U.S. patent number RE37,576 [Application Number 09/573,469] was granted by the patent office on 2002-03-12 for single phase motor with positive torque parking positions.
This patent grant is currently assigned to General Electric Company. Invention is credited to Wen Liang Soong, Charles M. Stephens.
United States Patent |
RE37,576 |
Stephens , et al. |
March 12, 2002 |
Single phase motor with positive torque parking positions
Abstract
A motor with positive torque parking positions. The motor
includes a rotor which is rotatable about an axis of rotation and a
stator in magnetic coupling relation with the rotor. The stator
includes a plurality of teeth each having a radially extending
shaft and an axially extending face. The faces of the stator teeth
define an aperture for receiving the rotor and the faces of the
stator teeth and the rotor define a air gap therebetween. Each
stator tooth has a notch in its face that is approximately at least
as wide as the shaft of the stator tooth so that the stator has a
magnetic configuration relative to the rotor for parking the rotor
in a rest position corresponding to a positive torque starting
position. The motor also includes a winding on the shafts of the
stator teeth and a control circuit for controlling current in the
winding whereby an electromagnetic field is produced for rotating
the rotor at a desired speed or torque during the operation of the
motor.
Inventors: |
Stephens; Charles M.
(Pattersonville, NY), Soong; Wen Liang (Fullarton,
AU) |
Assignee: |
General Electric Company
(Schenectady, NY)
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Family
ID: |
27487241 |
Appl.
No.: |
09/573,469 |
Filed: |
May 17, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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678524 |
Jul 9, 1996 |
|
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352393 |
Dec 8, 1994 |
|
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023790 |
Feb 22, 1993 |
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Reissue of: |
760755 |
Dec 5, 1996 |
05773908 |
Jun 30, 1998 |
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Current U.S.
Class: |
310/254.1;
310/156.01; 310/168; 310/179 |
Current CPC
Class: |
H02K
3/18 (20130101); H02M 5/458 (20130101); G05D
13/30 (20130101); H02P 6/08 (20130101); F23N
1/062 (20130101); H02K 1/148 (20130101); H02P
6/085 (20130101); H02K 29/03 (20130101); H02K
29/08 (20130101); H02P 6/20 (20130101); H02P
6/26 (20160201); H02K 1/146 (20130101); F23N
3/082 (20130101); H02P 25/04 (20130101); F23N
5/18 (20130101); F23N 2233/04 (20200101); F23N
2225/04 (20200101); H02K 2201/06 (20130101); F23N
2225/06 (20200101) |
Current International
Class: |
F23N
3/08 (20060101); F23N 1/06 (20060101); F23N
1/00 (20060101); F23N 3/00 (20060101); H02P
25/04 (20060101); G05D 13/30 (20060101); G05D
13/00 (20060101); H02P 6/20 (20060101); H02K
29/06 (20060101); H02M 5/458 (20060101); H02P
6/08 (20060101); H02P 25/02 (20060101); H02K
29/03 (20060101); H02K 1/14 (20060101); H02K
3/18 (20060101); H02M 5/00 (20060101); H02P
6/00 (20060101); H02K 29/08 (20060101); F23N
5/18 (20060101); H02K 001/12 () |
Field of
Search: |
;310/49R,156,254,264 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Enad; Elvin
Attorney, Agent or Firm: Welsh & Katz, Ltd. Wasserbauer;
Damian G. Horton; Carl B.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of commonly assigned
application Ser. No. 08/678,524, filed Jul. 9, 1996 (pending),
which is a continuation commonly assigned application Ser. No.
08/352,393, filed Dec. 8, 1994 (abandoned), which is a continuation
of commonly assigned application Ser. No. 08/023,790, filed Feb.
22, 1993 (abandoned), the entire disclosures of which are
incorporated herein by reference.
Claims
What is claimed is:
1. A motor comprising:
a rotor rotatable about an axis of rotation;
a stator in magnetic coupling relation with the rotor, said stator
including a plurality of teeth each having a radially extending
pole body and an axially extending face, said pole bodies of the
stator teeth each having a generally uniform thickness throughout
its radial extent, said faces of the stator teeth defining an
aperture for receiving the rotor, said faces of the stator teeth
and said rotor defining an air gap therebetween, each stator tooth
having a notch in its face which is approximately at least as wide
as the thickness of the pole body of the respective stator tooth,
said notch defining a modified air gap reluctivity between the
stator and the rotor for parking the rotor in a rest position
corresponding to a positive torque starting position;
a winding on the pole bodies of the stator teeth; and
a control circuit for controlling current in the winding whereby an
electromagnetic field is produced for rotating the rotor at a
desired speed or torque during operation of the motor.
2. The motor of claim 1 wherein each notch has a generally
rectangular cross section transverse to the axis of rotation.
3. The motor of claim 1 wherein each notch has a width and a depth
relative to the face of its respective stator tooth, said depth
being a function of the inverse of said width.
4. The motor of claim 1 wherein each notch has a width relative to
the face of its respective stator tooth between approximately
60.degree. (electrical) and 90.degree. (electrical).
5. The motor of claim 1 wherein each notch is offset relative to a
center line of its respective stator tooth thereby defining an
asymmetrical air gap relative to the center line.
