systems for die-cast rotors may include an
oxide coating on the laminations applied after punching the
laminations from sheet stock, or an iron fluoride insulation
coating, which is produced by a sulfur hexafluoride atmosphere
treatment. An iron oxide slurry core-plating of the lamination
stack may also be employed, as described in the above reference.
U.S. Pat. No. 5,488,984 describes a procedure for applying
insulation to the slots of rotor laminations using sodium
tetraborate decahydrate and sodium nitrite. However, none of these
prior art insulation systems provides ideal insulation for high
efficiency and minimum unbalanced pull, and none enables an
effective consumable pattern casting of the conductors with copper.
Other prior art insulating systems for fabricated rotors either
employ slot liners or use an insulating film that is applied to
conductive bars that are then inserted into the slots; however,
both of these approaches are expensive.
[0007] A prior art refractory insulation system for electrical
coils is disclosed in U.S. Pat. No. 5,634,434. This insulation
system is applied to wire during or just prior to winding of the
wire into a coil. No reference is made to applying a refractory
coating to any laminations comprising the core of the coil.
[0008] Casting Methods Applicable to Producing Rotors
[0009] The two most common consumable pattern casting methods are
lost wax and lost foam. The lost wax method is widely used in
jewelry and art casting and for casting difficult materials, such
as titanium, which is sometimes used in turbine engine rotors.
Because of its accuracy, the lost wax method is commonly used to
produce complex parts with very close tolerances. The typical
procedure is to produce a pattern of wax that is identical to the
desired finished part, except that the pattern is made a small
percentage oversize, to allow for the shrinkage of the cast metal
during cooling. Gates, which are usually formed as short,
small-diameter rods of wax, are added to the wax pattern to connect
a sprue to the pattern, usually at the lowest points of the
pattern. The sprue, also usually made of wax, defines the opening
and path through which molten material is channeled into a mold and
typically has a relatively large cross-sectional area to enable the
molten casting metal to be carried from a funnel-shaped port at the
top of the sprue, into the pattern area through the gates. Vents,
formed either using small rods of wax or tubes, are added to the
pattern at the high points and other locations, to provide a path
for gases to escape during casting. The complete assembly,
including pattern, gates, and sprue, is then coated with several
layers of a refractory, usually applied by spraying or dipping in a
water-solution, ceramic slurry that is allowed to air dry between
coats. The ceramic is removed from the ends of the vents of the
coated assembly, and the assembly is heated to melt away the wax
and strengthen the ceramic coating, resulting in a preheated
ceramic mold having the pattern cavity of the part to be cast. This
mold is placed in a vents-up position while still hot from
preheating, and molten metal is poured into the sprue cavity
through the funnel-shaped port to fill the pattern cavity from the
bottom up. Gases in the pattern cavity escape through the vents to
enable complete filling of the pattern cavity. The metal in the
gates freezes (solidifies) before the metal in the sprue, to
prevent molten metal from the pattern cavity being pulled back into
the sprue during the cooling process. After cooling, the ceramic
mold is broken away from the casting and the metal of the gates,
sprue and vents is removed, yielding the desired part.
[0010] An alternative to lost wax casting is lost foam casting,.
The lost foam casting technique is an industrial process that is
particularly useful for producing complex shapes, such as aluminum
engine blocks and cast iron electric motor shells. This process
uses a consumable pattern, gates, and sprue--all formed of expanded
polystyrene foam or other type of plastic foam. The foam pattern is
typically formed by casting separate pieces of the finished
assembly with expanded foam and gluing these pieces of foam
together to form the complete consumable foam assembly. The foam
assembly is then dipped into a water solution slurry containing a
suspended refractory material. This slurry coats the pattern and
leaves a thin heat-resistant, gas permeable layer after the coated
assembly air dries. The coated foam is placed in a flask (i.e., an
open top barrel), which is vibrated while sand is added. The sand
settles around the coated pattern within the flask and mechanically
supports the thin refractory layer before and during casting.
Molten metal is then poured into the sprue part of the mold. The
heat of the molten metal vaporizes the foam as the molten metal
rises in the mold, filling the void that is formed in the mold as
the foam vaporizes, to produce a nearly exact replica of the foam
pattern. The pattern is again made slightly larger than the desired
size of the metal part, to compensate for the shrinkage of the
metal that occurs during cooling. The gas permeable refractory
coating left behind after the foam has vaporized determines the
shape of the part and vents the gases produced by the vaporized
foam. Lost foam castings can optionally include metal inserts,
which are inserted into the cast foam and supported as necessary
before and during casting, so that the metal inserts become an
integral part of the cast metal part.
