U.S. patent application number 12/568742 was filed with the patent office on 2010-11-04 for generator with ferromagnetic teeth.
This patent application is currently assigned to American Superconductor Corporation. Invention is credited to Timothy MacDonald, Gregory L. Snitchler.
Application Number | 20100277136 12/568742 |
Document ID | / |
Family ID | 43029903 |
Filed Date | 2010-11-04 |
United States Patent
Application |
20100277136 |
Kind Code |
A1 |
Snitchler; Gregory L. ; et
al. |
November 4, 2010 |
GENERATOR WITH FERROMAGNETIC TEETH
Abstract
A stator assembly for use in a superconducting generator
operated at frequencies up to 10 Hz is disclosed. The stator
assembly includes a ferromagnetic stator winding support having a
plurality of teeth defining slots, the slots configured to receive
and support stator windings. The stator winding support is formed
so that the ratio of the sum of the widths of the slots to the sum
of the widths of the teeth and slots is in the range of 0.65 to
0.90.
Inventors: |
Snitchler; Gregory L.;
(Shrewsbury, MA) ; MacDonald; Timothy; (North
Grafton, MA) |
Correspondence
Address: |
Occhiuti Rohlicek & Tsao LLP
10 Fawcett Street
Cambridge
MA
02138
US
|
Assignee: |
American Superconductor
Corporation
Devens
MA
|
Family ID: |
43029903 |
Appl. No.: |
12/568742 |
Filed: |
September 29, 2009 |
Current U.S.
Class: |
322/59 |
Current CPC
Class: |
Y02E 40/625 20130101;
H02K 1/165 20130101; H02K 55/04 20130101; Y02E 10/725 20130101;
Y02E 40/60 20130101; H02K 2213/03 20130101; H02K 7/1838 20130101;
Y02E 10/72 20130101 |
Class at
Publication: |
322/59 |
International
Class: |
H02P 9/02 20060101
H02P009/02 |
Goverment Interests
GOVERNMENT RIGHTS STATEMENT
[0001] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of Contract No. 70 NANB7H7055 awarded by the National Institute of
Standards and Technology/Advanced Technology Program.
Claims
1. A stator assembly for use in a rotating electrical machine, the
stator assembly comprising: a stator core including a plurality of
teeth defining slots therebetween, the slots configured to receive
and support stator windings, the stator core being formed of a
ferromagnetic material, and wherein the ratio of the sum of the
widths of the slots to the sum of the widths of the teeth and slots
is in the range of 0.65 to 0.90.
2. The stator assembly of claim 1, wherein the slots are elongate,
radially extending openings formed in the stator core.
3. The stator assembly of claim 1, wherein the teeth extend
radially inward and are mutually circumferentially spaced
apart.
4. The stator assembly of claim 1, further comprising at least one
stator winding disposed in a slot; the stator winding including a
coil formed of plural bundles of conductive wires.
5. The stator assembly of claim 1, wherein the stator core includes
an assembly of laminated plates.
6. The stator assembly of claim 1, wherein the ratio of the sum of
the widths of the slots to the sum of the widths of the teeth and
slots is in the range of 0.70 to 0.90.
7. The stator assembly of claim 1, wherein the ratio of the sum of
the widths of the slots to the sum of the widths of the teeth and
slots is in the range of 0.75 to 0.90.
8. The stator assembly of claim 1, further comprising a stator
winding including a straight portion, wherein the straight portion
is disposed in one of the slots and the straight portion is
configured to substantially fully occupy the slot.
9. The stator assembly of claim 8, wherein the cross sectional area
of the straight portion is substantially the same as the area of
the slot
10. The stator assembly of claim 1, further comprising a stator
winding including multiple stator winding straight portions
disposed in each slot.
11. The stator assembly of claim 10 wherein the cross sectional
area of a straight portion is substantially half the area of the
slot, and two stator winding straight portions are disposed in each
slot.
12. The stator assembly of claim 1, further comprising at least one
stator winding, the stator winding comprising a cranked coil
winding.
13. The stator assembly of claim 1, further comprising at least one
stator winding including individual wire conductors arranged within
the slot so that a radial conductor dimension is at least a factor
of 1.5 larger than the circumferential conductor dimension.
