Ques. What voltages are ordinarily used on two phase lines of more than moderate length?
Ans. For transmission distances of more than two or three miles, pressures of from 1,000 to 2,000 volts or more are employed to economize in copper. For long distance transmission of over fifty miles, from 30,000 to 100,000 volts and over are used.
Ques. For long distance transmission at 30,000 to 40,000 volts, what additional apparatus is necessary?
Ans. Step up and step down transformers.
Fig. 2,153.—Diagram illustrating two phase three wire transmission. The third wire C is attached to the connector between one end of phase A, and phase B windings.
Ques. Explain the method of transmitting two phase current with three wires.
Ans. The connections at the alternator are very simple as shown in fig. 2,152. One end of each phase winding is connected by the brushes a and b', to one of the circuit wires, that is to A and B respectively. The other end of each phase winding is connected by a lead across brushes a' and b, to which the third wire C is joined.
The current and pressure conditions of this system are represented diagrammatically in fig. 2,153. The letters correspond to those in fig. 2,152, with which it should be compared.
As shown in the figure each coil is carrying 100 amperes at 1,000 volts pressure. Since the phase difference between the two coils is 90°, the voltage between A and B is √2 = 1.414 times that between either A or B and the common return wire C.