Note: Descriptions are shown in the official language in which they were submitted.
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ROTORS UTILIZING A STEPPED SKEW
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No.
60/090,773, filed June 26, 1998.
BACKGROUND OF THE IJNVENTION
This invention relates generally to electric motors and, more particularly, to
a line start magnetically salient rotor AC electric motor.
Line start permanent magnet motors include rotors having permanent
magnets and induction squirrel-cages. The induction squirrel cages enable
starting
on a conventional AC power source, and the permanent magnets improve motor
efficiency. Such rotors sometimes are referred to herein as divided magnet
rotors.
In an exemplary form, a divided magnet rotor includes a rotor core, a rotor
1o shaft, permanently magnetized locations, and secondary conductors. The
rotor shaft
extends through the rotor core and is coaxial with the rotor core axis of
rotation.
The secondary conductors also extend through the rotor core and are arranged
axially with respect to the rotor shaft. Such secondary conductors are offset
from
the outer circumference or periphery of the rotor core and are connected at
opposite
t s ends of the core by end rings. Notches at the outer periphery of the rotor
core
typically are radially aligned with at least one secondary conductor.
Permanent
magnets are located in the notches and the permanent magnets are magnetized to
form a selected number of poles.
To decouple stator slot order harmonics, the rotor bars in the squirrel cage
2o typically are skewed. Skewing is accomplished by slightly turning the rotor
laminations with respect to each other so that the passages formed by
overlapping
slots of the rotor laminations are generally helical in shape. In a divided
magnet
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rotor, skewing the laminations is difficult. Particularly, some magnetic
materials
that may be used for the permanent magnets are brittle and prevent such
skewing.
In addition, open slot rotors generally provide advantages over closed slot
rotors. In a closed slot rotor, the flux flows through the bridge (i. e. , the
region of
iron immediately tow ards the rotor outer diameter from the rotor bar) and
saturates
the bridge depending on the rotor current. The level of current at which the
bridge
saturates will be passed through four times per cycle, causing time harmonics
in the
stator current. These time harmonics create the basic forcing function for a
class of
noise. The leakage flux which causes the bridge to saturate reduces the torque
1o produced by the machine at that current level and in turn raises the losses
related to
current flow at a give torque. An open slot rotor does not provide a high
permeability path for this component of the leakage flux. Open slot rotors,
however, typically are more difficult to fabricate than closed slot rotors.
It would be desirable to provide a divided magnet rotor that includes
15 permanent magnets yet also decouples stator slot order harmonics. It also
would be
desirable to provide such a rotor which is not subject to rotor bridge
saturation.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention, a divided magnet rotor
includes a stepped skew rather than a helical skew. The stepped skew enables
the
use of straight magnet sections that can be inserted into the rotor core
notches
2o thereby eliminating the need to produce a helix from the rotor cage. The
stepped
skew is effective in decoupling stator slot order harmonics. In addition, the
stepped
skew rotor includes, in some embodiments, open slots so that the rotor is not
subject
to rotor bridge saturation.
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The divided magnet rotor includes a rotor core, a rotor shaft, permanently
magnetized locations, and secondary conductors. The rotor shaft extends
through
the rotor core and is coaxial with the rotor core axis of rotation.
The rotor core includes rotor laminations in a stack arranged in at least two
sets. The slots in the first set of laminations have skew portions extending
laterally
in a first direction, and the slots in the second set of laminations have skew
portions
extending laterally in a second direction opposite the first direction. The
radially
inner portions of corresponding slots in the first and second sets of rotor
laminations
overlap each other. Such a configuration forms a stepped skew.
to Notches, or channels, extend from an outer diameter (OD) of the rotor
laminations to the skew portion of each respective slot. The notches extend
axially,
and permanent magnets are located in the notches. Specifically, straight
magnet
sections of permanently magnetized material are inserted into the notches. The
straight magnet sections are magnetized to form a selected number of poles.
The
15 secondary conductors extend through the rotor core slots and are arranged
axially
with respect to the rotor shaft. The secondary conductors are offset from the
outer
circumference or periphery of the rotor core and are connected at opposite
ends of
the core by end rings.
The above described divided magnet rotor has a stepped skew rather than a
2o helical skew. The stepped skew enables the use of straight magnet sections
that can
be inserted into the rotor thereby eliminating the need to produce a helix
from the
rotor cage. The stepped skew is effective in decoupling stator slot order
harmonics.
