1996-08-01

Analysis of the Generation Mechanism of Chirp Noises in the Brushes of Blower Motors 961785

This paper describes the self-excited brush vibrations which induce chirp noises in the permanent-magnet direct-current blower motors (150 W) used in automobile air-conditioners.
Based on regeneration of the brush noises and analysis of the brush vibration measurements, we can distinguish two types of brush vibration: one is a high frequency mode, 10 - 20 kHz, which results from coupling the pitching mode and the radial-direction mode of brush; the other is a low frequency mode of the brush which vibrates together with the insulator disk holding the two brushes, 1 - 5 kHz, which results from coupling the yawing mode and the rolling mode. Dynamic models of these two types of vibration and the motion equations are constructed by analyzing the brush vibration measurements, and the mechanisms of generation are confirmed by the eigenvalues and eigenvectors obtained from the motion equations.
We found that the high frequency vibration was caused by the friction between the brush and commutator due to the phase difference between the pitching mode and the radial-direction mode; and that when the rigidity of the insulator disk was insufficient, the low frequency vibration could be caused by the frictional force between the brush and commutator due to the phase difference between the yawing mode and the rolling mode.
These vibrations are self-excited vibrations which occur despite the flat friction coefficient gradient for sliding velocity.

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