Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02774303 2012-10-11
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MAGNET SHAPING IN PERMANENT MAGNET SYNCHRONOUS
MACHINES
TECHNICAL FIELD
[0001] The
present invention generally relates to a permanent magnet
motor/generator and, more particularly, to shaping magnets in permanent magnet
synchronous alternating current (AC) machines.
BACKGROUND
[0002] Electric
machines may be classified as direct current (DC) or
alternating current (AC), the latter being categorized as either synchronous
or
induction.
Synchronous AC electric machines may be further classified as
brushless, sine wave, step, hysteresis and reluctance. In the sine wave
category
are those that use permanent magnets and those that use wound fields. The
permanent magnets of permanent magnet synchronous AC machines are either
surface-mounted, inset-mounted or interior-mounted.
[0003]
Permanent magnet synchronous AC electric machines may be used
for motors, generators and motor/generators. One particular application is a
flywheel energy storage system.
[0004] In a
flywheel energy storage system, it is often desirable to operate an
electric machine at high rotational speeds in a vacuum environment.
Frequently, a
structural element is used to support the magnets in the electric machine to
obtain
higher speed operation. This structural element is variously called an
overwrap,
sleeve, bandage, or sheath and can be made of non-ferrous metallic or
composite
materials. It is desirable to have magnets that provide uniform loading of the
sleeve.
For operation in a vacuum environment, it is desirable to minimize rotor
losses,
since heat generated on the rotor cannot be rejected with convective means.
Magnet shaping can be used to reduce heat generated in the magnets; however,
magnet shaping can also provide undesired, non-uniform loading of the sleeve,
and
may incur significant manufacturing cost increases. Accordingly, there is a
need for
an electric machine that provides low rotor losses and good structural
integrity
without incurring excessive manufacturing costs.
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SUMMARY
[0005] In general, the present invention provides a novel permanent magnet
shape for a permanent magnet synchronous machine. The permanent magnet has
a shaped body that is characterized by a radially outer surface that is
arcuate, a flat
inner surface, and first and second side surfaces. The first and second side
surface
face an opposite pole and a like pole, respectively. The first side surface
and the
flat inner surface subtend an angle 61 whereas the second side surface and the
flat
inner surface subtend an angle 62. Each of these angles is greater than 90
degrees.
This shaped magnet exhibits low rotor losses, has good structural integrity,
and is
also easy to manufacture.
[0006] Accordingly, one aspect of the present invention is a permanent
magnet for use in a permanent magnet synchronous machine, the permanent
magnet comprising a shaped body, the shaped body having an arcuate radially
outer surface, a flat inner surface, a first side surface facing an opposite
pole, and a
second side surface facing a like pole. The first side surface and the flat
inner
surface subtend an angle 61 that is greater than 90 degrees and the second
side
surface and the flat inner surface subtend an angle 62 that is also greater
than 90
degrees.
[0007] Another aspect of the present invention is a permanent magnet
synchronous machine comprising a stator, a rotor and a plurality of permanent
magnets mounted to the rotor in which each magnet has a shaped body
characterized by an arcuate radially outer surface, a flat inner surface, a
first side
surface facing an opposite pole, and a second side surface facing a like pole.
The
first side surface and the flat inner surface subtend an angle 61 that is
greater than
90 degrees and the second side surface and the flat inner surface subtend an
angle
62 that is also greater than 90 degrees.
[0008] Yet another aspect of the present invention is a flywheel energy
storage system comprising a flywheel and a permanent magnet synchronous
machine having a stator, a rotor and a plurality of permanent magnets mounted
to
the rotor, wherein each permanent magnet includes a shaped body characterized
by
an arcuate radially outer surface, a flat inner surface, a first side surface
facing an
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opposite pole, and a second side surface facing a like pole. The first side
surface
and the flat inner surface subtend an angle öi that is greater than 90 degrees
and
the second side surface and the flat inner surface subtend an angle 62 that is
also
greater than 90 degrees.
