Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-06-13
2003-05-27
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S156190, C310S156380, C310S156430, C310S156210, C310S156010
Reexamination Certificate
active
06570288
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an outer-rotor-type brushless direct current motor in which an annular outer rotor having a plurality of permanent magnets is disposed radially outside a stator so as to be rotatable.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 11-215749 discloses an outer-rotor-type brushless direct current motor in which an outer rotor having a plurality of permanent magnets placed in the circumferential direction is disposed radially outside a stator so as to be rotatable. This outer rotor type brushless direct current motor reduces noise by recessing the two end sections in the circumferential direction of each of the permanent magnets of the outer rotor, that is to say, the radially inner periphery of the end sections of adjacent permanent magnets that face each other, so as to make the cogging torque waveform almost sinusoidal.
SUMMARY OF THE INVENTION
In order to increase the maximum torque in an outer rotor type brushless direct current motor, it is a shortcut to increase the size of the permanent magnets, but this causes the external dimensions of the outer rotor to increase accordingly. In order to reduce the external dimensions without reducing the maximum torque, high performance rare earth magnets are used as the permanent magnets instead of ordinary ferrite magnets, or a material with a high magnetic flux density is used for the cores. Reduction in the size of the air gap between the outer rotor and the stator is also considered to be effective.
However, the use of rare earth magnets and the use of a high magnetic flux density material for the cores increase the cost. Further, when using ferrite magnets, if a narrow air gap is set, the performance is degraded at low temperatures as a result of a decrease in magnetization.
The present invention has been achieved in view of the above-mentioned circumstances, and it is an object of the present invention to reduce the external dimensions of an outer rotor type brushless direct current motor without reducing its maximum torque.
In order to achieve the above-mentioned object, an outer rotor type brushless direct current motor of the present invention includes a stator fixed so as to surround the outer periphery of a rotating shaft, and an outer rotor arranged so as to surround the outer periphery of the stator and able to rotate around the rotating shaft, the outer rotor being formed from an annular yoke and a plurality of permanent magnets attached along the inner periphery of the yoke so that the N magnetic poles face the S magnetic poles, wherein the radially outer periphery of the two end sections in the circumferential direction of each of the permanent magnets is reduced in thickness so as to form thin sections, and magnetic flux passage sections of the yoke providing connection between the thin sections of adjacent permanent magnets are increased in thickness.
In accordance with the above-mentioned arrangement, because the outer periphery of the two end sections in the circumferential direction of the permanent magnet is reduced in thickness so as to form thin sections, the volume of the permanent magnets can be reduced, thereby reducing cost. Because the magnetic flux passage sections of the yoke which provide connection between the thin sections of adjacent permanent magnets are made thick, the cross-section of the magnetic path in the magnetic flux passage sections is widened, thereby suppressing the occurrence of magnetic saturation and preventing a decrease in maximum torque. Moreover, because the magnetic flux passage sections of the yoke are expanded radially inwards, the external dimensions of the yoke do not increase. Furthermore, because the thick and thin sections of the permanent magnets are arranged along the circumferential direction, the cogging torque can be reduced, thereby reducing noise during operation. Because it is unnecessary to reduce the air gap in order to guarantee the maximum torque, there is no possibility of the performance being degraded at low temperature due to a decrease in the magnetization.
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Hosoda Masaharu
Kaizuka Masaaki
Okuda Kazuma
Takahashi Kazunari
Elkassabgi H. Y. M.
Honda Giken Kogyo Kabushiki Kaisha
Ramirez Nestor
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