In addition, the rotor has open slots so that the rotor is not subject to
rotor bridge
saturation.
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BRIEF DESCRIPTTON OF THE DRAWINGS
Figure 1 is an enlarged fragmentary view of a first embodiment of a rotor
core having slots which are skewed;
Figure 2 is a perspective view of the rotor core shown in Figure 1;
Figure 3 is an enlarged fragmentary view of a second embodiment of a rotor
core having slots which are skewed;
Figure 4 is a perspective view of the rotor core shown in Figure 4;
Figure 5 is an enlarged fragmentary view of a third embodiment of a rotor
core having slots which are skewed;
1o Figure 6 is a perspective view of the rotor core shown in Figure 5;
Figure 7 is an enlarged fragmentary view of a fourth embodiment of a rotor
core having slots which are skewed;
Figure 8 is a perspective view of the rotor core shown in Figure 7;
Figure 9 is an enlarged fragmentary view of a fifth embodiment of a rotor
1s core having open slots which are skewed;
Figure 10 is a perspective view of the rotor core shown in Figure 9;
Figure 11 is an enlarged fragmentary view of a sixth embodiment of a rotor
core having open slots which are skewed;
Figure 12 is a perspective view of the rotor core shown in Figure 11;
2o Figure 13 is an enlarged fragmentary view of a seventh embodiment of a
rotor core having slots which are skewed;
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Figure 14 is a perspective view of the rotor core shown in Figure 13; and
Figure 15 is a partial cross sectional view of a motor.
DETAILED DESCRIPTION OF THE INVENTION
Set forth below in more detail is a description of various exemplary
embodiments of divided magnet stepped skew rotors. The rotors may be used in
many different motor configurations including many different stator
configurations.
Generally, a divided magnet stepped skew rotor includes a squirrel cage with
permanent magnets secured at an outer periphery of the rotor laminations. The
stepped skew enables the use of straight sections of permanent magnets rather
than
requiring that the permanent magnets be skewed. This rotor configuration
facilitates
to the fabrication of divided magnet rotors since the stepped skew enables use
of
straight magnetic sections yet also is effective in decoupling stator slot
order
harmonics. In addition, and in some embodiments, the rotor has open slots so
that
the rotor is not subject to rotor bridge saturation.
Referring noa~ particularly to the drawings, Figure 1 is an enlarged
1 s fragmentary view of a first embodiment of a rotor core 20, and Figure 2 is
a
perspective schematic view of core 20. The schematic views set forth herein
are
intended only to illustrate various configurations of permanent magnets with
respect
to the rotor cores and do not illustrate each aspect of such cores. Core 20
has slots
22 which are skewed. Slots 22 include a radially inner portion 24 and first
and
2o second skew portions 26 and 28. Core 20 also includes a plurality of
laminations 30
having an outer periphery 32. A first set 34 of rotor laminations 30 has a
plurality
of slots 22 having first skew portions 26 extending in a first direction, and
a second
set 36 of rotor laminations 30 has a plurality of slots 22 having second skew
portions 28 extending in a second direction.
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Core 20 further includes a plurality of notches 38 having an open end at
outer periphery 32. In the embodiment shown in Figures 1 and 2, each notch 38
extends axially with respect to a center axis of rotor core 20, and each notch
38 is
coextensive with a respective one of slots 22. No bridge of lamination
material
extends between notches 38 and slots 22, and notches 38 have a substantially
rectangular cross sectional shape. As shown in Figure 1, a first notch 40 is
substantially aligned and coextensive with first skew portion 26, and a second
notch
42 is substantially aligned and coextensive with second skew portion 28.
Permanently magnetizable material 44 is located in notches 40 and 42.
to Figure 3 is an enlarged fragmentary view of a second embodiment of a rotor
core 50, and Figure 4 is a perspective schematic view of core 50. Core 50 has
slots
52 which are skewed. Slots 52 include a radially inner portion 54 and first
and
second skew portions 56 and 58. Core 50 also includes a plurality of
laminations 60
having an outer periphery 62. A first set 64 of rotor laminations 60 has a
plurality
is of slots 52 having first skew portions 56 extending in a first direction,
and a second
set 66 of rotor laminations 60 has a plurality of slots 52 having second skew
portions 58 extending in a second direction.