[0011] Typically, the die-cast conductive structure of prior art
rotors has been formed of aluminum metal, although, it would often
be preferable to use copper conductors and end caps, because of the
better electrical conductivity of copper compared to aluminum.
Die-cast copper conductors have not been used in induction machine
rotors because copper is cast at least at a 500-600.degree. F.
higher temperature than aluminum. The higher temperature used for
making die-cast rotor conductor assemblies of copper places too
much thermal stress on portions of the dies, resulting in very
short die life. Indeed, even if cast from aluminum or other
lower-temperature metal, the rotor conductor assemblies can place
too much thermal stress on the conventional oxide coatings used for
insulation and thereby cause electrical leakage currents.
[0012] Cast induction machine rotors therefore require a more
effective rotor lamination stack insulating system to improve
efficiency and reduce unbalanced magnetic pull, particularly, if it
is desired to use cast copper for the conductors in the slots and
for the end caps. Thus, an electrical insulating system that also
provides thermal insulation of the lamination stack from the molten
metal during casting enables lower pressure die casting or the use
of consumable pattern casting with copper to improve efficiency. A
more rugged lamination insulating system would also improve the
cost effectiveness of fabricated rotors. A better (i.e., lower cost
to fabricate and more effective) insulating system is clearly
essential if doubly-fed cage rotors are to be feasible in
commercial applications, since doubly-fed cage rotors will not
operate properly if there are leakage currents through the
insulation separating the conductors from the metal used for stack
of laminations.
SUMMARY OF THE INVENTION
[0013] In accord with the present invention, as shown in FIG. 3, a
thin layer, typically 0.005" to 0.010" thick, of a ceramic adhesive
204 including a ceramic binder and one or more powdered ceramic
materials is applied to all or a portion of the surface inside each
rotor slot 111 of rotor lamination stack 101, to insulate a slot
conductor 203 from the lamination stack. It is also contemplated
that the present invention can also be used on electrical machines
that include a rotor with two stacks of laminations. The entire
inside surface of each rotor slot is coated with the ceramic
adhesive when the conductor is subsequently to be cast in-place
within the slot. Alternatively, axial stripes of ceramic adhesive
applied along the length of each slot may be used for rotors in
which a fabricated conductor bar is to be inserted in the slot.
These stripes are arranged to guide the bar during insertion and
prevent the bar surface from contacting the uncoated lamination,
while providing a passage for later injecting additional insulating
material to fully fill the space between the bar and the inner
surface of the lamination stack slot.
[0014] Squirrel cage rotor conductors, including end rings 202 and
slot conductors 203, may be cast using standard casting techniques,
such as die casting or consumable pattern casting, employing slots
lined with ceramic adhesive 204 as part of the mold for the
conductors. If desired, only a majority of the slots may include a
cast conductor, e.g., by plugging the slots that are to remain free
of the conductor before the casting process is carried out. The
ceramic insulation provides thermal insulation between the molten
metal and the lamination stack 101 during casting, enabling longer
bars to be cast through the lamination stack than with other
insulation methods, and then serves as an electrical insulator to
prevent leakage currents from circulating in the laminations of the
rotor during operation. This approach enables consumable pattern
casting with copper, for reduced rotor losses, even though copper
casting may be done at temperature approaching or even exceeding
2500.degree. F.
[0015] Fabricated rotors can also be made by inserting conductive
bars into the ceramic adhesive lined slots, and then finishing the
rotor using standard fabricated rotor manufacturing techniques.
Again, it is possible that the copper conductor will be inserted in
a majority of the slots, leaving some slots open, without any
conductor. The ceramic insulation still has superior thermal and
electrical insulating properties, compared to conventional
insulation systems used on prior art induction machine rotors.
[0016] The conductive bars used in the slots of brushless
doubly-fed rotors, such as those described in U.S. Pat. No.
