The present application is based on and claims priority from Japanese Patent Application 2002-9931, filed Jan. 18, 2002, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ac generator that has a rotor having a plurality of claw-shaped pole pieces and a field coil held inside the pole pieces.
2. Description of the Related Art
It has been found that the magnetic noise of an ac generator is caused by the third harmonic wave of magnetic flux that is distorted during operation. In order to reshape the waveform of the magnetic flux into a sine wave, JP-B2-61-11066 proposes an ac generator in which a rotor has scalene trapezoidal claw-shaped pole pieces. That is, the surface of the pole pieces has a trapezoidal shape that is asymmetrical with respect to the center line of the pole piece that is perpendicular to the rotation direction of the rotor.
JP-B2-3223536 proposes a rotor having claw-shaped pole pieces each of which has a chamfered front surface that is formed at the front of the pole pieces in the rotation direction thereof and a chamfered rear surface that is formed at the rear of the pole pieces in the rotation direction and is larger than the chamfered front surface.
However, it is difficult to form the claw-shaped pole pieces disclosed in JP-B2-61-11066 by forging because they are asymmetrical with respect to the center line, which makes it difficult to evenly apply forging pressure on the entire surface of the pole pieces. As a result, the rotor can not have the pole pieces having an accurate uniform shape.
On the other hand, the ac generator disclosed in JP-B2-3223536 has a problem of a magnetic noise caused by sharp change of magnetic flux between the pole pieces. Because the chamfered rear surface of one pole piece and the chamfered front surface of the next pole piece extend from opposite end of the rotor to face each other, the magnetic flux changes sharply when the rotor rotates, thereby causing magnetic noise. In addition, the output power may be reduced because the chamfered rear surface becomes too large for the pole pieces to conduct effective magnetic flux.
Therefore, an object of the invention is to provide a powerful and noiseless ac generator.
An ac generator according to a feature of the invention includes a rotor having trapezoidal claw-shaped pole pieces that alternately extend in opposite directions to interleave each other, in which each of the pole pieces has a chamfered surface at the tip thereof that faces the inside surface of the stator. The chamfered surface has narrower area at the front half of the pole piece in the rotation direction than the rear half. Because such an asymmetrical chamfered surface is formed at limited area after the pole pieces are forged, the pole pieces having accurate size and shape can be provided. When the rotor with such pole pieces and a field coil is rotated in a predetermined direction to generate power, the magnetic flux sharply increases at the front half of the chamfered surfaces of the pole pieces. On the other hand, the magnetic flux gradually decreases at the rear half of the chamfered surfaces. Therefore, a magnetic wave that is similar to the wave provided by the scalene trapezoidal pole pieces of a prior art ac generator is formed. Such a magnetic wave is opposite in phase to the third harmonic wave, so that the magnetic noise caused by the third harmonic wave is canceled. In addition, because the area of the chamfered surface is comparatively smaller than the prior art, reduction in the output power is negligibly small.
In an ac generator according to another feature of the invention, a rear half surface of each of the pole pieces in the rotation direction has a concavity or a through hole at the tip portion thereof facing the inside surface of the stator core. The concavity or the through hole provides an asymmetrical chamfered surface at a limited area of the pole piece. Because the concavity or the through hole can be easily formed when or after the pole pieces are forged, the pole pieces having accurate size and shape can be provided. This feature brings about substantially the same effects as the above-described ac generator.
In the ac generator described above, each of the pole pieces may have a front chamfered surface and a rear chamfered surface that are approximately symmetrical at the opposite corners thereof. Therefore, there is little change in magnetic flux between the pole pieces. As a result, the magnetic noise caused by change in the magnetic flux is very low.