Core 50 further includes a plurality of notches 68 having an open end at
outer periphery 62. In the embodiment shown in Figures 3 and 4, each notch 68
2o extends axially with respect to a center axis of rotor core 50 and along an
entire
length of core 50. A bridge 70 of lamination material extends between each
notch
68 and respective slots 52. Notch 68 has a substantially rectangular cross
sectional
shape, and each notch 68 is substantially aligned with a radial axis of one of
slot
radial inner portions 5:1. Permanently magnetizable material 72 is located in
notches
25 68.
Figure 5 is an enlarged fragmentary view of a third embodiment of a rotor
core 80, and Figure 6 is a perspective schematic view of core 80. Core 80 has
slots
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82 which are skewed. Slots 82 include a radially inner portion 84 and first
and
second skew portions 86 and 88. Core 80 also includes a plurality of
laminations 90
having an outer periphery 92. A first set 94 of rotor laminations 90 has a
plurality
of slots 82 having first skew portions 86 extending in a first direction, and
a second
s set 96 of rotor laminations 90 has a plurality of slots 82 having second
skew
portions 88 extending in a second direction.
Core 80 further includes a plurality of notches 98 having an open end at
outer periphery 92. In the embodiment shown in Figures 5 and 6, each notch 98
extends axially with respect to a center axis of rotor core 80, and each notch
98 is
to coextensive with a respective one of slots 82. A bridge 100 of lamination
material
extends between notches 98 and slots 82, and notches 98 have a substantially
rectangular cross sectional shape. As shown in Figure 5, a first notch 102 is
substantially aligned and coextensive with first skew portion 86, and a second
notch
104 is substantially aligned and coextensive with second skew portion 88.
is Permanently magnetizable material 106 is located in notches 98.
Figure 7 is an enlarged fragmentary view of a fourth embodiment of a rotor
core 110, and Figure 8 is a perspective schematic view of some elements of
core
110. Core 110 has slots 112 which are skewed. Slots 112 include a radially
inner
portion 114 and first and second skew portions 116 and 118. Core 110 also
includes
2o a plurality of laminations 120 having an outer periphery 122. A first set
124 of
rotor laminations 120 has a plurality of slots 112 having first skew portions
116
extending in a first direction, and a second set 126 of rotor laminations 120
has a
plurality of slots 112 having second skew portions 118 extending in a second
direction.
2s Core 110 further includes a plurality of notches 128 having an open end at
outer periphery 122. In the embodiment shown in Figures 7 and 8, each notch
128
extends axially with respect to a center axis of rotor core 110, and each
notch 128
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extends the entire length of core 110. A bridge 130 of lamination material
extends
between notches 128 and slots ,112, and notches 128 have a substantially
rectangular
cross sectional shape. As shown in Figure 7, a first notch 130 is
substantially
aligned and coextensive with first skew portion 116, and a second notch 132 is
s substantially aligned and coextensive with second skew portion 118.
Permanently
magnetizable material 134 is located in notches 128.
Figure 9 is an enlarged fragmentary view of a fifth embodiment of a rotor
core 140, and Figure 10 is a perspective schematic view of core 140. Core 140
has
slots 142 which are skewed. Slots 142 include a radially inner portion 144 and
first
to and second skew portions 146 and 148. Core 140 also includes a plurality of
laminations 150 having an outer periphery 152. A first set 154 of rotor
laminations
150 has a plurality of slots 142 having first skew portions 146 extending in a
first
direction, and a second set 156 of tutor laminations 150 has a plurality of
slots 142
having second skew portions 148 extending in a second direction.
is Core 140 further includes a plurality of notches 158 having an open end at
outer periphery 152. In the embodiment shown in Figures 9 and 10, each notch
158
extends axially with respect to a center axis of rotor core 140, and each
notch 158 is
coextensive with a respective one of slots 142. A bridge 160 of lamination
material
extends between notches 158 and slots 142, and notches 158 have an irregular
cross
2o sectional shape. As shown in Figure 9, a first notch 161 is substantially
aligned and
coextensive with first skew portion 146, and a second notch 162 is
substantially
aligned and coextensive with second skew portion 148. Notches 158 are open and
do not include permanent magnets. Rotor core 140 is thus an open slot rotor.
