Shaded -Pole Motors
A shaded-pole induction motor is an indu ction molor with only a main winding.
Instead of having an auxiliary winding, it has salie nt poles, and one portion of
each pole is surrounded by a short-circuited coil called a shading coil (see Figure
1O- 22a). A time-varying flux is induced in Ihe poles by the main winding. When
the pole flux varies, it induces a voltage and a curre nt in the shading coil which
opposes Ihe original change in flUX.1llis opposition retards the flux changes under
the shaded portions of the coils and therefore produces a slighl imbalance between
Ihe two oppositely rotating stator magnetic field s. TIle net rolation is in Ihe
direction from the unshaded to the shaded portion of the pole face. The
torque-speed characteristic of a shaded-po le molar is shown in Figure IO- 22b.
Shaded poles produce less starting torq ue than any other type of induction
motor starting system. TIley are much less effi cie nt and have a much higher slip