Handbook of Electrical Installation Practice - BeKnowledge

Rotating Machines 425
Brushgear
Machines fitted with slip rings or a commutator have brushgear. To be effective the
slip rings or commutator must be in gook condition and free from surface defects.
Slip rings which are badly scored need to be skimmed in a lathe. For removal of
minor grooves a commutator stone can be used, rested on a brush arm as the rotor
is slowly driven round. Finally, the surface can be polished using a piece of 00 grade
glasspaper wrapped around the square end of a piece of wood as the rotor is turned
by hand.
A similar technique can be applied to commutators but ‘stoning’ is a dangerous
operation which should be carried out only by someone experienced in this
work. A commutator which is in good condition should have developed a smooth
brown surface patina. When this low friction patina is present the commutator
surface should not be stoned or polished. Raised commutator bars or mica are indicated
by heavy and uneven blackening and then skimming and undercutting are
required. When the commutator is uniformly black, cleaning and polishing should
be considered.
Longitudinal undercutting of the mica insulating between commutator segments
is always necessary after skimming in a lathe (Fig. 15.25). Although the bars of the
commutator should be chamfered lightly, the bar width must not be reduced.
It is sometimes impossible to achieve the brown surface patina which provides
long brush life. The cause is usually light load running and insufficient brush current
density. So often margins are built into the specified machine rating that light loadings
are inevitable. However, where this is the case, it is worth considering the
removal of some of the brushes. An equal number of brushes may be removed from
the same track on each brush arm so increasing the current density of the remaining
brushes. The machine manufacturer is always willing to give advice as to the
number of brushes required per arm for the actual loadings on site.
Inspection of the brushgear will show up excessive or uneven brush wear and the
presence of carbon dust. All carbon dust should be vacuumed out of the machine
at each inspection. The brushes should be checked for wear and for freedom of
movement in the brush holder. Although brushes may be allowed to wear down
almost to the metal of the pigtail, it is worthwhile replacing them much earlier. If
replaced when they are only reduced 50% in length the brushes are most unlikely
to require attention, or cause an unscheduled plant stoppage, before the next
routine inspection. Many d.c. motor manufacturers now offer brush wear detection
Fig. 15.25 Correct and incorrect undercutting of the mica.