BIPOLAR DIAGRAM 



155 



lort-circuit curve at the point corresponding to the actual armature 

 irrent /. The reason for this last step is obvious when it is recalled 

 lat when the machine in question is operated as generator on short 



circuit, E\ and R\ are zero and Ai = F. This is in fact the definition 



of Ai. 



Equation (5) gives the radii of the constant power circles in 



ampere-turns; but it is somewhat simpler to compute these radii 



from the formula given for the E.M.F. diagram, Eq. (19), Chapter II. 



This formula may be written 



Radius 



(6) 



FIG. 76. 



or in armature amperes, 



Radius (armature amperes) = 



(7) 



Designate by k af the ratio of short-circuit armature amperes to the 

 corresponding field amperes as taken from the short-circuit curve. 

 Then the radius in field amperes is 



Radius (field amperes) = 



2T 



(8) 



