The pressure across the two outer wires A and E is, as in the three wire system, √2 or 1.41 times that from either outer wire to the middle wire C, that is 141 volts.
The pressure across the two wires connected to the middle of the coils, that is, across B and D, is 50 × √2 = 70.5 volts.
Fig. 2,158.—Two phase four wire transmission and five wire distribution system. The relative pressures between the various conductors are indicated in the diagram.
Three Phase Systems.—There are various ways of arranging the circuit for three phase current giving numerous three phase systems.
1. With respect to the number of wires used they may be classified as
- a. Six wire;
- b. Four wire;
- c. Three wire.
Fig. 2,159.—Line connections of three phase three wire long distance transmission, and distribution system. The three phase alternator A, is driven by the water wheel B, and furnishes current at say 2,200 volts plus sufficient pressure to compensate for line drop. With 1:10 step up transformers C, this would give a transmission pressure of 22,000 volts plus line drop. It is this transformation that secures the copper economy of the system. At the distribution end are the step down transformers; one set reducing the voltage down to 2,200 volts, and supplying current direct to the synchronous motor, and through another set of other step down transformers, as L and K, to lighting and power circuits at 220 volts. Another set of step down transformers M reduce the pressure directly to 120 volts for power and lighting, the pressure being regulated by the regulators G. Arc lamps with individual transformers further reducing the pressure to 50 volts are connected to this circuit as shown.