156 SYNCHRONOUS MOTORS 



When P 2 = O the circle passes through and A, and the corre- 

 sponding radius is, 



n> 



Radius (field amperes, P2 = 0)= . =CO. 



This together with Ri determines the isosceles triangle of Fig. 76, 

 although the base angles 6 are the same as for the E.M.F. diagram 

 (Fig. 27) and can be computed from r and Z, the latter being deter- 

 mined in the usual manner from the saturation and short-circuit 

 curves. The two methods of constructing this diagram give the 

 same results when the value of Z employed is that corresponding to 

 the point E\ on the saturation curve. 



Thus the ampere-turn diagram is completely and easily deter- 

 mined by the same information required by the E.M.F. diagram; 

 namely, the armature resistance, the saturation and short-circuit 

 curves, and the impressed E.M.F. 



In the case of the polyphase machine, Pi, E\, and r should desig- 

 nate the per-phase values. 



The ampere- turn or M.M.F. diagram has several advantages over 

 the E.M.F. diagram. 



First, the substitution of an equivalent M.M.F. for the leakage 

 impedance E.M.F., involves less error than the substitution of an 

 equivalent E.M.F. for the armature M.M.F., because in the common 

 type of synchronous motor the armature M.M.F. is large as com- 

 pared with the leakage impedance E.M.F. 



Second, hi determining the excitation or phase characteristics 

 (Fig. 79) by means of the M.M.F. diagram, F is given directly 

 in field amperes, the quantity actually measured, and A\ is 

 given in equivalent field amperes, for which the corresponding armature 

 current can be taken directly from the short-circuit curve, which is 

 usually a straight line; whereas when taken from the E.M.F. diagram 

 the results are in terms of armature current and of 2, the latter 

 being a hypothetical E.M.F., which would be induced if there were 

 no armature reaction, and assumed constant for constant excitation. 



Third, the M.M.F. method gives a clearer picture of the actual 

 phenomena. E.g., if all the constants refer to two similar machines 

 (generator and motor) together with the connecting line, and if 

 these machines are of the revolving field type, R\ and F are respect- 

 ively the field M.M.F.'s of generator and motor, and their relative 



