Note: Descriptions are shown in the official language in which they were submitted.
CA 02604812 2007-10-24
HIGH SPEED ROTOR
FIELD OF THE INVENTION
This invention relates to electromechanical machines, and in particular, to a
permanent magnet electromechanical machine incorporating a high speed rotor
design.
BACKGROUND AND SUMMARY OF THE INVENTION
In order to meet the constant demand for efficient, power dense drivers for
industrial and commercial applications, high speed, permanent magnet electric
motors
and generators are required. Presently, however, there are very few permanent
magnet
electric motors or generators that are rated over several hundred kilowatts
(kW) and that
provide high speed shaft rotation. While shaft speeds of up to approximately
100,000
revolutions per minute (rpm) have been achieved in permanent magnet electric
motors
and generators having low power ratings, higher rated machines are typically
limited to
shaft speeds of several thousand rpms or less. In order to provide high speed,
permanent
magnet electric motors and generators, a rotor designed for high speed
rotation is
required.
Permanent magnet electric motors and generators typically incorporate a drum-
shaped rotor having permanent magnets located thereon to establish magnetic
poles. In a
first rotor construction, the permanent3nagnets are fastened on the outer
surface of the
rotor drum. This type of rotor construction is known as a "surface mounted"
permanent
magnet rotor. Alternatively, the permanent magnets may be embedded below the
surface
of the motor. This type of rotor construction is known as an "embedded"
permanent
magnet rotor. Both types of rotor constructions utilize rare earth magnets. As
is known,
rare earth magnets typically have poorer mechanical properties than the other
elements of
the rotor, and as such, cannot be used as load bearing elements in the rotor
design.
Further, rare earth magnets exhibit a weak resistance to corrosion, as well
as, to the flow
of electricity. Consequently, rare earth magnets can be de-magnetized by
exposure to
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corrosive environments or high teinperatures caused by eddy currents flowing
in the
magnets, or any other heat generating mechanism of the machine's operation.
Surface mounted permanent magnet rotors are conceptually simple, and
therefore,
perceived to be less costly. Typically, the magnets are retained on the outer
diameter of
the rotor in one of four ways. First, the magnets may be enclosed in a non-
ferromagnetic
holder that is attached to the rotor by mechanical means such as fasteners, a
version of
"tongue and groove" geometry, or a combination of both. Second, the magnets
may be
glued directly to the outer surface of the rotor. Third, the magnets may be
glued directly
to the outer surface of the rotor, and thereafter, a non-ferromagnetic, metal
sleeve is
shrink-wrapped around the magnets. Fourth, the magnets may be glued directly
to the
outer surface of the rotor, and thereafter, the rotor assembly is wrapped with
a high
strength, high modulus composite fiber/epoxy.
Each of the prior designs for surface mounted permanent magnet rotors has
certain shortcomings. For example, in the designs wherein the magnets are
shielded by a
metallic sleeve, the metallic sleeve is subjected to higher order harmonics in
the stator
due to the power supply and the stator slot geometry. As a result, eddy
currents are
generated in the metallic sleeve so as to cause heating of the rotor and the
magnets. At
very high frequencies, such as those experienced in machines running
significantly faster
than approximately 3600 rpms, the heating of the metallic sleeve can damage
the
magnets. As such, rotor thermal management is a significant design
consideration for
any high speed, permanent electric motor or generator using such a magnet
retention
means.
In the designs wherein the magnets are glued to the rotor or wherein a
composite
fiber/epoxy wraps is used to retain the magnets on the rotor, the electrical
properties of
the magnetic material allow eddy currents to flow, thereby heating the magnets
directly.
It can be appreciated that a composite wrap over the magnets makes the cooling
of the
magnets a greater challenge since the composite wrap also acts to thermally
insulate the
magnets. Alternatively, simply gluing the magnets to the rotor is not feasible
for high
speed applications as the mechanical properties of the magnets are not up to
the task of
holding together when subjected to the tensile loads that results from high
rotational
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speeds. Further, finding a suitable adhesive for gluing the magnets on the
rotor may be
difficult.