Figure 11 is an enlarged fragmentary view of a sixth embodiment of a rotor
2s core 170, and Figure 12 is a perspective schematic view of core 170. Core
170 has
slots 172 which are skewed. Slots 172 include a radially inner portion 174 and
first
and second skew portions 176 and 178. Core 170 also includes a plurality of
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laminations 180 having an outer periphery 182. A first set 184 of rotor
laminations
180 has a plurality of slots 172 having first skew portions 176 extending in a
first
direction, and a second set 186 of rotor 180 laminations has a plurality of
slots 172
having second skew portions 178 extending in a second direction.
Core 170 further includes a plurality of notches 188 having an open end at
outer periphery 182, In the embodiment shown in Figures 11 and 12, each notch
188 extends axially with respect to a center axis of rotor core 170, and each
notch
188 is coextensive with a respective one of slots 172. No bridge of lamination
material extends between notches 188 and slots 172, and notches 188 have an
1o irregular cross sectional shape. As shown in Figure 11, a first notch 190
is
substantially aligned and coextensive with first skew portion 176, and a
second
notch I92 is substantially aligned and coextensive with second skew portion
178.
Notches 188 are open and do not include permanent magnets. Rotor core 170 is
thus an open slot rotor. In one embodiment, the rotor assembly is fabricated
by
pouring a molten metal such as aluminum into slots 172 and notches 188 while
rotor
core 170 is maintained within a cast that prevents the molten aluminum from
freely
flowing out of notches 188. Rotor core 170 is then brushed to remove any
excess
aluminum from the outside of rotor laminations 180. In an alternative
embodiment,
the rotor assembly is fabricated by initially forming rotor core I70 with a
thin wall
of lamination material on the outside of notches 188. Molten aluminum is then
poured into slots 172 and notches 188. The thin wall of lamination material on
the
outside of notches 188 is then removed so that slots 172 and notches 188 form
open
slots.
Figure 13 is an enlarged fragmentary view of a seventh embodiment of a
rotor core 200, and Figure 14 is a perspective schematic view of core 200.
Core
200 has slots 202 which are skewed. Slots 202 include a radially inner portion
204
and first and second skew portions 206 and 208. Core 200 also includes a
plurality
of laminations 210 having an outer periphery 212. A first set 214 of rotor
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laminations 210 has a plurality of slots 202 having first skew portions 206
extending
in a first direction, a second set 216 of rotor laminations 210 has a
plurality of slots
202 having second skew portions 208 extending in a second direction, and a
third
set 218 of rotor laminations 210 has a plurality of slots 202 having first
skew
portions 206 extending in a first direction.
Core 200 further includes a plurality of notches 220 having an open end at
outer periphery 212. In the embodiment shown in Figures 13 and 14, each notch
220 extends axially with respect to a center axis of rotor core 200, and each
notch
220 is coextensive with a respective one of slots 202. No bridge of lamination
1o material extends between notches 220 and slots 202, and notches 220 have a
substantially rectangular cross sectional shape. As shown in Figure 13, a
first notch
222 is substantially aligned and coextensive with first skew portion 206, and
a
second notch 224 is substantially aligned and coextensive with second skew
portion
208. Permanently mab~netizable material 226 is located in notches 222 and 224.
Many variations of the above described rotor cores are possible. For
example, additional sets of rotor laminations can be added depending upon the
desired operating characteristics. In addition, the particular dimensions of
the slots
can be selected to provide desired operating characteristics. Dimensions of
such
slots are discussed, for example, in U.S. Patent No. 5,640,064, which is
assigned to
2o the present assignee and hereby incorporated herein, in its entirety, by
reference.
Additional details regarding divided magnet rotors are set forth, for example,
in
U.S. Patent No. 5,548.172, which is assigned to the present assignee and
hereby
incorporated herein, in its entirety, by reference. The rotor cores described
above
could also be fabricated without the notches located on the outer periphery of
the
rotor laminations. The rotor bar slots would still have a stepped skew and the
rotor
bar slots could either be open slots or closed slots.
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Figure 15 illustrates a motor 250 which may incorporate any of the rotors R
described above. Motor 250 includes a housing 252 having motor endshields 254
and 256 secured thereto. Motor endshields 254 and 256 include supports 258 and
260 for bearing assemblies 262 and 264. Rotor shaft S is coaxially aligned
with
bearing assemblies 262 and 264 and extends through openings 266 and 268 formed
in endshields 254 and 256.
