Single-Phase Induction Motors:

        Most home and business appliances operate on single-phase ac
power. For this reason, single-phase ac motors are in widespread use. 
Furthermore, induction motors account for a large percentage of these,
due to their rugged construction, maintenance-free operation, and low
cost.

        A single-phase induction motor operates on the principle of
induction, just as does a three-phase motor. However, in a single-phase
motor, the stator field does not rotate, but instead merely alternates
poles as the single sinewave voltage swings from positive to negative.
Because of this, the stator field remains lined up in one direction
with the poles changing position once each cycle. As a result, a single
phase induction motor will run once it has been started, but it has no
means for starting itself. A true spinning motion is not obtained from
a single-phase motor as it is in a three-phase motor. When a single-phase
motor is first turned on, the rotor may not be aligned perfectly with the 
unidirectional stator field. If not, then a small voltage will be induced
in the rotor, and it will tend to move into alignment. This similar to the
dead center position of a dc motor. Therefore we will call the alignment
position dead center. When the rotor reaches dead center, voltage
is no longer induced because no lines of force are cut in the stator field.
However, when the rotor is already spinning with some speed, inertia
carries it past dead center, just as with a dc machine, and voltage 
continues to be induced in the rotor. In this way, the rotor turns by
being attracted to the continually alternating stator poles.

        A single-phase motor could be started by mechanically spinning
the rotor, and then quickly applying power. However, most of these motors
use some sort of automatic starting. Single-phase induction motors are
often referred to by the starting method used. There are three basic
types, Shaded-Pole, Split-phase induction, and Split-phase capacitor
start Motors.
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