5,239,251, and in U.S. Pat No. 6,278,211 (the drawings and
specification of which are hereby specifically incorporated herein
by reference), can be also be insulated from the laminations with
this ceramic adhesive insulation method, to avoid the substantial
parasitic torques that arise as a result of conventional die
casting or non-insulated/poorly insulated inserted bar
construction. The interconnection conductors between bars for these
rotors can be cast simultaneously with the bars, using the
consumable pattern casting technique, and employing copper for the
conductors, to achieve reduced rotor losses.
[0017] Accordingly, claims for one aspect of the present invention
are directed to a squirrel cage rotor for use in an electrical
machine. The squirrel cage rotor includes at least one stack of
metallic laminations affixed to a shaft. A plurality of
spaced-apart openings are formed around a periphery of each
metallic lamination and these openings are generally aligned in the
stack, so as to define slots. These slots extend generally axially
along the stack. A ceramic insulating coating is applied to at
least a portion of an internal surface of each of the slots. The
ceramic insulating coating is a substantially better thermal
insulator and electrical insulator than the metallic laminations.
At least one electrical conductor extends axially within each of at
least a majority of the slots. Also, at least one electrically
conductive component is included at an axial end of the at least
one stack and is used for interconnecting electrical conductors
that extend axially from the slots in the at least one stack.
[0018] The ceramic insulating coating comprises a binder and a
ceramic powder, and the ceramic powder preferably includes at least
one of an alumina, a berylia, a magnesia, a thoria, a zirconia, a
zircon, a spinel, and a mullite powder. Most preferably the ceramic
powder comprises an alumina powder. The binder of the ceramic
insulating coating can comprise either an inorganic binder, or an
organic-based material that is processed after the slots are lined
(e.g., by heating) to substantially eliminate its organic
content.
[0019] The conductor that extends through a slot can comprise a
cast-in-place conductor that was formed in the slot using a molten
metal, or can be a metal bar that is inserted through the slot.
Similarly, the electrically conductive component that is provided
at least at one axial end of the stack also preferably comprises a
cast-in-place end cap that is formed using a molten metal, and
preferably, when the cast-in-place conductors are formed in the
slots. Because of its superior conductivity (compared to metals
such as aluminum), the metal used for the cast-in-place conductors
and the cast-in-place end cap preferably comprises copper.
[0020] In one embodiment, the plurality of slots are open on a
peripheral edge of the at least one stack of metallic laminations.
The squirrel cage rotor is adapted to be energized by one of a
singly-fed stator, a doubly-fed stator having two sets of windings
with different numbers of poles on a common lamination stack, or a
doubly-fed stator having one set of windings on each of two
lamination stacks, depending upon the type of electrical machine in
which the squirrel cage rotor will be employed.
[0021] In one preferred embodiment, the ceramic insulating coating
is applied over substantially an entire internal surface of each
slot. Also, in at least one embodiment, the ceramic insulating
coating is formed as a plurality of generally axially extending
ridges that are spaced apart around the internal surface of each of
the plurality of slots, These ridges are substantially thicker than
the ceramic insulating coating covering the internal surface of the
plurality of slots that is between the ridges.
[0022] Another aspect of this invention is directed to a method for
fabricating a squirrel cage rotor for use in an electrical machine.
The steps of the method are generally consistent with the
functionality of the components of the squirrel cage rotor
discussed above.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0023] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0024] FIG. 1A (prior art) is a side-elevational view of a die cast
cage rotor for a conventional induction machine employing
conventional slot insulation;
[0025] FIG. 1B (prior art) is an enlarged portion of a
cross-sectional view of the die cast cage rotor, taken along
section line 1B-1B, showing one slot;
[0026] FIG. 1C (prior art) is an end view of the die cast cage
rotor of FIG. 1A;
[0027] FIG. 2 (prior art) is a plan view of a lamination for a
conventional induction machine rotor designed for die casting of
the conductors that will be disposed in peripheral slots of the
lamination;
[0028] FIG. 3A is a side-elevational view of a cast cage rotor for
a conventional induction machine employing ceramic slot insulation
in accord with the present invention;
[0029] FIG. 3B is an enlarged portion of a cross-sectional view of
the cast cage rotor of FIG. 3A, taken along section line 3B-3B,
showing one slot with the ceramic insulation;
[0030] FIG. 3C is an end view of the cast cage rotor of FIG.
