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.
Run / Jog selector
Turn on JavaScript to press the buttons and run this circuit.
Do this
The selector sits in the seal-in branch. In Jog it opens that branch.
Jog with a control relay
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.
Jog with a control relay
Turn on JavaScript to press the buttons and run this circuit.
Do this
A run relay (CR) does the latching; the starter M only ever follows CR or the Jog button.
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.
On the job
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 people ask
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.
Spot something wrong on this page? Report it
Thanks — your report was sent.
That didn’t send. Please try again later.
A heads-up on tools that fit — by email, optional
Leave your email and tick the box for a short heads-up, at most every two weeks, about tools, drills and diagrams that fit what you've been working through — here and on bentright.com and fieldcalc.net. One list across the three, one click to leave, and nothing is sent until you confirm from your inbox.