Jogging circuit (inching)
Runs a motor in short bursts that last only while the button is held — the contactor deliberately doesn't seal in, so it stops the instant you let go.
What it does
Jogging — also called inching — drives a motor in short, controlled bursts for precise positioning. The motor runs only while the jog button is held and stops the moment it's released, so you can nudge machinery a little at a time instead of running it continuously.
How it works
A normal start/stop circuit latches on through a seal-in (hold-in) auxiliary contact wired across the start button. A jog circuit deliberately defeats that seal-in: the jog path energises the contactor coil but never brings the hold-in into play.
So the coil stays energised only while the button is pressed. Let go and the contactor drops straight out — no latch, no continuous run. That 'no seal-in' behaviour is the whole point of the circuit.
How it's wired
Three common builds: (1) a Run/Jog selector switch in series with the seal-in contact — in RUN the seal-in latches normally, in JOG it's open so the start button becomes a jog button (this is the version drawn above). (2) A jog pushbutton whose NC contact breaks the seal-in when pressed — simple, but with a real hazard: release it quickly while the contactor is still dropping out and the seal-in can re-make, latching the motor into an unexpected continuous run.
(3) A jog circuit with a dedicated control relay (CR) — the safest choice for frequent jogging. The start button picks up CR, which forms the seal-in and holds the main contactor; the jog button energises the main contactor directly, bypassing CR, so no seal-in can ever form and there's no timing race.
When and why it's used
Anywhere you need to inch machinery into position: threading material into presses, conveyors and packaging lines; aligning or indexing a load; positioning a machine for maintenance; bump-testing which way a motor turns before committing to a run; or clearing a jam.
Jogging gives precise incremental movement with the operator keeping a hold-to-run (dead-man) grip — nothing runs unattended, which is exactly what you want when someone is close to the machine setting it up.
Safety and contactor duty
Jogging is hard on the gear. Each jog makes and breaks near full locked-rotor/inrush current — that's AC-4 utilisation duty under AS/NZS 60947.4, far more punishing than the AC-3 of normal starting — so the contactor must be sized or derated for jog duty or its main contacts pit and weld. Repeated bursts also heat the motor with no time to cool.
Because jogging is usually a setup or maintenance mode with someone near moving parts, the contactor must drop out the instant the button is released. Never modify a jog circuit in a way that lets it seal in.
Safety first
Treat jogging as hold-to-run — it's a setup/maintenance mode close to moving parts. The contactor must drop out the instant the button is released; if a jog circuit ever latches into continuous run, stop and fix the seal-in defeat rather than work around it. Size or derate the contactor for AC-4 jog duty.
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 contacts welded closed — load won't switch off
The contactor won't drop out when the coil is de-energised. The load stays powered even with the control circuit off, because the main contacts have welded together.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
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.
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.
Related definitions
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.
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.