At the instant when the current a, flowing in at A, is at its maximum, two currents b and c, each one-half the value of a, will flow out B and C, thus producing a negative pole at A and a positive pole at B and at C. The resultant of the latter will be a positive pole at E, and consequently, the magnetic needle will point towards A.
Fig. 1,687.—Production of a rotating magnetic field in a two pole three phase motor. In order to obtain a uniformly rotating magnetic field, it is necessary to arrange the phase windings in the direction of rotation, in the sequence ACB, not ABC as indicated on the magnets. Thus poles 1 and 4 are connected in series to phase A, 2 and 5 in series to phase C, and 3 and 6 in series to phase B. The different phase windings are differently lined, and it should be noted that they have a common return wire, though this is not absolutely necessary. Since the phases of the three currents differ from each other by one-third of a period or cycle, each of the phase windings will therefore set up a field between its poles, which at any instant will differ, both in direction and magnitude, from the fields set up by the other phase windings. Hence, the three phase windings acting together will produce a resultant field, and if plotted out, the directions of this field for various fractions of the period is such that in one complete period the resultant field will make one complete round of the poles in a clockwise direction, as indicated by the curved arrow. The positions of the resultant field during one complete period may be tabulated as follows:
| One Cycle | ||||||
|---|---|---|---|---|---|---|
| 0° to 60° | 60° to 120° | 120° to 180° | 180° to 240° | 240° to 300° | 300° to 360° | |
| Polarity | 1N - 4S | 2N - 5S | 3N - 6S | 4N - 1S | 5N - 2S | 6N - 3S |
As the cycle advances, however, the mutual relations of the fluctuations of the pressures of the three currents, and the time of their reversals of direction will be such, that when a maximum current is flowing at any one of the points A, B, and C, two currents each of one-half the value of the entering current will flow out of the other two points, and when two currents are entering at any two points, a current of maximum value will flow out of the other point. This action will produce one complete rotation of the magnetic field during each cycle of the current.
Fig. 1,688.—Production of three phase rotating magnetic field with winding on laminated iron ring. The winding is divided into twelve sections, which are connected in three groups, A, B, and C, of four sections each, the sections in each group being evenly placed round the ring with the sections of the two other groups between them. One end of each group is to be connected to the line wire and the other end to the common junction J, from which it follows that the winding given is an example of "star" winding. With three phase currents the winding will give at every instant four N poles and four S poles round the ring, and in actual working these poles will be on the inner periphery because of the presence of an inner ring or cylinder of good magnetic iron placed, with the requisite clearance to allow of rotation, as close as is mechanically possible to the outer ring. Each one of these eight poles will make a complete revolution round the ring in four times the periodic time of the currents supplied. Thus, if the supply current has a frequency of 50, a complete revolution of the field will take place in .08 (=4/50) of a second, which corresponds to an angular velocity of 750 revolutions per minute in place of 3,000 revolutions per minute, which would be the angular velocity with a bipolar field at this periodicity. Similarly a continuously wound Gramme ring tapped at twelve points, joined in three groups of four each to the supply mains, would give an eight pole rotary field. In this case the grouping would be a "mesh" grouping, with each side of the mesh formed of four coils in parallel.
