To collect current from a rotating commutator or to feed current to it use is made of brush-gear which consists of:
ii. Brush holders
iii. Brush studs or brush-holder arms
iv. Brush rocker
v. Current-collecting busbars.
Brushes. The brushes used for D.C. machines are divided into five classes:
(i) Metal graphite
(ii) Carbon graphite
- The allowable current density at the brush contact varies from 5 A/cm2 in case of carbon to 23 A/cm2 in case of copper.
- The use of copper brushes is made for machines designed for large currents at low voltages. Unless, very carefully lubricated, they cut the commutator very quickly and in any case, the wear is rapid. Graphite and carbon graphite brushes are self-lubricating and, are, therefore, widely used. Even with the softest brushes, however, there is a gradual wearing away of the commutator, and if mica between the commutator segments does not wear down so rapidly as the segments do, the high mica will cause the brushes to make poor
contact with segments, and sparking will result and consequent damage to commutator.
So to prevent this, the mica is frequently ‘undercut’ to a level below the commutator surface by means of a narrow milling cutter.
Brush holders. Box type brush holders are used in all ordinary D.C. machines. A box type brush holder is shown in Fig. 14. At the outer end of the arm, a brush box, open at top and bottom is attached. The brush is pressed on to the commutator by a clock
spring. The pressure can be adjusted by a lever arrangement provided with the spring. The brush is connected to a flexible conductor called pig tail. The
flexible conductor may be attached to the brush by a screw or may be soldered.
- The bush boxes are usually made of bronze casting or sheet brass. In low voltage D.C. machines where the commutation conditions are easy galvanised steel box may be used.
- Some manufacturers use individual brush holders while others use multiple holders, i.e., a number of single boxes built up into one long assembly.
Brush rockers. Brush holders are fixed to brush rockers with bolts. The brush rocker is arranged concentrically round the commutator. Cast iron is usually, used for brush rockers.