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Commutation: Graphite brushes (GB) or Precious metal brushes (EB)

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Urs Kafader
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Properties of brushed DC motor commutation systems

On small DC motors one finds two types of brush systems which are presented here for comparison. (In one motor there is only one type!)

  Graphitb_rste.jpg
  Graphite brushes (GB) 

  Edelmetallbuerste.jpg
  Precious metal brushes (EB)
  • suits better for high motor currents which are found in larger motors
  • also for high and frequent current peaks
  • in servo applications -> typical start-stop operation, reversed operation, positioning
  • not suited for small currents
  • higher friction, higher no-load current
  • higher audible noise
  • higher electromagnetic emissions
  • higher costs
  • less expensive than the graphite commutation
  • easier realized in small dimensions -> smaller motors
  • very low friction -> high efficiency
  • low audible noise
  • low electromagnetic interference
  • small and constant voltage drop over the commutation system
  • restricted ability to take up high motor currents
  • in the smallest motors and in applications which demand only for small currents -> typical: continuous operation at high speeds, e.g. fans

Application fields

The following table gives general tendencies and experience values. Because of the complexity of the involved processes deviations may occur in the concrete application case.

  • GB: graphite brush commutation
  • EBCLL: precious metal brush commutation with CLL

 Criteria,

 Qualification

Graphitb_rste.jpgGB

Edelmetallbuerste.jpgEBCLL
 continuous operation  good life expectancy  good life expectancy
 start-stop operation  suited  reduced life expectancy
 reversed operation  suited but reduced life
 expectancy 		  reduced life expectancy
 relative load current for
 high life expectancy		  higher (50-80% of max.
 continuous current) 		  smaller (<50% of
 max. continuous current)
 absolute load current   high  small
 peak currents  well suited, grace to
 Cu-content in brush		  less suited
 max. winding temperature  125°C (...155°C)
 (glass fiber bondage) 		  85°C
 (scotch bondage) 
 atmosphere (humidity,
 inert gases, vacuum)		  sensitive  not sensitive
 high ambient temperature  sensitive to strongly
 enhanced humidity		  not sensitive
 low ambient temperature  sensitive to strongly
 reduced humidity		  not sensitive
 noise, vibration  louder  quieter
 price  higher  lower

 

Remember

Precious metal commutation

  • in smaller motors
  • for continuous operation at low currents

Graphite commutation

  • in bigger motors
  • for continuous operation at high currents
  • for start-stop and reversed operation

 

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