Three-Phase Rotating Fields

The three-phase induction motor also operates on the principle of a rotating magnetic field. The following discussion shows how the stator windings can be connected to a three-phase AC input and has a resultant magnetic field that rotates.

Views A to C in the following figure show the individual windings for each phase and View D shows how the three phases are tied together in a Y-connected stator. The dot in each diagram indicates the common point of the Y-connection. You can see that the individual phase windings are equally spaced around the stator. This places the windings 120° apart.

FIGURE

The three-phase input voltage to the stator of the above figure is shown in the graph of the following figure. Use the left-hand rule for determining the electromagnetic polarity of the poles at any given instant. In applying the rule to the coils, consider that current flows toward the terminal numbers for positive voltages and away from the terminal numbers for negative voltages.

FIGURE

The results of this analysis are shown for voltage points 1 through 7 in the figure. At point 1, the magnetic field in coils 1 -1A is maximum with polarities as shown. At the same time, negative voltages are being felt in the 2-2A and 3-3A windings. These create weaker magnetic fields, which tend to aid the 1-1A field. At point 2, maximum negative voltage is being felt in the 3-3A windings. This creates a strong magnetic field, which is aided by the weaker fields in 1 -1A and 2-2A. As each point on the voltage graph is analysed, it can be seen that the resultant magnetic field is rotating in a clockwise direction. When the three-phase voltage completes one full cycle (point 7), the magnetic field has rotated through 360°.