180 PRINCIPLES OF ELECTRICAL DESIGN 



per square inch between contact surfaces has been assumed in 

 order to avoid the plotting of a large number of curves. For 

 high-voltage machines, the harder quality of carbon will gener- 

 ally be found most suitable. For low voltages, economy may 

 frequently be effected by using the graphitic brushes with cur- 

 rent densities as high as 60 amp. to the square inch of contact 

 surface. It is an interesting, but not very clearly explained, 

 fact that the temperature rise of the negative brushes is greater 

 than that of the positive brushes. In other words, the watts 

 lost are greater when the current flow according to the popular 

 conception is from carbon to copper, than when it is from 

 copper to carbon. The resistances given in Fig. 68 have been 

 averaged for the + and brushes. 



On low-voltage dynamos, when the current to be collected is 

 very large, copper brushes must be used. The resistance of 

 the contact between brush and commutator is then much lower 

 than with carbon brushes and the current density may be as 

 high as 200 amp. per square inch of contact surface. The con- 

 tact-surface resistance may be anything between 0.0007 and 

 0.0028 ohm per square inch, a safe figure for the purpose of 

 calculating the brush losses being 0.002 ohm. If the current 

 density at the contact surface is 150 amp. per square inch (a 

 very common value), the total loss of pressure at the brushes 

 will be 0.004 X 150 = 0.6 volts, instead of about 2 volts, which 

 is usual with carbon brushes. 



The degree of hardness of the copper used for the commutator 

 segments is a matter of importance; an occasional soft bar will 

 invariably lead to sparking troubles because of unequal wear. 

 A perfectly true cylindrical commutator surface is essential 

 to sparkless running. The different sets of brushes should be 

 " staggered" in order to cover the whole surface of the commu- 

 tator and so prevent the formation of grooves. For the same 

 reason, and also to ensure more even wear of the journals and 

 bearings, some end play should be allowed to the shaft. In 

 large machines it is not uncommon to provide some device, in 

 the form of an electromagnet with automatically controlled 

 exciting coil, to ensure that the desirable longitudinal motion 

 of the rotating parts shall be obtained. 



Owing to the hardness of the mica insulation relatively to that 

 of the copper bars, there is a tendency for the mica to project 

 slightly above the surface of the copper. This naturally leads 



