I 



DIAGRAM FOR COUPLED SYNCHRONOUS MOTORS 169 



internal e.m.f.'s generated in the two alternators by the direct flux, 

 i.e., the flux along the field axis. Let Si and 82 represent these 

 two e.m.f.'s, and let them be drawn as ordinates on the total charac- 

 teristic curve (Fig. 85). The corresponding abscissas om\, and om,2, 

 represent the excitation ampere-turns necessary to generate these 

 e.m.f.'s at no load. We add to them, respectively, the counter 

 ampere-turns of the armature, m\n\ and W22, obtained, respectively, 

 oy projecting (Fig. 84) the vector A I on the perpendiculars to OA i 

 and OAz, since these projections represent the reactive components 

 of the armature ampere-turns. They are 





"V/2 



in which NI and A T 2 represent the numbers of peripheral armature- 

 conductors per pair of poles in the two machines, and K is the utili- 

 zation-coefficient for the particular winding. We thus obtain the 

 e.m.f.'s EI and 2; but these are not yet altogether exact, because we 

 have neglected the small increases of excitation which are necessary to 

 compensate for the increase in saturation of the field magnet cores 

 resulting from the increase in magnetic leakage between the pole- 

 pieces. The correction necessary is very easily made, if the permeance 

 Bj, of the leakage path is known and if the excitation-characteristic of 

 the magnetic field cores alone (OQ) has been drawn, as shown in Fig. 

 85, by plotting it reversed, to the left of the axis of ordinates, taking 

 the excitation ampere-turns as abscissas and the magnetic flux 

 through the magnets and yoke as ordinates. This characteristic curve 

 should be drawn according to a scale of ordinates such that the 

 magnetic fluxes may be represented by the electromotive forces 

 which they would generate in the armature if they were to thread 

 through it. Let @ be the angle of the tangent to the curve OQ 

 at the point c. Let us now draw at MI an angle bM\a = v., whose 

 tangent represents the permeance Bj. of the magnetic leakage path. 

 Then the Vertical segment ab, intercepted on the line N\n\, repre- 

 sents the additional magnetic leakage flux. Let us draw two hori- 

 zontal lines ac and bd, and take their intersection with the field- 

 characteristic (at the left in Fig. 85), and let us then find the cor- 



tan a 



responding abscissas, p\q\. The segment P\qi = . 7,m\n\ represents 



tan (j 



and measures the supplementary ampere-turns required. Let us 