An additional drawback to surface mounted permanent magnet rotors is the cost
of the magnets. The surface mounted magnets are necessarily shaped to closely
fit the
outer surface of the rotor. Shaping the surface mounted magnets involves the
precision
grinding of each magnet at its interface with the rotor, usually before
magnetization,
followed by the use of special tooling to energize the magnets after they are
installed on
the rotor. These manufacturing steps can add significantly to the overall cost
of the final
product. Finally, surface mounted rotors are more susceptible to damaging the
magnets
in "off-design" operating conditions, such as pole slips or stator short
circuits.
While rotors that incorporate magnets embedded below the surface of the rotor
are more complex in appearance, this type of rotor constructions has proved to
be
relatively simple to design, manufacture and assemble. In such embedded magnet
rotor
configuration, the rotor is made of a non-ferromagnetic material and the
magnets are
arranged so that the direction of magnetization is perpendicular to an axial
point passing
through the middle of each installed magnet and the rotor center line.
Laminated pole
pieces are installed on the sides of each magnet, with the polarity of the
magnets arranged
to have the same polarity on each side of a particular pole piece. As a
result, a magnetic
pole is formed on the outside diameter of the rotor. The embedded magnet rotor
configuration has the advantage of shielding the magnets from the stator
harmonics that
can cause eddy current heating in the magnets, as well as, damage to the
magnets from
the high flux transients and reversals resulting from stator short circuits or
pole slipping
during operation. In addition, the laminated pole pieces effectively limit
eddy currents in
the poles, and thus, the heating of the rotor in total. Further, in Embedded
magnet rotor
configurations, the magnets are usually simple rectangular shapes and are
installed
magnetized. As a result, a manufacturer does not have to invest in unique
magnetizing
tooling for each rotor diameter being produced. This, in turn, significantly
reduces the
cost of the final product. In view of the foregoing, it can be appreciated
that the
embedded magnet rotor configuration offers greater design freedom since the
burden of
cooling the rotor is limited and/or eliminated.
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Heretofore, in embedded magnet rotor configurations, the magnets ar=e
restrained from movement in the radial direction by the pole pieces. For
example,
wedges or other blocking features may be used to restrain radial movement of
the
magnets. These wedges or bloclcing features are attached to the rotor by keyed
tangs,
"fir-tree" tongue and groove geometry and composite fiber/epoxy materials
wound
around the outside diameter, or any combination of the above. Alternatively,
the magnets
may have a trapezoidal cross section with the pole pieces being in contact
with the
magnets. If the magnets move radially away from the rotor center, the magnets
and the
pole pieces are loaded in compression by their respective geometries. In most
circumstances, these arrangements for embedding the magnets within the rotor
are
adequate. However, the mechanical properties of the magnet materials and pole
pieces
limit the surface speeds such machines can achieve, making them most suitable
for low
RPM, -high torque/power design.
In accordance with one aspect of the present invention there is provided an
electromagnetic machine which has a stator extending along the longitudinal
axis and
having an inner surface defining a rotor receipt cavity. A rotor extends along
and is
rotatable about the longitudinal axis and is positioned within the rotor
receipt cavity. A
plurality of ring assemblies is supported on the rotor, wherein each of the
ring assemblies
includes magnetic retention means. A plurality of magnets are
circumferentially spaced
about the rotor and extend through the ring assemblies. Each magnet is
generally parallel
to the axis of the rotor and is disposed in relation to a corresponding magnet
retention
means. The magnet retention means prevents outward radial movement of the
magnet
during operation of the machine.
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A rotor assembly for an electromechanical machine may include a rotor
connectable to a shaft for rotational and movement therewith. The rotor
extends along an
axis and has first and second circumferentially spaced lobes projecting
radially therefrom.
