POLYPHASE POWER 401 



the possible output would be about 50% greater than as 

 if the single-phase winding were used. 



The single-phase induction motor is more expensive 

 than a three-phase motor of the same capacity, not only 

 because it weighs more but because some special starting 

 devices must be furnished. As pointed out before, the single- 

 phase motor has no starting torque. 



The single-phase converter is practically never used; its 

 capacity is much less than that of a three-phase machine 

 of the same size, and also the single-phase machine has a 

 great tendency to hunt. From the standpoint of safe heat- 

 ing, the three-phase machine would have a capacity more 

 than 50% greater than that of the single-phase machine. 



The synchronous motor is always wound for polyphase 

 connection, not only because its capacity is thereby increased, 

 but, also, because of the possibility of starting the poly- 

 phase machine as an induction motor. The single-phase 

 synchronous motor would necessarily be equipped with 

 some special starting device. 



The efficiency of practically any piece of machinery 

 is greater when designed for polyphase power than when 

 designed for single-phase power. 



Hence we may conclude that polyphase machinery is prefer- 

 able to single-phase machinery because of its less cost and 

 weight and also because of its better starting and operating 

 characteristics. 



128. Grouping of Apparatus on a Three-phase System. 

 When a three-phase generator is used for supplying the 

 current for a lamp load the three-phase system is generally 

 divided into three separate single-phase systems. This 

 is shown in Fig. 263. Three separate feeders are connected 

 to the three-phase line and these feeders supply the neigh- 

 boring districts. 



Balancing of Loads. It is necessary to keep the loads 

 on the three phases nearly equal; when unequal the load 

 is said to be unbalanced. The unbalance in the current 



