378 Electrical Power Systems Technology
nent capacitor motor. Its circuit is shown in Figure 14-21. This motor has
no centrifugal switch, so its capacitor is permanently connected into the
circuit. These motors are only used for very low-torque requirements, and
are made in small fractional-horsepower-size units.
Both split-phase motors and capacitor motors may have their direc-
tion of rotation reversed easily. Simply change the relationship of the start
winding and the run winding. When either the start winding connections
or the run winding connections (but not both) are reversed, the rotational
direction will be reversed.
Shaded-pole Induction Motors—Another method of producing torque
by a simulated two-phase method is called pole shading. These motors
are used for very low-torque applications, such as fans and blower units.
They are low-cost, rugged, and reliable motors that ordinarily come in low
horsepower ratings, from 1/3000 to 1/30 hp, with some exceptions.
The operational principle of a shaded-pole motor is shown in Figure
14-22. The single-phase alternation shown is for discussion purposes only.
The dotted lines represent induced voltage into the shaded section of the
field poles. Note the shading coils in the upper right and lower left of the
two poles. The shaded pole is encircled by a heavy copper conductor and
is actually a part of the main field pole. This closed-loop conductor will
cause current to be induced into the shaded pole when AC is applied to
the field.
When an AC voltage is applied to the stator windings, the magnetic
Figure 14-20. Voltage/current relationships in a capacitor-start, single-phase in-
duction motor