How do a contactor and an overload relay work? The seal-in circuit
The heart of every motor panel: a contactor that runs a motor drawing tens of amps from a small button current, and an overload relay that stops the motor before it burns. Half of fault-finding is knowing these two parts and their numbers.
What does it do? A contactor is a relay built for big currents: when coil A1–A2 is energised, three main contacts (1/L1–2/T1, 3/L2–4/T2, 5/L3–6/T3) and the auxiliaries move together. An auxiliary contact's last digit gives its type: 3-4 is NO (13-14), 1-2 is NC (21-22). A thermal overload has one bimetal per phase; if the motor current stays above the setting for long enough, the bimetals heat, bend, open the 95-96 NC contact and close the 97-98 NO contact. In a seal-in circuit, contact 13-14 wired in parallel with the start button keeps feeding the coil after the button is released; stop (NC) or the overload stops the motor by breaking that path. When power fails and returns, the motor doesn't restart by itself.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/4: Stopped
Start (S1) is open and the seal-in contact 13-14 is open: no voltage reaches K1's coil. The main contacts are open and the motor is stopped. K1's NC auxiliary 21-22 is closed, so the 'ready' lamp H2 is lit.
1. Coil voltage (A1–A2)
The full coil voltage while pulled in (230 V AC here; 24 V AC/DC and 400 V coils exist too, check the label). A low supply makes the contactor buzz or chatter.
On the scope
Multimeter230 V AC (pulled in)
🔍 How to spot it on a board
•A DIN-rail box marked A1/A2, 1/L1…6/T3 and 13NO/14NO on the front; the coil voltage printed on it (e.g. 230 V 50 Hz, 24 V DC)
•An overload plugged straight under the contactor, with a current dial (e.g. 9–13 A), a test button and a (blue) reset button on the front
•Green start (NO) and red stop (NC) buttons and fault / running lamps on the panel door
🔧 How to measure it
•Isolate, lock off and prove dead by measuring: there's 400 V in the panel.
•Coil resistance A1–A2: OL means an open coil. Push the main contacts in with the test button and check ≈0 Ω.
•Running, clamp all three phase currents: are they equal, where are they against the overload setting?
•If the motor won't run, follow the voltage down the control chain: fuse → 95-96 → S0 → S1/13-14 → A1.
⚠ Common faults
•Burnt or pitted main contact: the motor single-phases, hums and trips the overload.
•Seal-in contact (13-14) not conducting: the motor only runs while start is held.
•Overload keeps tripping: setting too low, the motor genuinely overloaded, phase loss or a winding fault; turning the dial up is not a fix.
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