402 ELECTRICAL MACHINERY 



taken by the different feeders results in more or less unbal- 

 ancing in the voltages of the three phases and thus unsatis- 

 factory illumination results. 



If the load is unbalanced, the voltage cannot be regulated 

 by such a device as the Tirrill regulator; this can work 

 on one phase only and hence the voltage can be maintained 

 constant on one phase only. 



In grouping the apparatus on the three-phase system 

 it is not only necessary to connect the same capacity of 

 apparatus on each phase but the grouping must be done 

 in such a way that the load is balanced at all times. It 

 is not advisable to run single-phase motors from one phase 



Lamps 



tips Lamps 



1 LL 



TT 



Generate* Lampg or8mall Larg; motor 

 induction motors 



Fia. 263. A Single Three-phase Distribution System. 



of a three-phase system; too much unbalancing results 

 unless the motor is very small. 



129. Polyphase Power Transformation. It is not possi- 

 ble to transform a single-phase system into a polyphase 

 system by means of ordinary transformers, but it is possible 

 to transform one polyphase system into another, also to 

 raise or lower the voltage of a given system with or without 

 changing the number of phases. 



Two-phase to Three-phase Transformation. The trans- 

 formation of a two-phase system into a three-phase system 

 (or vice versa) is accomplished by two transformers grouped 

 according to the Scott connection. Fig. 264 shows how 

 the connection is made. Transformer A has a ratio of 

 1 to 1, and transformer B has a ratio of 1 to 0.866, so that the 

 voltage 1-4 is only 86.6% of the voltage 2-3. If it is desired 



