178 ALTERNATING CURRENTS 



as to make the average values of all the losses equal to those which 

 occur at full load. 



The short-circuit curve of the alternator being supposed known, and 

 being assumed to be a straight line, we can find from it the short- 

 circuit current which would be obtained with the normal full-load 

 excitation. Let the ratio of this short-circuit current to the normal 

 full-load current be denoted by o. If we reduce the exciting current 



to - of its full-load value, then (the short-circuit curve being a straight 



line) the short-circuit current will become equal to the normal 

 full-load current. Thus in order to produce a short-circuit current 

 equal to the normal full-load current, the exciting current must be 



made equal to - times its full-load value. 

 a 



t\ duration of open-circuit runs 



Let ?/ = - - = - -iTi-;? n~r i so that 1 ?/ = 



ti + t 2 total time of test 



duration of short-circuit runs , . . . , , . , 



- - , then it is evident that n the average 

 total time of test 



armature copper watts during the test are to be equal to the actual 

 full-load armature copper watts, the short-circuit current during the 



short-circuit runs must equal -7, ------ times the full-load current. 



V 1 y 



Hence the exciting current corresponding to this short-circuit current 



must be / . - times the full-load exciting current, so that the 

 VI - y 



exciting watts = - . - 2 times the full-load exciting watts 



""* & ^ 



_ - _ w e . If T = total duration of test, in hours, then the watt- 

 ' 



hours dissipated in the field coils during the short-circuit runs are 



But we must make the total watt-hours dissipated in the field coils 

 equal to those which would be dissipated under full-load conditions, 

 i.e. to w e T. Thus the difference 



m 1Verr\ ,. I 1 Yp 



^ e T - - Z T W e \l - ^ 



must correspond to the watt-hours dissipated in the field coils during 

 the open-circuit runs, and since the duration of these runs is yT 

 hours, the field watts during these runs must be 



X 1 " 



