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Claims  |
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What is claimed is:
1. A rotor for a permanent magnet synchronous motor having a stator
developing a rotating direct axis flux comprising, in combination,
a shaft journalled about an axis,
magnetically permeable inner and outer portions of said rotor defining a
plurality of magnet apertures,
a plurality of magnets in said magnet apertures,
said inner portion lying between said magnets and said shaft and said outer
portion lying between said magnets and the outer periphery of said rotor,
said magnets being magnetized to establish an even plurality of magnetic
poles on the periphery of said rotor by the flux of said magnets and with
the flux of said magnets being additive to the direct axis flux of the
stator at no load,
and a plurality of reinforcing ribs disposed substantially along the
magnetically neutral axes intermediate the magnetic poles mechanically
interconnecting said inner and outer portions for resisting centrifugal
force on said outer portion during rotation.
2. A rotor as set forth in claim 1, including a plurality of conductor bar
apertures around the periphery of each of said laminations,
and metallic conductor means in said conductor bar apertures.
3. A rotor as set forth in claim 2, including first flux barrier walls in
said rotor connecting the inner end of a first conductor bar aperture to
an end of a first of said magnet apertures and defining a first flux
barrier to leakage flux of a first of said magnets.
4. A rotor as set forth in claim 3, including metallic conductor means in
the space of said flux barrier.
5. A rotor as set forth in claim 4, wherein said metallic conductor means
in said flux barrier space engages said first magnet in said first magnet
aperture to aid in holding said first magnet in place.
6. A rotor as set forth in claim 3, including second flux barrier walls in
said rotor connecting the inner end of a second conductor bar aperture to
an end of said first magnet aperture and defining a second flux barrier to
leakage flux of said first magnet.
7. A rotor as set forth in claim 6, wherein said first magnet aperture,
said first and second flux barrier spaces and said first and second
conductor bar apertures define a substantially continuous aperture
embracing a pole on said rotor from part of the rotor periphery to another
part of the rotor periphery.
8. A rotor as set forth in claim 3, including a third conductor bar
aperture adjacent said first conductor bar aperture,
third flux barrier walls in said rotor connecting the inner end of said
third conductor bar aperture to an end of a second magnet aperture and
defining a third flux barrier to leakage flux from a second magnet in said
second magnet aperture.
9. A rotor as set forth in claim 8, wherein said first and third conductor
bar apertures are on opposite sides of one of said ribs,
and the minimum circumferential width of said combined first and third flux
barrier is greater than the maximum width of said one of said ribs to
inhibit leakage flux circumferentially across said first and third flux
barriers from the series combination of said first and second magnets.
10. A rotor as set forth in claim 1, wherein said magnetically permeable
outer portion is formed from a stack of annular laminations.
11. A rotor as set forth in claim 10, wherein said magnetically permeable
inner portion is formed from a stack of annular laminations.
12. A rotor as set forth in claim 1, wherein said magnetically permeable
inner and outer portions are formed from a stack of unitary laminations.
13. A rotor as set forth in claim 1, wherein said ribs have a length
greater than the radial dimension from the outer periphery of said rotor
to said magnets.
14. A rotor as set forth in claim 1, wherein each of said ribs has a radial
length substantially equal to the radial dimension from the periphery of
said shaft to the periphery of said rotor.
15. A rotor as set forth in claim 1, wherein each of said ribs is
magnetically permeable.
16. A rotor as set forth in claim 15, wherein the width of each of said
ribs is narrow relative to the radial length thereof to restrict the flow
of leakage flux from said magnets.
17. A rotor as set forth in claim 15, wherein the leakage flux from the
magnet on one side of one rib is lengthwise through that rib in opposition
to the leakage flux from the magnet on the other side of that rib to have
a minimum net leakage flux lengthwise through that rib.
18. A rotor as set forth in claim 1, wherein said shaft is magnetically
permeable.
19. A rotor as set forth in claim 1, including a non-magnetic lamination in
said rotor having an absence of magnet apertures in order to increase the
resistance to centrifugal force of said rotor.
20. A rotor as set forth in claim 19, wherein said non-magnetic lamination
has a plurality of apertures communicating with said magnet apertures to
permit introduction of metallic conductor means thereinto.