First and second sets of laminated pole pieces are provided. Each set of
laminated pole
pieces is receivable on a corresponding lobe. A magnet is disposed between the
sets of
pole pieces. The rotor assembly includes a magnet retention ring for
preventing radial
movement of the magnet. The magnet retention ring has a radially outer edge
and
includes a backing plate and a magnet retention element. The backing plate has
first and
second cutouts therein for receiving corresponding lobes therethrough. The
magnet
retention element projects from a first side of the backing plate and extends
between the
first and second sets of laminated pole pieces.
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In a preferred embodiment, each lobe projecting from the rotor includes a neck
terminating at an enlarged head. Each laminated pole piece is generally c-
shaped and
includes first and second ends separated by a predetermined distance for
accommodating
the neck of a corresponding lobe therebetween. The magnet retention element
includes a
radially outer surface extending between first and second sets of laminated
pole pieces
and an inner surface directed towards the magnet. A shim may be positioned
between the
inner surface of the magnet retention element and the magnet for preventing
radial
movement of the magnet during rotation of the rotor.
Each set of laminated pole pieces includes a plurality of first pole pieces
having a
first radial thickness and a plurality of second pole pieces having a second
radial
thickness. The plurality of first pole pieces of a corresponding set of
laminated pole
pieces are positioned adjacent each other to form a first stack and the
plurality of second
pole pieces of the corresponding set of laminated pole pieces are positioned
adjacent each
other to form a second stack. It is contemplated that the first radial
thickness be greater
than the second radial thickness to control end leakage of flux.
In accordance with a further embodiment, a rotor assembly may include a rotor
connectable to a shaft for rotational movement therewith. The rotor extends
along an
axis and has a plurality of circumferentially spaced lobes projecting radially
therefrom.
A plurality of ring assemblies are supported on the rotor. Each ring assembly
includes a
plurality of circumferentially spaced poles supported on corresponding lobes.
A plurality
of magnets are circuniferentially spaced about the rotor and extend through
the ring
assemblies. Each magnet is generally parallel to the axis of the rotor and is
disposed
between corresponding pairs of poles of each ring assembly.
Each ring assembly includes a magnet retention ring for preventing radial
movement of the plurality of magnets. Each magnet retention ring has a
radially outer
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edge and includes a backing plate and a plurality of circumferentially spaced
magnet
retention elements projecting from a first side of the backing plate. The
backing plate has
a plurality of cutouts therein for receiving corresponding lobes therethrough.
It is
contemplated that each magnet retention element extends between corresponding
pairs of
poles and has a retaining bar projecting from the terminal thereof. Each
backing plate
includes a second side having a plurality of circuniferentially spaced
retaining bar receipt
cavities forrned therein. Each retaining bar receipt cavity is adapted for
receiving a
corresponding retaining bar of an adjacent magnet retention ring in a mating
relationship.
Each of the poles of each ring assembly includes a plurality of laminated pole
pieces. The rotor includes first and second ends wherein one of the plurality
of ring
assemblies is positioned adjacent the first end of the rotor. The laminated
pole pieces of
each of the poles of the one of the plurality of ring assemblies positioned
adjacent the
first end of the rotor includes a plurality of first pole pieces having a
first radial thickness
and a plurality of second pole pieces having a second radial thickness. The
first radial
thickness of the first pole pieces is greater than the second radial thickness
of the second
pole pieces and are positioned adjacent the first end of the rotor. It is
contemplated that
all of the laminated pole pieces include a generally arcuate, radially outer
edge. In
addition, all of the laminated pole pieces include a leading edge and a
trailing edge which
are asymmetrical.
The rotor of the electromagnetic machine according to the invention may
include
a plurality of circumferentially spaced lobes projecting radially therefrom.
Each ring
assembly includes a plurality of circumferentially spaced poles supported on
corresponding lobes. Each of the poles of
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each ring assembly includes a plurality of laminated pole pieces. The rotor
includes first
and second ends and one of the plurality of ring assemblies is positioned
adjacent the first
end of the rotor. The laminated pole pieces of each of the poles of the one of
the plurality
of ring assemblies positioned adjacent the first end of the rotor includes a
plurality of first
pole pieces having a first radial thickness and a plurality of second pole
pieces having a
second radial thickness. It is contemplated that the first radial thickness of
the first pole
piece be greater than the second radial thickness of the second pole pieces.
