Ques. How are they employed in each case?
Ans. For lighting purposes the phases are isolated in separate circuits, that is, each is used as a single phase current. For driving motors the circuits are combined.
Fig. 1,374.—Elementary four pole three phase alternator. There are three sets of inductors, each set connected in series and spaced on the drum with respect to each other two-thirds pole pitch apart. As shown, six collector rings are used, but on actual three phase machines only three rings are employed, as previously explained. The inductors have distinctive coverings for the different phases. The arrows indicate the direction in which the induced pressures tend to cause currents.
Ques. Why are they combined for power purposes?
Ans. On account of the difficulty encountered in starting a motor with single phase current.
Ferarris, of Italy, in 1888 discovered the important principle of the production of a rotating magnetic field by means of two or more
Fig. 1,375.—Elementary four pole three-phase alternator and sine curves showing current or pressure conditions for one revolution. Six collector rings are shown giving three independent circuits. The pitch of the inductors for each phase is the same as the pole pitch, and the phase difference is equal to two-thirds of the pole pitch, giving the sequence of current or pressure waves as indicated by the sine curves. The waves follow each other at ⅓ period, that is, the phase difference is 120 degrees. Inductors A, B, and C, the beginning of each phase winding, are lettered to correspond with fig. 1,374, with which they should be compared.