At the 180° point of the cycle, b reverses in direction and produces a negative pole at D, and as the fluctuation of the pressure of the two currents during the second half of the cycle, from 180° to 360°, bear the same relation to each other as during the first half, the resultant poles of the rotating magnetic field thus produced carry the needle around in continuous rotation so long as the two phase current traverses the windings of the ring.
Fig. 1,684.—Moving picture method of showing motion of a rotary magnetic field. A number of sheets of paper are prepared, each containing a drawing of the motor frame and a magnetic needle in successively advancing angular positions, indicating resultant directions of the magnetism. The sheets are bound together so that the axis of the needle on each sheet coincides. When passing the sheets in one way the revolving field will be seen to rotate in one direction, while, when moving the sheets backward, the rotation of the magnetic field is in the opposite direction, showing that the reversal of the order of the coils has the effect of reversing the rotation of the magnetic field.
Production of Rotating Magnetic Field by Three Phase Current.—A rotating magnetic field is produced by the action of a three phase current in a manner quite similar to the action of a two phase current. Fig. 1,685 shows a ring suitably wound and supplied with a three phase current at three points A, B, C, 120° of a cycle apart.
Fig. 1,685.—Production of a rotating magnetic field by three phase current. A ring wound as shown is tapped at points A, B, and C, 120° apart, and connected with leads to a three phase alternator. As described on page [1,304], a rotating magnetic field is produced in a manner similar to the two phase method.
Fig. 1,686.—Diagram of three phase, four pole Y connected field winding.