130 ELEMENTS OF ELECTRICAL ENGINEERING. 



When a dynamo runs at constant speed with constant field 

 excitation, the stray power loss is approximately constant, irre- 

 spective of the current output of the machine if it is acting as a 

 generator, and irrespective of the load on the machine if it is act- 

 ing as a . motor. Therefore the stray power loss of a shunt 

 generator driven at constant speed or of a shunt motor supplied 

 from constant voltage mains, is approximately constant irrespec- 

 tive of the load on the machine. The stray power loss of a series 

 generator or motor varies greatly with its load. 



It is often desired to find the stray power S", in a dynamo 

 (generator or motor), when run at such speed and field excita- 

 tion as to give an armature voltage E a ", having given the experi- 

 mentally determined value S r of the stray power loss of the 

 dynamo when run at such speed and field excitation as to give an 

 armature voltage a f . Now the various component parts of the 

 stray power loss are very different functions of field excitation 

 and speed, and the dependence of 5 upon field excitation and 

 speed cannot be accurately represented by any simple formula. 

 For small variations of field excitation and speed, however, it is 

 sufficiently exact to assume that the stray power loss is propor- 

 tional to the electromotive force induced in the armature, from 

 which we obtain the equation : 



s" = s'|v (30) 



64. Experimental determination of stray power loss. The 



stray power loss of a dynamo cannot be satisfactorily calculated 

 from any easily obtainable data. The simplest method for deter- 

 mining the stray power loss of a dynamo at a prescribed speed 

 and prescribed degree of field excitation, is to observe the power 

 P required to drive the dynamo as a motor, without any load, at 

 the prescribed speed, and with its field excited to the prescribed 

 degree. This power P exceeds the stray power loss 5 by a 

 very small amount, which can be calculated as explained 

 below. 