14. (canceled)
15. The stator assembly of claim 1, further comprising at least one
stator winding including individual wire conductors arranged within
the slot so that a radial conductor dimension is at least a factor
of 3 larger than the circumferential conductor dimension.
16. A rotating electrical machine comprising: a rotor assembly; and
a stator assembly, the stator assembly including a stator core
including a plurality of teeth defining slots, the slots configured
to receive and support stator windings, the stator winding support
being formed of a ferromagnetic material, and wherein the ratio of
the sum of the widths of the slots to the sum of the widths of the
teeth and slots is in the range of 0.65 to 0.90.
17. The machine of claim 16, wherein the rotor assembly includes a
rotor having high temperature superconducting windings.
18. The machine of claim 16, wherein the rotor and stator
assemblies are configured to operate at frequencies up to 10
Hz.
19. (canceled)
20. (canceled)
21. The machine of claim 16, wherein the stator core is configured
to have a tooth flux density during operation that is greater than
1.8 T.
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. The machine of claim 16, wherein the stator core is configured
to have a tooth flux density during operation that is greater than
2.8 T.
27.-30. (canceled)
31. (canceled)
32. (canceled)
33. A wind turbine comprising: a rotating electrical machine, said
rotating electrical machine having a rotor assembly; and a stator
assembly, the stator assembly including a stator core including a
plurality of teeth defining slots, the slots configured to receive
and support stator windings, the stator winding support being
formed of a ferromagnetic material, and wherein the ratio of the
sum of the widths of the slots to the sum of the widths of the
teeth and slots is in the range of 0.65 to 0.90.
Description
BACKGROUND OF THE INVENTION
[0002] Power generation continues to be an important application of
rotating electrical machines. Wind energy is one of the fastest
growing sources of electricity in the United States and around the
world, and wind turbines employing rotating electrical machines are
used to convert wind energy to usable power. The generator
component of a wind turbine includes the electrical generator,
control electronics, and an optional gearbox for converting the low
speed incoming rotation to high speed rotation suitable for
generating electricity. In a wind turbine, the generator component
may be approximately one third of the overall wind turbine
cost.
[0003] In some conventional non-superconducting generators, stator
windings are surrounded by a back iron that acts as a magnetic flux
path. The back iron is often in the form of stacked laminated
plates, the plates including iron teeth that extend between the
stator windings to provide a flux path and to support the stator
windings, which are positioned in slots formed between the teeth.
In such machines, which often operate at high frequencies of 60 Hz
or greater, the teeth carry the magnetic flux, and the ratio of the
area of slots to the area of teeth is about 50 percent. This ratio
is required in conventional machines to accommodate the magnetic
flux generated in these machines. In some conventional machines,
tooth area is actually increased relative to slot area as a means
to reduce the cost of the machine, due to relative differences in
cost between iron and copper.
[0004] Superconducting generators have been under development since
the early 1960s. The use of superconducting windings in these
machines has resulted in a significant increase in the
magnetomotive forces generated by the windings and increased flux
densities in the machines. The flux densities were increased to
such an extent that there were high losses due to saturation of the
iron teeth, as well as due to eddy current losses. As a result,
superconducting machines have been developed to operate without
iron teeth between the stator coils since the flux density would
result in high dissipation in these members. In some cases,
fiber-composite teeth, rather than iron teeth, are used to support
stator coils in these machines. However, such fiber composite teeth
are very expensive to manufacture.
SUMMARY
[0005] In some aspects, a stator assembly for use in a rotating
electrical machine is provided. The stator assembly includes a
stator core including a plurality of teeth defining slots, the
slots configured to receive and support stator windings. The stator
core is formed of a ferromagnetic material, and the ratio of the
sum of the widths of the slots to the sum of the widths of the
teeth and slots is in the range of 0.65 to 0.90.
[0006] The stator assembly may include one or more of the following
features: The slots are elongate, radially extending openings
formed in the stator core. The teeth extend radially inward and are
mutually circumferentially spaced apart. The stator assembly
further includes at least one stator winding disposed in a slot,
and the stator winding includes a coil formed of plural bundles of
conductive wires. The stator core includes an assembly of laminated
plates. The ratio of the sum of the widths of the slots to the sum
of the widths of the teeth and slots is in the range of 0.70 to
0.90. The ratio of the sum of the widths of the slots to the sum of
the widths of the teeth and slots is in the range of 0.75 to 0.90.
