Shaded-Pole Motors:

        A shaded-pole motor is so  named because it uses a small shorted
Shading Coil wound in a small notch in the stator pole piece. In
some motors this coil consists of a single copper ring or copper band.
When the electromagnetic field builds up around the main coil, the flux
cuts across the conductors of the shading coil. Since the shading coil
is shorted, current flows which produces a field Opposite that of
main field. The main field is then strongest on the side away from the 
shading coil. However, the field through the shading coil reaches maximum
intensity much later - at a time when the main field is already decreasing.
The electromagnetic field in the pole piece then appears to be stronger on
the side nearest (or through) the shading coil. This produces a sweeping
motion from side to side in the stator pole piece, although small, this
motion is enough to maintain an induced voltage in the rotor and start
the rotor turning.
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