Motor 250 also includes a stator 270 having a stator core 272 and stator
windings 274 Stator windings 274 include a start winding and a first and a
second
main winding. The first winding is wound to form a first, lower, number of
poles
1o and the second main winding is wound to form a second, higher, number of
poles.
The start winding is wound to form a number of poles equal to the number of
poles
of the first main winding. Stator core 272 forms a tutor bore 276 Rotor shaft
S is
concentrically arranged axially of stator core 272, and rotor core RC is
positioned
concentrically with rotor shaft S.
15 A switching unit 278, shown in phantom, is mounted to endshield 254.
Switching unit 278 includes, in one form, a movable mechanical arm 280 A
centrifugal force responsive assembly 282, also shown in phantom, is mounted
to
rotor shaft S and includes a push collar 284 which engages mechanical arm 280
Push collar 284 is slidably mounted on rotor shaft S. Assembly 282 also
includes a
2o weighted arm and spring (not shown in detail) secured to rotor shaft S. The
weighted arm is calibrated to move from a first position to a second position
when
the rotor speed exceeds a predetermined speed. When the weighted arm moves to
the second position, push collar 284 also moves from a first position to a
second
position. As a result, mechanical arm 280 of switching unit 278 moves from a
first
2s position to a second position, which causes switching unit 278 to switch
from a first
circuit-making position to a second circuit-making position. Switching unit
278 is
utilized separately in some applications (without arm 280) and switching unit
278
and assembly 282 are utilized in combination in other applications. Switches
used
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to control energization of start and main windings are well known.
Synchronized
switching apparatus and methods which may be utilized in motor 250 are
described,
for example, in U.S. Patent Application Serial No. 09/042,374, filed March 13,
1998, and hereby incorporated herein, in its entirety, by reference.
s In one specific embodiment, the first main stator winding is wound to form
four poles and the second main stator winding is wound to form six poles.
Motor
rotor permanent magnets M are magnetized to form six poles. Switching unit 278
is
coupled to an external control, such as a furnace control. Centrifugal force
responsive assembly 282 is not utilized in this particular application.
Switching unit
0 278 causes the first main winding to be energized for the high fire mode and
the
second main stator winding to be energized for the low fire mode.
In operation, and at motor start-up, the stator start winding and the first
main
winding are energized. The magnetic fields generated by such windings induce
currents in squirrel cage conductors C of motor rotor R, and the magnetic
fields of
15 such windings and conductors C couple and rotor R begins to rotate. Since
the start
winding and first main winding form four poles, the magnetic fields of the
such
windings do not effectively couple to the magnetic fields of rotor permanent
magnets
M configured to form six poles.
Once rotor R has sufficient speed, the start winding is de-energized. If the
2o furnace is to operate in the high fire mode, switching unit 278 causes the
first main
winding to remain energized. As a result, motor 250 operates as an induction
motor
in a relatively higher speed, four pole mode of operation. If the furnace is
to
operate in the low fire mode, however, switching unit 278 energizes the second
main winding and the first main winding is de-energized. As a result, the
rotor
2s speed decreases.
When the rotor speed equals the six pole synchronous speed, i. e. , 1200 rpm,
the magnetic fields of rotor permanent magnets M couple with, and "lock" into,
the
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magnetic fields generated by the second main winding. Rotor R then rotates at
substantially the synchronous speed for the six pole configuration, i.e., 1200
rpm.
If the furnace is required to later operate in the high fire mode, switching
unit 278
energizes the first main winding and de-energizes the second main winding.
Motor
250 then operates as an induction motor and the rotor speed increases.
In another application, and as in the embodiment discussed above, the first
main stator winding a wound to form four poles and the second main stator
winding
is wound to form sia: poles. Motor rotor permanent magnets M are magnetized to
form six poles. In this particular application, motor 250 operates as a single
speed
io motor. Centrifugal force responsive assembly 282 is utilized and is
calibrated to
transition from the first position to the second position when the rotor speed
exceeds
1200 rpm, i.e., six pole synchronous speed. When switching unit 278 is in the
first
circuit-making position, the first main winding is energized, i.e., the lower
pole
mode. When unit 278 is in the second circuit-making position, the second main
~ 5 winding is energized. i. e. , the higher pole mode. Centrifugal force
responsive
assemblies and switches are well known and are described, for example, in more
detail in U. S. Patent ~los. 4, 726,112 and 4, 856,182, both of which patents
are
assigned to the present assignee.