The stator assembly further includes a stator winding including a
straight portion, wherein the straight portion is disposed in one
of the slots and the straight portion is configured to
substantially fully occupy the slot. The cross sectional area of
the straight portion is substantially the same as the area of the
slot. The stator assembly further includes a stator winding
including a straight portion, wherein there are multiple stator
winding straight portions disposed in each slot. The cross
sectional area of the straight portion is substantially half the
area of the slot, and two stator winding straight portions are
disposed in each slot. The stator assembly further includes at
least one stator winding, the stator winding including a cranked
coil winding. The stator assembly further includes at least one
stator winding including individual wire conductors arranged within
the slot so that a radial conductor dimension is at least a factor
of 1.5 larger than the circumferential conductor dimension. The
stator assembly further includes at least one stator winding
including individual wire conductors arranged within the slot so
that a radial conductor dimension is at least a factor of 2 larger
than the circumferential conductor dimension. The stator assembly
further includes at least one stator winding including individual
wire conductors arranged within the slot so that a radial conductor
dimension is at least a factor of 3 larger than the circumferential
conductor dimension.
[0007] In other aspects, a rotating electrical machine is provided.
The rotating electrical machine includes a rotor assembly and a
stator assembly. The stator assembly includes a stator core having
a plurality of teeth defining slots, the slots configured to
receive and support stator windings. The stator winding support is
formed of a ferromagnetic material, and the ratio of the sum of the
widths of the slots to the sum of the widths of the teeth and slots
is in the range of 0.65 to 0.90.
[0008] The rotating electrical machine may include one or more of
the following features: The rotor assembly includes a rotor having
high temperature superconducting windings. The rotor and stator
assemblies are configured to operate at frequencies up to 10 Hz.
The rotor and stator assemblies are configured to operate at
frequencies up to 3 Hz. The rotor and stator assemblies are
configured to operate at a frequency of about 2 Hz. The stator core
is configured to have a tooth flux density during operation that is
greater than 1.8 T. The stator core is configured to have a tooth
flux density during operation that is greater than 2.0 T. The
stator core is configured to have a tooth flux density during
operation that is greater than 2.2 T. The stator core is configured
to have a tooth flux density during operation that is greater than
2.4 T. The stator core is configured to have a tooth flux density
during operation that is greater than 2.6 T. The stator core is
configured to have a tooth flux density during operation that is
greater than 2.8 T. The machine further includes a stator winding
having a straight portion, wherein the straight portion is disposed
in one of the slots and the straight portion is configured to
substantially fully occupy the slot. The machine further includes a
stator winding having a straight portion, wherein there are
multiple stator winding straight portions disposed in each slot.
The machine further includes at least one stator winding having
individual wire conductors arranged within the slot so that a
radial conductor dimension is at least a factor of 1.5 larger than
the circumferential conductor dimension. The machine further
includes at least one stator winding, the stator winding including
a cranked coil winding. The machine further includes an air gap
between the stator assembly and the rotor assembly that is greater
than 15 mm. The machine further includes an air gap between the
stator assembly and the rotor assembly that is greater than 20
mm.
[0009] Superconducting electric machines are ideally suited for use
in wind turbine applications as a wind-driven direct-drive
generator. Due to the low frequency (10 Hz or less) output of a
wind turbine, a low cost superconducting generator can be provided
that includes stator winding support formed of a ferromagnetic
material, without incurring the large power losses associated with
high operating frequency generators. Use of a stator winding
support formed of a ferromagnetic material results in a relatively
low cost generator.
[0010] As will be described in greater detail below, the inventive
stator assembly has features which contribute toward increasing the
overall performance, as well as reducing the overall manufacturing
cost of a HTS generator. In particular, the low frequency
superconducting generator permits a stator support design in which
the ratio of overall slot area to overall support area is greater
than 60 percent. Due to the relative large slot size, the cross
sectional area of the conductor within the slot is much greater
than in a conventional machine, providing increased power
generation.
