In previous posts, we briefly discussed vibration caused by rotor factors. Today, Ms. will summarize the vibrations caused by end cover axial vibration and brushes. In actual manufacturing processes, vibration and noise are two problems that go hand in hand, especially mechanical noise, the root cause of which is vibration. In other words, controlling mechanical noise should start with eliminating mechanical vibration.
Axial vibration and noise of end cap
Axial vibration of the end cover is one of the sources of mechanical noise, mainly excited by bearing vibration, and is particularly important in small motors. The smaller the axial dynamic stiffness of the end cover, the easier it is to generate larger vibration velocities and noise.
Vibration and noise of the brush assembly
Vibration and noise in the brush assembly are caused by structural and technological factors such as poor commutator surface condition, excessive gap between the brush and brush holder, insufficient brush pressure or improper application of pressure causing brush misalignment, and insufficient rigidity of the brush holder, brush frame and brush rod.
The sliding contact condition of the brushes and commutator during DC motor operation, as well as the cuprous oxide film formed on the commutator surface and the graphite film and dust particles covering it, not only affect the commutation performance of the motor, but also affect vibration and noise.
Practice has shown that under no-load conditions, the aforementioned thin film is difficult to form due to the low temperature of the sliding contact surface, and the dry friction between the brushes and the commutator increases noise. For example, when the no-load operation cycle of a rolling mill DC motor accounts for more than 50% of the total time, the noise under no-load conditions is 6-10 dB higher than under load. This vibration is different from the vibration caused by mechanical reasons on the commutator surface, which can be detected when the motor is running at low speed (a vibration can be felt by lightly touching the brush). The frequency spectrum of brush vibration noise generated by sliding contact is generally in the range of 1000~-8000Hz, and it does not change much with the change of motor speed, which is one of the characteristics that distinguishes it from mechanical causes.
The vibration noise generated by sliding contact brushes is also related to brush polarity. For example, the positive brushes of a DC generator vibrate less than the negative brushes because the positive brushes can separate graphite and carbon crystals, absorbing moisture on the commutator surface to form a lubricating film, while the negative brushes remove this. The formation of the sliding contact film is also related to the brush grade. The selection of the grade should first ensure good commutation performance, but also take into account the brush vibration and noise.
Besides mechanical noise, aerodynamic noise is also a major challenge in motor noise control, especially for high-speed motors, where the problem is more concentrated. In the following post, Ms. will share some of the content with everyone.
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