21. A rotor as set forth in claim 1, wherein said ribs are disposed
substantially radially.
22. A rotor as set forth in claim 1, wherein said magnets are magnetized
substantially radially to establish a substantially radially oriented
direct axis flux to said magnetic poles.
23. A rotor as set forth in claim 2, including
a primary winding on the stator energizable to establish a rotating
magnetic field for cooperation with said metallic conductor means as a
secondary winding for starting of the rotor as an induction motor and for
cooperation with said magnetic poles of said rotor to run said rotor at
synchronous speed.
24. A rotor as set forth in claim 1, wherein two adjacent magnets have
M.M.F.'s disposed in series per pair of poles.
25. A rotor as set forth in claim 1, wherein one of said ribs lies between
two adjacent magnets with a leakage path from one magnet lying lengthwise
through said one of said ribs and the leakage path from the adjacent
magnet lying lengthwise in opposition through that same rib.
26. A rotor as set forth in claim 1, wherein said ribs have a constant
circumferential width throughout the length thereof.
27. A rotor as set forth in claim 1, including a magnetic bridge bridging a
portion of one of said magnet apertures from said inner to said outer
portion of said rotor and dividing said one magnet aperture into at least
two parts.
28. A rotor as set forth in claim 2, including a magnetic tooth between
adjacent conductor bar apertures effecting a radial extension of each said
rib,
said magnetic tooth having a larger circumferential width than said ribs.
29. A rotor as set forth in claim 2, including interlocking projections
between said metallic conductor means and said ribs.
30. A rotor as set forth in claim 2, wherein each said magnet aperture is a
continuous aperture having at least two angularly related pockets,
and separate magnets disposed in said at least two pockets.
31. A rotor as set forth in claim 30, including a non-magnetic lamination
in said rotor,
and apertures in said non-magnetic lamination to establish said metallic
conductor means in the volume between said separate magnets in said two
pockets.
32. A rotor as set forth in claim 1, wherein said magnet apertures are
circular arcs.
33. A rotor as set forth in claim 1, including a non-magnetic lamination in
said rotor included in said inner and outer portions of said rotor,
and a plurality of magnet apertures in said non-magnetic lamination to
accommodate said plurality of magnets.
34. A rotor for a permanent magnet synchronous motor having a stator
developing a rotating direct axis flux comprising, in combination,
a shaft journalled about an axis,
magnetically permeable inner and outer portions of said rotor defining a
plurality of magnet apertures,
a plurality of magnets in said magnet apertures,
said inner portion lying between said magnets and said shaft and said outer
portion lying between said magnets and the outer periphery of said rotor,
said magnets being magnetized to establish an even plurality of magnetic
poles on the periphery of said rotor by the flux of said magnets and with
the flux of said magnets being additive to the direct axis flux of the
stator at no load,
said magnet apertures being elongated and considerably more narrow in a
dimension along a rotor radius than the dimension parallel to a tangent to
the shaft,
said magnet apertures being positioned relatively closely to said shaft to
establish said outer portion of said rotor with a larger radial dimension
than that of said inner portion,
said outer portion forming a large volume relative to said inner portion
for a large pole shoe on each pole to carry the flux of said magnets,
and said pole shoe having a wide arc in the order of 160 electrical
degrees.
35. A rotor as set forth in claim 34, including a squirrel cage conductive
winding on said rotor with conductor bars in said outer portion of said
rotor,
said conductor bars occupying a majority of the volume near the periphery
of the rotor.
36. A rotor as set forth in claim 35, wherein said conductor bars are
substantially uniformly spaced around the periphery of said rotor.
37. A rotor as set forth in claim 35, wherein said conductor bars are
substantially uniformly spaced on each pole shoe and throughout
substantially the complete arcuate expanse of each pole shoe.
38. A rotor as set forth in claim 34, wherein said magnets are magnetized
along direct axes which are substantially radial,
and the dimension of said outer portion along one said direct axis exceeds
the combined dimensions of said magnet aperture and inner portion along
said one direct axis.
39. A rotor as set forth in claim 38, wherein the dimension of said outer
portion along said one direct axis is approximately double the combined
dimensions of said magnet aperture and inner portion along said one direct
axis.
