214 ELECTRICAL ENGINEERING 



across them. This loss cannot be calculated but its presence is 

 shown by the heating of solid pole faces. To reduce it the pole 

 faces in direct-current machines should always be laminated. 



(f) The brush friction loss in foot pounds er second is equal to 

 the product of the total brush pressure in pounds, the peripheral 

 speed of the commutator in feet per second and the coefficient of 

 friction between the brush and the commutator. With carbon 

 brushes the pressure should be from 1.5 to 2 Ibs. per square inch; 

 this value multiplied by the area of all the brushes gives the total 

 brush pressure in pounds. In railway motors, where there is a 

 great deal of vibration, pressures up to 5 Ibs. per square inch are 

 used in order to insure good contact. The coefficient of friction 

 between a carbon brush and the commutator is about 0.3. The 

 brush friction loss varies directly as the speed but is independent 

 of the load. 



(g) The journal friction loss increases as the (speed)! but is 

 independent of the load. 



(h) The windage loss depends on the shapes of the rotating 

 parts of the machine and cannot be accurately determined. It 

 varies almost as the cube of the speed but is usually very small. 



The losses may be divided into two groups, the constant losses 

 and the variable losses. 



The constant losses are those which do not vary to any great 

 extent under load and include the shunt-field copper loss, the iron 

 losses and the friction and windage losses. 



The variable losses are those which increase with load, namely, 

 the armature copper loss and the series-field copper loss. 



All the losses in a machine appear as heat and raise the tem- 

 perature of the various parts. 



131. Efficiency. The efficiency of a machine may be va- 

 riously expressed as, 



output irkr . 



T) = - - 100 per cent 

 input 



output 



100 per cent 



output + losses 



input losses 



= - - 100 per cent. . . . (223) 



input 



The efficiency varies with the output ; at light loads it is low on 

 account of the constant losses; between f load and full load it is 



