Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-06-04
2004-04-27
Ponomarenko, Nicholas (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S156120, C310S112000
Reexamination Certificate
active
06727629
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to rotary electric motors, more particularly to permanent magnet motors comprising a plurality of axially spaced rotor and stator poles, the axially spaced rotor magnets or stator magnets being shifted from axial alignment with each other.
BACKGROUND
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics. In a vehicle drive environment, for example, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience. The above-identified copending related U.S. applications describe formation of electromagnet core segments as isolated magnetically permeable structures configured in an annular ring. With such arrangements, flux can be concentrated to provide advantageous effects as compared with prior art embodiments.
As described in the above-identified Maslov et al. applications, isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched. The lack of additional poles within the group avoids any such effects within a group. Further benefits are described from utilization of three dimensional aspects of motor structure, such as a structural configuration wherein axially aligned stator poles and axially aligned rotor magnets provide highly concentrated flux density distribution in the active air gap of the machine. Such configuration provides a greater number of poles with the same individual active air gap surface areas and/or greater total active air gap surface area than conventional motors having the same air gap diameter.
In addition to benefits of flux concentration obtainable with the configurations described above, recently introduced neodymium-iron-boron (NdFeB) magnetic materials can produce larger flux densities than other permanent magnetic materials previously used in brushless machines, thus increasing torque output capacity. The use of high density producing permanent magnets in motors which comprise a great number of poles presents a concern for ameliorating undesired effects that can be introduced by cogging torque. Cogging torque is produced by magnetic attraction between the rotor mounted permanent magnets and those stator poles that are not in a selectively magnetized state. This attraction tends to move the rotor magnet to an equilibrium position opposite a stator pole to minimize the reluctance therebetween. As the rotor is driven to rotate by energization of the stator, the magnitude and direction of the cogging torque produced by magnet interaction with non-energized electromagnet segments changes periodically to oppose and increase the torque produced by the energized stator segments. In the absence of compensation, cogging torque can change direction in an abrupt manner with the rotation of the rotor. If cogging torque is of significant magnitude, it becomes a rotational impediment, as well as a source of mechanical vibration that is detrimental to the objectives of precision speed control and smooth operation.
As an illustration of the development of cogging torque, a motor such as disclosed in the copending application Ser. No. 09/826,422, is considered. The disclosure of that application has been incorporated herein.
FIG. 1
is an exemplary view showing rotor and stator elements. Rotor member
20
is an annular ring structure having permanent magnets
21
substantially evenly distributed along cylindrical back plate
25
. The permanent magnets are rotor poles that alternate in magnetic polarity along the inner periphery of the annular ring. The rotor surrounds a stator member
30
, the rotor and stator members being separated by an annular radial air gap. Stator
30
comprises a plurality of electromagnet core segments of uniform construction that are evenly distributed along the air gap. Each core segment comprises a generally u-shaped magnetic structure
36
that forms two poles having surfaces
32
facing the air gap. The legs of the pole pairs are wound with windings
38
, although the core segment may be constructed to accommodate a single winding formed on a portion linking the pole pair. Each stator electromagnet core structure is separate, and magnetically isolated, from adjacent stator core elements. The stator elements
36
are secured to a non-magnetically permeable support structure, thereby forming an annular ring configuration. This configuration eliminates emanation of stray transformer flux effects from adjacent stator pole groups.
FIG. 2
is a partial plan layout of two adjacent stator core elements
36
, with pole faces
32
denominated A-D, in relation to the rotor magnets, denominated 0-5, during motor operation. The positions of the rotor magnets are depicted at (A)-(C) for three instants of time (t
1
-t
3
) during a period in which the rotor has moved from left to right. At time t
1
, the winding for the A-B stator pole pair is energized with current flowing in a direction to form a strong south pole at A and a strong north pole at B. The winding for the C-D stator pole pair is not energized. The position of the rotor is shown at (A). North magnet
1
and south magnet
2
overlap stator pole A. South magnet
2
and north magnet
3
overlap stator pole B. At this time magnet
3
is approaching an overlapping position with pole C. South magnet
4
is in substantial alignment with pole C and north magnet
5
is in substantial alignment with pole D. At this time motoring torque is produced by the force of attraction between south pole A and north pole magnet
1
, the force of attraction between north pole B and south pole magnet
2
, and the force of repulsion between north pole B and north pole magnet
3
. Poles C and D have respective weak north and south magnetization caused by the attraction of magnets
4
and
5
. This attraction, which seeks to maintain minimum reluctance is in opposition to motor driving torque.
At time t
2
, the rotor has moved to the position shown at (B). The energization of the pole pair A-B windings has been commutated off. Windings of the C-D pole pair are not energized. Magnets
1
and
2
are substantially in alignment with poles A and B respectively. North magnet
3
and south magnet
4
overlap pole C. South magnet
4
and north magnet
5
overlap pole D. Poles A and B have weak south and north magnetization respectively. The stator poles C and D are influenced by both north and south rotor magnets. Pole C is in a flux path between north pole magnet
3
and south pole magnet
4
. Pole D is in a flux path between south pole magnet
4
and north magnet pole
5
. A cogging torque thus has developed that opposes the motor driving torque and changes in magnitude as the rotor magnets move from direct alignment with the non-energized stator poles to partial alignment
At time t
3
, the rotor has moved to the position shown at (C). Energization of the A-B pole pair windings has been reversed, causing a strong north pole at pole A and a strong south pole at B. Windings of the C-D pole pair are not energized. North magnet
1
and south magnet
2
overlap stator pole B. South magnet
0
and north magnet
1
overlap stator pole A. At this time south magnet
2
is approaching an overlapping position with pole C. North magnet
3
is in substantial alignment wit
Benson Mark A.
Maslov Boris A.
Soghomonian Zareh
Hanh Nguyen
McDermott & Will & Emery
Ponomarenko Nicholas
Wavecrest Laboratories LLC
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