How to test the noise level of a brushless geared motor?

The noise level of a brushless geared motor can be tested using the following methods:

Test conditions: The motor needs to run for more than 5 minutes to ensure it reaches a stable operating state, avoiding the influence of unstable factors during the startup phase on the test results.

Test distance: When using a decibel meter or other noise measuring instrument, the instrument should be placed 30 cm away from the motor. This distance effectively captures the noise generated by the motor's operation, while avoiding the influence of local sound fields on the measured value due to being too close, or excessive noise attenuation due to being too far, thus failing to accurately reflect the motor's noise level.

Test results: Under the above conditions, the measured decibel value is the noise level of the brushless geared motor. For example, after running for 5 minutes, a brushless planetary geared motor measured a decibel value of 56 dB at a distance of 30 cm.

Sources and reduction methods of brushless geared motor noise:

Noise sources:

Electromagnetic noise: The alternating magnetic field in the air gap causes vibration of the stator, rotor, and the entire motor structure, generating low-frequency noise. The main source of noise is the periodically varying radial force or unbalanced magnetic field generated by the magnetic field (including the fundamental magnetic field and various higher harmonic magnetic fields) in the air gap. This causes hysteresis contraction and vibration in the stator and rotor cores, accounting for approximately 20% of the total motor noise.

Methods to reduce noise:

Improve machining accuracy: Brushless planetary geared motors have complex shapes, with many machined surfaces consisting of planes and holes. These surfaces are mostly assembly reference surfaces, requiring high machining accuracy in dimensional precision, surface roughness, shape, and position. Especially in the machining of the shafts of the bearing holes, dimensional and shape errors can cause poor fit between the bearings and holes, thus affecting the smoothness of motor operation and generating noise. Therefore, improving machining accuracy and ensuring the fit accuracy of each component can effectively reduce noise.