40. A rotor as set forth in claim 34, wherein the volume of magnetically
permeable material in said outer portion exceeds the volume of
magnetically permeable material in said inner portion. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Synchronous motors are utilized in textile and glass inudstries and other
applications requiring precise synchronization of multiple motors. Often
these multiple motors are energized from variable frequency sources to
provide high or low speed. In many prior art applications these multiple
motor drives have used synchronous motors of the reluctance type such as
in U.S. Pat. Nos. 3,126,493 or 3,652,885. These did eliminate the need for
DC excitation power to the motor field, yet such motors had poor power
factor and tended to be slightly unstable at low speeds, i.e., certain low
frequency operating points.
The prior art synchronous permanent magnet motors have usually been
constructed with either Alnico or Ferrite magents in the rotor. The Alnico
magnets have a fairly high induction density but unfortunately have a very
low coercive force so that the magnets are readily demagnetized by the
M.M.F. from the primary winding on the stator of the motor. Conversely,
Ferrite magnets have good coercive force but have low induction density.
With such magnets, the rotor volume must be increased considerably to
obtain acceptable power factor and horsepower rating. Thus the motor is
large for its horsepower rating and uneconomical to manufacture as well as
having an inherent lower maximum speed before disintegration. A motor of
this type is illustrated in U.S. Pat. No. 3,492,520. Permanent magnet
synchronous motors currently manufactured by some manufacturers are ones
wherein the magnets are disposed in the rotor with the long dimension
radially and magnetized circumferentially and thus two magnets act in
parallel to supply the flux for a given pole on the rotor. This has the
disadvantage that the shaft must be non-magnetic or must have a
non-magnetic sleeve in order to avoid degradation of performance by
leakage flux from one face of the magnet through the shaft and back to the
other face of the magnet. Also leakage flux can have a path from one face
of the magnet to the other through a magnetic bridge along the outer
periphery of the rotor which further bleeds flux away from the useful flux
crossing the air gap to the stator.
The problem to be solved therefore is how to construct a permanent magnet
synchronous motor which has a small physical size, is economical to
manufacture for its horsepower rating, will withstand high rotational
speeds yet one which has high power factor and efficiency.
SUMMARY OF THE INVENTION
This problem is solved by a permanent magnet synchronous motor having a
rotor comprising, in combination, a shaft journalled about an axis,
magnetically permeable inner and outer portions of said rotor defining a
plurality of magnet apertures, a plurality of magnets in said magnet
apertures, said inner portion lying between said magnets and said shaft
and said outer portion lying between said magnets and the other periphery
of said rotor, said magnets being magnetized to establish an even
plurality of magnetic poles on the periphery of said rotor by the flux of
said magnets, and a plurality of reinforcing ribs disposed substantially
along the magnetically neutral axes intermediate the magnetic poles
mechanically interconnecting said inner and outer portions for resisting
centrifugal force on said outer portion during rotation.
Accordingly, an object of the invention is to provide a new and improved
permanent magnet synchronous motor, namely, a motor which provides greater
torque, high power factor, and higher efficiency than prior art
synchronous induction motors of the same size.
Another object is to employ superior performance Rare Earth magnets to
achieve optimum design performance for synchronous motors.
Another object of this disclosure is to overcome the deterioration of motor
performance from magnet demagnetization caused by high transient current
at pull-out from synchronism and on acceleration from standstill with
highest anticipated line voltage.
Another object is to provide an increased amount of direct axis flux in the
rotor. This is achieved through the use of Rare Earth magnets that provide
high induction along the direct axis, and the use of a rotor structure
which minimizes leakage flux by utilizing large flux barriers.
Another object is to provide a high ratio of quadrature axis reactance to
direct axis reactance in the synchronous motor rotor so that maximum power
output and torque are produced.
Another object is to provide simple means for attaining superior mechanical
strength of permanent magnet rotors to allow safe operation at high
operating speeds.
Another object is to provide for modular rotor design so that axially short
rotor modules containing suitable short magnets can be assembled to
provide for several overall rotor lengths and thereby be suitable to
achieve multiple ratings in the same machine diameter.
