152 SYNCHRONOUS MOTORS 



The triangle E\E^E of Fig. 74 is the E.M.F. equivalent of the 

 M.M.F. triangle R\FA\ of Fig. 73; or, if r and x be neglected, it 

 would reduce to the E.M.F. equivalent of the triangle RFA of 



Fig. 73- 



This is the basis of the diagrams of Chapter II, which are essen- 

 tially E.M.F. diagrams. 



The M.M.F. or ampere-turn diagram is similarly obtained from 

 the general diagram by substituting for the E.M.F.'s Ir and Ix their 

 equivalent M.M.F.'s Ni T and Ni x ; i.e., Ni r and Ni x are the M.M.F.'s 

 which would produce the fluxes $ T and $ x , the cutting of which by 

 the armature conductors would induce E.M.F.'s equal and opposite 

 to Ir and Ix (see Fig. 73), where RI and $1 are the M.M.F. and 

 flux corresponding to the impressed E.M.F. E\, and A\ the M.M.F. 



No.2 



FIG. 74. 



corresponding to the E.M.F. 7Z. Thus the M.M.F. rectangle A\R\F 

 is similar and equivalent to the E.M.F. rectangle IZEiEp, and the 

 M.M.F. triangle A\FR\ of Fig. 75 is similar and equivalent to the 

 E.M.F. triangle EE<2,E\ of Fig. 74. 



It is thus evident that with constant impressed E.M.F., constant 

 load and variable excitation, the point B, Figs. 75 and 76, will lie 

 on the arc of the circle as does Az in Fig. 27; but it will be more 

 interesting to go back to fundamental principles and to make this 

 proof independent of that for Fig. 27. 



In any direct-current dynamo-electric machine the electro- 

 magnetic torque is proportional to the product of the ampere-turns 

 on the armature and the flux upon which the armature current reacts. 

 In an alternating current machine the space phase of the armature 



