Shading coil

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AC relay with shading coil (arrow head) for operating with AC

A shading coil or Shading ring (Also called Frager spire or Frager coil) is a single (or a few) turn of electrical conductor (usually copper or aluminum) located in the face of the magnet assembly or armature. The main alternating magnetic flux induces currents in the Frager coil. So, these induced currents create an auxiliary magnetic flux. This auxiliary flux is out of phase from the main flux[1].

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. .

The property of property allows the armature sealed-in when the main flux falls to zero (e. 100 (or 120) times / second with 50 (or 60) cycles AC).

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 (e.g. reducing the bouncing[3] of relay or power contacts and the chatter)[4]

Shaded-pole AC motors[edit]

A Shaded Pole motor is an AC single phase induction motor. The auxiliary winding, is composed of a copper ring, called a shading ring (or shading coil when more shorted turns)[5].

Shaded-pole motors are provided with a main winding connected to the single-phase electric supply. In addition, that kind of AC motors have a permanently short-circuited winding, known as a shading coil and located ahead of the main winding in the direction of rotation. This shading coil consists of one (e.g. shading ring) or more (e.g. shading coil) shorted turns. The shading coil allows to delay the establishment of magnetic flux in the region that it encircles and producing thereby a small component of rotating field at standstill[6].


  1. ^ "Figure 2. Sketch of the contactor's electromagnet, dimensions in..." ResearchGate.
  2. ^ "".
  3. ^ Riba, J. R.; Espinosa, A. G.; Cusido, J.; Ortega, J. A.; Romeral, L. (2008). "Design of Shading Coils for Minimizing the Contact Bouncing of AC Contactors". 2008 Proceedings of the 54th IEEE Holm Conference on Electrical Contacts. pp. 130–136. doi:10.1109/HOLM.2008.ECP.34. ISBN 978-1-4244-1901-2.
  4. ^
  5. ^ "Principle of Operation - 德昌电机".
  6. ^ "Electric motor - Capacitor induction motor". Encyclopedia Britannica.

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