2-B] EFFICIENCY. 53 



gives the iron losses (hysteresis and eddy current) of the generator. 

 (3) The brushes of the generator are lifted, the diminution in motor 

 input giving brush friction. (4) The belt is next thrown off, the 

 diminution in motor input now giving the generator journal friction, 

 windage and the belt loss. 



The iron losses may be found for various excitations at normal 

 speed. These losses should be determined for an increasing excita- 

 tion; the losses with a decreasing excitation would be more. 



For obtaining iron losses alone, this method with the machine 

 driven mechanically is better than the method (7) with the ma- 

 chine driven electrically; for it gives iron losses directly, separate 

 from friction, and it is not necessary to go through any separation 

 of losses as in 20. This avoids error due to extrapolation and 

 makes no assumption that friction and windage are directly propor- 

 tional to speed. 



On account of belt tension, journal friction will be more than in 

 the no-load test with the belt off (7). Belt losses are also included 

 with friction and windage. This may sometimes be desirable, since 

 it is the usual condition of operation. In a test of the motor per se, 

 these losses ought not to be included, but they cannot be simply sepa- 

 rated (243). 



If the loss found by lifting the brushes is more when the machine 

 is excited than when not excited, the brushes are not in the neutral 

 position, thus causing additional loss by current circulating through 

 an armature coil and brush. 



If it is desired to separate the iron losses into components, hyster- 

 esis loss and eddy current loss, runs are made with varying speed and 

 a constant excitation for each run. For each run plot iron-loss -f- 5* 

 as a straight line, similar to ac in Fig. 2. For any speed, the 

 product be X S gives watts eddy current loss for the particular ex- 

 citation ; db X gives watts hysteresis loss. 



negligible, it should be taken into account. Belt loss cancels out and 

 does not enter into the determination of iron losses or brush loss. 





