How does a 120VAC coil relay work?

[Steve-R]

Here is an easy one for someone:

Obviously a plain electromagnetic coil will just vibrate with AC voltage. So how does a 120VAC coil relay work? Does it have an internal diode to change to DC, and a capacitor to keep a minimum DC voltage on the coil so it stays shut?

Thanks!

 

[Jraef]

Look up the term "shading coil", that will explain it for you.

Reader's Digest version:
A coil of conductive wire embedded in the pole face of the steel laminations of the armature creates a secondary magnetic field that is expanding and colapsing like the main field, but in a phase shift just behind the main coil. The residual magnetism of this secondary field keeps the electromagnet from buzzing.

 

[GoldDigger]

Look up the term "shading coil", that will explain it for you.

Reader's Digest version:
A coil of conductive wire embedded in the pole face of the steel laminations of the armature creates a secondary magnetic field that is expanding and colapsing like the main field, but in a phase shift just behind the main coil. The residual magnetism of this secondary field keeps the electromagnet from buzzing.

The key point that allows the shading coil to work is that the two magnetic fields generated act on the relay arm at slightly different physical positions, so that the force on the arm will never be zero. If the shading coil affected the entire pole equally, it would not help.
A shading coil can also be used to create a time delay in a DC relay. Telco made heavy use of that property.