Electric motor

Details Electric motor Electric motor

1999-03-19

2001-04-24

LaBalle, Clayton (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S156030, C310S261100

Reexamination Certificate

active

06222288

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an electric motor comprising a stator and a rotor, one of the two parts being a slotted armature, with a number of slots equal to R, the other part comprising a number 2P of magnetic poles.
In electric motors, the advantage of the slots, from the magnetic point of view, is that the paths of the magnetic field through the air are shortened because they can enter the cogs of the armature directly from the pole pieces across a relatively small air gap and, from the mechanical point of view, is that the windings placed in the slots are prevented from moving laterally by the flanks of the cogs delimiting the slots.
However, slotted armatures have a drawback known as the “cogging torque” brought about by the interaction between the magnets and the slots of the armature.
This cogging torque and a solution to moderate it have been described, for example, in the article by Messrs Ackermann, Janssen, Sottek and van Steen published in IEE PROCEEDINGS-B vol. 139, No 4, July 1002, pages 315 to 320 and entitled “New technique for reducing cogging torque in a class of brushless DC motors”. In this article, the measures proposed for reducing the cogging torque in permanent-magnet brushless motors, if the number of magnetic poles is fairly close to the number of armature slots, consist in carefully adjusting the size of the poles or the size of the slots.
In order to reduce this disadvantageous effect, it is also known practice, in motors with a long rotor and a radial air gap, to have on the shaft of the motor several magnets juxtaposed axially but slightly offset from one another in the peripheral direction.
SUMMARY OF THE INVENTION
The object of the invention is to propose another measure for modifying the variation in magnetic flux as a function of the angle of rotation, in such a way that the fluctuations caused by the presence of the slots are minimized.
For this purpose, the electric motor according to the invention as described in the preamble of claim
1
, is one wherein, of the 2P angular gaps between the centers of the polar faces of two adjacent magnetic poles, (2P−1) gaps are each equal either to
a
1
=(360°/2
P
)−
b
  (1)
or to
a
2
=(360°/2
P
)+
b
  (2)
where the angular distance b has the value:
b=
360°/(2
PR
)  (3),
R being the number of slots.
Thus, instead of positioning the magnetic poles in the customary manner with a uniform distribution with all the gaps equal, namely with polar spacings equal to 360°/2P, (2P−1) gaps are reduced by the constant angle b or, alternatively, they are increased by this constant angle b in such a way that the remaining gap is, applying formula (I):
c
1
=360°−(2
P−
1)
a
1
  (4),
or, by applying formula (2):
c
2
=360°−(2
P−
1)
a
2
  (5)
This remaining gap is therefore either larger or smaller than the others. In the case of a motor according to formula (2), the motor must of course be designed in such a way that the remaining gap c
2
leaves enough space for the two magnetic poles which limit this gap c
2
to be fitted satisfactorily.
The solution proposed by the present invention makes it possible to reduce the fluctuations in magnetic flux as a function of the angle of rotation because in this way it is possible to avoid all the magnetic poles passing by the slots simultaneously.


REFERENCES:
patent: 4700098 (1987-10-01), Kawashima
patent: 4769567 (1988-09-01), Tuneya et al.
patent: 5396137 (1995-03-01), Masayuki et al.
patent: 6025666 (2000-02-01), Kliman
patent: 41 24 425 (1992-01-01), None
patent: 5-137304 (1993-01-01), None
patent: 6-133616 (1994-05-01), None
International Search Report in SN 0745/98 CH.

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