The first pole
pieces are positioned adjacent the first end of the rotor. Each laminated pole
pieces
includes a leading edge and a trailing edge which are asymmetrical.
The stator of the electromagnetic machine includes a plurality of laminated
stator
pieces laminated along an axis generally parallel to the longitudinal axis.
The laminated
stator pieces are radially spaced from the poles of the rotor assemblies. The
stator may
include a plurality of radially extending cooling channels extending
therethrough. The
cooling channels communicate with the rotor receipt cavity in order to cool
the rotor.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings furnished herewith illustrate a preferred construction of the
present
invention in which the above advantages and features are clearly disclosed as
well as
others which will be readily understood from the following description of the
illustrated
embodiment.
In the drawings:
Fig. 1 is a cross-sectional view of an electromechanical machine incorporating
a
rotor assembly in accordance with the present invention;
Fig. 2 is an exploded, isometric view of the rotor assembly of the present
invention;
Fig. 3 is an exploded, isometric view of the ring assembly for the rotor
assembly
of the present invention;
Fig. 4 is a cross-sectional view of a rotor assembly of the present invention
taken
along line 4-4 of Fig. 1;
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Fig. 5 is a cross-sectional view of the rotor assembly of the present
invention
taken along line 5-5 of Fig. 4;
Fig. 6 is an enlarged, cross-sectional view showing a portion of the
electromechanical machine of Fig. 1;
Fig. 7 is a cross-sectional view of the electromechanical machine taken along
line
7-7 of Fig. 6;
Fig. 8 is an enlarged, cross-sectional view of a portion of the rotor assembly
of
Fig. 4 showing an alternate embodiment of the pole pieces for the rotor
assembly of the
present invention; and
Fig. 9 is an enlarged, cross-sectional view of a portion of the rotor assembly
of
Fig. 4 showing the second alternate embodiment of the pole pieces for the
rotor assembly.
DETAILED DESCRIPTION OF THE INVENTION
Refening to Fig. 1, an electromechanical machine incorporating a rotor
assembly
in accordance with the present invention is generally designated by the
reference numeral
10. Electromechanical machine 10 includes an enclosure 12 which defines an
interior 14
for receiving stator 16 and rotor assembly 18, as hereinafter described.
Stator 16 includes
a plurality of stator stacks 20a-20e which are positioned adjacent each other
and
supported by stator frame 22. Radial cooling channels 24 extend between
corresponding
pairs of stator stacks 20a-20e so as to allow air or an alternate coolant to
pass
therethrough into air gap 26 defmed between the radially inner surfaces 28 of
stator
stacks 20a-20e and radially outer surface 30 of rotor assembly 18, Fig. 6. As
best seen in
Figs. 1 and 6, stator stacks 20a-20e are formed by a plurality of stator pole
pieces 32
laminated together.
Referring to Figs. 2-3, rotor assembly 18 includes an end plate 34 having
first and
second opposite sides 34a and 34b, respectively. A rotatable stub shaft 36 is
operatively
connected to second side 34b of end plate 34 in any conventional manner so as
to
translate rotation of shaf136 to end plate 34. Rotor 38 extends from first
side 34a of end
plate 34 and is operatively connected thereto in any conventional manner. It
is
contemplated to form rotor 38 from various types of materials including
magnetic,
ferromagnetic, non-ferromagnetic, and high strength materials. However, it can
be
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appreciated that rotor 38 may be formed of other materials without deviating
from the
scope of the present invention.
Rotor 38 is generally cylindrical in shape and includes a radially outer
surface 40
defining a plurality of circumferentially spaced, flat lands 40a-40d, Fig. 4.
A plurality of
circumferentially spaced lobes 42 extend from radially outer surface 40 of
rotor 38
between corresponding pairs of lands 40a-40d. Each lobe 42 includes neck
portion 42a
extending from outer surface 40 of rotor 38 and an enlarged head 42b on the
terminal end
of neck portion 42a. It is contemplated to round the intersections 44a and 44b
of
enlarged head 42b and neck 42a of each lobes 42 in order to avoid stress at
such
intersections 44a and 44b during operation of electromechanical rinachine 10.
