DIRECT CURRENT GENERATORS 65 



Then total CR 



-\-Ci\Ru 

 Wi = Core loss watts, taken from the core loss curve 



corresponding to VL+CR for each load 

 H 7 2 = Watts brush friction from core loss test. 



If the value taken from test appears inconsistent, 

 calculate W 2 by the formula : 



33000 



F = Circumference of commutator in feet 

 N =R.p.m. 



B = Number of brushes 

 L = Lb. pressure per brush 



ju = Coefficient of brush friction for the particular 

 type of brush used. 



In the case of engine-driven machines or those which 

 are furnished without base, shaft or bearings, the bearing 

 friction is omitted from the total losses, and is charged 

 against the prime mover. 



In nearly every case it is preferable to use the calcu- 

 lated brush friction instead of that obtained from test. 

 During a short test, the commutator and brush contact 

 surfaces cannot get into such good condition as that 

 which obtains after a long period of commercial oper- 

 ation. Consequently, the brush friction test does not 

 represent the conditions that will exist after the machine 

 has been in operation for some time. The coefficient 

 of friction determines the value of brush friction, which 

 in turn is, determined by the condition of the com- 

 mutator and brush contact surface. This coefficient 

 varies considerably at first and only reaches a constant 

 value after a considerable period of operation. The 

 coefficient used in the above formula for the calculation 

 of brush friction was obtained by means of exhaustive 

 tests on carbon brushes of certain types with various 

 pressures and commutators. These tests extended 

 over a long period to obtain constant and satisfactory 

 conditions for both brush and commutator surface. 

 The resulting values of brush friction can, therefore, 



