214 ALTERNATING CURRENTS 



" V" or open-delta connection, Fig. 204. Under balanced loads, 

 the voltages may become slightly unbalanced. This is not seri- 

 ous in commercial transformers, as their regulation is seldom 

 greater than 2 or 3 per cent. 



At first thought it might appear that the V-connection would 

 have two-thirds the capacity of the delta-connection. Both 

 transformers work at a reduced power-factor when connected in 

 V, even though the power-factor of the load remains fixed. 

 Therefore, the kv-a. capacity of the V-connection is less than 

 two-thirds of the kv-a. capacity of the delta-connection having 

 individual transformers of equal rating. The ratio of the V- 

 capacity to the delta-capacity is l/\/3 = 58 percent, rather than 

 66% per cent. This can be proved as follows: 



Let / be the rated current of each transformer and E the line 

 voltage. The power, at unity power-factor, Fig. 204, is 



P! = V3EI 



As the transformer rating is determined by the current, the 

 output of three of these transformers in delta would be 



P 2 = 3EI 

 Therefore, 



P! V3EI 1 



PT -SET = V or 58 per cent. 



Oftentimes in practice a V-bank of transformers is first in- 

 stalled. The third transformer is added when the increase in 

 load on the system warrants it. The rating of the bank is then 

 increased 73 per cent, with an investment increase of but 50 

 per cent. 



93. The Scott or T-connection. By means of the Scott or 

 T-connection it is possible to transform not only from three-phase 

 to three-phase by means of two transformers, but also from three- 

 phase to two-phase or from two-phase to three-phase. The method 

 of connecting for three-phase to three-phase transformation is 

 shown in Fig. 205 (a) and (6). Two transformers having pri- 

 maries ad and be and secondaries a'd' and b'c' are used. The 

 middle point d of the winding be and d r of the winding b'c' must 

 be accessible. One end d of the primary winding ad is connected 

 to the middle point d of the primary be. The respective ends of 

 the three coils are connected to the three-phase supply abc. The 