Similarly,
the intersections 46a and 46b of neck portion 42a of each lobe 42 and radially
outer
surface 40 of rotor 38 are rounded so as to reduce stress at such intersection
46a and 46b
during operation of electromechanical machine 10. The number and locations of
such
intersections may be increased and distributed along the radial interface of
the lobe 42
and the laminated pole pieces, 70, described hereinafter.
Rotor assembly 18 further includes a plurality of ring assemblies 48 supported
on
rotor 38. Referring to Fig. 3, each ring assembly 48 includes a magnet
retention ring 50
for preventing radial movement of magnets 52a-52d, as hereinafter described.
Each
magnet retention ring 50 includes first and second opposite sides 50a and 50b,
respectively. A plurality of circumferentially spaced retaining bar receipt
cavities 54,
Fig. 5, are provided in second side 50b of magnet, for reasons hereinafter
described.
A plurality of circumferentially spaced magnet retention elements 56 project
from
first side 50a of magnet retention ring 50. Each magnet retention element 56
includes a
generally, flat radially inner surface 58 and a generally arcuate radially
outer surface 60.
Inner surface 58 and outer surface 60 of magnet retention element 56 are
separated by
first and second generally arcuate sides 62 and 64, respectively. Sides 62 and
64 of each
magnet retention element 56 are shaped to form a mating relationship with the
leading
and trailing edges 66 and 68, respectively, of corresponding laminated pole
pieces 70, as
hereinafter described.
Each magnet retention element 56 terminates at a generally flat terminal end
surface 72. Retaining bar 74 projects axially from end surface 72 of each
magnet
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retention element 56. Each retaining bar 74 is axially aligned with a
corresponding
retaining bar receipt cavity 54 formed in second side 50b of magnet retention
ring 50
such that retaining bar 54 may be axially received within a corresponding
retaining bar
receipt cavity 54 formed in second side of magnet retention ring 50 of an
adjacent ring
assembly 48a, Figs. 2 and 5, so as to interlock adjacent ring assemblies 48
and 48a.
Magnet retention ring 50 further includes a plurality of circumferentially
spaced
lobe-shaped cutouts 76 which are dimensioned to allow magnet retention ring 50
to be
slid axially onto lobes 42 of rotor 38. In addition, magnet retention ring 50
includes a
plurality of circumferentially spaced magnet cutouts 78 therein for allowing
corresponding magnets 52a -52d to pass therethrough, as hereinafter described.
Ring assemblies 48 further include a plurality of stacks 80 of a predetermined
number of laminated pole pieces 70. Each laminated pole piece 70 (and hence,
each
stack 80 of laminated pole pieces) is generally c-shaped and has an arcuate,
radially outer
edge 82 and inner edge 84 which defines a lobe-shaped gap 86 therethrough for
allowing
each stack 80 of laminated pole pieces 70 to be received on a corresponding
lobe 42.
Each laminated pole piece 70 terminates at first and second end 88 and 90,
respectively,
which are separated by a predetermined distance to accommodate neck portion
42a of a
corresponding lobe 42 therebetween. This feature is intended to retain the
pole pieces in
position during operation and therefore, may be altered to include as many
lobed lands
along the radial interface between the laminated pole pieces 70 and the rotor
lobe 42.
Stacks 80 of laminated pole pieces 70 are positioned on first sides 50a of
magnet
retention ring 50 to form ring assemblies 48. When stacks 80 of laminated pole
pieces 70
are positioned, leading edges 66 of laminated pole pieces 70 engage first
sides 62 of
corresponding magnet retention elements 56 and trailing edges 68 of laminated
pole
pieces 70 engage second sides 64 of corresponding magnet retention elements
56. In
addition, lobe-shaped gaps 86 defined by the laminated pole pieces 70 are
axially aligned
with a corresponding lobe-shaped cutouts 76 in magnet retention ring 50 of
corresponding ring assemblies 48.
