58 ELECTRICAL MACHINERY 



brushes would not serve at all. For machines of voltages 

 of 100 and more carbon brushes must always be used. 



Contact Area of Brushes. The brushes and commutator 

 form a moving contact surface across which all of the 

 current which flows from the armature to the external 

 circuit must pass. To keep this contact surface in good 

 condition, i.e., smooth and free from dirt, requires more 

 care than any other task connected with the operation 

 of dynamo-electric machinery. The area of the brush 

 where it comes in contact with the commutator surface 

 is called the contact area of the brush. 



Safe Current. The current which can be carried safely 

 by a square inch of contact area depends upon the two 

 materials forming the contacting surfaces. If a copper 

 brush is bearing on the commutator surface and the brush 

 has one sq.in. of contact area, 150 to 200 amperes may 

 safely be carried from the commutator by the brush; if 

 the brush is made of carbon and has one sq.in. of contact 

 area, not more than 40-60 amperes may be safely carried. 

 If these values are exceeded, the brush and commutator 

 will get too hot. 



If either the commutator or brush surface becomes 

 rough, the contact area is very much diminished. If a 

 piece of dirt works into the contact surface of a brush 

 and so makes a projection on this surface, it is evident 

 that the brush will touch the commutator only at this 

 projecting place unless the brush is very flexible. 



Flexibility of Brushes. A brush made of copper leaves 

 or wires is very flexible while one made of a carbon block 

 is not flexible at all. So that if a slight projection on the 

 commutator lifts off from the commutator one part of 

 a copper brush contact surface, the rest of the brush may 

 stay in contact with the commutator owing to its flexi- 

 bility. But if one corner of a carbon brush is lifted from 

 the commutator, the whole brush leaves the commutator 

 surface and the circuit is practically opened. 



