Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1994-09-29
2001-01-30
LaBalle, Clayton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S156030
Reexamination Certificate
active
06181035
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to permanent magnet electric motors and, more particularly to the reduction or elimination of cogging torque in permanent magnet electric motors, particularly in brushless motors, by changing the shape and arrangement of the magnets.
Conventional permanent magnet motors are subjected to a reluctance torque, also known as a cogging torque, that results in an oscillation of the torque produced by the motor itself. This torque is due to periodic interactions between the magnetic field in the air gap created by the inductor (in this case by the permanent magnets) and because of the non-uniformity of the permeance of the magnetic circuit of the armature framework or the stator which is typically provided with channels or slots for housing the windings.
Generally, cogging torque exists because the magnetic energy that is stored in the air gap varies according to the rotor's angular position with respect to the stator. Since cogging torque is naturally undesirable, the designers of brushless motors have developed systems to reduce it. Typically, this reduction in cogging torque is accomplished by disrupting the periodicity of the magnetic field. It is known that the cogging torque may be reduced by making the slots in the armature extend obliquely along the motor's main rotation axis, such as by staggering the laminated iron sheets forming the armature. It is also possible to reduce the cogging torque by varying the width and spacing of the permanent magnets or the slots in the rotor. Although methods of this type are effective in reducing cogging torque, they have the disadvantage of also reducing the torque produced by the motor and increasing the motor's production cost.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a permanent magnet motor having a reduced or eliminated cogging torque while having a cost and complexity similar to conventional motors.
A permanent magnet motor is provided comprising a rotor rotatable about a central motor axis and a stator armature supporting the motor windings disposed coaxially with the rotor and separated from the rotor by a circumferential air gap. The rotor carries a plurality of permanent magnet elements arranged to provide an angular distribution of alternating magnetic polarity about the rotor. The stator armature has a number of slots formed therein facing the air gap and disposed at equally spaced intervals about the stator as in conventional practice, the slots house the motor windings. In accordance with one aspect of the present invention, the permanent magnet elements are shaped and arranged such that the distribution of the density of magnetic energy that is stored in the air gap in the absence of stator slots, as a function of a linear coordinate taken along a circumference disposed within the air gap and coaxial to the motor axis, exhibits an increasing transition in magnitude in a first angular position near a first end of a magnetic polarity and a decreasing transition in magnitude in a second angular position near a second end of the magnetic polarity, the transitions being asymmetrical with respect to each other and the density of magnet energy being substantially constant between consecutive transitions. Additionally, the slots of the stator are disposed in positions whereby during operation, when a first slot is in a first angular position corresponding to the first angular position of the increasing transition in magnitude of the distribution of magnetic energy density, a second slot is in a second angular position that corresponds to the second angular position of the decreasing transition in magnitude of the distribution of magnetic energy density, each slot functioning to modify the distribution of the density of magnetic energy in the air gap according to a modulation function which has a local development symmetrical to a radial plane passing through the center of the slot.
In a further aspect of the present invention, each of the permanent magnet elements is shaped as an arcuate segment subtending an angular aperture, &bgr;, where:
&bgr;=K(360°
c
)+(4/p)(1−1/(5p
(3P−2)/2)
)
wherein:
p is the number of pairs of magnet elements;
n
c
is the number of slots; and
K=1,2, . . . ((n
c
/2p)−l), and
n
c
/2p is an integer greater than 1.
Further, the permanent magnet elements advantageously have radially inward corners which are bevelled at an angle of about 45 degrees and, preferably, are bevelled to a depth of about one and one-half times the radial width of the air gap. In a further aspect of the present invention, the permanent magnet elements are disposed circumferentially about the rotor and spaced apart at equal intervals of spacing, the interval of spacing between adjacent permanent magnet elements being approximately equal to the quantity (kp−3w), where k is a selected positive integer, p is the center to center spacing between adjacent slots, and w is the radial width of the air gap.
In a still further aspect of the present invention, each of the permanent magnet elements comprises an N pole and an S pole disposed at opposite ends of the magnet element and has a polarity transition region between the N pole and the S pole, and an axially extending groove disposed within the polarity transition region and facing the air gap. In a still further aspect of the present invention, each of the permanent magnet elements comprises an N pole and an S pole disposed at opposite ends of the magnet element and has a polarity transition region between the N pole and the S pole, and a magnetic flux conductor, such as for example a steel plate, disposed on the face of the magnet element facing the air gap so as to extend across the polarity transition region.
Most advantageously, each of the permanent magnet elements comprises an N pole and an S pole disposed at opposite ends of the magnet element and has a polarity transition region between the N pole and the S pole, an axially extending groove disposed within the polarity transition region and facing the air gap, and a magnetic flux conductor element disposed on the face of the magnet element facing the air gap so as to extend across the groove.
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Electric Motors & Electronic Motor-Control Techniques; by I.M. Gottlieb; (No Month) 1976; H.W. Sams and Co. Inc.; Bobs-Merrill Co. Inc.; Indianapolis, Kansas City, New York; (p. 17).
Howard & Howard
LaBalle Clayton
Motors Acquisition Corp.
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