Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.
What it does
A start/stop circuit lets a momentary press of a start button keep a motor running, and a press of stop turn it off — without holding the button down. It's the classic motor control circuit.
How it works
Press start and the contactor coil energises. The contactor has an auxiliary contact wired in parallel with the start button — the 'seal-in' or 'hold-in' contact. As the contactor pulls in, that aux contact closes and now feeds the coil itself.
So when you release the start button, the coil stays energised through the seal-in contact. The circuit has 'latched'. Pressing stop (a normally-closed button in series) breaks the coil circuit, the contactor drops out, the seal-in opens, and it stays off.
Why it matters
The seal-in is also a safety feature: if the supply is lost, the contactor drops out and won't restart by itself when power returns — you have to press start again. The overload's contact sits in this circuit too, so an overload trip drops the latch.
Safety first
Never strap out the seal-in or stop to 'make it work' — you lose no-volt protection and a working stop. Find why it won't latch instead.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Contactor drops out on its own / won't stay latched
The contactor pulls in when you press start but drops out the moment you release the button, or randomly during running — the seal-in (latch) isn't holding it.
Auxiliary contact not making — seal-in or interlock fails
The contactor pulls in, but an auxiliary contact (used for seal-in, interlock, or status) doesn't change state, so the circuit won't latch, an interlock misbehaves, or status feedback is wrong.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
Related definitions
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.