TAILED STUDY OF OPERATION WITH NORMAL LOAD 47 



Current-limit Circle. It is known that the armature cannot indef- 

 initely withstand a current exceeding a certain limiting value I m . If 

 we draw, around the pole A\ as a center, a circle of radius precisely 

 equal to I m (by the scale of amperes), all the points A^ situated out- 

 side that circle will give values 1=A\A^ !>7 TO ; whereas, inside that circle 

 the value of I will be < 7 m . The circle drawn is therefore a limit 

 beyond which the load-point must not go, for continuous operation. 



Lines of Equal Phase. In the same way that lines of equal power 

 have been drawn, lines of equal current-phase can be easily drawn, 

 such that, at the loads represented by their various points, the phase- 

 angle between the E.M.F. and the current will always be the same. 



If the phase of the current 7 is measured from the E.M.F. EI, 

 these lines are evidently straight lines issuing from the point A\. The 

 right line A\N corresponds to "zero" phase-angle. All the loads to 

 the right thereof correspond to a lag in phase, and all the loads to the 

 left correspond to a lead in phase. (Fig. 27.) 



To obtain lines corresponding to currents of equal phase with 

 respect to the E.M.F. 2, we note that all the points for any current 

 value whatever, among these lines, should present a constant angle 

 OA2Ai. These lines are therefore circles having OA\ as a chord. 

 If, for example, we wish the line of phase corresponding to the lag 

 a between E% and 7, we construct, on OA 1 as a chord, a circle admitting 

 of the angle a f (since the true phase of the current lags by the angle f 



with respect to the line AiA 2 ). 



Only the circle corresponding to an angle a=7t has been drawn, 

 in the example represented in the diagram. This circle represents 

 the exact opposition in phase between the current and the internal 

 E.M.F. E%. It is easily seen that this circle is tangent to the two 

 lines OA T and AN. 



For all loads corresponding to points situated inside this circle, 

 the phase is >TT, outside the circle, on the contrary, it is <TT. 



[It will be noted, in passing, that the circle of zero power 7*2 =, 

 passing through the point A { is nothing more than the circle of phase 



, because the power is zero every time that the current is in quad- 

 rature with the E.M.F.] 



Numerical Example. Let us return to the example of power-trans- 

 mission by means of two Mordey alternators, for which the numerical 

 data have already been given (page 38), and let us apply the second 

 diagram to this case. 



