Fig. 2,155.—Diagram of two phase system with four wire transmission and three wire distribution. In the three wire circuits the relative pressures between conductors are as indicated; that is, the pressure between the two outer wires A and B is 141 volts, when the pressure between each outer wire and the central is 100 volts.
Ques. Describe another method of transmission and distribution with two phase current.
Ans. The current may be transmitted on a four wire circuit and distributed on three wire circuit as in fig. 2,155.
The four wire transmission circuit is evidently equivalent to two independent single phase circuits.
In changing from four to three wires, it is just as well to connect the two outside wires A and B together (fig. 2,152), as it is to connect a´ and b. It makes no difference which two secondary wires are joined together, so long as the other wires of each transformer are connected to the outside wires of the secondary system.
Ques. For what service is the two phase three wire system adapted?
Ans. It is desirable for supplying current of minimum pressure to apparatus in the vicinity of transformers. It is more frequently used in connection with motors operating from the secondaries of the transformers.
Ques. How should the third or common return wire be proportioned?