A contactor holds in only as long as its coil keeps getting enough voltage through a complete control circuit. It drops out because something opened that circuit, or because the coil voltage sagged far enough that the magnet let go.
Split the problem in two before you do anything else. A contactor that drops out cleanly and stays out has lost its seal-in path or had a protective device open it. A contactor that chatters — buzzing, rattling, pulling in and releasing rapidly — has marginal coil voltage or a mechanical problem with the armature, and chattering is destructive: it hammers the contacts and can weld or burn them in a short time.
The second useful question is what else happens at the same moment. If the contactor drops out every time a large motor starts elsewhere in the building, the control supply is sagging. If it drops out when a door opens, a guard moves, a float rises or a temperature climbs, an interlock is doing its job and the real problem is out in the process. If it drops out at random, look at connections, worn contacts and vibration.
A control circuit is usually 120 V or 24 V from a control transformer, and on 347/600 V equipment the control transformer is often the first thing to suspect when a circuit behaves as though the voltage is not quite there.
The most common reason a motor contactor drops out is that the overload relay opened its contact, or a thermostat, pressure switch, float, limit switch, flow switch or safety interlock in the control string opened. The contactor is doing exactly what it was told. Before chasing the starter, find out whether the control circuit was interrupted on purpose. If it is the overload, the question becomes why the motor drew too much current — that is covered in why a motor keeps tripping the overload.
Coils will hold in over a range of voltage, but not much below it. An undersized control transformer, a long control run in small conductors, several coils energized from one transformer, or a large motor starting on the same supply can drop control voltage momentarily. The symptom is a contactor that drops out at the instant something else starts, or during a brown-out. This is very common on farm yards and shops where a welder, a large compressor or a grain leg starts on the same service.
Contactors are consumables. After enough operations the main contacts erode, the contact springs weaken and the armature faces get dirty or pitted. Dirt, rust and mineral dust on the pole faces prevent the armature from seating fully, which leaves an air gap, which drops holding force. The result is chattering or random dropout under vibration. Coils also fail — an overheated or partly shorted coil draws more current, runs hot and loses holding force as it warms.
Vibration and years of thermal cycling loosen control terminals, push-on spade terminals and terminal-block screws. Corrosion does the same in damp or agricultural environments. The circuit is intact until vibration or heat opens it briefly. These are maddening intermittent faults and they are found by inspection and by measuring while the machine runs, not by guessing.
In a standard start/stop circuit, an auxiliary contact on the contactor holds the coil energized after the start button is released. If that auxiliary contact is worn, dirty, loose or misadjusted, the contactor pulls in while the button is pressed and drops out the moment it is released. That symptom — runs while held, drops when released — points almost directly at the seal-in path or the stop circuit.
Pushbuttons and selector switches wear, fill with dust, take water, and get damaged. A stop button with a cracked contact block, a mushroom head that is slightly out, or an e-stop that is not fully reset will open a control circuit intermittently. Field stations on equipment outdoors or in wash-down areas are the usual offenders.
A control transformer running near its limit sags on inrush, and coil inrush is several times the holding current. Add a second or third contactor to the same transformer and the first one drops out when the others pick up. A control fuse that is marginal, or a fuse holder with poor contact pressure, produces the same behaviour.
Where a PLC output, a timer or an interposing relay drives the coil, the fault can be upstream of the contactor entirely. A failing output, a relay with worn contacts, or a controller resetting will cycle the contactor. On packaged equipment this often shows up together with a fault code on the machine controller. See electrical controls.
A cracked housing, a broken shading coil on the armature face, a bent arc shield, debris in the mechanism, or a contactor mounted where it takes constant vibration can all prevent a clean seal. A broken shading coil in particular produces a loud AC buzz and rapid contact damage.
Safety: anything that involves opening enclosures or taking readings on live equipment is work for a qualified electrician. The checks below are the ones that do not.
A chattering contactor should be shut down, not watched. Every cycle hammers the contacts and the motor, and a motor being rapidly reconnected is being restarted repeatedly with full inrush, which is hard on the motor and can produce a partial-phase condition — see phase loss on a motor. Stop as well if there is a smell of hot insulation, visible scorching, a buzz that got louder over days, or a contactor that has become hot enough to discolour.
Do not jumper out a device that keeps opening the control circuit. That device is usually a safety or a protection, and bypassing it removes the only thing standing between an intermittent fault and a serious one. Intermittent control faults are found with readings taken while the machine misbehaves, which is what electrical troubleshooting and control panel service work is for. Photos of the starter, the nameplates and any panel schematic sent through the request form shorten the call considerably.
Almost always the seal-in path: a worn or loose auxiliary contact on the contactor, or a break in the holding circuit. It is a small repair once the circuit is traced, but it is inside the enclosure.
It is destructive, and it should be shut down. The buzz means the armature is not seating, which means the contacts are not making properly, which means heat at the contacts and repeated restarts for the motor. It also frequently means a broken shading coil or a marginal control voltage that needs to be found.
Sometimes that is the answer, but only after ruling out the control circuit and coil voltage. A new contactor on a sagging control supply chatters exactly like the old one did, and a new contactor of the wrong rating for the duty fails early.
Voltage sag. The compressor inrush pulls the supply down far enough that the control transformer output drops below the coil hold-in range. The fix is usually on the supply or control transformer side, not the contactor.
Yes, and it often is. An overload relay opens the control circuit, which drops the contactor, which looks like a contactor problem. Check whether the overload needs resetting before assuming the starter has failed.
Shut it down before the contacts weld, then get the control circuit traced.
