256 



ALTERNATORS. 



average power absorbed by the losses at a speed whose 

 average value was 168 revs, per minute, was therefore 

 545 / (1722- 1642) 



30 



watt8 ' 



From a series of such values a curve was plotted with 

 watts vertical and speeds horizontal, showing the watts lost 

 at various speeds, the watts lost at full speed being then 



18O 



16O 



14O 



= 120 



a 100 



4O 



20 



1234 



Minutes. 

 FIG. 121. RETARDATION CURVES OF AN ALTERNATOR. 



I. Unexcited. 

 II. Excitation 27 amperes. 



III. 

 IV.- 



:M 



If, 



taken from the point where the curve would cut the ordinate 

 corresponding to full speed. It is to be noted that such a 

 set of readings gives the watts in terms of 7, the moment 

 of inertia of the rotating parts. In order to eliminate this 

 unknown factor a separate observation is made to determine 

 the total friction and iron losses at full speed. This is usually 

 done in the manner already alluded to. by running the 

 alternator as a synchronous motor and measuring the power 

 absorbed by means of a wattmeter. This observation then 

 gives the value of the total losses at full speed, and enables the 

 actual value of all the other proportional readings to be 

 assigned. The losses due to iron losses and to friction can 

 thus be obtained at any speed. By taking readings with 



