The brush holder, as an impressive but important component, acts as fixing the carbon brush in DC motor and AC motor with its brush, which can ensure the motor work normally.
What does the brush holder look like?
The brush holder is generally comprised of metallic cover (copper alloy or stainless) and pressure spring. The metallic cover remains a precise "box" or "tube" to place the carbon brush; and, the spring continuously pressures the force from the top to the bottom to "push" the carbon brush to the surface of rotating commutator (or the slip ring)
Simply, the brush holder just looks like a clamp that can not only firmly hold the carbon brush but also closely connect with rotating metallic surface.
How does the brush holder work?
In conclusion, the operation principle of the brush holder can summarize the following three points:
1. Fixing and Guiding--to ensure the carbon brush not shaking
If we need the carbon brush to freely slide within its brush holder frame along with its axial direction without any shaking, its precise gap will be keep within 0.1-0.3mm between the inner cavity of the brush holder and its carbon brush. If too insufficient gap, the carbon brush will be jammed; and, if too excessive gap, the carbon brush will shake in the process of its operating, causing spark or even erosion. Therefore, the precision of the brush holder will directly affect its motor operation stability.
2. Constant Pressure Contact-smooth current flowing
Once running, the current needs to be transmitted to its rotating commutator from the carbon brush. And, the spring on the top of the brush holder should continuously force the pressure (normally 15-25 kPa), ensuring close and constant contact between the carbon brush and the commutator. With the wear of the carbon brush, its length will be shortened and its spring will be gradually spreaded to compensate. That's why the brush holder should be required with enough travel space.
3. Heat Dissipation Protection-not to let the heat to destroy anything
When the carbon brush connects with the commutator, the friction will be generated to cause friction heat and arc heat. At that time, the metallic cover of the brush holder can be regarded as a radiator to let the heat out through the carbon brush, protecting the carbon brush against fragmentation or oxidation speeding up due to its heat. And, some types of the brush holders are designed with its ventilating slot or fins for the purpose of lifting its heat dissipation efficiency.
Why the carbon brush will be worn, but the brush holder will not?
The reason is that the carbon brush is a quick-wear part to meet the requirement for its design. The carbon brush is selected graphite-based materials with lower hardness than that of the metallic commutator. If worn, the engineer just needs to replace a new carbon brush, which can reduce the maintenance cost than to replace overall rotator. And, the brush holder, as a bearing structure, is generally designed with wear-resistant metallic materials with a longer designed life than that of the carbon brush.
In conclusion, in spite of small structure, the brush holder is regarded as the footstone for reliable parameters for the motor. With the three main mechanisms of precise cooperation, constant pressure contact and heat dissipation protection, the carbon brush can stably and continuously transmit the current within its high-speed operating. When you use the electric tool next time, you can imagine that small component hidden in the deep position of the motor, finishing its millisecond-level-accuracy mission.
Generally, the service life of the carbon brush remains 500-3000 hour that is suitable for different motor types, specific load and conditions and the environment as well. When the carbon brush length is worn to its 1/3 to 1/2 original one, the spring pressure will be insufficient and respectively its contact resistance will be increased, needing to be replaced.
