Fig. 440.—Graph showing the e.m.f. changes of a single-phase current for one "cycle."
440. Three-phase Currents.—Now suppose we have three coils as in Fig. 441, the coils being evenly spaced, or 120 degrees apart, at A, B, and C. If the coils are rotated in a magnetic field, each will produce an electromotive force. The result produced by three such coils is called a three-phase current. Ordinarily six wires, or three circuits, would be required to carry the current produced by three separate coils; for when coil "C" is in the 90 degree position, where its e.m.f. is a maximum, coil "B" is 120 degrees past its maximum, and coil "A" is 240 degrees past its maximum. The graph (Fig. 441) shows the maximum points of the three e.m.f's. separated by intervals of 120 degrees. In practice, however, it is found possible to use three wires instead of six, as explained in Art. 441.
Fig. 441.—Graph showing the e.m.f. changes of a three-phase current for one "cycle."
441. Three-wire Transmission.—The currents produced in the three coils just described undergo precisely the same changes as those represented in the graph (Fig. 441) for the three electromotive forces. Careful examination of the graph will show that at any point the sum of the plus e.m.f's. equals the sum of the minus e.m.f's. In other words the algebraic sum of the three e.m.f's. is zero. Therefore if we properly connect a transmission line of three wires to the generator, the sum of the currents leaving the generator will equal the sum of the currents returning to it. Since the algebraic sum of the currents produced by the three coil combination described in Art. 440 is always zero, it is possible to use three wires on three-phase transmission lines. Fig. 442 shows a "tower" carrying three, three-wire transmission lines. Long distance, high tension transmission lines are generally three-wire lines carrying three-phase a.-c. currents.
Fig. 442.—A "tower" supporting three, three-phase circuits of a high tension transmission line.
442. Alternators.—A dynamo which delivers alternating current is known as an alternator. Commercial alternators have many pairs of poles in the field and as a rule the field rotates while the armature is stationary. The field must be supplied with direct current for the polarity of each coil in the field must remain unchanged. Usually a separate "exciter" is used, which is a small direct current generator. The current from this exciter is fed into the rotating field by means of slip rings. Fig. 439 shows a d.-c. (direct current) exciter on the end of the armature shaft of the large alternator.