6. The motor of claim 1 wherein the stator comprises a generally
cylindrical stator core having two axially facing ends and a stator
reluctance section positioned axially adjacent one of the ends of
the stator core.
7. The motor of claim 6 wherein the pole bodies of the stator teeth
extend radially from the stator core so that each stator tooth has
two axially facing surfaces and wherein the stator reluctance
section has a plurality of legs, each leg of the stator reluctance
section corresponding to one of the stator teeth and being
positioned axially adjacent one of the axially facing surfaces of
its corresponding stator tooth.
8. The motor of claim 7 wherein a portion of each leg of the stator
reluctance section is substantially coterminous with the pole body
of its corresponding stator tooth.
9. The motor of claim 8 wherein another portion of each leg of the
stator reluctance section extends into the air gap between the
stator and the rotor at the notch of its corresponding stator
tooth.
10. The motor of claim 6 wherein the stator includes a non-magnetic
spacer section between the stator core and the stator reluctance
section.
11. The motor of claim 6 wherein the rotor comprises a generally
cylindrical rotor core having two axially facing ends and a rotor
reluctance section positioned axially adjacent one of the ends of
the rotor core.
12. The motor of claim 11 wherein the rotor comprises a plurality
of permanent magnetic elements situated radially on an outer
surface of the rotor core so that each permanent magnet element has
two axially facing ends substantially level with the ends of the
rotor core and wherein the rotor reluctance section has a plurality
of legs, each leg of the rotor reluctance section corresponding to
one of the permanent magnet elements and being positioned axially
adjacent one of the axially facing ends of its corresponding
permanent magnet element.
13. The motor of claim 12 wherein each leg of the rotor reluctance
section overlaps at least in part one of the ends of the permanent
magnet elements and extends into the air gap between the stator and
the rotor.
14. The motor of claim 12 wherein each leg of the rotor reluctance
section has a width less than that of its corresponding permanent
magnet element.
15. The motor of claim 12 wherein each leg of the rotor reluctance
section is situated on one of the ends of its corresponding
permanent magnet element at a predetermined angular position
relative to the permanent magnet element.
16. The motor of claim 11 wherein the rotor has an end cap on each
of its ends and wherein the rotor reluctance section is positioned
on one of the end caps.
17. The motor of claim 11 wherein the rotor and stator reluctance
sections are positioned radially adjacent each other.
18. The motor of claim 1 wherein the rotor comprises a generally
cylindrical rotor core and a plurality of permanent magnet elements
situated radially on an outer surface of the rotor core along a
helical path which traverses a skew angle .theta. with respect to
the axis of rotation.
19. The motor of claim 18 wherein the skew angle 0 is between
approximately 60.degree. (electrical) and 90.degree.
(electrical).
20. The motor of claim 1 comprising a single phase, single winding,
electronically commutated dynamoelectric machine.
21. A stationary assembly for a motor, said motor having a rotor
which is rotatable about an axis of rotation, said stationary
assembly being in magnetic coupling relation with the rotor, said
stationary assembly comprising:
a stator core having a plurality of teeth, said teeth each having a
radially extending pole body and an axially extending face, said
pole bodies of the stator teeth each having a generally uniform
thickness throughout its radial extent, said faces of the teeth
defining an aperture for receiving the rotor, said faces of the
teeth and said rotor defining an air gap therebetween, each tooth
having a notch in its face which is approximately at least as wide
as the thickness of the pole body of the respective stator tooth,
said notch defining a modified air gap reluctivity between the
stator core and the rotor for and the rotor in a rest position
corresponding to a positive torque starting position; and
a winding on the pole bodies of the teeth, said winding being
adapted to be energized for producing an electromagnetic field to
rotate the rotor at a desired speed or torque during operation of
the motor.
22. The stationary assembly of claim 21 wherein each notch has a
generally rectangular cross section transverse to the axis of
rotation.
23. The stationary assembly of claim 21 wherein each notch has a
width and a depth relative to the face of its respective tooth,
said depth being a function of the inverse of said width.
24. The stationary assembly of claim 21 wherein each notch has a
width relative to the face of its respective tooth between
approximately 60.degree. (electrical) and 90.degree.
(electrical).
25. The stationary assembly of claim 21 wherein each notch is
offset relative to a center line of its respective tooth thereby
defining an asymmetrical air gap relative to the center line.
26. The stationary assembly of claim 21 wherein the stator core is
generally cylindrical and has two axially facing ends and further
comprising a reluctance section positioned axially adjacent one of
the ends of the stator core.
27. The stationary assembly of clam 26 wherein the teeth extend
radially from the stator core so that each tooth has two axially
facing surfaces and wherein the reluctance section has a plurality
of legs, each leg of the reluctance section corresponding to one of
the teeth and being positioned axially adjacent one of the axially
facing surfaces of its corresponding tooth.
28. The stationary assembly of claim 27 wherein a portion of each
leg of the stator reluctance section is substantially coterminous
with the pole body of its corresponding stator tooth.
29. The stationary assembly of claim 28 wherein another portion of
each leg of the stator reluctance section extends into the air gap
between the stator and the rotor at the notch of its corresponding
stator tooth.
30. The stationary assembly of claim 26 wherein the stator includes
a non-magnetic spacer section between the stator core and the
stator reluctance section..Iadd.
31. A washing machine, comprising in combination:
a rotatable component mounted for rotation about an axis, the
rotation of said rotatable component during operation of said
washing machine causing a washing process to occur; and
a single-phase brushless DC motor that is directly coupled to said
rotatable component so that the rotational speed of a moveable
component of said motor is substantially identical to the
rotational speed of said rotatable component during operation of
said washing machine, said motor including:
a stator including a plurality of radially extending pole shoes,
wherein each one of said pole shoes is generally T-shaped and
comprises a first radial part of relatively narrow circumferential
extent and a second radial part of relatively larger
circumferential extent, the second radial parts of adjacent pole
shoes being circumferentially spaced from each other by an
intermediate gap, and wherein the circumferential extent of each of
said intermediate gaps is small compared to the circumferential
extent of each of said second radial parts,
a winding including a plurality of coils disposed on said pole
shoes, wherein each one of said coils is wound around a
corresponding one of said pole shoes,
a bearing and shaft assembly including a shaft aligned on an axis
and bearings surrounding said shaft,
a rotor that is rotatable about said axis via said bearings and
includes a permanent magnetic ring affixed thereto such that a
generally cylindrical air gap is defined between adjacent surfaces
of said pole shoes and said permanent magnetic ring,
a rotor position detector that generates an output signal that is
generally representative of the position of said rotor with respect
to said stator, wherein changes of state of said output signal are
in a fixed relationship with zero crossing points of the back EMF
generated by the rotation of the rotor with respect to the stator,
and
control circuit that is electrically connected to said rotor
position detector and receives said output signal, said control
circuit selectively energizing said coils to cause said coils to
operatively interact with said permanent magnetic ring and thereby
cause said shaft and said rotating component to rotate at
substantially identical rotational speeds during operation of said
washing machine..Iaddend..Iadd.
32. The washing machine of claim 31 wherein said rotor position
generates said output signal at least in part by sensing a flow of
magnetic flux..Iaddend..Iadd.
33. The washing machine of claim 32 wherein said rotor position
detector generates said output signal at least in part by sensing a
flow of magnetic flux directly from said permanent magnetic
ring..Iaddend..Iadd.
34. The washing machine of claim 33 wherein said rotor position
detector comprises a Hall effect device..Iaddend..Iadd.
35. The washing machine of claim 31 wherein said rotor position
detector comprises an optical switch assembly..Iaddend..Iadd.
36. The washing machine of claim 31 wherein the changes of state of
said output signal substantially coincide with the zero crossing
points of the back EMF generated by the rotation of the rotor with
respect to the stator..Iaddend..Iadd.
37. The washing machine of claim 31 wherein said shaft is rotatably
mounted on said stator..Iaddend..Iadd.
38. The washing machine of claim 31 wherein said permanent magnetic
ring has a plurality of permanent magnetic poles defined therein,
and wherein a pole gap is defined between each adjacent pair of
said permanent magnetic poles..Iaddend..Iadd.
39. The washing machine of claim 38 wherein said pole gaps are
skewed..Iaddend..Iadd.
40. The washing machine of claim 31 wherein said rotatable
component comprises an agitating wheel..Iaddend..Iadd.
41. The washing machine of claim 31 wherein the surface of said
pole shoes adjacent said air gap are contoured..Iaddend..Iadd.
42. The washing machine of claim 31 wherein said rotatable
component comprises a rotatable washing
container..Iaddend..Iadd.
43. The washing machine of claim 31 wherein said pole shoes
coaxially surround said permanent magnetic ring..Iaddend..Iadd.
44. The washing machine of claim 31 wherein the second radial parts
of each one of said pole shoes includes a notch, each one of said
notches defining a modified air gap reluctivity between the stator
and the rotor for parking the rotor in a rest position
corresponding to a positive torque starting
position..Iaddend..Iadd.
45. The washing machine of claim 31 wherein all of said coils when
energized are energized substantially simultaneously..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to motors and stationary
assemblies therefor and, particularly, to an improved stationary
assembly for providing positive torque parking positions in a
single phase electronically commutated motor for use in a
horizontal axis washing machine.
In general, a motor such as an electronically commutated or
brushless motor has permanent magnets mounted on its rotor. The
stator of such a motor has a plurality of teeth and wire-wound
coils on the teeth which, when energized with current, interact
with the permanent magnet rotor to produce positive or negative
torque, depending on the direction of the current with respect to
the polarity of the magnets. The polarity of the magnets relative
to the stator winding alternates when the rotor moves
unidirectionally. Thus, it is necessary to alternate the direction
of the stator current in synchronism to maintain a constant
direction of torque. An electronic inverter bridge typically
controls energization of the stator winding for controlling the
direction and amount of torque produced by the motor as well as the
rotor shaft speed.
As is known in the art, single phase brushless motors typically
have starting problems. The magnetic saliencies formed by the
stator teeth cause a cogging torque which forces the permanent
magnet rotor to rest, or park, at particular angular positions in
the absence of external electrical or mechanical stimulus. This
cogging torque is also referred to as an indenting or parking
torque. In a single phase motor, the rotor's parking positions can
coincide with positions of zero electromagnetic torque production
which makes it difficult to start the motor. This problem can also
make it more difficult to reverse the rotor's direction of
rotation.
One approach to overcome this problem is to provide a starting
winding which disadvantageously increases the cost and complexity
of the motor.
Another approach is to provide parking cuts in the stator teeth or
to provide additional parking magnets or parking laminations.
Although several motor configurations are known for parking a
motor's rotor in a particular position, these motor configurations
increase the cost of the motor and/or fail to provide parking
positions with sufficient starting torque, especially for low
torque motors such as single phase electronically commutated
motors.
In general, brushless DC motors are disclosed in, for example, U.S.
Pat. Nos. 5,423,192, 4,933,584 and 4,757,241, all of which are
commonly assigned with the present invention described herein and
the entire disclosures of which are incorporated herein by
reference. In particular, single phase motors are disclosed in, for
example, U.S. Pat. Nos. 5,483,139, 5,465,019, 5,140,243, 4,724,678,
4,635,349, 4,626,755, 4,313,076 and 3,134,385, all of which are
commonly assigned with the present invention described herein and
the entire disclosures of which are incorporated herein by
reference.
SUMMARY OF THE INVENTION
Among the several objects of this invention may be noted the
provision of an improved motor which provides a positive torque
parking position; the provision of such a motor which is
particularly well suited for use in a horizontal axis washing
machine; and the provision of such a motor system which is
economically feasible and commercially practical.
Briefly described, a motor embodying aspects of the present
invention includes a rotor which is rotatable about an axis of
rotation and a stator in magnetic coupling relation with the rotor.
The stator includes a plurality of teeth each having a radially
extending shaft and an axially extending face. The faces of the
stator teeth define an aperture for receiving the rotor and the
faces of the stator teeth and the rotor define an air gap
therebetween. Each stator tooth also has a notch in its face that
is approximately at least as wide as the shaft of the stator tooth
so that the stator has a magnetic configuration relative to the
rotor for parking the rotor in a rest position corresponding to a
positive torque starting position. The motor also includes a
winding on the shafts of the stator teeth and a control circuit for
controlling current in the winding whereby an electromagnetic field
is produced for rotating the rotor at a desired speed or torque
during operation of the motor.
In another embodiment, the invention is directed a stationary
assembly for a motor having a rotor which is rotatable about an
axis of rotation and which is in magnetic coupling relation with
the stationary assembly. The stationary assembly includes a stator
core that has a plurality of teeth each having a radially extending
shaft and an axially extending face. The faces of the teeth define
an aperture for receiving the rotor and the faces of the teeth and
the rotor define an air gap therebetween. Each tooth also has a
notch in its face that is approximately at least as wide as the
shaft of the tooth so that the stator core has a magnetic
configuration relative to the rotor for parking the rotor in a rest
position corresponding to a positive torque starting position. The
stationary assembly also includes a winding on the shafts of the
teeth that is adapted to be energized for producing an
electromagnetic field to rotate the rotor at a desired speed or
torque during operation of the motor.
Other objects and features will be in part apparent and in part
pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a motor system according to a
preferred embodiment of the invention.
FIG. 2 is a top view of portions of the motor of FIG. 1 including a
stator and a rotor having permanent magnets situated thereon.
FIG. 3 is a perspective view of the rotor illustrating skew of its
magnets.
FIG. 4 is a graph illustrating an electromagnetic characteristic
during steady state performance for a motor with a conventional
stator.
FIG. 5 is an enlarged, partial top view of the stator having
notches for preferentially parking the rotor according to a
preferred embodiment of the invention.
FIG. 6 is an enlarged, partial, top view of the stator having
notches for preferentially parking the rotor according to another
preferred embodiment of the invention.
FIG. 7 is a graph illustrating an electromagnetic characteristic
during steady state performance for a motor with a notched stator
according to the invention.
FIG. 8 is an enlarged, partial view of the stator and rotor having
a reluctance section.
FIG. 9 is a cross section of the stator and rotor taken along the
line 9--9 in FIG. 8.
Corresponding reference characters indicate corresponding parts
through the drawings.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings, FIG. 1 shows a motor system 100
according to a preferred embodiment of the present invention. The
system 100 includes a motor, generally designated 102, having a
stationary assembly, or stator, 104 and a rotatable assembly, or
rotor, 106 in magnetic coupling relation to the stator 104. In the
embodiment described herein, the motor 102 is a single phase,
permanent magnet motor. It is to be understood, however, that
aspects of the present invention may be applied to electronically
controllable motors or dynamoelectric machines such as single phase
permanent magnet motors, external rotor motors (i.e., inside out
motors), single and variable speed motors, selectable speed motors
having a plurality of speeds, brushless dc motors and
electronically commutated motors. Such motors may also provide one
or more finite, discrete rotor speeds selected by an electrical
switch or other control circuit.
