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ELECTRICAL MACHINERY 



differ in phase from each other by 120; this winding is 

 called a three-phase winding, and the system of wires to 

 which such an armature supplies power is called a three- 

 phase system. 



It might seem that a three-phase system must consist 

 of six wires; as a matter of fact, there are never more than 

 three wires. The coils are always so inter-connected in the 

 armature that only three slip-rings are required on the 

 armature and hence only three wires in the three-phase 

 distributing system. 



FIG. 141. Two Ways of Interconnecting the Coils of a, Three-ph* 



Armature. 



Three-phase Y -connection. There are two ways of 

 making this inter-connection of the coils; sometines one is 

 used and sometimes the other, but in either case only three 

 rings are used on the armature and only three wires are 

 used in the outside circuit. Fig. 141 (a) and (6) show the 

 two methods of connecting a three-phase armature. In 

 (a) all of the beginnings of the coils are connected together 

 and the ends of the coils are connected to the three slip- 

 rings. The junction of the beginnings of the coils is not 

 connected to any ring. An armature so connected is said 

 to be Y-connected because a schematic diagram of the 

 winding resembles the capital letter Y, 



