Control hardware · beginner
3-Wire Motor Starter
A classic start/stop control circuit. A momentary START button pulls in the contactor, its own auxiliary contact keeps it on, and STOP breaks that holding path.
Why it exists
Make a motor stay on after START is released, stop reliably, and remain off after a power failure until an operator starts it again.
Quick answer
What is a 3-wire motor starter?
A three-wire motor starter uses a normally-closed STOP path, a momentary normally-open START button and a contactor auxiliary contact wired as a seal-in branch. Pressing START energises the coil; the auxiliary contact then keeps the coil energised after START is released. Opening STOP, the overload contact or the control supply drops the coil and the circuit stays off when power returns.
Inside the control system
How the PLC relates to it
A PLC implementation should preserve the same safe state behaviour. STOP, E-stop and overload conditions are healthy-state permissives; START creates a run request; and loss of a required permissive removes that request. When a physical contactor is used, the program should distinguish MOTOR_RUN_CMD from CONTACTOR_FB and alarm if they disagree. Automatic restart after a power cycle or safety reset is normally undesirable unless the machine risk assessment and operating specification explicitly allow it.
Cause and effect
How it works, step by step
- 01
Healthy STOP and overload contacts complete the control path.
- 02
Pressing START briefly energises the contactor coil.
- 03
The contactor auxiliary contact closes around START to seal in the coil.
- 04
STOP, E-stop, overload, or lost power opens the path and drops the motor out.
01 / Open the case
Watch the mechanism do the work
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Know the terminals
- STOP — NC
- START — NO
- 13–14 — seal-in auxiliary
- A1 / A2 — contactor coil
Recognise the faults
- Seal-in contact missing or miswired
- STOP programmed with the wrong healthy-state assumption
- Motor restarts automatically after power returns
- Overload contact bypassed
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Selection checks
- Control voltage and contactor coil rating.
- Correct normally-closed STOP and overload contact use.
- Auxiliary contact availability for seal-in and PLC feedback.
- No-voltage release and restart behaviour required by the machine.
- Short-circuit, overload, isolation and emergency-stop coordination.
Commissioning sequence
- 1With control power isolated, trace STOP and overload contacts in series with the contactor coil path.
- 2Press START and confirm the coil energises and the seal-in auxiliary contact closes around the momentary button.
- 3Release START and verify the contactor remains energised without relying on the button.
- 4Operate STOP, overload and E-stop individually and prove each one drops the coil.
- 5Cycle control power and confirm the motor remains off until a fresh start command is given.
Troubleshooting answers
Frequently asked questions
Why is it called a three-wire control circuit?
The traditional control station uses separate stop, start and holding paths rather than a maintained two-wire run command. The name describes the control method, not the three motor phases.
What is a seal-in contact?
It is a normally-open auxiliary contact that closes when the contactor energises and provides an alternate current path around the momentary START button.
Why does the motor stay off after a power failure?
Loss of power drops the contactor and opens its seal-in contact. When power returns, the momentary START path is still open, so a new deliberate start is required.
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