In a preferred embodiment of the invention, a motor shaft 108
mechanically connects the rotor 106 to a particular device to be
driven, such as a rotatable component 110. For example, the
rotatable component 110 comprises a basket 116 which is part of a
horizontal axis automatic washing or laundry machine, generally
indicated 118. Preferably, rotatable component 110 also includes a
connection mechanism 120 for coupling the basket 116 to the shaft
108. The connection mechanism 120 may comprise a fixed ratio speed
reducer, such as a gear box or a pulley arrangement or, in some
applications, shaft 108 of motor 102 may be directly coupled to
basket 116. Although disclosed for use with basket 116, it is to be
understood that motor 102 may be part of a number of different
systems for driving a rotatable component. For example, rotatable
component 110 may be an agitator and/or basket of a vertical axis
washing machine or a fan, blower, compressor or the like. Commonly
assigned U.S. Pat. Nos. RE 33,655, 5,492,273, 5,418,438, 5,423,192,
and 5,376,866, the entire disclosures of which are incorporated
herein by reference, describe various rotatable components for
which the present invention is suited for use.
In laundering apparatus such as the laundry machine 118, basket 116
is rotatable within a tub (not shown) which holds water for washing
the fabrics to be laundered. Basket 116 first agitates water and
fabrics to be laundered and then spins them to cause a centrifugal
displacement of water from the tub.
Preferably, a user interface, or system control, 122 provides
system control signals in the form of motor commands to a control
circuit 124 via line 126. In this instance, the system control 122
provides signals representing desired washing times, desired
washing cycles, and the like. As represented by the block diagram
of FIG. 1, the control circuit 124 provides motor control, or
commutation, signals via line 128 for electronically controlling a
plurality of gate drives 130. In turn, the gate drives 130 provide
drive signals via line 132 for switching a plurality of power
switches 134, such as IGBT's, BJT's or MOSFET's. In addition to
providing drive signals which have been shifted from, for example,
5 volts to 15 volts for driving the power switches 134, gate drives
130 also condition the signals provided by control circuit 124 via
line 128 for optimal operation of power switches 134.
As shown in FIG. 1, a power supply 136 provides high voltage DC
power via line 138 to power switches 134. By selectively switching
the power supply 136 in connection with the winding (see FIG. 2)
included in stator 104, power switches 134 provide power via line
140 to motor 102. Preferably, power switches 134 energize the motor
winding in a preselected sequence for commutating motor 102 in
response to control circuit 124. In this instance, control circuit
124 selectively activates power switches 134 to control rotation in
motor 102 as a function of a commutation signal. It is to be
understood that power supply 136 may also provide power to operate
control circuit 124.
Referring further to FIG. 1, a position sensor 142 provides control
circuit 124 with feedback via line 144 representative of the
angular position of rotor 106 relative to stator 104. In a
preferred embodiment, the position sensor 142 comprises one or more
Hall sensors providing a rotor position feedback signal which has a
predefined angular relationship relative to the motor back
electromotive force (EMF) (e.g., in phase or 90.degree. out of
phase with the back EMF). Other position sensors, such as optical
sensors, may also be used to provide rotor position feedback
instead of or in addition to the Hall sensors. Commonly assigned
application Ser. No. 08/680,010, filed Jul. 15, 1996, the entire
disclosure of which is incorporated herein by reference, describes
a quadrature winding suitable for generating a position signal in a
single phase motor according to the invention.
Preferably, control circuit 124 generates its commutation signals
as a function of the zero crossings of the back EMF of the winding.
As such, the product of the current and the back EMF determines
torque production in motor 102. In order to sustain positive
torque, it is necessary to energize the winding when the back EMF
has crossed zero in the direction that will oppose the voltage
energizing it. Since it is desired that motor current crosses zero
at the time the motor back EMF also crosses zero, control circuit
124 preferably commutates motor 102 at an angle relative to the
next back EMF zero crossing. In other words, control circuit 124
estimates subsequent back EMF zero crossings based on the sensed
position of rotor 106 and generates gate drive signals at line 128
for driving power switches 134 coincident with or in advance of the
estimated back EMF zero crossings. Thus, control circuit 124
generates the commutation signals as a function of the sensed
position of rotor 106 as represented by the position signal. As an
example, commonly assigned U.S. Pat. No. 5,423,192, the entire
disclosure of which is incorporated herein by reference, describes
one preferred means for detecting zero crossings.
In operation, control circuit 124 generates commutation signals via
line 128 in response to the system control signals. The commutation
signals cause system 100 to produce a motor current that matches
the load torque demand as a function of a regulated current
reference level. By matching torque load with produced torque,
motor 102 is able to operate at a desired torque or speed. The
commutation signals preferably include a series of pulse width
modulated cycles, wherein each cycle causes a corresponding
switching event of power switches 134. In turn, the current in the
winding produces an electromagnetic field for rotating the rotor
106 of motor 102. To control the speed of rotatable component 110,
system 100 preferably controls the speed of motor 102 by
controlling the power delivered to the load. In particular, system
100 regulates current in motor 102, which in turn regulates torque,
to obtain the desired motor speed by matching the load and motor
loss demand torque at the desired speed. Preferably, control
circuit 124 is embodied by a microprocessor or microcontroller
and/or an application specific integrated circuit (ASIC) or
universal electronically commutated motor integrated circuit (UECM
IC).