Another object is to secure the permanent magnets by casting around the
magnets when die casting the rotor squirrel cage with aluminum or other
suitable casting material. Uninterrupted flux barrier and magnet slots
continuing radially to rotor slots near the quadrature axes permit the
cage material to flow around the magnets.
Another object is to eliminate the necessity for a stainless steel shaft or
a non-magnetic sleeve around the usual magnetic steel shaft.
Another object of the invention is to provide radially directed reinforcing
ribs along the quadrature axes to hold the rotor together by resisting
centrifugal force during rotation.
Another object of the invention is to provide ribs which are sufficiently
wide circumferentially to have the requisitive strength yet sufficiently
narrow to effectively inhibit leakage flux.
Another object of the invention is to provide a motor which utilizes much
tooling from the high production induction motor art for economy of
manufacture.
Another object of the invention is to provide a permanent magnet induction
motor with a high pull-in and pull-out torque.
Other objects and a fuller understanding of the invention may be had by
referring to the following description and claims, taken in conjunction
with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an end elevational view of a rotor, minus the shaft, constructed
in accordance with the present invention;
FIG. 2 is a longitudinal sectional view taken on the line 2--2 of FIG. 1;
FIG. 3 is an enlarged cross-sectional view of the rotor and part of the
stator of the invention;
FIG. 4 is an enlarged cross-sectional view of a non-magnetic lamination
which may be used in the rotor of the invention in embodiments where the
rotor includes a plurality of axially disposed magnets;
FIGS. 5 and 6 are partial views, similar to FIG. 3, but of different
modifications;
and FIGS. 7 and 8 are cross-sectional views to a smaller scale of still
further modifications of the invention;
FIG. 9 is a cross-sectional view of a two-pole rotor constructed according
to the invention;
FIG. 10 is a cross-sectional view of the two-pole rotor of FIG. 9 showing a
non-magnetic lamination therein;
and FIG. 11 is a partial view, similar to FIG. 3, of a further modification
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1-4 illustrate a motor 12 which is a permanent magnet synchronous run
and induction start motor. This motor has a rotor 13 and a stator 14
cooperable across a cylindrical air gap 15. The stator 14 has a stator
primary winding 16 which is energizable with an alternating voltage to
provide a rotating direct axis field or primary M.M.F. The primary winding
16 may be similar to that found in the usual induction motor.
The rotor 13 is mounted on a shaft 18 having an axis 19 and journalled in
any suitable manner, not shown, relative to the stator 14. This shaft is
not shown in FIGS. 1 and 2. The rotor 13 may be constructed with any even
number of poles and a four pole rotor is shown in FIG. 3. The rotor
includes magnetically permeable inner and outer portions 21 and 22 which
together define walls for four magent apertures 23-26. A plurality of
magnets are disposed in the magent apertures in various ways. (The magnets
may be axially, tangentially or outwardly disposed.) As shown, this may
include only one magnet 27-30 per aperture 23-26, respectively. The inner
portion 21 lies between the magnets 27-30 and the shaft 18 and the outer
portion 22 lies between the magnets and the outer periphery of the rotor
13. The magnets 27-30 are magnetized in a generally outwardly direction to
establish inwardly and outwardly disposed north and south poles on the
magnets as indicated on the drawing. This means that adjacent magnets
cooperate in providing M.M.F.'s in series in order to establish alternate
north and south poles on the periphery of the rotor 13. These magnets
establish the direct axis flux such as along axis 35 which is additive to
the stator winding direct axis flux at no load. The inner and outer
portions 21 and 22 carry the flux of the magnets 27-30 across the air gap
15 to the stator 14 as shown by a path 31 and this path may include the
shaft 18 which may be magnetically permeable rather than non-magnetic as
in many prior art designs. The outer portion 22 includes conductor bar
apertures 32 containing conductor bars 33 which may be individual copper
bars, for example, or preferably are cast in place, such as with aluminum
or other suitable casting material. The conductor bars 33 form a part of a
squirrel cage winding which includes end rings 34 for a short circuited
secondary winding on the rotor 13.
