66 COMMERCIAL ELECTRICAL TESTING 



be relied on to give accurate and final results for that 

 type of brush. 



W 3 = Bearing friction from core loss test 



Wb = Watts output = C L X VL 



The brush contact resistance, R 5 , is that taken from a 

 curve made for the type of brush employed, and corres- 

 ponds to the brush current density per square inch at 

 any given load. 

 Brush current density per square inch = 



c, 



\ total brush area 



^ u 1/4.1. IXwXsXt 



One-half the total brush area = --- - 



fi 



where / = Length of brush parallel to the shaft 

 w = Width of brush 

 5 = Number of studs 

 t = Number of brushes per stud 



For reasons similar to those just given, extensive 

 tests have been made to determine the contact resistance 

 of different types of brushes, from which curves have 

 been plotted with brush current densities as abscissa 

 and either brush contact resistance per square inch or 

 CR drop in brush contact as ordinates. In order to 

 measure the contact resistance directly the commu- 

 tator would have to be short circuited and the voltage 

 drop measured from the commutator to the surface 

 of each brush. This would be a long operation entailing 

 considerable expense. The results also could not be 

 reliable owing to the newness of commutator and 

 brushes. It is therefore preferable to use the brush 

 contact resistance obtained from the curves mentioned. 



If W 3 = bearing friction from core loss test, 

 then total loss in watts = 2W= 



The quantity C G V L -C<?R G = C 2 R loss in the shunt 

 field rheostats. 



The watts input W a will then be 



W a = W b + 2W, where W b = watts output = C L V L 



Wb 

 The efficiency E = - 