In one embodiment, the regulated current reference level is a peak
regulated current for normal motoring operation communicated by a
pulse width modulated signal having a variable duty cycle
representative of the desired level. For example, the duty cycle of
may vary from 0% to 100% where 100% corresponds to a maximum peak
regulated current value and the duty cycle is proportional to the
desired current in motor 102. In the alternative, control circuit
124 generates a variable voltage signal, the magnitude of which
represents the desired current. Other suitable means for providing
the peak regulated current level include a simple resistor circuit
or potentiometer.
Commonly assigned application Ser. No. 08/647,694, filed May 15,
1996, the entire disclosure of which is incorporated herein by
reference, discloses a system for regulating motoring current and
controlling circulating currents in a single phase motor. Commonly
assigned application Serial No. (TO BE ASSIGNED) entitled "Single
Phase Motor for Laundering Apparatus," filed Dec. 5, 1996, the
entire disclosure of which is incorporated herein by reference,
describes a suitable motor and control for use with the present
invention.
FIG. 2 illustrates portions of motor 102 which includes stator 104
and rotor 106. In a preferred embodiment, stator 104 and rotor 106
are magnetically coupled and rotor 106 rotates about a central axis
of rotation (see FIG. 3) coaxial with shaft 108. Stator 104
includes a stator core 150 having a plurality of teeth 152 which
are wrapped by a winding, portions of which are generally indicated
154. As such, electrically energizing winding 154 generates an
electromagnetic field for rotating rotor 106. Although motor 102 is
shown in a standard configuration with rotor 106 within stator 104
and with the stator teeth 152 extending radially inwardly, it is
contemplated that the invention may be used on an inside-out motor
wherein stator 104 is within rotor 106.
In view of the shape of stator teeth 152, the conventional way to
wind the pole is to "sew" the wire constituting winding 154 around
each stator tooth 152 for the required number of turns. Commonly
assigned application Ser. No. 08/678,524 describes an alternative
method of installing winding 154 on stator teeth 152 using a high
speed bobbin coil winding machine.
Preferably, the stator core 150 is a stack of steel laminations
held together by winding 154 itself, welding, adhesive bonding or
another suitable means. Alternatively, stator core 150 is an
integral piece of steel. Those skilled in the art will understand
all suitable means for holding the laminations together. While
stator 104 is illustrated for purposes of disclosure, it is
contemplated that other stationary assemblies of various other
constructions having different shapes or winding patterns and with
different numbers of teeth may be utilized within the scope of the
invention so as to meet at least some of the objects thereof.
In one preferred embodiment, rotor 106 includes a number of
permanent magnet elements 156. In the illustrated embodiment,
twelve permanent magnet elements 156 are situated on a rotor core
158 of rotor 106. Energizing winding 154 establishes magnetic poles
which provide a radial magnetic field relative to the permanent
magnets 156. When the field intersects with the flux field of the
magnet poles, rotor 106 rotates relative to stator 104 according to
the relative polarity of the field and magnet poles to develop a
torque in a desired direction. The developed torque is a direct
function of the intensities or strengths of the magnetic fields.
For example, in an electronically commutated motor, winding 154 is
commutated without brushes by sensing the rotational position of
rotor 106 as it rotates within stator core 150. Power switches 134
control the direction of current flow through winding 154 and,
thus, control the direction of the magnetic field generated by
stator 104. Because the position of rotor 106 is known via position
sensor 142, control circuit 124 is able to control the magnetic
field in stator 104 to cause rotor 106 to rotate in a desired
direction.
FIG. 3 illustrates a preferred embodiment of rotor 106 employing a
skewed magnet imprint. As shown, the transitions between the north
and south poles of adjacent magnets 156 follow a generally helical
path resulting in a magnet imprint skew of .theta.. For example, in
a twelve pole motor, a skew of .theta.=15.degree. (mechanical)
corresponds to a skew of 90.degree. (electrical). Also, rotor 106
rotates about an axis 160 coaxial with shaft 108.
A factor in designing a single phase, single winding motor,
concerns potential problems during start up or reversal. The
magnetic saliencies formed by the stator teeth cause a cogging
torque which forces the permanent magnet rotor to rest, or park, at
particular angular positions. This cogging torque is also referred
to as an indenting or parking torque. In a single phase motor, the
parking positions can coincide with positions of zero
electromagnetic torque production which makes it difficult to start
the motor. Similarly, a zero torque parking position makes it more
difficult to reverse the direction in which the motor rotates. FIG.
4 illustrates an exemplary load gravitational moment curve 162
relative to an exemplary cogging, or parking, torque curve 164 for
a conventional single phase motor in a horizontal axis washer.
In this example, the gravitational moment is associated with
laundry in the horizontal axis washer's basket at a given instant
in time. It is to be understood that the actual gravitational
moment changes as the basket rotates. Generally, the gravitational
moment is defined with respect to the basket by:
W * r.sub.d * sin (.theta..sub.d)
where W is the effective weight of the load (i.e., the wet laundry)
in the basket; r.sub.d is the effective radius of the load (i.e.,
the distance from the center of the basket to the load's center of
mass); and ed is the angle between the load's center of mass and
the center of the basket where 0.degree. is at the lowest vertical
position. As is well known in the art, the above expression is
reflected into a motor coordinate system as a function of the
pulley ratio .rho. of the connection mechanism coupling the motor
to the basket, the number of pole pairs of the motor and an
arbitrary intercept which varies from one tumble to the next.