The rotor 13 includes a plurality of reinforcing ribs 37-40 which are
substantially radially directed along the neutral or quadrature axes
intermediate the magnetic poles of the rotor. Pole shoes 36, four in this
case, are established between these ribs. These pole shoes are quite wide,
over 160 electrical degrees, for good flux conduction to the stator. A
quadrature axis 41 is shown in FIG. 3. These ribs 37-40 mechanically
interconnect the inner and outer portions 21 and 22, respectively, for
resisting centrifugal force on the outer portion during rotation.
The outer portion 22 may be laminated, and in the preferred embodiment the
rotor 13 is formed from magnetically permeable laminations 42, as shown in
FIG. 2 and each of these laminations has a plurality of these ribs 37-40.
The rotor 13 may also optionally include non-magnetic high strength
laminations 43 as shown in FIGS. 2 and 4. Such laminations, if used,
contain the conductor bar apertures 32 so that the conductor bars 33 may
be contained therein and may be continuous throughout the length of the
rotor 13. Also such high strength laminations may include the magnet
apertures 23A-26A so longer longitudinal length magnets may be used rather
than magnets of a length only sufficient to coincide with a module or
group 44 of the laminations 42.
The rotor 13 also includes flux barriers to prevent or minimize leakage
flux. These flux barriers are non-magnetic spaces and are formed by
barrier walls 47 on the outer portion 22 and by barrier walls 48 on the
ribs 37-40. These barrier walls 47 and 48 define a flux barrier space. In
the four pole construction shown in FIG. 3 there are eight such flux
barrier spaces in the preferred embodiment. The inner end of a first
conductor bar aperture 49 is connected by a first flux barrier space 50 to
one end of the magnet aperture 24. The other end of this magent aperture
is connected by a second flux barrier space 51 to the inner end of a
second conductor bar aperture 52. A third conductor bar aperture 53 is one
which is on the side of the rib 37 opposite the first conductor bar
aperture 49. The inner end of this conductor bar aperture 53 is connected
by a third flux barrier space 54 to one end of the magnet aperture 23. The
other end of this magnet aperture is connected by a flux barrier space 55
to the inner end of a conductor bar aperture 56. This is continued around
the rotor and it will be noted that conductor bar apertures 49 and 52,
flux barrier spaces 50 and 51 and the magnet aperture 24 form a continuous
slot or aperture spanning or embracing the south pole on the periphery of
the rotor 13. Such continuous slot extends to near the outer periphery of
the rotor at the magnetic bridges 57 and 58. There is a similar magnetic
bridge 59 at the outer end of the conductor bar aperture 53.
The magnetic bridges 69 and the outer ends of the squirrel cage conductor
bars 33 intermediate the continuous barrier slots do not significantly
contribute to the quadrature axis leakage flux, i.e., the flux through
ribs 37-40, and may be radially wider than the magnetic bridges 57 and 58.
The ribs 37-40 are sufficiently circumferentially wide for the requisite
mechanical strength of the rotor yet sufficiently narrow to allow for a
flux barrier 50 and 54 of acceptable width to inhibit excessive leakage
flux. This does not necessarily mean that the ribs are of constant width,
but that the flux barriers 50, 54 become wider close to the magnets. The
flux barriers may also extend further inwardly from the inner magnet face.
During manufacture of the motor 12, the unmagnetized pieces to become the
magnets 27-30 are slipped in place in the stack of laminations 44. After
that the non-magnetic lamination 43 is added and another stack of
laminations 44 plus the magnets 27-30. These stacked sections or modules
are added as required to make the desired horsepower rating of the motor.
When the entire stack is complete on a dummy shaft, it is placed in the
die casting machine and the conductor bars 33 and the end rings 34 are
cast in place. At that time the metallic conductor forming the squirrel
cage enters the flux barrier spaces and fills them to engage the ends of
the magnets 27-30 and securely fasten them in place. The magnets are
preferably Rare Earth magnets, such as Samarium-Cobalt magnets and these
are very hard and brittle, and must be ground with extreme care and
equipment as the fine grinding particles can ignite spontaneously. As a
result it is not practical in production to grind the magnets to closely
fit within the magnet apertures and they must be a few thousandths of an
inch undersize. The squirrel cage material therefore securely holds these
magnets in place. After completion of the rotor as shown in FIG. 2, then
the entire rotor assembly is preferably placed in a magnetizing fixture to
magnetize the bars of magnetic material 27-30 so that they exhibit
permanent magnet characteristics. This is not to say that the rotors may
not be assembled with the bars 27-30 already magnetized.