[0011] In some embodiments, by selecting a particular stator
winding configuration, a single winding is disposed in a
corresponding slot. As a result, the cross sectional area of the
conductor within the slot is further increased relative to other
stator winding configurations, in which a single slot is occupied
by leg portions of two or more stator windings, and in which
insulation disposed between individual leg portions reduces overall
conductor cross sectional area.
[0012] A generator includes a stator core formed of ferromagnetic
laminations and including plurality of teeth provided to support
stator coil windings. Use of ferromagnetic material to form the
stator support reduces manufacturing costs relative to use of fiber
composite materials. Although the ferromagnetic teeth of the stator
support become highly saturated, the generator is operated at
relatively low frequency, whereby the power losses associated with
higher frequency machines due to teeth saturation, as well as due
to copper and heat generation, are minimized.
[0013] The superconducting synchronous generator can be shown to
produce stator tooth flux densities that are greater than 2.8 Tesla
at generator operating frequencies of 10 Hz or less. In addition,
employing an air gap greater than 15 to 20 mm between the stator
and rotor assemblies in these machines further reduces losses.
[0014] Modes for carrying out the present invention are explained
below by reference to an embodiment of the present invention shown
in the attached drawings. The above-mentioned object, other
objects, characteristics and advantages of the present invention
will become apparent from the detailed description of the
embodiment of the invention presented below in conjunction with the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic side sectional view of a
generator.
[0016] FIG. 2 is an end perspective view of the stator and rotor
assemblies of the generator of FIG. 1.
[0017] FIG. 3 is an partial sectional view of the stator core of
the generator of FIG. 1 without stator windings.
[0018] FIG. 4 is perspective view of a cranked stator winding where
each side occupies a complete slot.
[0019] FIG. 5 is a partial sectional view of the stator core of the
generator of FIG. 1 with cranked stator windings disposed in the
slots.
[0020] FIG. 6 is perspective view of the end turns of the cranked
stator windings within the stator core.
[0021] FIG. 7 is an open circuit magnetic flux profile for the
generator of FIG. 1.
[0022] FIG. 8 is a graph of arc length (meters) versus flux density
(Tesla), representing air gap flux density in a generator having
non-ferromagnetic teeth and a slot area to support area ratio
greater than 0.60.
[0023] FIG. 9 is a graph of arc length (meters) versus flux density
(Tesla), representing air gap flux density in a generator having
ferromagnetic teeth and a slot area to support area ratio greater
than 0.60.
[0024] FIG. 10 is a graph of arc length (meters) versus flux
density (Tesla), representing air gap flux density in a generator
having ferromagnetic teeth, a slot area to support area ratio
greater than 0.60, and an air gap of greater than 15 mm.
[0025] FIG. 11 is a perspective view of a diamond stator
winding.
[0026] FIG. 12 is a partial sectional view of the stator core of
the generator of FIG. 1 with diamond stator windings disposed in
the slots.
[0027] FIG. 13 is a perspective view of a flat stator winding
portion of a single layer stator winding.
[0028] FIG. 14 is a perspective view of a upset-end winding portion
of a single layer stator winding.
[0029] FIG. 15 is a perspective view of an assembly of multiple
single layer stator windings.
DETAILED DESCRIPTION
[0030] Referring now to FIG. 1, generator 10 is a rotating
superconducting machine that includes a rotor assembly 40 mounted
within a stator assembly 100. As will be described in greater
detail below, the generator 10 is configured for use in low
frequency applications of 10 Hz or less. For example, when
generator 10 is used as a generator in a wind turbine, the rotor
and stator assemblies 40, 100 are configured to operate at about 2
Hz.
