POWER LOSSES IN GENERATORS AND MOTORS. 129 



the term RJ?. The field loss of a series dynamo (generator or 

 motor) varies greatly with the load. 



(ft) Armature loss. The power losses in the armature of a 

 dynamo include the losses in the armature core due to eddy 

 currents and hysteresis. These core losses are, however, usu- 

 ally included under the term " stray power" loss. By armature 

 loss is here meant simply the power which is consumed in the 

 armature windings in accordance with Joule's law, together with 

 the power lost at the brushes on account of contact resistance. 

 The armature loss in watts may be calculated from the equation : 



A = RJ: (29) 



in which R a is the resistance of the armature from positive to 

 negative brushes ; R a includes the resistance (hot) of the arma- 

 ture windings, the contact resistances of the brushes, and the 

 resistances of the brushes themselves ; and I a is the total cur- 

 rent leaving the armature at the positive brushes, or entering the 

 armature at the negative brushes. 



Armature loss varies greatly with the current output of the 

 generator, and with the amount of load on a motor. 



(c) Stray power loss. This term includes all losses * which 

 cannot be satisfactorily calculated from simple data. These 

 losses are 



(1) Eddy current and hysteresis losses, chiefly in the armature 

 core, due to reversals of magnetization as the armature rotates. 

 Considerable eddy current loss may occur in the massive copper 

 conductors of a bar-wound armature is explained in Art. 32. 



(2) Friction losses in the bearings and at the brushes, and 



(3) Air friction loss, or windage, as it is called, due to the fan- 

 like action of the rotating armature. 



These losses cannot be separately measured with accuracy. 

 The stray power loss which is the sum of these losses can, how- 

 ever, be quite accurately determined for a given machine by an 

 experimental test. 



*That is, all losses except RI* losses. 

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