The outer portion 22 may be formed from a stack of annular laminations with
such annular outer portion secured by the reinforcing ribs to the inner
portion 21. Also the inner portion 21 may be formed from a stack of
annular laminations and connected by the ribs to the outer portion 22. The
ribs may be unitary with either the inner portion or the outer portion or
may be completely separate from each and interlocked with each. If
separate ribs are used, merely interlocked with the inner and outer
portions, then such ribs may be of non-magnetic material such as high
strength stainless steel. Where the ribs are unitary with the inner or
outer portions, then the ribs are inherently of magnetically permeable
material. The preferred embodiment shown in FIGS. 1-3 is one wherein the
ribs 37-40 are unitary with the inner and outer portions 21 and 22. Each
lamination is a unitary lamination held together by the ribs 37-40 and the
magnetic bridges such as 57-59, in addition to the squirrel cage 33, 34.
OPERATION
Compared with the highest coercive force Alnico magnets, Samarium-Cobalt
magnets have about the same level of induction density but with about five
times the coercive force and over three times the peak energy product; or
compared with the best Ferrite magnets, Samarium-Cobalt magnets have more
than twice the induction density with about two and one-half times the
coercive force and more than five times the peak energy product.
Therefore, for the same rating, Rare Earth magnets will be thinner and
also require less area. Of major importance is the fact that Rare Earth
magnets have practically straight line B-H curves with unsurpassed maximum
resistance to demagnetization.
An understanding of the principles involved in the theory of synchronous
machines will illustrate the objectives of this disclosure. The classical
power formula for synchronous motors, omitting stator resistance, which is
small, and friction, and windage and iron losses is:
##EQU1##
where m is the number of phases, V is the phase voltage, Eo is voltage
induced in the primary winding by the D-c field, .delta. is the torque or
load angle between V and Eo phase voltages and Xd, Xq are the direct and
quadrature axis reactances per phase respectively. Thus it is seen that
for the synchronous motor, where Xd is inherently large, that maximum
power occurs with the second term when the quadrature reactance is as
small as is possible.
Similarly, for reluctance synchronous motors, where Eo is zero without
rotor excitation, the power equation is derived from the Synchronous motor
equation by omitting the first term, or
##EQU2##
Thus, the reluctance synchronous motor also requires as small a value of Xq
as possible and a large value of Xd to develop optimum power. An example
using magnets to oppose quadrature axis flux and minumize Xq is U.S. Pat.
No. 3,126,493. However, power factor is necessarily poor since with no
rotor M.M.F., the gap flux must be supplied entirely by primary M.M.F.
However, permanent magnet motors cannot be designed with high direct axis
reactance (Xd) since the high coercive force magnets required to prevent
demagnetization have permeabilities that are low when compared with
electrical steel used in Synchronous Machines. Rearrangement of the
Synchronous motor power equation will make it more suitable for the
permanent magnet synchronous motor, or
##EQU3##
Then, with high magnet induction making Eo large and with low Xd, the first
term will be large. The second term will be maximum and additive when
.delta. = 135 degrees and Xq is as large as possible. Accordingly, maximum
power for the permanent magnet motor is obtained when the load angle is
greater than 90 electrical degrees and less than 135 electrical degrees
and with Xd small and Xq large. High power factor requires a high Eo
value, resulting from high magnet induction, large magnet area, or large
number of effective stator coil turns.
Since it is undesirable to increase machine volume and number of stator
winding turns, the present invention utilizes Rare Earth magnets to
achieve larger ratings and higher power factors in usual frame sizes for
inductance motors and, moreover, provide much greater resistance to
demagnetizing forces. The present invention discloses improvements in
rotor construction required for the embodiment of Rare Earth magnets.
Moreover, since motors for textile drives operate over a wide speed range,
by variable frequency control, to 12,000 RPM or higher, the present
structure attains the necessary mechanical strength for these speeds when
required without resorting to complicated parts or extra operations.
