Motor control · lesson 6 of 12
Jogging (inching)
Jogging means running a motor only while a button is held. Every jog circuit does the same thing: it takes the seal-in out of the path. Here are two ways to do it.
A Run/Jog selector
The simplest version puts a selector switch contact in series with the seal-in. In Run the contact is closed and the circuit latches as normal. In Jog it is open, so the seal-in branch can never complete and Start becomes a hold-to-run button.
Sélecteur marche/à-coups
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À faire
Le sélecteur est dans la branche du contact de maintien. En position À-coups, il ouvre cette branche.
Marche par à-coups avec un relais de commande
A better arrangement uses a small control relay, CR, to do the latching. Run and Stop latch CR on rung 1. The starter M on rung 2 is fed by either a CR contact or the Jog button — and nothing on rung 2 seals anything in. So a jog can never turn into a run by accident.
Marche par à-coups avec un relais de commande
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À faire
Un relais de marche (CR) assure le maintien; le démarreur M ne fait que suivre CR ou le bouton À-coups.
You will still see the "Jog button with an NC contact in the seal-in branch" circuit on older machines. It works most of the time — but it depends on the button's NC contact reclosing after the starter's seal-in has dropped out, and a quick jab can beat it. A motor that runs away when someone only meant to nudge it is the reason the relay version exists.
Sur le chantier
Jogging exists so somebody can inch a machine into position. Everything that goes wrong with it comes from the seal-in getting back in when it should not.
Common mistakes
- A jog contact that breaks the seal-in too late. On a selector-type jog the contact has to be open before the coil pulls in, or the starter seals itself and the machine runs away from the operator.
- Jogging with the selector left in Run. The button then latches the motor instead of inching it. Label the selector and put the pilot light where the operator actually stands.
- Jogging a starter it was not chosen for. Repeated inching is hard duty: heavy inrush over and over, with no running time to cool. Whether a given contactor is rated for it is a manufacturer question, and the answer is on their sheet.
- Using a control relay's NC contact for the seal-in break without checking its timing — a slow relay gives the seal-in a moment to catch.
On a real panel
- The control-relay version is the safer pattern: the relay decides, so the seal-in path is broken by a contact that is already open before Start does anything.
- A jog station is usually near the machine, not at the panel — that is the whole point. Its enclosure and cord entry have to survive where it lives.
- Expect guards, interlocks or a hold-to-run device around a jog function on production machinery. The machine's own standard, not this page, says what is required.
Prove it before you leave
- In Jog, press and release the button ten times with the motor disconnect open. The starter must drop out every single time the button is released.
- In Run, press and release once. It must seal in and hold.
- Switch from Run to Jog while it is running. Decide with the machine's owner what should happen, then make sure that is what does.
- Only then close the disconnect and inch the real machine, with your hand on Stop.
Questions fréquentes
What does jog mean in motor control?
Running a motor only while a button is held, with no seal-in — to inch a machine into position, line up a belt or thread material. Let go and it stops.
Why use a control relay for jogging?
The quick way — putting a jog button’s NC contact in the seal-in branch — can race: if that NC contact recloses before the starter’s seal-in has dropped out, the motor latches and keeps running. A separate run relay means the jog path simply has no latch in it.
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