4-6
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 have
a resultant magnetic field that rotates.
Figure 4-4, views A-C show the individual windings for each phase. Figure 4-4, 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 120º apart.
Figure 4-4.—Three-phase, Y-connected stator.
The three-phase input voltage to the stator of figure 4-4 is shown in the graph of figure 4-5. Use the
left-hand rule for determining the electromagnetic polarity of the poles at any given instant. In applying
the rule to the coils in figure 4-4, consider that current flows toward the terminal numbers for positive
voltages, and away from the terminal numbers for negative voltages.