Referring further to FIG. 4, the rotor will tend to come to rest
at, for example, parking positions 166, 168, 170 without any
external electrical or mechanical stimulus. As shown, the parking
positions 166, 168, 170 occur at the intersections of the parking
torque curve 164 and the load gravitational moment curve 162. In
this instance, stable parking positions can occur where parking
torque curve 164 has a negative slope and intersects gravitational
moment curve 162, i.e., parking positions 166, 170. On the other
hand, position 168 is unstable and the rotor is not likely to park
in this position. Thus, as shown in FIG. 4, the single phase motor
can park in an undesirable parking position which corresponds to
zero electromagnetic torque (e.g., at a rotor position of
180.degree. (electrical)) resulting in a stalled rotor.
According to the invention, motor system 1 overcomes the potential
start up problem by forcing the parking positions of the permanent
magnet rotor 106 away from the electromagnetic torque nulls. As
those skilled in the art recognize, a stator has a plurality of
teeth having faces defining an aperture for receiving a rotor. The
faces of the stator teeth and the rotor define an air gap
therebetween. In the present invention, a relatively large notch is
cut into the stator teeth which affects the air gap reluctivity
between the stator and the rotor. This change in the air gap causes
the rotor to parking a new position relative to the electromagnetic
torque of the motor.
FIG. 5 illustrates a portion of stator 104, particularly stator
core 150, according to a preferred embodiment of the invention.
Each tooth 152 has a relatively slender shaft 172 and a relatively
wide tip 174. As shown, a radially facing surface 176 on each tooth
tip 174 defines the air gap between stator 104 and the permanent
magnets 156 of rotor 106. The wide tip 174 spreads the flux into
the energy conversion air gap region between rotor magnet 156 and
stator tooth 152. According to the invention, a notch, generally
indicated 178, in tooth tip 174 provides a modified air gap
reluctivity between rotor magnets 156 and stator tooth 152.
The parking positions of rotor 106 and corresponding start up
torque may be determined from magnetic field calculations based on
a filamentary magnetic circuit approach. Using such an approach,
the periphery of rotor magnet 156 and the air gap are divided into
a plurality of filaments (not shown). In general, the filaments are
spatial entities having equal peripheral span but different radial
length depending on the location of the filament in the magnetic
geometry. For each filament, magnetic circuit lengths in the air
gap and in rotor magnet 156 are determined. The classical magnetic
circuit equation for a permanent magnet is then solved to obtain
the flux density and the magnetic energy in each filament. The flux
linkage at position .theta.=.alpha. may be calculated by the
following summation over all the filaments: ##EQU1##
Likewise, the magnetic energy at a position .theta.=.alpha. is
calculated by a similar summation over the filaments. These field
calculations are performed repeatedly at regular increments in the
rotor position over an entire electrical cycle. Numerical
processing produces cyclic waveforms of the flux linkage, magnetic
energy, flux linkage differential, and cogging torque.
Referring further to FIG. 5, the notch 178 in each stator tooth 152
is relatively large and is offset relative to a center line 180
through tooth 152. In a preferred embodiment, the width w of notch
178 is approximately as wide as the shaft 172 of tooth 152. In
certain applications, it may be desired for width w to be
significantly wider than shaft 172. Preferably, the width w and the
depth d of notch 178 are inversely related so that as the width w
is increased, the depth d is decreased. As such, the motor geometry
of FIG. 5 provides a desirable parking torque characteristic
according to the present invention.
As an example, the shaft 172 of each stator tooth 152 is
approximately 0.275 inches wide and the diameter of stator core 150
from surface 176 of one tooth 152 to surface 176 of an opposite
tooth 152 is approximately 3.125 inches. In this example, notch 178
has a desired width of approximately 60.degree. (electrical) which
corresponds to approximately 10.degree. (mechanical). Thus, the
width w of notch 178 is approximately 0.273 inches. The motor
configuration of FIG. 5 also employs a relatively large skew of the
magnet imprint (e.g., 90.degree. (electrical)) to obtain the
desired parking torque characteristic.
FIG. 6 illustrates another preferred motor geometry providing a
desirable parking torque characteristic according to the invention.
In this instance, notch 178 is relatively shallow but wider than
the notch configuration of FIG. 5. As an example of this
alternative embodiment, the shaft 172 of each stator tooth 152 is
approximately 0.275 inches wide and the diameter of stator core 150
from surface 176 of one tooth 152 to surface 176 of an opposite
tooth 152 is approximately 3.125 inches. In this example, notch 178
has a desired width of approximately 90.degree. (electrical) which
corresponds to approximately 15.degree. (mechanical). Thus, the
width w of notch 178 is approximately 0.409 inches. Preferably,
rotor 106 employs less magnet skew (e.g., 60.degree. (electrical))
in the embodiment of FIG. 6 than in the embodiment of FIG. 5 to
obtain the desired parking torque characteristic. Further, notch
178 of FIG. 6 need not be offset with respect to the center line
180 of tooth 152. This geometry beneficially provides improved
electromagnetic torque production characteristic.
