As the cycle continues, however, the strength of a will diminish and that of b increase, thus shifting the induced pole toward C, until b attains its maximum and a falls to its minimum at 90° or the end of the first quarter of the cycle, when the needle will point toward C. At 90°, the phase a current reverses in direction and produces a negative pole at B, and as its strength increases from 90° to the 180° point of the cycle, and that of phase b diminishes, the resultant negative pole is shifted past C toward B, until a attains its maximum and b falls to its minimum at 180°, and the needle points in the direction of B.
Fig. 1,663.—Diagram of two phase, eight pole field winding. The winding is divided into 16 groups (equal to the product of the number of poles multiplied by the number of phases). Each group such as at A comprises a number of coils in series, each coil being located in a separate pair of slots, the end of one being connected to the beginning of the next. When the currents are in the same direction, the currents circulate in the same direction in two adjacent groups, a pole then with this arrangement being formed by two groups, both phases contributing to the formation of the pole. After ½ cycle when the current in each phase reverses, the pole advances the angular distance, covered by two groups; hence the field completes one revolution in eight alternations of current.
Figs. 1,664 to 1,683.—Sine curves of two phase current and diagrams showing the physical conception of a two phase rotating magnetic field. The alternating magnetizing current is assumed to be of such strength that, at its maximum strength, the field produced may be represented by 10 lines of force as indicated by the parallel lines. At the beginning of the rotation, fig. 1,664, phase A magnetization, according to sine curve is zero, indicated by the solid black poles, while phase B is of strength 10 with
current in the direction to produce a south pole at B. Similarly, in fig. 1,665, the strength of A is 4 lines, and of B, 9 lines, the resultant magnetization having rotated 22½°. The direction of the resultant magnetization is indicated by the arrow in each figure. It should be noted in fig. 1,669, that the polarity of B is reversed, the current curve now being above the zero line. By following the arrow through the successive positions the rotation of the resultant magnetization is clearly seen.