HOBART AND PUNGA'S METHOD 



179 



At,'; i in, the full-load iron losses amounting to Wi, it follows that 

 since the duration of the open-circuit runs is yT, the iron losses 



//' * 



* 



luring these runs must be * if the average iron losses during the test 



t/ 



air to be equal to the normal iron losses at full load. We thus find, 

 considering the open-circuit runs 



excitation or field copper loss 

 iron loss 



mm l\ 

 " *' 



/- 1 \u>c 

 ' V u*'Wi 



y 



Now a, w e , and w { being known, the right-hand member of the 

 above equation represents a known quantity. In order, therefore, to 

 find the excitation and iron losses during the open-circuit runs, we 

 take the curve connecting these losses (Fig. 125), and from the 



OPEM-CIRCt/IT IRON LOSS B 



FIG. 125. Relation connecting Iron with Excitation Loss. 



origin draw a line making an angle with the horizontal axis (the axis 

 of open-circuit iron loss) whose tangent is equal to f 1 --- J ^ It 



is obvious that the intersection A of this straight line with the 

 curve of losses gives us a point on the curve the ratio of whose 

 ordinate to its abscissa satisfies the above equation. Thus we find 

 OB = iron loss during open-circuit runs, BA = excitation loss 



1/0' 



during open-circuit runs, y = -, and the other quantities immedi- 



ately follow. 



As regards the actual values of t\ and fa, we may make ti -f fa 

 anything between 10 and 20 minutes, say. If, for example, we find 

 y = 0'6, and we make ti -f fa = 15, the duration of each open-circuit 

 inn will be 9 minutes, and that of each short-circuit run 6 minutes. 



As in other heat tests, the most reliable method of ascertaining 

 the rise of temperature is the increase of resistance method ( 93). 