In yet another alternative embodiment, notch 178 may be sized to
have a variable, or stepped, depth. In this instance, the notch,
stepped air gap and magnet skew are sized to optimize parking
torque in various applications.
FIG. 7 illustrates an exemplary load gravitational moment curve 190
relative to exemplary cogging, or parking, torque curves 192, 194
for motor 102 as used in horizontal axis washer 118 according to
the present invention. In FIG. 7, the curve 192 represents the
parking torque for the motor configuration of FIG. 5 and the curve
194 represents the parking torque for the motor configuration of
FIG. 6.
With respect to parking torque curve 192, rotor 106 will tend to
come to rest at, for example, parking positions 196, 198, 200
without any external electrical or mechanical stimulus. As shown,
the parking positions 194, 196, 200 occur at the intersections of
the parking torque curve 192 and the load gravitational moment
curve 190. In this instance, stable parking positions can occur
where parking torque curve 192 has a negative slope and intersects
gravitational moment curve 190, i.e., parking positions 196, 200.
On the other hand, position 198 is unstable and rotor 106 is not
likely to park in this position. Advantageously, the motor
configuration of FIG. 5 causes parking torque curve 192 to be
shifted so that the stable parking positions 196, 200 do not
intersect the positions of zero electromagnetic torque production.
In other words, the entire negative slope portions of curve 192 is
between the torque nulls. Thus, as shown in FIG. 7, motor 102 will
not park where the electromagnetic torque is zero and a stall
situation is much less likely to occur.
With respect to parking torque curve 194, parking positions 202,
204 occur at the intersections of the parking torque curve 194 and
the load gravitational moment curve 190. In this instance, a stable
parking position can occur where parking torque curve 194 has a
negative slope and intersects gravitational moment curve 190, i.e.,
parking position 204. On the other hand, position 202 is unstable
and rotor 106 is not likely to park in this position.
Advantageously, the motor configuration of FIG. 6 causes parking
torque curve 194 to be shifted so that the stable parking position
204 does not intersect the positions of zero electromagnetic torque
production. In other words, the entire negative slope portions of
curve 194 is also between the torque nulls. Thus, as shown in FIG.
7, motor 102 will not park where the electromagnetic torque is zero
and a stall situation is much less likely to occur.
As is known in the art, Coulomb friction associated with, for
example, sliding of the water seals inside laundry machine 118, may
affect the gravitational moment of the load in certain situations.
As a result, the rotor 106 of motor 102 may park at an unstable
parking position that coincides with a position of zero
electromagnetic torque production. The present invention provides
further improvements to the motor configurations of FIGS. 5 and 6
that cause rotor 106 to park at a stable position of non-zero
torque even when Coulomb friction is problematic.
FIG. 8 illustrates portions of stator 104 and rotor 106. In
particular, FIG. 8 is a fragmentary top view of a stator reluctance
section 208 having a plurality of legs 210, each corresponding to
one of the stator teeth 152. Also shown in FIG. 8 is a fragmentary
top view of a rotor reluctance section 212. According to the
present invention, the reluctance sections 208, 212 comprise a
relatively thin layer of a low reluctivity material such as iron
positioned generally axially adjacent to stator core 150 and rotor
core 158, respectively, for providing additional electromagnetic
torque at selected positions, such as the torque nulls of the
magnet section of motor 102.
With respect to stator reluctance section 208, each leg 210 is
approximately the same width as the shaft 172 of stator tooth 152
and overlies an axially facing surface of a corresponding one of
the stator teeth 152. During construction of this embodiment of
motor 102, winding 154 is wrapped around both stator teeth 152 and
reluctance section leg 210.
Similarly, rotor reluctance section 212 overlies an end of each
permanent magnet 156 of rotor 106. In a preferred embodiment, rotor
reluctance section 212 includes a plurality of axially projecting
legs 214 which extend into the air gap defined between the surface
176 of tip 174 and magnet 156 Preferably, reluctance sections 208,
212 are aligned to provide electromagnetic torque at the positions
where motor 102 would otherwise produce no electromagnetic torque,
should the situation arise in which Coulomb friction causes rotor
106 to become stuck at a torque null. Although illustrated as being
centered on magnet 156, it is to be understood that the reluctance
section 212 may be rotated relative to magnets 156 before it is
secured to rotor 106 to position each leg 214 at a predetermined
position relative to its corresponding permanent magnet 156.
Referring now to FIG. 9, rotor 106 preferably includes a
non-magnetic end plate 218 which separates rotor reluctance section
212 from rotor core 158 and magnets 156. For this reason, stator
104 includes a nonmagnetic spacer stack 220 which separates stator
reluctance section 208 from stator core 150. As an example, FIG. 9
illustrates magnet 156 having an axial length of approximately 1.65
inches, end plate 218 and spacer stack 220 each having an axial
length of approximately 0.1 inches, and reluctance section 208, 212
each having an axial length of approximately 0.075 inches.
In view of the above, it will be seen that the several objects of
the invention are achieved and other advantageous results
attained.
As various changes could be made in the above constructions and
methods without departing from the scope of the invention, it is
intended that all matter contained in the above description or
shown in the accompanying drawings shall be interpreted as
illustrative and not in a limiting sense.
* * * * *