[0009] A
further aspect of the present invention is a permanent magnet for
use in a permanent magnet synchronous machine that includes two shaped magnet
halves formed as an integral structure. This
permanent magnet comprises a
shaped body formed of two magnet halves or portions. The shaped body has an
arcuate radially outer surface and an inner surface composed of two flat
angled
sections for each of the two magnet halves or portions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Further
features and advantages of the present technology will
become apparent from the following detailed description, taken in combination
with
the appended drawings, in which:
[0011] FIG. 1
is a plan view of a permanent magnet for use in a four-pole
magnet array in accordance with an embodiment of the present invention;
[0012] FIG. 2
is a plan view of a four-pole magnet array comprising eight
magnets of the type depicted by way of example in FIG. 1;
[0013] FIG. 3
is a plan view of a six-pole magnet array comprising twelve
magnets in accordance with another embodiment of the present invention;
[0014] FIG. 4
is a plan view of an eight-pole magnet array comprising sixteen
magnets in accordance with another embodiment of the present invention;
[0015] FIG. 5
is a schematic depiction of a permanent magnet synchronous
machine having permanent magnets in accordance with embodiments of the
present invention; and
[0016] FIG. 6
is a schematic depiction of a flywheel energy storage system
incorporating having shaped permanent magnets in accordance with embodiments
of the present invention.
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[0017] It will be noted that, throughout the appended drawings, like
features
are identified by like reference numerals.
DETAILED DESCRIPTION
[0018] In general, the present invention is an electric machine comprising
a
permanent magnet having a novel shape that exhibits superior performance
characteristics in terms of low rotor losses and good structural integrity
while being
easy to manufacture.
[0019] FIG. 1 illustrates an example of a shaped magnet generally
designated
by reference numeral 10 for use in a four-pole magnet array in an electric
machine.
This shaped magnet 10 is defined geometrically by an origin (0), a radius (R),
a gap
(G), a base length (L) and an angle (8).
[0020] As illustrated in FIG. 1, the magnet 10 is characterized by an
arcuate
radially outer surface 12 (which may be of a constant radius R) as defined
from the
origin 0. As depicted in FIG. 1, the radially inwardly facing surface
comprises a flat
inner surface 14 defining a base of the magnet. A centerline of base length L
(which
line is defined as being a line extending orthogonally from the origin 0 to
the flat
inner surface) divides the flat inner surface 14 (i.e. the base of the magnet)
into first
and second sections of length b1 and b2, respectively.
[0021] As illustrated in FIG. 1, the shape of the magnet body 10 is
characterized further by a first side surface 16 facing an opposite pole (N-S,
S-N)
and a second side surface 18 facing a like pole (N-N, S-S). In the embodiment
illustrated in FIG. 1, the magnet 10 is a North magnet. The first side surface
16 thus
faces a South magnet whereas the second side surface 18 faces a North magnet.
In the embodiment depicted in FIG. 1, the first side surface 16 is spaced
apart by a
gap G from the adjacent magnet of opposite polarity whereas the second side
surface abuts the adjacent magnet of like polarity.
[0022] In embodiments of the invention, the first side surface 16 and the
flat
inner surface 14 subtend an angle 61 that is greater than 90 degrees. The
second
side surface 18 and the flat inner surface 14 subtend an angle 152 that is
also greater
than 90 degrees.
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[0023] As illustrated in FIG. 1, the angle 0 subtends the orthogonal
centerline
of length L and the quadrature axis (denoted "q axis"). The q axis is offset
from the
direct axis ("d axis") by a rotation of 90 degrees magnetic.
[0024] In the embodiment depicted in FIG. 1, the magnet angles are defined
as follows:
[0025] 0 is the angle subtended by the q-axis and a line orthogonal to an
adjacent magnet base (i.e. flat inner surface) which line also passes through
the
origin 0 of the radius R of the arcuate radially outer surface.
[0026] 61 is the angle subtended by the base and side of a magnet where the
side of the magnet faces an opposite pole (S-N, N-S).
[0027] 62 is the angle subtended by the base and side of a magnet where the
side of the magnet faces a like pole (N-N, S-S).
[0028] As noted above, a characteristic of this shaped magnet is that 61
and
62 are each greater than 90 degrees. In most embodiments, 61 is not equal to
62. In
some embodiments, 61 is greater than 62 as illustrated by way of example in
FIG. 1.
In the specific example presented in FIG. 1, 61 is 120 degrees and 62 is 105
degrees, although it should be understood that these angles will vary
depending on
the exact geometry and size of the magnets and rotor.
