Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
What it is
A contactor is a heavy-duty, remotely-operated switch. A small control signal energises its coil, and the coil's magnetic pull closes a set of main contacts that carry the load current.
It lets a low-power control circuit (a button, a PLC output, a timer) switch a high-power load safely and at a distance.
How it works
Energise the coil and it becomes an electromagnet, pulling the moving armature so the main contacts close. Remove the coil voltage and a spring snaps the contacts back open.
Most contactors also carry auxiliary contacts — small extra contacts (normally-open or normally-closed) used for seal-in latching, interlocks, and status feedback rather than load current.
Where it's used
Motor starters, heater banks, lighting contactors, and anywhere a control circuit needs to switch real power. They're rated by the current and duty they can switch and break.
Common faults: a coil that reads voltage but won't pull in, contacts that weld closed, chatter from low coil voltage, and auxiliary contacts that don't make for the seal-in.
Safety first
A welded contactor can't switch its load off by normal means — isolate upstream. Treat the load side as live until proven dead.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Contactor has voltage at the coil but won't pull in
You measure the rated control voltage (e.g. 24V) across the coil terminals, but the contactor refuses to energise — no clunk, no pull-in, contacts stay open.
Contactor contacts welded closed — load won't switch off
The contactor won't drop out when the coil is de-energised. The load stays powered even with the control circuit off, because the main contacts have welded together.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
Related definitions
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.
Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.