In operation, and at motor start-up, switching unit 278 is in the first
circuit-
2o making position and the first main winding and the start winding are
energized. The
magnetic fields generated by such windings induce currents in squirrel cage
conductors C of motor rotor R. The magnetic fields of such windings and rotor
secondary conductors C couple and rotor R begins to rotate. Since the first
main
winding and start winding are energized to form four poles, the magnetic
fields of
25 such windings do not effectively couple to the magnetic fields of permanent
magnets
M which are magnetized to form six poles.
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Once the speed of rotor R exceeds 1200 rpm, the weighted arm of assembly
282 causes push collar 284 to move to the second position. Push collar 284
causes
mechanical arm 280 to move to the second position, and switching unit 278
switches
to the second circuit-making position. The second main winding is then
energized.
s As a result, the speed of rotor R decreases. When the rotor speed equals the
six
pole synchronous speed, i.e., 1200 rpm, the magnetic fields of the rotor
permanent
magnets M couple with, and "lock" into, the magnetic fields generated by the
second main winding. Rotor R then rotates at substantially the synchronous
speed
for the six pole configuration, i.e., 1200 rpm. As described above, rotor R is
to "dragged" or "coasts" into synchronous speed rather than "pushed" into
synchronous speed. Enabling rotor R to coast into synchronous speed is~much
easier than attempting to "push" rotor R into synchronous speed with a lower
pole
induction winding, which is typical in known line start synchronous A. C.
motors.
Additional details relating to starting and running divided magnet motors are
set
1s forth, for example, in U.S. Patent No. 5,758,709, which is assigned to the
present
assignee and hereby incorporated herein, in its entirety, by reference.
Many modifications and variations of motor 250 illustrated in Figure 3 are
possible and contemplated. For example, motor 250 could be configured to
operate
as a two pole / four pole motor, a six pole / eight pole motor, or some other
two
2o mode motor. The specific structure of motor 250, such as the type of
bearing
assemblies 262 and 264 and motor frame, of course, may also vary. Switches
other
than centrifugal force responsive switches can be used for the one speed unit.
For
example, a rotor speed sensor and switch mounted to stator 270 or optic based
controls could be utilized.
2s With respect to the manufacture and assembly of rotor R, laminations are
stamped from steel. As is well known, each lamination may be annealed or
otherwise treated so that a coating of insulating material is formed thereon.
Laminations are then stacked, for example in two sets, to a desired height to
form
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the rotor core. Rotor laminations are stacked so that radially inner portions
of the
slots are aligned, and the skew portions in the first set are offset from the
skew
portions in the second set.
Once laminations are stacked to the selected height and aligned, as described
above, permanent magnets M are formed or located in the notches at the rotor
core
outer periphery using, for example, an injection molding process.
Particularly,
magnets may be formed from neodymium iron using injection molding.
Neodymium iron in a form suitable for injection molding is commercially
available
from the Magnaquench division of General Motors located in Anderson, Indiana.
Alternatively, magnets M could be manufactured using alternative techniques
such
as extrusion, casting and sintering processes, and then secured to the rotor
core.
Squirrel cage conductors C and rotor end rings ER are then formed using an
aluminum die cast process. Rotor shaft S is then inserted through aligned
openings
in each lamination and the end rings. Rotor shaft S is secured to the end
rings by
15 welding, for example. Magnets M may then be magnetized. Additional details
regarding assembly of a rotor and a motor are set forth, for example, in U.S.
Patent
Nos. 4,726,112 and 5,548,172, which are assigned to the present assignee.
The above described divided magnet rotors have a stepped skew rather than a
helical skew, and the stepped skew enables the use of straight magnet sections
that
2o can be inserted into the rotor thereby eliminating the need to produce a
helix from
the rotor cage. The stepped skew also is effective in decoupling stator slot
order
harmonics. In addition, the rotors may have open slots so that in at least the
open
slot configuration, the rotor is not subject to rotor bridge saturation.
While the invention has been described in terms of various specific
25 embodiments, those skilled in the art will recognize that the invention can
be
practiced with modification within the spirit and scope of the claims.