3A;
[0031] FIG. 4 is a cross-sectional along a rotor centerline through
the rotor slots and fan blades of a conventional induction machine
rotor lamination stack that includes the ceramic slot insulation,
in accord with the present invention, applied to all slot surfaces,
and with foam components added to enable lost foam casting of
conductors;
[0032] FIG. 5 is an enlarged portion of the cross-sectional view of
FIG. 4, showing part of the end ring section;
[0033] FIG. 6 is an enlarged portion of the cross-sectional view of
FIG. 4, after coating the end ring section with a refractory
material, in preparation for lost foam casting of the rotor
conductors;
[0034] FIG. 7 is an enlarged cross sectional view of one rotor slot
after the ceramic slot insulation has been applied in axial stripes
in the slots, in accord with this invention;
[0035] FIG. 8 is an enlarged cross sectional view of one open rotor
slot (from a rotor having open instead of closed slots) after the
ceramic slot insulation has been applied in axial stripes in the
open slots, in accord with one embodiment of the present invention;
and
[0036] FIG. 9 is a cross-sectional view of an assembly for casting
a rotor like that of FIG. 4, except the rotor includes two stacks
of laminations that are spaced-apart to accommodate interconnection
conductors that extend between the conductors in the slots of each
stack of laminations.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] A preferred embodiment of a conventional singly-fed
induction machine squirrel cage rotor 200 with ceramic adhesive
insulated slots, in accord with the present invention, is shown in
FIGS. 3A, 3B, and 3C. Ceramic adhesive coating 204, which is
applied on the inside surface of the closed slots in lamination
stack 101, thermally and electrically insulates cast slot
conductors 203 from the lamination stack. End rings 202
electrically connect all conductors 203 at each end of the rotor.
Coating 204 provides substantial thermal insulation of the molten
metal from the large mass of the steel lamination stack during the
brief (about one second) casting period, but does not substantially
reduce the transfer of heat from conductors 203 to the lamination
stack during subsequent operation of the induction machine. Rotor
200 is generally identical to a conventional squirrel cage rotor,
like that shown in prior art views of FIGS. 1A, 1B, and 1C, except
that it includes ceramic adhesive coating 204, which is used for
the slot insulation, and except for the use of copper (instead of
aluminum) to form end rings 202 and conductors 203.
[0038] The ceramic adhesive coating used in embodiments of the
present invention is selected because it has the following
properties:
[0039] 1. suitable for application on steel laminations as a
coating that may range in thickness from about 0.005" to
0.010";
[0040] 2. coefficient of thermal expansion similar to steel;
[0041] 3. minimum electrical insulation of about 10.sup.8
ohm-cm;
[0042] 4. no outgassing when exposed to molten metal during casting
of the conductor;
[0043] 5. strong enough to withstand subsequent manufacturing
processes and operating stresses; and
[0044] 6. thermal conductivity in the range from about 3% to about
10% that of steel. The lower conductivity aids in casting by
reducing heat loss from the molten metal into the laminations
during the brief casting period, while adequately conducting heat
from the conductors to the lamination stack during operation of the
electrical machine in which the rotor is used.
[0045] A wide range of ceramic adhesives that generally meet the
above criteria and are readily available include a ceramic binder
and one or more powdered ceramic materials. Typical ceramic
materials that are usable include alumina, berylia, magnesia,
thoria, zirconia, zircon, spinel, and mullite. Ideal binders are
inorganic, although partially organic binders may be used provided
they are heat treated at sufficiently high temperatures to drive
off the organic materials prior to casting. For example, an
alumina-based ceramic adhesive, Resbond 989.TM., from Cotronics
Corp. provides a good combination of the desired properties.
[0046] The desired coating properties are attained by a procedure
that includes the steps of:
[0047] 1. cleaning the interior surface of the slots with a
solvent;
[0048] 2. coating the interior surface of the slots with a
water-adhesive thinner mixture to fill gaps in the lamination
stack;
[0049] 3. coating the interior surface of the slots with a first
coating of the Resbond 989 ceramic adhesive thinned slightly with
an appropriate water-adhesive thinner, typically in a pattern
similar to that shown in the cross-sectional view of FIG. 7; ridges
205 in each slot 111 serve to guide a tool that is also used to
apply the second coating, as described below, in order to shape the
coating on the interior surface of the slots to a desired thickness
and shape;
[0050] 4. air drying the first ceramic adhesive coating;
[0051] 5. coating the interior surface of the slots with a second
coating of a suitable ceramic adhesive, such as Cotronics ceramic
adhesive, Resbond 989.TM., thinned slightly with the water-adhesive
thinner, to achieve a constant thickness in the range of about
0.005" to about 0.010", using the tool; the tool is guided by the
ridges of the first coating, to center the coating and shape the
interior surface of the coating to the desired uniform
thickness;
[0052] 6. air drying the second ceramic adhesive coating; and
[0053] 7. heat treating the assembly at a temperature of
approximately 450.degree. F., to attain a maximum available coating
strength.
