Fig. 1,372.—Elementary four pole two phase alternator. The winding consists of one inductor per phase per pole, that is, four inductors per phase, the inductors of each phase being connected in series by the "connectors" and terminating at the collector rings. This arrangement requires four collector rings, giving two independent circuits. The pitch of the inductors of each phase is equal to the pole pitch, and the phase difference is equal to one-half the pole pitch, that is, phase B winding begins at B, a point half-way between inductors A and A' of phase A winding. Hence when the current or pressure in phase A is at a maximum, in the ideal case, when inductor A for instance is under the center of a pole, the current or pressure in B is zero, because B is then half-way between the poles.
Polyphase Alternators.—A multiphase or polyphase alternator is one which delivers two or more alternating currents differing in phase by a definite amount.
For example, if two armatures of the same number of turns each be connected to a shaft at 90 degrees from each other and revolved in a bipolar field, and each terminal be connected to a collector ring, two separate alternating currents, differing in phase by 90 degrees, will be delivered to the external circuit. Thus a two phase alternator will deliver two currents differing in phase by one-quarter of a cycle, and similarly a three phase alternator (the three armatures of which are set 120 degrees from each other) will deliver three currents differing in phase by one-third of a cycle.
In practice, instead of separate armatures for each phase, the several windings are all placed on one armature and in such sequence that the currents are generated with the desired phase difference between them as shown in the elementary diagrams 1,372 and 1,373 for two phase current, and figs. 1,374 and 1,375 for three phase current.
Fig. 1,373.—Developed view of elementary two phase four pole alternator and sine curves, showing the alternating current or pressure generated during one revolution of the armature. The complete winding for the three phases are here visible, the field magnets being represented as transparent so that all of the inductors may be seen. By applying Fleming's rule, as the inductors progress under the poles, the directions and reversals of current are easily determined, as indicated by the sine curves. It will be seen from the curves that four poles give two cycles per revolution. Inductors A, and B are lettered to correspond with fig. 1,372, with which they should be compared.
Ques. What use is made of two and three phase current?
Ans. They are employed rather for power purposes than for lighting, but such systems are often installed for both services.