210 POLYPHASE CURRENTS. [Exp. 



capacity for the same power than apparatus in which currents 

 and voltages are in phase. 



26. A Comparison. In comparing the relative advantages of 

 transformer-connections, it is to be borne in mind that three 

 transformers (even though of somewhat smaller aggregate 

 capacity) will usually cost more than two. The F-connection 

 gives the least voltage per transformer and the least insulation 

 strain, particularly if the neutral is grounded; for this reason it 

 is to be preferred on high potential lines, say, 20,000 volts or 

 over. On the other hand, the delta-connection has the advan- 

 tage that, if one transformer breaks down, the remaining two 

 will operate F-connected; at moderate voltages (say, under 

 20,000 volts) the delta-connection is accordingly to be preferred. 



In the delta-connection, if one transformer breaks down, each 

 remaining transformer will have instead of J of the whole 

 power and will have to carry the line current instead of the 

 delta current. By what per centages are current and power in 

 each transformer thus increased? This increase would cause 

 abnormal heating. For the same heating (same current) show 

 that the two transformers F-connected will carry 57$ per cent, 

 as much load as the three delta-connected transformers. 



With transformers delta-connected, the voltage of the system 

 can be increased by using the same transformers F-connected. 

 In a new system, the delta-connection is sometimes installed with 

 a view to changing later to a F-connection and a higher voltage. 



A single 3-phase transformer requires less material than three 

 single-phase transformers of the same aggregate capacity, and 

 is more efficient. (See Handbooks.) The three single-phase 

 transformers may be cheaper or more readily obtained because 

 more nearly standard, and in case of breakdown one third and 

 not all the equipment needs be replaced; in other respects the 

 single 3-phase transformer is preferable and is coming more and 

 more into use. 



27. Six-Phase Circuits. A 6-phase circuit is a 6-wire cir- 



