Mechanical Systems 383
per phase is the same as that of the DC stepping motor shown in Figure
14-11. Refer now to Figure 14-12. In the diagram shown, poles N1-53 and
N5-57 represent one phase, while poles N2-54 and N6-58 represent the
second phase. There are places for 48 teeth around the inside of the stator.
One tooth per pole has been eliminated, however, to provide a space for
the windings. Five teeth per pole, or a total of 40 teeth, are formed on the
stator. The four coils of each phase are connected in series to achieve the
correct polarity.
The rotor of the synchronous motor is an axially magnetized perma-
nent magnet. There are 50 teeth cast into its form. The front section of the
rotor has one polarity, while the back section has the opposite polarity. The
physical difference in the number of stator teeth (40) and rotor teeth (50)
means that only two teeth of each part can be properly aligned simultane-
ously. With one section of the rotor being a north pole and the other sec-
tion being a south pole, the rotor has the ability to stop very quickly. It can
also produce complete direction reversals without hesitation, because of
this gear-like construction.
A circuit diagram of a single-phase synchronous motor is shown in
Figure 14-24. The resistor and capacitor of this circuit are used to pro-
duce a 90° phase shift in one winding. As a result, the two windings are
always out of phase, regardless of whether the switch is in the clockwise
(cw) or counterclockwise (ccw) position. When power is applied, the four
coils of one phase produce an electromagnetic field. The rotor is attracted
and aligns itself to these stator coils. Then, 90° later, the four coils of the
second phase produce a corresponding field. The stator is again attracted
to this position. As a result of this action, the rotor “sees” a moving force
across first one phase and then the other. This force gives the rotor theFigure 14-24. Circuit diagram of a single-phase synchronous motor