The motor 12 of FIGS. 1-4 starts as an induction motor and runs at
synchronous speed. The stator winding 16 is energizable with an
alternating voltage to establish a rotating primary field. This co-acts
with the squirrel cage winding 33-34 to start the rotor 13 under induction
motor principles. It will be noted from FIG. 3 that the squirrel cage
winding is one with wide and relatively large conductor bars for a very
low resistance squirrel cage. The conductor bars 33 are wider than the
teeth 65 so that the conductor bars occupy a majority of the volume near
the periphery of the rotor. This gives good starting torque and high
sub-synchronous speed so that this sub-synchronous speed is very close to
synchronous speed for good pull-in torque. It also provides very good
pull-out torque upon overload. This low resistance squirrel cage is aided
by the filling of the flux barriers such as 50, 51, 54 and 55 with the
metallic conductor of the squirrel cage. The design of the rotor as
illustrated in FIG. 3 provides in each pole shoe 36 almost a full 180
electrical degrees of pole span for the magnetic flux of the magnets to
depart from and return to the rotor 13 across the air gap 15. This
provides minimum reluctance for good flux linkage with the stator 14. As
illustrated in FIG. 3 the conductor bar apertures or slots 32 may be wider
than the magnetic teeth therebetween. Also it will be noted that in FIG. 3
all of the conductor bar apertures 32 are identical which utilizes a
maximum of conventional induction motor tooling and production machinery
for economy of manufacture of the motor, although it is understood that
equal spacing of conductor bar apertures is not necessary for the practice
of the invention.
In FIG. 4 the high strength non-magnetic laminations 43 may have small
apertures, if desired, at the radial and peripheral positions
corresponding with the flux barrier spaces 50, 51, 54 and 55. Such small
apertures are shown as apertures 116 in FIG. 10, described below. This
will help permit flow of metallic casting material into these flux barrier
spaces, such as 50, 51, 54 and 55.
The reinforcing ribs 37-40 are significant novel features of the invention.
First, these ribs provide mechanical strength to the outer portion 22.
This outer portion 22 by itself is an annular magnetically permeable
portion connected by the magnetic bridges 57-59. The ribs 37-40 are placed
under tension during rotation of the rotor and resist centrifugal force on
the outer portion 22. The ribs 37-40 impart sufficient strength to the
entire rotor structure so that operation at 1800 rpm with a 3.65 inch
diameter rotor does not require any high strength non-magnetic laminations
43. These laminations are primarily used at high speed applications such
as 8,000 to 12,000 rpm.
A second feature of the ribs is that they establish the laminations as
unitary laminations rather than several separate pieces in one lamination
layer. This makes the unitary lamination easy to handle during initial
punching, trimming, stacking on an arbor, insertion of the magnet slugs
27-30, and placing in the die-casting machine for forming the squirrel
cage 33-34.
A third feature of the ribs is that they establish a minimum degradation of
the magnetic and electrical performance of the motor. FIG. 3 illustrates
first, second, third and fourth leakage paths 60-63, respectively, whereby
flux from one face of a magnet may leak to the opposite face without being
a working flux; namely, without crossing the air gap 15 to the stator 14.
The leakage path 61 is that from magnet 28 radially outwardly through the
rib 38. The leakage path 62 is from the magnet 29 radially inwardly
through this same rib 38. Assuming the theoretically perfect condition of
equal strength of the magnets 28 and 29, then these two opposing fluw
leakage paths will be equal and in opposition for a zero net leakage flux
lengthwise along the ribs 37-40. The ribs 37-40 are narrow in
circumferential width compared to their length. In one motor actually
constructed in accordance with the present invention, the rotor was 3.650
inches in diameter, the magnetic bridges 57-59 were 0.010 inches in radial
dimension, the ribs 37-40 were 0.080 inches wide, the magnetic tooth width
65 between conductor bar apertures 33 was 0.104 inches, and the conductor
bar apertures 32 had a minimum width of 0.102 inches as shown at dimension
66. Considering then the leakage flux path 63 wherein the M.M.F.'s of the
two magnets 28 and 29 are in series, this leakage flux must bridge two of
the conductor bar apertures 32 and the rib 38, or its extension, the
magnetic tooth 67. The rib 38, which includes the magnetic tooth 67, has a
circumferential width which is less than the minimum width of the two flux
barriers established at the inner end of the conductor bar apertures. The
fact that this leakage path 63 has to jump two flux barriers, one on each
side of the rib 38, means that this leakage path contains only a minimum
amount of leakage flux. This contributes considerably to the high
efficiency and high power factor of the motor 12. In the motor constructed
in accordance with the invention, the efficiency was 86.7 percent and the
power factor was 82.5 percent. This gave an apparent efficiency of the
product of the two of 71.5 percent. This was with 60 hertz energization
and 1800 rpm operation on a two horsepower motor which had a pull-out
torque of 11.85 lb. feet and a pull-in torque of 7.3 lb. feet.