[0029] In a case in which the base of the magnet is arcuate, then 6 is the
angle subtended by the side of the magnet and the tangent of the base at the
intersection of the side.
[0030] The angles 61, 62 and 0 are defined as follows:
= 90 + 360 x
2n
82 = 90 +360 /
2n
n 360
= ¨ = x
2n
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where n is the number of magnetic poles and, preferably,
2
x=
[0031] However, x may take any value in a range of
7
¨<x<-
9 9
[0032] In some
4-pole embodiments of the magnet, the mechanical angle
theta 8 is between 25 and 35 degrees.
[0033] In the
embodiment depicted in FIG. 1, the base dimension b1 left of the
centerline L (i.e. the length of the first section) is greater than the base
dimension b2
right of the centerline (length of the second section). In the same
illustrated
embodiment, the thickness t1 of the first side facing the magnet of opposite
polarity
is less than the thickness t2 of the second side facing the magnet of similar
polarity.
In other words, the surface area of the first side surface is less than that
of the
second side surface. This
geometric asymmetry is one characteristic of the
magnet. However,
it should be appreciated that the shaped magnet may be
asymmetrical or symmetrical about the direct axis.
[0034] The
geometric asymmetry may also be expressed in terms of a ratio of
asymmetrical length to total length, i.e. (bi-b2)/(b1+b2). For a four pole
magnet
configured as shown by way of example in FIG. 1, the ratio of asymmetrical
length
to total length would be approximately +35% but, in other embodiments, this
could
range between, for example, 0% and 75%.
[0035] The
electric machine comprising this novel magnet shape can be
operated as a motor, as a generator or as a motor/generator. One application
of
this electric machine is in a flywheel energy storage system although it will
be
understood that the electric machine may be incorporated into any other
suitable
electromechanical system. The electric machine may be a high-speed electric
machine or in a low-speed electric machine. When these shaped magnets are
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incorporated into an electric machine such as a flywheel energy storage
system,
they provide low rotor losses and good structural integrity by virtue of
uniform
distribution of centrifugal loading on the sleeve without incurring excessive
manufacturing costs.
[0036] As will
be readily appreciated by those of skill in the art, the geometry
of the shaped body of the magnet may be varied, i.e. the R, L, G and 8
parameters
may be varied to achieve different effects. Tuning of the variables R, L, G
and 8 will
thus enable the electric machine designer to adapt the performance
characteristics
for specific applications or uses.
[0037] The two
adjoining magnets of like pole may be, in one embodiment,
formed or manufactured as a single integral magnet. In such an embodiment, the
magnet would be defined by a common arcuate radially outer surface, two
inclined
flat surfaces, and a pair of side surfaces. The magnets would thus define a
line (or
plane) of symmetry along the d-axis. The angle 61 would be defined as before
(i.e.
the angle between the first side surface and the flat inner surface). The
angle 61
would be greater than 90 degrees as shown in FIG. 1. However, the angle
62would,
in this particular embodiment, be re-defined as the angle subtended by the
flat inner
surface and the line of symmetry or d-axis. As shown in FIG. 1, the magnet
body
may be formed of two symmetrical halves (two symmetrical portions) that are
symmetrical about the plane of symmetry defined by the d-axis. The thickness
t2 of
the magnet along the plane of symmetry is greater than the thickness t1 along
each
of the side surfaces. In an
electric machine, these magnets are arranged with
alternating poles with a gap between each adjacent magnet.
[0038] In
another embodiment, each magnet may be segmented into magnet
components or subcomponents to facilitate manufacturing or assembly
operations.
In other words, each magnet may comprise a plurality of components or
subcomponents that are joined, attached, connected or otherwise assembled to
form a single consolidated magnet.
[0039] The
present invention may be applied to 2-pole, 4-pole, 6-pole or 2n-
pole magnet arrays as shown by way of various examples depicted in FIGS. 2-4.
For example, in a 2-pole (1 pole pair) magnet array, 8 = 60 . In other words,
each
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angled section of each permanent magnet defines an angle e (where e = 60
degrees in this particular configuration) between a quadrature axis and a line
that is
orthogonal to one of the flat angled sections and that intersects the origin 0
of the
radius of curvature of the arcuate radially outer surface.
