Magnetic material for electromagnetic relays

For the core, yoke and armature of an electromagnetic relay a soft magnetic material is chosen, such as soft iron, low-carbon steel, silicon steel, or Ni-Fe alloys (permalloy) for sensitive relays.

How a relay works

       spring
   +----/\/\--+
   |    armature (soft iron)
   |  ====*===========  contacts
   |  |   ^ pulled when coil on
   |  |  +--+
   |  |  |##| coil on soft-iron core
   +--+--+--+--- yoke

When current flows in the coil, the core is magnetized and pulls the armature, closing (or opening) the contacts. When the current stops, the core must lose its magnetism immediately so the spring can pull the armature back.

Justification

1. High permeability: a small coil current (low power) produces enough flux to pull the armature; fast pick-up.
2. Low coercivity and low retentivity: on switching off, almost no residual magnetism remains, so the armature releases at once and does not stick. A hard material would keep the armature held.
3. Narrow hysteresis loop / low hysteresis loss: suitable for frequent operation and for AC relays; less heating.
4. High saturation flux density: strong pull from a small core.
5. Good mechanical workability and low cost: soft iron is easy to shape into cores and armatures.

A small non-magnetic shim (residual pin) is often fixed on the armature to keep a tiny air gap, further preventing sticking due to any residual magnetism.