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
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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
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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.
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.
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