The sliding effect of a brush is mainly a mechanical problem: permanent contact with the rotating part and at the same time avoiding arc and sparking. The coefficient of friction "µ" is depending on different facts:grade of carbon brush and patina condition
peripheral speed "r" of the slipring body (m/s) - µ is large at low speed and reaches a minimal value at rising speed.
Current density "S" (A/cm2)
It influences µ. If "S" is increased, µ is reduced but will rise again at higher current density.
Temperature of commutator or slipring "t°C".
The friction will be reduced if patina and temperature are increased. At excessive temperature a dry friction is produced and the friction increases. The same stands for temperature and wear.
Carbon brush pressure "p" (KP):
Higher pressure means higher friction. Consequently higher carbon wear, higher heating of the slipring body. If the brush pressure is below its minimum, brush sparking and vibration are caused, i.e. increased wear of brush and slipring material.
Absolute humidity of wear (g water/m3 air):
Controls friction power - patina will reduce and friction power will increase in dry air (from < 3g water/m3 air) or in chemically aggressive atmosphere.
Result: chattering and vibration of brushes - breakage to follow - wear very high.
The optimum values are in between 8 - 15g water/m3 air.
At too greater humidity (e.g. > 25g water/m3 air) interferences are to be expected as well. Results: Difficult patina, spotted patina, grooving, increased wear.
Surface condition of collector/slipring:
It should neither be too rough nor too smooth; should form good patina.
Grade of roughness "Ra".
Should be in between 0,8 µ < Ra < 1,2 µ m; it can be obtained by means of suitable abrasive.
The electrical problem must be divided into 4 categories, in order to explain the selection of a suitable material for brushes, commutator and slipring. In a logic sequence the following table contains the 4 groups of current transforming functions:
|DC- current||AC-current||DC- current||AC-current|
|Exciting rings on||Inductive machines||DC generator||AC serial motors|
|Alternators||Converters||Motors||Single and multiples|
|Synchronous motors||Schrage motors||Converters||Phase AC-motor|
|AC generators||Exciters||Repulsion motors|
Current transformation and slipring size are the only features with slipring machines, the number and dimensions of brushes depend exactly on mechanical and thermal conditions.
In the case of commutation machines, the brushes have to fulfill other functions besides the current transformation.
Each brush covers commutator bars, current flows from bar to bar across the brush contact surface together with the load current.
Commutator and slipring must be smooth and true, poor surface must be turned or ground. Check whether mica is well undercut (1), if motor has recessed mica, carefully remove flakes of mica and rough spots. Ensure the beveled segment edges below 45° (2) or 1/25 of the segment width b (3).
The copper segments must not be high or low. Ignorance will cause brush chatter, sparking or breakage incl. high brush water.
Dimensions and shape of carbon brushes depend mainly on electrical facts (e.g. an improvement is possible if sandwich, twin or triple brushes are used).
The contact voltage drop "Uc" (V) is not of pure omic nature but is influenced by external facts and by the carbon brush material - e.g. specific brush pressure "p" - current density "S" (A/cm2) - peripheral speed "r" (m/s).
Further influences: Absolute humidity of air (g/m3 air), condition of commutator/slipring surface, temperature of the moving parts, gases, vapours, brush holders, polarity and patina.