Motor start/stop station wiring
Motor: off
Tap Start: the coil pulls in and the holding contact closes across Start, so the coil stays fed after you let go — that is the seal-in. Tap Stop and it breaks the rung and drops out. The copper highlight is the live path, not a wire colour.
A start/stop station uses a momentary Start, a momentary Stop, and a holding contact so a brief press keeps a motor running until you tell it to stop. Start energizes the coil; the coil's own holding contact seals it in; Stop breaks the rung and it drops out. Tap the buttons above and watch the seal-in do the work.
How this circuit works
Follow the control rung. Power comes in through the normally-closed Stop button to a junction. From there, two things in parallel can carry it on to the coil: the Start button while it is pressed, and the holding contact. At rest both are open, so the coil is dead. Press Start and current reaches the coil; the coil pulls in and closes its holding contact across Start. Now let go of Start — the holding contact is already carrying the current, so the coil stays energized. That is the seal-in.
To stop it, you have to break the rung. That is the Stop button's job: press it and the whole rung opens for an instant, the coil loses power, the holding contact falls open, and there is no longer any path to re-energize the coil. It stays off until Start is pressed again. This drop-out-and-stay-out behaviour is exactly why the arrangement is trusted around machinery.
Questions people ask
Why does the motor keep running after I let go of Start?
Because of the holding contact — the seal-in. When the coil energizes, one of its own contacts closes across the Start button and keeps the coil fed. So Start only has to be pressed for a moment; the seal-in takes over and holds the circuit in until something breaks the rung.
What makes it stop, then?
The Stop button. It is a normally-closed button sitting in series in the control rung. Press it and the rung opens, the coil drops out, the holding contact falls open with it, and the circuit cannot re-seal until Start is pressed again. Try it in the diagram — one tap of Stop and it is out for good.
What is a three-wire control circuit?
The name for exactly this Start/Stop/seal-in arrangement. It is called three-wire because of how the pushbutton station connects, and its defining feature is that it drops out and stays out on a loss of power — it will not restart on its own when power returns, which is a safety feature.
What does the coil actually do?
It is an electromagnet. Energize it and it pulls in a set of contacts — the holding contact in the control rung and the larger power contacts that feed the motor. A little control current commands a lot of load current. Drop the coil and everything it was holding springs open.
Related
- Contactor wiring diagram — the coil-and-contact idea on its own.
- Glossary: inrush — why a starting motor pulls so hard.
- All wiring diagrams
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