METHODS OF TESTING ALTERNATORS 



273 



reaction, since the angle a is known experimentally, and the values 

 of U and / can be consequently measured. This gives 



^ lJ B=wL 1 /=7tan-, 



2 



from which L' is known as a function of a, U and I. 



The same method permits varying the angle a successively, and 

 repeating the operation, commencing each time with the same voltage 

 at the terminals U, and thus tracing the entire characteristic of an 

 alternator operating upon a dead resistance. 



The above method gives immediately the values of the direct 

 and transverse armature reactions. As to the coefficient of self-induction 

 of the stray field cos, it may be determined for any given alternator 

 by the method indicated later on. 



A test may then be made of the two alternators coupled together, 

 without any angular difference of phase between them. The e.m.f.'s. EI 

 and 2 are then in simple opposition of phase, and the difference EI E% 

 will produce a resultant current which may be regulated in strength 

 by regulating the difference of excitation, and which current is dephased 

 by nearly 90. The diagram is given in Fig. 2 1 , where OC\ and OC<i 



O 



FIG. 21. 



are the two internal e.m.f.'s. The difference C\C<z represents the 

 fall of potential due to the impedance of the circuit of the armatures, 

 and which can be decomposed into two rectangular straight lines, 

 C\F representing the total ohmic drop (Ri +-#2)-^ due to the current, 

 and C<zF the total reactive drop in the armatures. Projecting F upon 

 C\Ci, a vector C^F' is obtained, which differs but little from C^Ci, 

 and which represents the fall of potential of the two machines due to 

 direct reaction. If the characteristic of total excitation be then drawn 

 as in Fig. 22, this drop will represent the sum of the two drops due 

 to the reactive current, of which one CC\ is positive and the other CC 2 



