The connections for a three phase motor using two transformers is shown in fig. 2,021 and is identical with the previous arrangement, except that one transformer is left out and the other two made correspondingly larger.
The copper required in any three wire three phase circuit for a given power and loss is 75 per cent. that necessary with the two wire single phase or four wire two phase system having the same voltage between lines.
Fig. 2,020.—Three phase motor transformer connections; the so-called Delta connected transformers.
The connections of three transformers for a low tension system of distribution by the four wire three phase system are shown in fig. 2,022. The three transformers have their primaries joined in delta connection and the secondaries in "Y" connection. The three upper lines of the secondary are the three main three phase lines, and the lowest line is the common neutral.
Fig. 2,021.—Three phase motor connections using two transformers.
The voltage across the main conductors is 200 volts, while that between either of them and the neutral is 115 volts; 200 volt motors should be joined to the mains while 115 volt lamps are connected between the mains and neutral. The arrangement is similar to the Edison three wire system and the neutral carries current only when the lamp load is unbalanced.
The voltage between the mains should be used in calculating the size of wires, and the size of the neutral wire should be made in proportion to each of the main conductors that the lighting load is to the total load.
