| 摘要 |
PROBLEM TO BE SOLVED: To lower unpleasant resonance sound and make it possible to prevent the emission of the resonance sound in another frequency range by providing air exhaust opening of a fan with which the diagonal line pitch of a joint device is not covered. SOLUTION: A plurality of the air exhaust holes 4 for cooling air driven by a fan incorporated inside a self-ventilated motor are provided only in half the outside circumference of a cylindrical frame 1. If the aperture length of the air holes is longer than the diagonal line pitch of a joint device, a phenomenon of resonance occurs and emits unpleasant resonance sound when the wavelength or half-wave length of air ripples in rotation agrees with the aperture length of the air holes. To prevent this, these air holes 4 are arranged so that air holesγ1,γ2 come nearly or the diagonally line and the diagonal line pitchβ1 of the quadrangle formed by the jointing device is not covered with a joint and a bending plate. This structure makes it possible to prevent the emission of the unpleasant resonance sound.
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