Contactor Coil
The electromagnet winding that pulls the contactor's moving core and closes the power contacts. In a radio remote control installation this is usually the load the receiver relay drives directly, and because of its inductive character it must be assessed under category AC-15 or DC-13 when sizing the contact.
A coil operates at two different current levels. At the moment of pull-in the air gap is large, so the inductance is low and the current rises to roughly 10 times the holding value. Once the core has seated, the current falls to the holding level. What matters when sizing the relay contact is the pull-in value.
In contactor catalogues, coil data is usually given as two separate lines in VA: pull-in power and holding power. To find the current, divide this value by the coil voltage. For example, on a medium-sized contactor with a 230V AC coil the pull-in power is on the order of tens of VA, and the resulting current must stay within the inductive budget of a 10A rated relay contact.
- AC coil: arcing is reduced with an RC type suppression circuit.
- DC coil: a reverse-parallel freewheeling diode is used.
- Long control line: the voltage drop along the cable makes it harder for the coil to pull in; do not skip the cross-section calculation.
If the coil hums or heats continuously, the supply voltage may be low or the core face may be dirty; this turns into a contact failure before long.
The context in which this term is used: Wireless remote control installation: pairing, relays, safety