404 ELECTRICAL MACHINERY 



Special Cases. In addition to the four possible connec- 

 tions for three transformers given above, it is possible to 

 use only two transformers. In the A-A connection for 

 instance, the transformation would be a true three-phase 

 one, even though one of the transformers is removed. 



This open A, or " V " connection, gives balanced three- 

 phase voltages, but two transformers so connected have a 

 capacity of only 57% of the capacity of three transformers 

 connected in A. If, for example, two 1000 kv-a. trans- 

 formers were connected in V, they would have a combined 

 capacity (sometimes called " group " capacity) of only 

 1730 kw.; the addition of a third 1000 kv-a. trans- 

 former would raise the group capacity to 3000 kw. 



In addition to the few simple connections shown, there 

 are many more complicated groupings possible. With one 

 scheme, called the " double A," it is possible to operate a 

 six-phase, synchronous converter from a three-phase supply 

 line. Thus the efficiency of the three-phase transmission 

 is combined with the increased capacity of a six-phase 

 converter. 



130. Metering Power in a Polyphase Circuit. In the 

 polyphase circuit the power and power factor cannot be 

 obtained as easily as for the single-phase circuit. In 

 general, if a polyphase circuit consists of n wires, it is 

 necessary to have n-l wattmeter readings in order to deter- 

 mine the power being supplied by the circuit. The con- 

 nection of the wattmeters for a three-phase circuit is shown 

 in Fig. 265. If the load is balanced and its power factor 

 is unity, the two wattmeters will read alike, but if the load 

 is unbalanced or the power factor is less than unity the 

 two meters will read differently. In any case, whatever 

 the balance and power factor, the sum of the two watt- 

 meter readings gives the true value of the power. 



Circuits with Low-power Factor. In metering three- 

 phase power, however, caution must be observed as one 

 meter may be reading negatively. This occurs if the 