The above discussion illustrates that the ribs provide a fourth feature;
namely, that the leakage flux in path 63 must jump two flux barriers
rather than only one, and this is a relatively wide flux barrier for
minimum flux leakage. A fifth feature is that the width of these two flux
barriers, at dimension 66, is a combined width greater than the width of
the rib at dimension 64 or at dimension 65.
An optional locating step 68 is shown on the inner portion 21 to locate the
magnets 27-30, but may be on the outer portion 22 or omitted entirely.
The ribs 37-40 provide a sixth feature of the invention; namely, that they
establish a continuous magnet slot or aperture which spans the pole. This
establishes the pole as being in excess of 160 electrical degrees in width
for a maximum amount of permeable material to conduct the direct axis
flux, even considering the relatively narrow teeth 65 or 67 and the
relatively wide slots 32. This continuous slot spanning the pole may be
made quite wide especially at the flux barriers 50, 51, 54 and 55 in order
to minimize the flux leakage and provide a maximum of useful flux from the
magnets.
The structure of the rotor is peculiarly adapted to take advantage of the
best properties of the Rare Earth permanent magnets. These magnets are
relatively wide compared to their thickness permitting a small rotor
diameter in turn permitting high speeds. Further, the thin plate-like
magnets permit ample volume outboard of the magnets to accommodate a low
resistance squirrel cage and adequate flux yoke for high starting torque.
The plate-like magnets are thin in a dimension along the direct axis 35 and
wide in a dimension perpendicular thereto. The magnets are positioned
close to the shaft and flux passes through this shaft. The dimension of
the outer portion 22 along one of the direct axes is approximately double
that of the combined dimensions of the magnet aperture and inner portion
21 along that same direct axis. This provides a large volume to the outer
portion 22.
There will also be a magnetic flux leakage path 60 through the magnetic
bridges 57 and 59, for example, around the end of the rib 37. Since these
magnetic bridges may be made relatively thin such as 0.010 inches, these
bridges will readily saturate and minimize the leakage flux in this path.
It is evident from the drawing of FIG. 3 that with low permeability Rare
Earth magnets compared with the permeability of the rotor steel and with
radially wide magnet apertures 23, that the direct axis reactance is low
relative to the primary M.M.F. while the flux paths between quadrature
axes outboard of the magnets in the outer portion 22 are not impeded and
yield a high quadrature reactance and a large ratio of X.sub.q to X.sub.d.
According to equation (3) above, this yields a high pull-out torque for an
improved performance motor.
FIG. 5 is a modification of the rotor and shows a partial view similar to
the sectional view of FIG. 3. In FIG. 5 the reinforcing rib 38A is
illustrative of all ribs in the motor and this exemplary form of rib has a
constant circumferential width all the way from the inner portion 21 to
the outer portion 22, including the magnetic tooth between the conductor
bar apertures 72 and 73. These apertures have been made slightly wider in
order to increase the flux barrier width. Also the next adjacent conductor
bar apertures have been beveled at the corners as at numeral 74 in order
to accommodate the wide flux barrier spaces 75 and 76 and yet provide
magnetically permeable material for adequate conduction of flux between
the beveled corner 74 and the flux barrier spaces 75 and 76. Whereas in
FIG. 3 the conductor bar apertures 32 were all uniform in size to aid in
the ease of manufacture of that lamination, the embodiment of FIG. 5 does
enlarge two of the conductor bar apertures 72 and 73 and modifies the
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