DESIGN OF ALTERNATORS 



245 



The load may be connected across one, two, or three, phases; 

 but in practice, especially in the case of power circuits, the 

 three-phase load is usually balanced] i.e., each phase winding of 

 the machine provides one-third of the total output. 



(A- 



FIG. 92. Diagram of connections for delta-connected three-phase 

 generator. 



Fig. 92 is a diagram of connections referring to a delta- 

 connected three-phase generator, and Fig. 93 is the correspond- 

 ing vector diagram, showing how the current in the external 

 circuit may be expressed in terms of the armature current. The 

 current leaving the terminal 

 A (Fig. 92) is /i - 7 2 , and 

 since there will be a difference 

 of 120 electrical degrees be- 

 tween the currents I\ and /2, 

 the vector construction of Fig. 

 93 gives 01 as the line cur- 

 rent. Its value is I = 21 a 

 cos 30, or \/3^o, where I a is 

 the current in the armature 

 conductors. The assumptions 

 here made are that the load 

 is balanced and that the cur- 

 rent variations follow the sim- 

 ple harmonic law. It is well 

 to bear in mind that vectors and vector calculations can be used 

 only when the variable quantities follow the sine law ; when used 

 in connection with irregular wave shapes, they must be supposed 

 to represent the "equivalent" sine function, because under no 

 other condition can the phase angle have any definite meaning. 



FIG. 93. Vector diagram of current 

 relations in delta-connected three-phase 

 generator. 