[0031] The rotor assembly 40 includes rotor windings 42 formed of a
high-temperature superconductor (HTS), a torque transfer system 50,
and an electromagnetic shield 60. The rotor windings 42 are
supported by a rotor winding support structure 44 within a cryostat
34. Although other configurations are possible, the rotor windings
42 of this embodiment include several HTS sub-coils formed in a
racetrack configuration. U.S. Pat. No. 6,509,819, the entire
contents of which are incorporated herein by reference, discusses
exemplary rotor coil configurations in more detail. The torque
transfer system 50 transfers the rotational forces generated by the
rotor assembly to an output shaft 30, and is also disposed in the
cryostat 34. The system 50 includes a torque tube 52 connected at
one end to the rotor winding support structure 44, and at the other
end to the output shaft 30 via an end plate 54. The electromagnetic
shield 60 surrounds the cryostat 34 and consists of a conductive,
non-magnetic material that shields rotor windings by attenuating
asynchronous fields produced by the stator currents. In addition,
the electromagnetic shield 60 shields the cryostat from heat
generated in the stator assembly.
[0032] The generator 10 also includes a brushless exciter 16 to
provide the current to the rotor windings 42. The exciter 16
consists of a transformer and associated electronics (not shown) to
condition and control the power for the rotor windings 42. The
rotor windings 42 are conduction cooled through the rotor support
structure 44 using gaseous helium, which circulates inside the
cryostat 34 to cool the HTS rotor windings 42. The coolant supply
lines 48 that permit inward and outward flow of the gaseous helium
to the rotor assembly 40 pass through the coaxial helium transfer
coupling 14, which is a stationary-to-rotating union. Bearings 32
are mounted in a frame 24 to support opposed ends of the output
shaft 30, whereby the rotor assembly 40 is rotatably and coaxially
supported within the stator assembly 100.
[0033] Referring to FIG. 2, the stator assembly 100 includes a
stator core 102 and stator windings 130. The stator core 102 is a
hollow cylindrical body 104 that is an assembly of stacked
laminated ferromagnetic plates 106. To form the stator core 102,
the ferromagnetic plates 106 are punched to provide the desired
toothed shape, and the plates 106 are stacked to form the hollow
cylindrical body 104 and fixed by conventional means such as
epoxy.
[0034] Referring also to FIG. 3, the stator core 102 includes a
back iron area 108 adjacent to and including the outer diameter of
the stator core 102, and multiple stator teeth 112 extend radially
inward from the back iron area 108 to form slots 114. Each tooth
112 has a tooth width w.sub.T, and all teeth 112 have approximately
the same tooth width w.sub.T. The number of teeth 112 provided
depends in part on the configuration of the stator winding 130 to
be supported in the stator core 102. In the illustrated embodiment
of generator 10, the stator core 102 is provided with 216 stator
teeth 112.
[0035] As viewed in cross section, the stator teeth 112 are
regularly and mutually circumferentially spaced apart, the spacing
between adjacent teeth 112 defining a stator slot 114. The stator
slots 114 are rectangular in shape and have a slot width w.sub.S
corresponding to the (circumferential) distance between adjacent
teeth 112. In this embodiment, the rectangular shape of the stator
slot 114 reflects the rectangular peripheral shape of the straight
portion of the stator winding 130 to be received in the slot 114.
The radially innermost edges 112a of the teeth 112 jointly define
the cylindrical rotor assembly-receiving bore 116 of the stator
core 102. Slot openings 120 are defined between each pair of
adjacent teeth 112, through which stator windings 130 are inserted
into the slot.
[0036] The rotor assembly 40 is supported within the bore 116 so
that the rotor and stator assemblies 40, 100 are coaxial. With this
arrangement, an air gap 118 is provided between an inner surface of
the stator core 102 (e.g., bore surface 116) and an outer surface
62 of the electromagnetic shield 60.
[0037] In the illustrated embodiment of generator 10, the stator
assembly includes 108 stator windings 130 arranged in a three or
nine phase configuration. For a three phase configuration, 36
stator windings 130 per phase are provided, resulting in a 24-pole
arrangement.
[0038] Referring to FIGS. 4-6, the stator windings 130 are formed
of transposed wire cables 132, in which the individual copper wire
conductors (157) are twisted and/or woven to form a pattern which
reduces eddy current losses. The transposed cables 132 may include
Litz wire, Rutherford wire, Robel wire, or any other suitable
transposed wire. The effect of transposition can also be achieved
by the use of end transpositions between wires or selecting designs
that use single wires in series for each coil and operated in
parallel at the end coil connections.
