PARTS OF A DYNAMO-ELECTRIC MACHINE 63 



of the contact becomes high, hence the PR loss at this 

 place becomes too great and the brush will overheat. If 

 too much pressure is exerted by the brush on the com- 

 mutator, the power used up due to mechanical friction of 

 the brushes becomes too great and the brushes and com- 

 mutator will get hot from this cause. It has been found 

 that with carbon brushes the best results are obtained when 

 the springs are adjusted to give a brush pressure of about 1.4 

 Ibs. per sq.in. of contact surface; this value may, however, 

 be anywhere between 1 Ib. and 5 Ibs., depending upon the 

 brush, speed, etc. 



Resistance of Brush Contact. The resistance of the 

 contact surface of a carbon brush and commutator is a 

 variable depending upon the current density at the contact 

 surface. As the current density increases the resistance 

 decreases; the variation of the resistance with the current 

 takes place in such a manner that the IR drop at the contact 

 surface is nearly constant, <ni<l not dependent upon the current. 

 While this IR drop is slightly different with different types 

 of brushes and with the different grades of carbon employed, 

 it is safe to assume that on the average c-c. machine, 

 with the commutator in good condition, the drop is one 

 volt per brush contact. As then 1 are always two brush 

 contacts in series the total brush contact resistance drop in 

 any c-c. machine is about two volts. 



Brushes on Low-voltage Machines. It is because of this 

 contact resistance drop that carbon brushes are never 

 used on very low-voltage machines. We shall show in a 

 later chapter that the efficiency of any electric generator 

 must be less than the ratio of the terminal voltage to the 

 generated voltage. 



The terminal voltage = E -IR a , (16) 



where E ff = the generated voltage of the machine; 



IR a = the total " drop " in the armature circuit (always 

 greater than the brush contact resistance drop) ; 