[0040] FIG. 2 is a plan view of a four-pole magnet array comprising eight
magnets of the type depicted by way of example in FIG. 1. In the 4-pole (2-
pole
pair) magnet array depicted in FIG. 2, e = 30 .
[0041] FIG. 3 is a plan view of a six-pole magnet array comprising twelve
magnets in accordance with another embodiment of the present invention. In the
6-
pole (3-pole pair) magnet array depicted in FIG. 3, 0 = 20 .
[0042] FIG. 4 is a plan view of an eight-pole magnet array comprising
sixteen
magnets in accordance with another embodiment of the present invention. In the
8-
pole (4-pole pair) magnet array depicted in FIG. 4, 0 = 15 .
[0043] Any of the curved surfaces shown by way of example in FIG. 1 may be
replaced by linear approximations of the curved surfaces. The arcuate radially
outer
surface may thus be replaced by a segmented surface composed of a plurality of
segments that approximate an arc.
[0044] Likewise, a curved implementation of any of the linear surfaces may
be possible in a variant. For example, the flat inner surface may be replaced
by a
slightly curved surface.
[0045] In another embodiment, the flat inner surface may be segmented into
two or more flat sub-sections having slightly different angles.
[0046] The magnets disclosed in this specification and illustrated in the
drawings may be used to construct a novel electric machine such as the one
illustrated by way of example only in FIG. 5.
[0047] In the embodiment schematically depicted by way of example in FIG.
5, a permanent magnet synchronous machine comprises a stator, a rotor and a
plurality of permanent magnets 10 mounted to the rotor. As described above,
each
permanent magnet has a shaped body characterized by an arcuate radially outer
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surface, a flat inner surface, a first side surface, and a second side surface
such that
that the angles 61 and 62 are both greater than 90 degrees, as described
above.
The machine may incorporate a 2-pole, 4-pole, 6-pole, 8-pole, or 2n-pole
magnet
array, where n = any even integer.
[0048] As depicted in FIG. 5, the 4-pole machine includes a stator core
52, a
motor shaft 54, a sleeve 56, a winding end turn 58 and eight permanent magnets
10
having the novel shape described above.
[0049] The permanent magnet synchronous machine may be part of a novel
flywheel energy storage system such as the one illustrated by way of example
only
in FIG. 6.
[0050] In the embodiment schematically depicted by way of example in FIG.
6, a flywheel energy storage system 100 comprises a flywheel 110, a housing
112,
and a permanent magnet synchronous machine having a stator, a rotor and a
plurality of permanent magnets mounted to the rotor. Upper and lower bearing
mounts 120, 130 are shown. Each permanent magnet includes a shaped body
characterized by an arcuate radially outer surface, a flat inner surface, a
first side
surface, and a second side surface such that that the angles 61 and 62 are
both
greater than 90 degrees, as described above.
[0051] The electric machine incorporating the novel shaped magnets may
itself have many variants. For example, the electric machine may have a sleeve
or
no sleeve, the magnets may be glued (bonded by adhesive) or not. It is noted
that
the magnets in low-speed sleeveless machines are bonded to the rotor. The
permanent magnets may be plated or unplated, or they may be coated or
uncoated.
The magnets may be axially straight or skewed, i.e. stepped or with a
continuously
varying helical angle. The magnets may be surface-mounted to the rotor or
inset-
mounted or interior-mounted. The rotor may be constructed of a solid ferrous
material or of a laminated material (e.g. laminated rotor iron). The rotor of
the
electric machine may be constructed as a straight cylinder or as a tapered
cone.
The stator of the electric machine may be constructed in any number of
variants
including air cored or laminated, with magnet wire or litz wire, or in any
suitable
design known in the art.
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[0052] Although the shaped permanent magnets are primarily designed for
use in a permanent magnet synchronous machine in a flywheel energy storage
system, these shaped permanent magnets may also be used in many other
different
electromechanical systems, including motors, generators and motor/generators.
[0053] The embodiments of the invention described above are intended to be
exemplary only. As will be appreciated by those of ordinary skill in the art,
to whom
this specification is addressed, many obvious variations, modifications, and
refinements can be made to the embodiments presented herein without departing
from the inventive concept(s) disclosed in this specification. The scope of
the
exclusive right sought by the applicant is therefore intended to be limited
solely by
the appended claims.
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