[0039] If the conductors 157 are rectangular in cross section, the
conductor radially-directed dimension (ie, the dimension aligned
with the slot depth d.sub.S) is longer than its circumferential
dimension (ie, the dimension aligned with the slot width w.sub.S)
in order to lower the eddy current losses. A typical conductor 157
has a radial dimension to circumferential dimension aspect ratio in
a range of 1.5:1 to 5:1. It should be noted that the aspect ratio
described here is opposite of what is typical in conventional
machines where the eddy current losses are dominated by cross-slot
leakage in which magnetic flux lines jump between two adjacent
teeth. Multistrand Litz wires typically have a 1:1 aspect
ratio.
[0040] The transposed cables 132 have an outer insulative sheath,
whereby insulation layer 152 is provided between turns, and the
outer periphery of the stator winding 130 includes a layer of
ground insulation 150.
[0041] The transposed cables 132 are wound around an axis
transverse to the longitudinal axis of the stator assembly 100 to
form an elongated multi-turn winding 130 including a parallel pair
of straight portions 136 connected by end turn portions 138. In
this case, the stator winding 130 is a cranked winding (FIG. 4) in
which the straight portions 136 are linear elongated members that
extend in parallel. The end turn portions 138 include a U-shaped
turn 140 that lies in a plane transverse to the plane in which the
straight portions 136 lie. As a result, one end winding leg portion
138a is located radially outward of the plane, and the return leg
end winding portion 138b is located in the plane. The particular
shape of the end portion 138 permits the stator windings 130 to
extend between respective slots 114 and to accommodate the presence
of the respective end portions 138 of the other stator windings 130
(FIG. 6).
[0042] The straight portion 136 of the stator winding 130 is
inserted into the slot 114 through the slot opening 120, and a slot
wedge 156 is used to retain the stator winding 130 within the slot
114. In some embodiments, an insulative slot liner 154 is
positioned in the slot 114 between the stator winding 130 and one
side wall 114a of the slot 114. In this configuration, the slot
liner 154 serves as packing within the slot to ensure even loading.
Alternatively, a slot liner 154' (not shown) may be configured to
surround the stator winding 130 on three sides, insulating the
winding 130 from all slot walls. Use of a cranked winding 130 is
advantageous since the rectangular cross section of the straight
portions 136 are well suited for use in the relatively wide slots
114 provided in the stator core 102. In particular, the
cross-sectional area of the straight portion 136 is substantially
the same as the slot area A.sub.S , where slot area A.sub.S is
defined as slot width w.sub.S multiplied by slot depth d.sub.S
(FIG. 3), and the cranked stator winding 130 substantially fully
occupies the slot 114 (FIG. 5).
[0043] Referring again to FIG. 1, the stator windings 130 are
electrically connected to an external power converter 25 via power
supply lines 22, and when torque is applied to the rotor causing
rotation of the output shaft 30, alternating magnetic flux is
produced which extends through the air gap 118 from the HTS rotor
windings 42 and interacts with the stator 130 to generate
power.
[0044] The stator windings 130 are cooled by conventional means.
For example, in some embodiments, the stator windings 130 may be
conduction cooled. In this example, the straight portions 136 can
be conduction cooled through the back-iron 108, and the heat can be
extracted by air flow or a liquid cooling jacket (not shown) on the
outer diameter of the stator core 102. The end turns 138 can be
cooled by forced convection. In other embodiments, the stator
windings 130 may be air cooled. For example, lamination spacing
(not shown) can be fabricated into the stator core 102, and air can
be ventilated from the inner diameter to the outer diameter of the
stator core 102 with forced convection. The end turns 138 can also
be cooled by forced convection. In other embodiments, the stator
windings 130 can be cooled by direct liquid cooling. For example,
the stator windings 130 can be cooled by direct contact with a
dielectric fluid with the addition of narrow cooling passages (not
shown) within the stator slot area. In still other embodiments, the
stator windings 130 can be fabricated with internal tubes built
into windings, and deionized water can be circulated in the tubes
to provide direct cooling. In still other embodiments, the stator
windings 130 can be fabricated with external water cooling tubes,
specifically, having tubes built outside the ground insulation of
the windings. Deionized or fresh water can be circulated in the
tubes to provide stator winding cooling. In this case, copper fins
can be potted with the tubes to improve cooling on the outside
surface of the ground plan insulation.
