CHAPTER V. 



POWER LOSSES IN GENERATORS AND MOTORS. 

 EFFICIENCY. 



63. Motor and generator losses. The losses of power in a 

 generator or in a motor are : the field loss F t the armature loss 

 A, and the stray power loss 5. The field loss is due to the 

 heat generated in the field windings by the field current, that is, 

 no power is required to excite the field magnet except the power 

 used to overcome the resistance of the field winding. The arma- 

 ture loss, as here defined, is due to the heat generated by the 

 armature current in the brushes, in the brush contacts, and in the 

 armature windings. Field and armature losses can be accurately 

 calculated as explained below. The stray power loss includes 

 eddy current and hysteresis losses, chiefly in the armature core, 

 and losses due to friction in the bearings and at the brushes, and 

 to air friction. Stray power loss cannot be accurately calculated 

 from any simple data. This loss is usually determined by 

 experiment. 



(a) Field loss. A certain amount of power is consumed in 

 maintaining the field current of a dynamo. All this power goes 

 to heat the field coils in accordance with Joule's law, that is, no 

 power would be consumed in field excitation if it were possible 

 to make the field windings of a material having zero resistance. 

 The field loss in watts may be calculated by means of the follow- 

 ing equations. 



( I ) For a shunt dynamo. 



F=R s r (270) 



or 



F=EI (276) 



x 



or 



127 



