Carbon brush and brush materials-electrical carbon (2)

How carbon brushes work?

What is commutator film: you can see Fig.3-oxide layer

Fig.3-oxide layer

Fig.3-oxide layer

To enhance film formation, the commutator should be turned to a surface roughness of 35 to 60 uin. A film will not form properly if the commutator surface is too smooth.

A thin, uniform film having a brown/grey patina is desired.

  • Overfilming- a thick, black film will increase contact drop and cause excess spaking and high electrical wear.
  • Underfilming- a very thin or non-existent film will result in high frictional wear and may degrade commutation.

Base filming characteristics is normally controlled by the brush grade.

Contact drop vs current density:

What is brush wear?

What is electrical wear?

fig.4-friction vs moisture

fig.4-friction vs moisture

  • Voltage pulses cause rapid increases in temperature at the conducting spots in the brush.
  • Small pieces of the brush are expelled, causing wear.
  • Higher voltages cause an electric arc to form, causing extensive damage to brush and commutator. The contact spots are heated to 2500-3000°

Wear rate increases with current density.

Frictional wear:

  • Shearing of graphite during sliding of brush face over commutator.
  • Rapid during initial film formation.
  • Greatly increased under some atmosphere and temperature conditions.
  • Higher for high-metal content brushes having little graphite.

Both electrical and friction wear are increased by high commutator bar-to-bar profile and high TIR.

Effect of atmosphere on friction of graphite, see fig.4-friction vs moisture.

Low temperature:

Thermal stability of copper graphite brush is key to long life and stable performance.

fig.5-brush face temperature

fig.5-brush face temperature

Under low temperature, it is difficult to build good film between brush and commutator surface, which caused fast mechanical friction; at the same time, the sparkle condition is bad.

Low temperature- low humidity- brush wear rapidly

Low temperature means low humidity, low humidity means the sub-pressure of water vapour is to low that the H2O molecular complete desorption from film.

High coefficient of friction- wear quickly.

High humidity- over filming.

 

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CFC CARBON CO., LTD/ HTMAGROUP
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