246 PRINCIPLES OF ELECTRICAL DESIGN 



Star Connection of Three-phase Armature Windings. If the 

 starting ends of all the phase windings of a polyphase generator 

 are connected to a common junction, or neutral point, the arma- 

 ture windings are said to be star-connected. In the three- 

 phase machine this is also referred to as the Y connection. 

 The outgoing lines being merely a continuation of the phase 

 windings, it follows that, with a star-connected machine, the 

 line current is exactly the same as the current in the armature 

 windings. The voltage between terminals is, however, no longer 

 the same as the phase voltage, as in the case of the previously 

 considered mesh-connected machine. Referring to the vector 

 diagram Fig. 94, we see that the voltage between lines 1 and 2 

 is Ei E 2 , which leads to the relation E = -\/3E a where E is 



FIG. 94. Vector diagram showing voltage relations in Y-connected three- 

 phase generator. 



the terminal voltage, and E a the phase voltage as measured 

 between any one terminal and the neutral point. 



There is little to be said in regard to the choice of armature 

 connections in a three-phase generator, except that, for the 

 higher pressures, the Y connection has the advantage of a lower 

 voltage per phase winding, and, for heavy current outputs, the 

 A connection has the advantage of a smaller current per phase 

 winding. 



Effect of Star and Delta Connections on Third Harmonic. 

 There is one difference resulting from the method of connecting 

 the phase windings of a three-phase generator which should be 

 mentioned. This has reference to the wave shape of the e.m.f. 

 The wave shape of the terminal voltage is not necessarily the 



