Shading coil

AC relay with shading coil (red arrow near top) 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 of an alternating current solenoid. The alternating current in the energized primary coil induces an alternating current in the shading coil. This induced current creates an auxiliary magnetic flux which is 90 degrees out of phase from the magnetic flux created by the primary coil.<ref>"Figure 2. Sketch of the contactor's electromagnet, dimensions in..." ResearchGate.</ref>

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<ref>"Chegg.com". prod.cheggstudy.prod2.cheggnet.com.</ref> and mechanical destruction of the magnet and power contacts. .

Because of the 90 degree phase difference between the current in the shading coil and the current in the primary coil, the shading coil maintains a magnetic flux and hence a force between the armature and the assembly while the current in the primary coil crosses zero. 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 <ref>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. S2CID 38248177.</ref> of relay or power contacts.<ref>"Shading Ring - CR4 Discussion Thread".</ref>

Shaded-pole AC motors

A shaded pole motor is an AC single phase induction motor. Its includes an auxiliary winding composed of a copper ring called a shading ring (or shading coil with more than one turn).<ref>"Principle of Operation - 德昌电机". www.johnsonelectric.com.</ref>

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.<ref>"Electric motor - Capacitor induction motor". Encyclopedia Britannica.</ref> These devices are typically used as low-cost motors for microwave oven fans.

External links

• The short film AC MOTORS AND GENERATORS (1961) is available for free viewing and download at the Internet Archive.
• The short film AC MOTORS (1969) is available for free viewing and download at the Internet Archive.
• Engineer's Relay Handbook, 5th edition, published by the Relay and Switch Industry Association (RSIA) – formerly NARM
• Information about relays and the Latching Relay circuit
• "Harry Porter's Relay Computer", a computer made out of relays
• "Relay Computer Two", by Jon Stanley