Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates to electric motors and/or achrators. In particular,
there is a
unitary pole shoe that minimizes flux leakage for an electric motor that
replaces multiple
pole shoes.
2. DESCRIPTION OF THE RELATED ART
Electromechanical rotary actuators and motors that operate with air-gap
magnetic flux in the axial direction usually have a soft-magnetic stator with
salient
poles- The tops of the poles face the air gap that separates the rotor from
the stator.
On the stator, the space between consecutive poles is determined by the
dimensions of
the insulated coils which, when energized with electric current, convert the
stator poles
into electromagnets.
The rotary actuator or motor develops torque that is directly related to the
coils'
current over a rotation angle that corresponds, approximately, to the arc
spanned by
one salient pole. To increase this angle, individual soft-magnetic "pole
shoes" are
placed on top of each stator pole- At the air gap, the shoes present an arc
that is wider
than the arc of the salient poles.
The pole shoes have to be placed, one over each stator pole, accurately, and
then secured in place pemnanently_ Common numbers of pole shoes are 2 and 4.
although larger numbers, such as 8 and 10, are also used. The placement and
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attachment of multiple pole shoes adds complexity and cost to the production
of electric
motors.
If the pole shoes are not evenly spaced, then the pertormance of the rotary
actuator or motor deteriorates, and the expelled useful rotation range may not
be
S possible. For example, pole shoes that are too close to each other favor
excessive
magnetic leakage between them, thus helping saturate their stator poles with
magnetic
flux that does not contribute to the development of any rotational torque. The
leaking
flux also contributes to a more inefficient motor.
Therefore, there is a current unmet need for a pole shoe that is readily
manufacturable and assembled into an electric motor at a low cost and that
minimizes
magnetic flux leakage between stator poles.
SUMMARY OF THE INVENTION
It is a feature of the invention to provide a unitary pole shoe for mounting
adjacent
a stator in an electric motor. The pole shoe includes several pole shoe
sections, each pole
shoe section is mounted adjacent a stator pole. Several bridges join the pole
shoe
sections. The bridges are located out of a main flux path between any two
adjacent stators
poles such that flux leakage between the adjacent stator poles is minimized.
The invention resides not in any one of these features per se, but rather in
the
particular combination of ail of them herein disclosed and claimed. Those
skilled in the art
will appreciate that the conception, upon which this disclosure is based, may
readily be
utilized as a basis for the designing of other structures, methods and systems
for cartying
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out the several purposes of the present invention. further, the abstract is
neither intended
to define the invention of the application, which is measured by the claims,
neither is it
intended to be limiting as to the scope of the invention in any way.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of the preferred embodiment of the unitary axial
flux
pole shoe with a stator.
Fig. 2 is a top view of the unitary axial flux pole shoe of figure 1.
Fig. 3 is a cross sectional view of figure 2 along section lines A-A.
to Fig. 4 is a top view of an alternative embodiment of the unitary axial flux
pole shoe.
It is noted that the drawings of the invention are not to scale. The drawings
are
intended to depict only typical embodiments of the invention, and therefore
should not be
considered as limiting the scope of the invention. In the drawings, like
numbering
represents like elements between the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Refen~ing to figures 1,2 and 3 the prefer-ed embodiment of the unitary axial
flux pole
shoe 30 is shown. A stator assembly 10 has a stator 12 with a unitary pole
shoe 30
mounted thereon. Stator assembly 10 is part of a rotary actuator or rotating
electric motor.
Stator 12 has a base 13 with four stator poles 14 extending therefrom, and a
stator cavity
15. Stator gaps 16 are located between the stator poles 14_ Holes 17 are
located at the
top of poles 14. Holes 17 receive fasteners 24 to attach pole shoe 30 to
stator 12. Several
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recesses 18 are located on the stator poles 18 to help align and secure pole
shoe 30.
Stator 12 is preferably formed from a sintered powdered metal. A wire winding
20 is
wound around stator pole 14. An electrical potential is applied to winding 20
in order to
generate a flux field 22 extending axially to the stator poles in the
direction shown by the
arrows in figure 1. By regulating either the direction of current passing
through the winding
or by changing the direction of the winding around the poles, each column can
become 'a
north or south elector-magnet. The flux field 22 acts upon a rotor (not shown)
that contains
magnets. The flux field causes an opposing field on the magnet to be biased
away from
the poles and to cause the rotor to rotate. During operation, the stator poles
generate flux
l0 22 that moves perpendicular to the plane of the pole shoe 30.
Disc shaped pole shoe 30 has four quarter circle shaped sections 32. Pole shoe
30 is preferably stamped from conventional carbon steel sheet metal. Prior art
pole shoes
are formed by pressing and sintering powdered metal. The pole shoe 30 is
mounted on
stator 12. Each pole shoe section 32 is mounted adjacent a stator pole 14.
Bridges 34 are
formed between sections 32 and link the pole shoe sections together. The
bridges 32 are
located out of the main flux path between any two adjacent stators poles such
that flux
leakage between the adjacent stator poles is minimized. The sections 32 define
a slot 35
between adjacent sections. A shaft aperture 36 passes through the center of
the pole
shoe 30. A shaft (not shown) would pass through aperture 36. Several notches
38 are
located on an inner circumferential surface 40 adjacent aperture 36. The
purpose of
notches 38 is to reduce the cross-section of bridges 34. Several outer
alignment notches
42 are located on an outer circumferential surface 48. Chamfered holes 44
allow fasteners
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24 to pass through. The alignment notches facilitate accurate and fast
assembly of the
rotary actuator. A thrust bearing support 46 is formed around the aperture.
The thrust
bearing support 46 carries a thrust bearing (not shown) that supports the
shaft.
The present invention allows the prior art multiple set of pole shoes to be
replaced by one single or unitary part, preferably one stamped out of sheet-
metal. The
unitary pole shoe incorporates the individual pole shoes that are joined by
bridges and
also incorporates a thrust bearing support. These bridges have to be
relatively thin and
out of the main path of flux that links two consecutive poles. This avoids the
problem of
excessive flux leakage between adjacent pole shoes that are too close_
While the pole shoe was shown fastened to the stator poles with screws, other
fastening methods can also be used. For example, the pole shoe can be welded
to the
stator pole or can be attached with rivets or an adhesive.
The advantages of the present invention are:
1. The assembly is greatly simplified because there is only one single pole
shoe to be
positioned instead of 2, 4, 6, 8, 10, etc. individual pole shoes.
2. Even spacing of the pole shoes is guaranteed by the stamping operation.
3_ The cost of materials and assembly is reduced.
4. The stamped part can incorporate features on its outer and inner edges that
help
the mechanical assembly of the rotary actuator or motor while locating the
pole
piece on the stator and or while locating the actuator housing around the pole
shoe.
5. A concave "thrust bearing support" can be formed at the center of the
stamped part,
thus incorporating what normally is another separate component of the actuator
or
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motor.
Referring to figure 4, an alternative embodiment of the unitary axial flux
pole shoe
60 is shown. Pole shoe 60 is similar to pole shoe 30 except that the bridges
62 are
oriented diagonally between the pole shoe sections 32. Bridges 62 form an
inner slot 64
and an outer slot 66. The bridges 62 connect the pole shoe sections 32, while
at the same
time being located out of the main flux path so as to minimize flux leakage.
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