Shading coil

Lore & Background

The shading coil is a simple yet effective device used in alternating current solenoids and relays. It consists of one or more turns of copper or aluminum placed on the face of the magnet assembly, encircling only part of the magnetic path. When the primary coil is energized with alternating current, it induces a current in the shading coil, which in turn produces an auxiliary magnetic flux that is 90 degrees out of phase with the primary flux. This phase shift ensures that a magnetic flux and thus a force between the armature and the assembly is maintained even when the primary current crosses zero. Without the shading ring, the armature would tend to open each time the main flux goes through zero, causing noise, heat, and mechanical damage to the magnet faces. The shading coil thus reduces bouncing or chatter of relay or power contacts.

In the context of shaded-pole AC motors, the shading coil serves as an auxiliary winding composed of a copper ring (or multiple turns). This auxiliary winding produces a secondary magnetic flux that, together with the flux from the primary coil, forms a rotating magnetic field suitable for applying torque to and rotating the rotor. These motors are typically used as low-cost motors for microwave oven fans.

Reader's Guide

The shading coil is significant primarily for its function in alternating current solenoids and relays, where it prevents the armature from opening during zero crossings of the primary current. This reduces noise, heat, and mechanical damage to the magnet faces, and minimizes bouncing or chatter of relay or power contacts. The article notes that without the shading ring, the armature would tend to open each time the main flux goes through zero, leading to these undesirable effects. In shaded-pole AC motors, the shading coil acts as an auxiliary winding that helps create a rotating magnetic field, enabling the motor to apply torque to the rotor. These motors are described as low-cost devices, with a typical application being microwave oven fans. The legacy of the shading coil lies in its simplicity and effectiveness in improving the reliability and quiet operation of AC solenoids and low-cost induction motors, as described in the source article.

Did You Know?

• A shading coil is also called a Frager spire or Frager coil.
• The induced current in the shading coil creates an auxiliary magnetic flux 90 degrees out of phase from the primary flux.
• Without a shading ring, the armature of an AC solenoid would tend to open each time the main flux goes through zero.
• Shaded-pole AC motors with shading rings are used as low-cost motors for microwave oven fans.