[0045] Referring again to FIG. 3, the stator core 102 is formed so
that the overall tooth width w.sub.T is small relative to the
overall slot width w.sub.S. In particular, the ratio of the sum of
the widths of the slots to the sum of the widths of the teeth and
slots is in the range of 0.65 to 0.90, where the sum of the widths
of the slots is defined as
Sum.sub.wS=.SIGMA.w.sub.S1+w.sub.S2+w.sub.S3+ . . . +w.sub.SN,
the sum of the widths of the teeth and slots is defined as
[0046] Sum.sub.wS+wT=w.sub.S1+w.sub.S2+w.sub.S3+ . . .
+w.sub.SN+w.sub.T1+w.sub.T2+w.sub.T3+ . . . +w.sub.TN, and N is the
number of slots in the stator core 102. In the illustrated
embodiment, N=216, but the stator core 102 is not limited to this
number of slots.
[0047] In other embodiments, the ratio of the sum of the widths of
the slots to the sum of the widths of the teeth and slots is in the
range of 0.70 to 0.90. In still other embodiments, the ratio of the
sum of the widths of the slots to the sum of the widths of the
teeth and slots is in the range of 0.75 to 0.90.
[0048] In some embodiments of the generator 10, the slot width
w.sub.S may be at least twice as large as the tooth width w.sub.T,
and the main function of the teeth in this device is to provide
support for the stator windings 130. A stator core formed having a
ratio of the sum of the widths of the slots to the sum of the
widths of the teeth and slots as disclosed here is novel since in
high frequency machines such slot to tooth proportions are
associated with high losses and associated limits in power output.
In contrast, in the generator 10, the relatively large slot area
permits use of additional conductor within the slot, which is
advantageous since power losses decrease with increased conductor
area particularly when the conductor is subdivided and effectively
transposed. Also advantageously, although the stator teeth 114 of
generator 10 mainly serve to support the stator windings 130, use
of the ferromagnetic material therein also results in a modest
increase (about 10 percent) in air gap flux.
[0049] In addition, because the generator 10 is operated at low
frequencies (up to 10 Hz), the eddy current losses associated with
ferromagnetic teeth and the copper conductors in the slots are
reduced.
[0050] Further advantageously, use of a ferromagnetic material to
form the stator teeth 114 permits the generator 10 to be
inexpensively manufactured.
[0051] When generator 10 is operated at a low frequency, for
example at about 2 Hz, the tooth flux density is greater than 1.8
Tesla. As seen in FIG. 7, which illustrates calculated flux density
over a portion the stator and rotor assemblies the tooth flux
density is shown to be greater than 3.0 Tesla. Thus, the generator
10 is configured to operate with flux saturated teeth, and during
operation, teeth 114 of generator 10 are at a much higher flux
density than in conventional superconducting generators, which
generally operate with teeth having a flux density of less than 1.8
Tesla. However, due to the low frequency operation of generator 10,
although the teeth 114 are saturated, the resulting losses (as
scaled from ARMCO tables) are less than 10 kW.
[0052] Referring now to FIG. 8, the flux density in the air gap, as
measured at the surface 62 of the electromagnetic shield 60, is
shown for a superconducting generator that includes a stator core
formed with non-ferromagnetic teeth (ie, composite or stainless
steel) but including a stator core in which ratio of the sum of the
widths of the slots to the sum of the widths of the teeth and slots
is in the range of 0.65 to 0.90. This graph shows a flux density
that is greater than 1 Tesla in the air gap. This flux density is
greater than in some conventional machines, which have an air gap
flux density of about 1 Tesla. This graph illustrates that more
power is produced in the air gap relative to a conventional machine
due to the use of HTS rotor coils, and further illustrates that the
inventive stator construction, including a ratio of the sum of the
widths of the slots to the sum of the widths of the teeth and slots
in the range of 0.65 to 0.90, does not prevent adequate power
production.
