Magnetic contactor and overload relay
A PLC output can switch about half an amp at 24 V. A 7.5 kW motor needs 15 A at 415 V. The part in between is the magnetic contactor: a heavy-duty relay whose small coil is driven by the PLC and whose big contacts switch the motor.
How a contactor works
A contactor has an electromagnet (the coil) and a set of contacts. When the coil is energised, it pulls in an armature that closes the main contacts. When the coil is de-energised, a spring opens them again. It is a relay built for high currents and frequent switching.
Terminal markings
| Terminals | Function |
|---|---|
| A1 / A2 | coil (e.g. 24 V DC from the PLC output, or 230 V AC) |
| 1/L1, 3/L2, 5/L3 | main contacts, supply side |
| 2/T1, 4/T2, 6/T3 | main contacts, motor side |
| 13–14 (x3–x4) | auxiliary NO contact (feedback, seal-in, run lamp) |
| 21–22 (x1–x2) | auxiliary NC contact (interlocks) |
Choose the coil voltage to match your control circuit. With a PLC, a 24 V DC coil driven directly by a transistor output (with a suppression diode) is the simplest.
The thermal overload relay
A contactor does not protect the motor. The overload relay mounts under the contactor and measures motor current with bimetal strips (or electronics). If the current stays too high for too long, for example a jammed conveyor, it trips.
- Set the dial to the motor's full-load current from its nameplate.
- 95–96: NC contact, opens on trip. Put it in series with the contactor coil, and/or wire it to a PLC input.
- 97–98: NO contact, closes on trip, for a "motor overload" alarm lamp or PLC input.
- Reset is manual (blue button) or automatic; manual is safer.
A contactor plus overload relay is called a magnetic starter or DOL (direct on-line) starter. Short-circuit protection comes from a fuse or a motor protection circuit breaker (MPCB) in front of it.
Control with a PLC
- PLC output Q0.0 → contactor coil A1; A2 → 0 V.
- Overload 95–96 (NC) → PLC input "OL OK".
- Contactor aux 13–14 → PLC input "motor running" feedback.
- In the program:
Motor := (Start OR Motor) AND Stop_OK AND OL_OK, plus an alarm if the output is on but the feedback does not come within 1 s.
Other starters
- Reversing: two contactors swap two phases, mechanically and electrically interlocked so both can never close.
- Star-delta: three contactors reduce starting current for large motors.
- VFD: a variable frequency drive replaces the contactor for speed control and soft starting.
Build a DOL starter, a star-delta starter and a PLC-controlled compressor with magnetic switches in the PLC Sim Lab Playground.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →