42 



COMMERCIAL ELECTRICAL TESTING 



load at which it is desired to make the test. The value 

 of this excitation should be^calculated from the saturation 

 and synchronous impedance curves. With phases A 

 and A' connected together, the voltage across phases 

 b and b' is read, the circuit closed, and the value of the 

 current flowing observed. Knowing the voltage between 

 phases a and b, a! and b' and b and b', the angle of 

 phase displacement may be readily obtained. Should 

 the resulting armature current be considerably greater 

 or less than that desired, a further trial will be necessary. 



Fig. 15. Shifting of Phases Shown Diagrammatically 



The current value will vary nearly as the angle of 

 displacement, so that an approximate value of the angle 

 desired can be found from the value of current and 

 angle previously ascertained. When the value of this angle 

 has been ascertained, the phase displacement should 

 be changed, so as to obtain as closely as possible the 

 desired value of current. With the machines still 

 connected together as they were originally, the angle 

 of phase displacement previously found will be increased 

 120 electrical degrees by connecting a' b. If a' c are 

 connected, a still further displacement of 120 degrees 

 is obtained. If with any of these connections, the 