[0054] An exemplary preferred embodiment uses lost foam casting to
form the rotor bars and end rings from copper. A completed rotor
assembly 300, prior to casting, is shown in FIG. 4, and an enlarged
view of an upper end ring 302 of this rotor assembly is illustrated
in FIG. 5.
[0055] Lamination stack 101 is clamped on a mandrel 301, and a
ceramic adhesive coating 204 is applied to the interior surface of
slots 111, as described above. Foam end ring patterns 302 and 303
each include fan blades and balance posts, as is typical in die
cast rotors. A short protrusion (typically about 0.10" in length),
which extends into each rotor slot 111, serves to position the end
ring relative to the lamination stack. Upper end ring pattern 302
includes vents 306 from each slot 111 to enable gas to escape
through vent tubes 307 during the casting process. The end ring
patterns are adhesively secured to lamination stack 101. The
interior volume of closed slots 111 is left open, except for the
protrusions described above. FIG. 5 is an enlarged cross-sectional
view through end ring pattern 302. This end ring includes the fan
blades, vent 306 through the foam, and vent tube 307. The vent tube
is supported by a collar 308.
[0056] The rotor assembly of FIG. 4 is prepared for lost foam
casting in the conventional way, with foam gates 304 attached to
lower end ring 303 to feed molten metal into the end ring from the
larger cross-sectional area of a sprue 305. The assembly is then
coated with a gas permeable layer 309 of clay or other material and
air dried. FIG. 6 illustrates an enlarged view of end ring 302, as
shown in FIG. 5, after the end ring has been coated with gas
permeable layer 309. Completed rotor assembly 300, which is now
coated with gas permeable layer 309, is placed in a flask, oriented
with upper end ring 302 at the top end. Sand is then added so that
it surrounds the completed rotor assembly, while the flask if
vibrated to settle the sand in close and supporting conformance to
the completed rotor assembly shape.
[0057] Molten copper is then poured into sprue 305 and flows
through gates 304. The heat of the molten copper vaporizes the foam
and the copper flows into the slots and into the voids thus formed
within the mold by the lost foam, as well as into the voids formed
inside the coating of the ceramic adhesive within the slots, and
forms the conductors and end rings. Ceramic adhesive 204, which
coats the interior surface of slots 111, forms part of the mold for
the cage structure. The ceramic adhesive coating insulates the
molten copper from the much cooler laminations during casting, to
enable longer conductors to be cast through the laminations, since
if excess heat were lost into the stack of laminations, the molten
copper would solidify before completely forming the conductors in
each slot. The lost foam casting process employed to form the
copper conductors and end ring(s) is thus generally conventional,
except for the use of the ceramic adhesive coating the surface of
the slots, as part of the mold, which has not previously been done
in the art.
[0058] An alternate casting method involves coating complete rotor
assembly 300, prepared as described above, with a ceramic coating
that covers the assembly and is typically applied in several layers
by dipping the complete rotor assembly in a ceramic slurry that is
allowed to dry between dips. The complete rotor assembly is then
heated to about 1200.degree. F. to vaporize the foam, leaving a
preheated mold. Molten copper is then poured into the resulting
voids within the mold while the lamination stack is still hot from
the step of preheating. The ceramic coating of the slots and the
end rings form the mold for the conducting structure, so that the
molten metal is shaped into the desired form for the conductors and
end ring(s). An advantage of this alternative method is that the
preheated lamination stack reduces premature cooling of the molten
copper and enables the molten copper to flow through even longer
stack lengths than is possible with the lost foam method described
above. This alternative method is similar to conventional lost wax
casting, except that the consumable pattern is formed of expanded
polystyrene foam rather than wax, and the ceramic adhesive coated
slots are used as part of the mold to define the shape of the cast
copper.
