Contactor

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.

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.

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.

Coils fail open — correct volts at A1–A2, no clunk, often a burnt smell — or develop shorted turns, which shows as humming, chattering, or a coil too hot to touch. Chatter also comes from low control voltage or a loose control connection: the coil can't hold the armature in cleanly.

Main contacts pit and burn until a pole stops conducting — the motor single-phases even though the contactor operates — or they weld shut so the load won't stop. Auxiliary contacts wear too: a seal-in aux that doesn't make is the classic 'runs while I hold the button' fault. Dust and gunge can jam the armature outright.

Isolate and prove dead — control and power. For the coil, lift one coil lead and measure resistance across A1–A2: expect a definite reading (roughly hundreds of ohms for a 230 V coil, much less for 24 V — it varies by brand and frame size, so compare a known-good one). OL means an open coil; near zero means shorted turns.

Live, if you're licensed and it's safe to work that way: rated volts across A1–A2 with no pull-in condemns the coil or mechanism. Under load, measure the volt-drop across each closed main pole — a healthy pole drops a few millivolts; a pole reading hundreds of millivolts (or running hotter than its mates) is burnt inside.

Measure the coil across A1–A2, not A1 to earth — a floating reading can look healthy and means nothing. And lift a lead before ohming: parallel paths through the control circuit or interposing electronics will lie to you.

A 'clunk' proves the armature moved, not that power flows — burnt mains can be open on a contactor that operates perfectly. If it drops out with one pole welded, the motor doesn't stop clean; it single-phases and cooks. And match replacement coils on voltage AND type — 24 V AC and 24 V DC coils look identical on the shelf.