Shading coil

A shading coil or shading ring (Also called Frager spire or Frager coil) is one or more turns of electrical conductor (usually copper or aluminum) located in the face of the magnet assembly or armature of an alternating current solenoid.

This induced current creates an auxiliary magnetic flux which is 90 degrees out of phase from the magnetic flux created by the primary coil.

Without this shading ring, the armature would tend to open each time the main flux goes through zero and create noise, heat and mechanical damages on the magnet faces, so it reduces bouncing or chatter[3] of relay or power contacts.

[5] The auxiliary winding produces a secondary magnetic flux which, along with the flux from the primary coil, forms a rotating magnetic field suitable for applying torque to and rotating the rotor.

[6] These devices are typically used as low-cost motors for microwave oven fans.

AC relay with shading coil (red arrow near top) for operating with AC
Frager spire or shading coil's purpose is to provide sizeable phase-shifted magnetic field (in blue) to keep the contactor on when the main coil flux (in red) passes through zero, avoiding unwanted chatter [ 2 ] and mechanical destruction of the magnet and power contacts. .