As best seen in Figs. 1 and 2, rotor assembly 18 is constructed by axially
sliding
rotor assemblies 49 onto rotor 38 such that lobes 42 of rotor 38 extend
through a
corresponding lobe-shaped cutouts76 in magnet retention ring 50 and gaps 86
defined by
CA 02604812 2007-10-24
laminated pole pieces 70 of corresponding stacks 80 of ring assemblies 48.
Retaining
bars 74 projecting from end surfaces 72 of magnet retention elements 56 are
seated
within corresponding retaining bar receipt cavities 54 in second sides 50b of
magnet
retention ring 50 of adjacent ring assemblies 48a, Figs. 2 and 5, as
heretofore described,
in order to interlock adjacent ring assemblies 48. Magnets 52a-52d are slid
axially along
corresponding lands 40a-40d of outer surface 40 of rotor 38 such that magnets
52a-52d
extend through corresponding magnet cutouts 78 in ring assemblies 48 and such
that
magnets 52a-52d are disposed between corresponding pairs of stacks 80 of
laminated
pole pieces 70 in ring assemblies 48. It is contemplated to provide shims 92
between
magnets 52a-52b and radially inner surfaces 58 of corresponding magnet
retention
elements 56 of each ring assembly 48 so as to prevent radial movement of
magnets 52a-
52d during the rotation of rotor assembly 18, and thermal growth of the rotor
18 as the
machine warms during operation.
After ring assemblies 48 and magnets 52a-52d are assembled on rotor 38, second
end plate 96 is secured to terminal end 38a of rotor 38 to maintain ring
assemblies 48 and
magnets 52a-52d thereon. Second end plate 96 includes a plurality of
circumferentially
spaced bolt openings 98 which are axially aligned with corresponding bolt
receipt
apertures 100 formed in terminal end 38a of rotor 38. Bolts 102 extend through
corresponding openings 98 in second end plate 96 and into bolt receipt
apertures 100 in
terminal end 38a of rotor 38 so as to interconnect second end plate 96 to
rotor 38.
Second end plate 96 includes a central aperture 104 for allowing stub shaft
106,
operatively connected to terminal end 38a of rotor 38 in any conventional
manner, to pass
therethrough. As best seen in Fig. 1, stub shafts 46 and 106 pass through
corresponding
bearings 108 and 110, respectively, in sidewalls 112 and 114, respectively, of
enclosure
12 so as to rotatably support rotor assembly 18. In its assembled condition,
it can be
appreciated that electromechanical machine 10 may be utilized as an electric
motor or a
generator.
Referring to Figs. 1 and 6, in order to limit the end leakage of the magnet
flux
provided by permanent magnets 52a-52d during operation of electromechanical
machine
10, it is contemplated to reduce the reluctance of the radial flux path across
air gap 26.
This is accomplished by forming stacks 80 of ring assembly 48b adjacent
terminal end
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38a of rotor 38 from a first set 71 a of laminated pole pieces 70 having a
radial thickness
T and a second set 71 b of laminated pole pieces 70a having a thickness T'.
Except for
the radial thickness, laminated pole pieces 70a are identical in structure to
laminated pole
pieces 70, and as such, the prior description of laminated pole pieces 70 can
be
understood to describe laminated pole pieces 70a, as if it is fully described
herein.
Referring to Figs. 8 and 9, additional alternate laminated pole pieces 70a may
be
provided adjacent terminal end 38a of rotor 38. By way of example, referring
to Fig. 9, it
is contemplated to shape inner edge 84 of laminated pole pieces 70a to the
outer surface
of a corresponding lobe 42 so as to help control the face flux distribution
under various
operating loads of electromechanical machine 10. Alternatively, referring to
Fig. 8, it is
contemplated to provide recesses 68a and 68b in the leading and trailing edges
66 and 68,
respectively, of laminated pole pieces 70a.
Various modes of carrying out the invention are contemplated as being within
the
scope of the following claims particularly pointing out and distinctly
claiming the subject
matter which is regarded as the invention.
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