[0059] A further alternative casting method is the traditional lost
wax casting process, where complete rotor assembly 300 is again
prepared as described above, except that the conductors are formed
of wax rather than polystyrene foam, and the ceramic adhesive
coated slots are used as part of the mold.
[0060] The preferred embodiment that was initially described above
is applicable in fabricating brushless doubly-fed induction machine
rotors of the type described in U.S. Pat. No. 5,239,251, which uses
one rotor lamination stack with an end ring on one end, and
insulated connections between the rotor bars on the opposite end.
These rotors require electrical insulation between the rotor bars
and the laminations. The rotors may use either open or closed
lamination slots, depending upon the design, with closed slots
normally preferably being used for cast rotors, and open slots
preferably being used with rotors that have conductors inserted
through the slots. A preferred embodiment for casting these rotors
is as described above, except that upper end ring 302 in FIG. 4 is
replaced with foam conductor patterns that when replaced with metal
during the casting process, provide the desired rotor bar
interconnections. Vents are added to these conductor patterns as
required for successful casting.
[0061] In addition, the preferred embodiment initially described
above is applicable to brushless doubly-fed induction machine
rotors of the type described in U.S. Pat. No. 6,278,211, which uses
two rotor lamination stacks on a common shaft, and which has an end
ring on the outer end of each stack of laminations used for each
rotor, and insulated interconnections between the inner ends of the
conductive bars extending through the slots of one lamination stack
and inner ends of the conductive bars extending through the slots
of the other lamination stack. The specification and drawings of
U.S. Pat. No. 6,278,211 are hereby specifically incorporated herein
by reference. These brushless doubly-fed induction machine rotors
require electrical insulation of the rotor bars relative to the
laminations, except at the end rings. They may use either open or
closed lamination slots, depending upon the design, with closed
slots normally preferably being used for cast rotors, and open
slots preferably being used with rotors that have conductors
inserted through the slots. The preferred embodiment for casting
these rotors is the lost foam method of consumable pattern casting
using an assembly 500 as shown in FIG. 9. Two lamination stacks 502
are mounted on a shaft (or mandrel) 501 and are spaced apart to
provide space for interconnection conductor patterns 503. The end
rings, gates, vents and sprue are as described for FIG. 4, except
that a ring 506 at the top of a sprue 504 feeds molten metal to
each interconnection conductor through additional gates 505. These
additional gates provide the required flow of molten metal to the
upper stator and interconnection conductors. Other added sprue and
gates may be required on larger rotors. To avoid adding vents, the
preferred embodiment casting method is the alternate casting method
described above, where assembly 500 is coated with several layers
of ceramic adhesive and is heated to about 1200.degree. F. to
vaporize the foam and preheat the mold. Then, molten metal, such as
copper, is poured into the sprue while the assembly is still
hot.
[0062] It will be understood that the insulation method of the
present invention is also thus applicable in providing insulation
on open slot rotors that are used on either conventional singly-fed
or doubly-fed, fabricated rotor induction machines. FIG. 8 shows a
cross-sectional view of an open slot 402 (having an opening 406) in
a lamination stack 401 that has been insulated with a ceramic
adhesive coating. This coating covers the entire internal surface
of the slots, but is applied in axial stripes or ridges 205, with
thinner areas 206 disposed between the ridges, as described above,
in connection with FIG. 7. A copper conductor 404 that has been
inserted into slot 402 is shown generally centered in the slot, but
may instead be offset from the center. Insulating axial stripes or
ridges 205 prevent copper conductor 404 from contacting lamination
stack 401 in slot 402, while providing gaps 408 that are defined
between thinner insulation areas 206 and the outer surface of
copper conductor 404. Additional insulation (not shown) may
optionally be injected into gaps 408 later in the manufacturing
process. An alternate method is to provide a constant thickness
insulation layer 204 on the internal surface of the slots, as shown
in FIG. 3.
[0063] Although the present invention has been described in
connection with the preferred form of practicing it and
modifications thereto, those of ordinary skill in the art will
understand that many other modifications can be made to this
invention within the scope of the claims that follow. Accordingly,
it is not intended that the scope of the invention in any way be
limited by the above description, but instead be determined
entirely by reference to the claims that follow.
* * * * *