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Motor control · lesson 3 of 12

Start/stop and the seal-in

This is the circuit every other motor control circuit is built from: a normally closed Stop, a normally open Start, and a seal-in contact in parallel with Start. Press Start and the coil pulls in; the coil closes its own seal-in; let go and it stays in; press Stop and it drops out for good. There is also a stand-alone start/stop diagram with the power rung drawn out and the three classic wiring mistakes.

Three-wire start/stop with seal-in

X1X21StopStartMOLM1, 2, 3Motor starter2MRunning3MStopped

Turn on JavaScript to press the buttons and run this circuit.

Do this

Rung 1 is the control. Rungs 2 and 3 are pilot lights run by M contacts.

Trace rung 1 out loud

  1. From X1, through Stop. It is normally closed and nobody is pressing it, so go through.
  2. A fork. Top branch: Start, open. Bottom branch: the M seal-in, open because M is not energized. Neither goes through — the coil is dead.
  3. Press Start. The top branch closes: through it, through the overload contact (closed unless tripped), into coil M, to X2. M pulls in.
  4. Every M contact moves. The seal-in closes. The Running light's NO contact closes; the Stopped light's NC contact opens.
  5. Let go of Start. The top branch opens — but the bottom branch is now closed. Still a complete path. That is the seal-in.
  6. Press Stop. The rung is broken before the fork, so both branches lose their supply. M drops out, the seal-in opens, and letting go of Stop changes nothing: the fork is open again.

Meter it

The last step of the lab hands you a meter with the black lead on X2. Every dot on the drawing is a place you can put the red lead. You read control voltage wherever the point connects back to X1, and 0 V wherever something between X1 and the point is open. On a real panel that is exactly how you find a problem: start at X1 and move along the rung until the voltage disappears. The part you just stepped over is open. The troubleshooting lesson is built around it.

Questions people ask

What is a seal-in contact?

A normally open contact of the motor starter wired in parallel with the Start button. When the coil pulls in, the contact closes and keeps the coil fed after Start is released. It is also called a holding contact, auxiliary contact or maintaining contact.

Why is the Stop button normally closed?

Because it has to break a path that is already carrying current. A normally closed button in series lets current through until it is pressed. It also fails safe in one important way: if a wire to the Stop button breaks, the circuit stops rather than becoming unstoppable.

Why do pilot lights use contacts instead of wiring across the motor?

A contact off the starter tells you the starter is pulled in. On a control print you usually see a NO contact for a Running light and a NC contact for a Stopped light, so exactly one is on at a time.

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