[0053] There are limitations to the use of ferromagnetic saturated
teeth. For example, the electromagnetic shield 60 can have
additional losses due the field changes associated with tooth
passing. Referring now to FIG. 9, the flux density in the air gap,
as measured at the surface 62 of the electromagnetic shield 60, is
shown for a superconducting generator that includes a stator core
formed with ferromagnetic teeth and including a stator core in
which the ratio of the sum of the widths of the slots to the sum of
the widths of the teeth and slots is in the range of 0.65 to 0.90.
This graph shows a flux density that includes some harmonic content
as represented by localized peaks at about 0.09 m and 0.18 m. Such
harmonic content is undesirable since it results in heating on the
electromagnetic shield 60, and thus power loss in the rotor
assembly 40. However, as seen in FIG. 10, such harmonic content can
be avoided by increasing the air gap 118. For example, in some
embodiments, generator 10 can include an air gap of greater than 15
mm. In other embodiments, generator 10 can include an air gap of
greater than 20 mm. For purposes of comparison, in a conventional
generator of comparable size, an air gap of 5 to 10 mm is often
used. Thus, a relatively large air gap 118 is beneficial for use in
a low speed system as embodied by generator 10.
[0054] The generator 10 has been described here as employing
cranked stator windings 130. However, the generator 10 is not
limited to use of the cranked stator winding 130, and it is well
within the scope of the invention to use windings of other
configurations, such as, but not limited to, diamond windings 230
or single layer windings 330.
[0055] Referring now to FIG. 11, a diamond winding 230 includes
transposed cables 132 wound in a generally diamond shape. The
diamond windings 230 are arranged within the stator slots 114 by
overlapping straight portions 236 of adjoining windings in the same
phase, whereby each slot 114 receives two windings 230. In
particular, one straight portion 236a occupies the radially inward
location of one slot 114, and the other straight portion 236b
occupies the radially outward location of another slot 114.
Standard diamond windings have an end geometry that makes it
difficult to achieve a greater slot fill than about 50 percent. As
seen in FIG. 12, the cross-sectional area of the straight portion
236 is about half of the slot area A.sub.S, and the straight
portion 236 of the diamond winding 230 occupies about half of the
slot 114. Due to the presence of ground insulation 150 on each
diamond winding, a double layer of insulation is formed between the
two windings 230 disposed in the slot 114, having the effect that
the amount of conductor in the slot 114 when using a diamond
winding 230 is less than when using a cranked winding 130.
[0056] Referring now to FIGS. 13-15, a single layer winding 330
includes a flat stator winding 330a (FIG. 13) used in combination
with a upset-end stator winding 330b (FIG. 14). In the flat stator
winding 330a, the end turn portion 338a is U-shaped and lies in the
same plane as the straight portion 336a. In the upset-end stator
winding 330b, the end turn portion 338b is U-shaped but extends
upward from the plane defined by the straight portions 336b. When
assembled together (FIG. 15), the straight portion 336b of the
upset-end stator winding 330b resides between, and in the same
plane as, the straight portions 336a of the flat stator winding
330a. In addition, the end turn portion 338b of the upset-end
stator winding lies parallel to, and partially overlies, the end
turn portion 338a of the flat stator winding 330a. Like the cranked
winding 130, the cross-sectional area of the straight portion 336a,
336b is substantially the same as the slot area A.sub.S, and the
single layer winding 330 substantially fully occupies the slot 114.
In addition, the single layer winding has a better conductor
packing factor than the cranked winding 130 due to differing
insulation requirements. However, the single layer coil 330 may be
more difficult to install and support than the single layer winding
130.
[0057] A selected illustrative embodiment of the stator assembly
100 for use in the generator 10 is described above in some detail.
However, the stator assembly as described herein is not limited to
use in a generator. For example, the stator assembly can be used in
other types of rotating electrical machines, including high torque,
low speed motors.
[0058] In addition, it should be understood that only structures
considered necessary for clarifying the present invention have been
described herein. Other conventional structures, and those of
ancillary and auxiliary components of the system, are assumed to be
known and understood by those skilled in the art. Moreover, while a
working example of the present invention has been described above,
the present invention is not limited to the working example
described above, but various design alterations may be carried out
without departing from the present invention as set forth in the
claims.
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