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.

Trace it yourself

Trace it at rest first, then press the buttons in your head in order. The holding contact is the whole lesson, so watch what it does at each step.

Rails: L1 on the left is the hot side, L2 on the right the return. Top rung is the control, bottom rung is the power.
At rest. Finger on L1, through Stop — it is normally closed, so the blade lies flat: go through. You arrive at a fork. Start is above, drawn open. The holding contact is below, drawn open. Two open paths: stop here. The coil is dead, and so is the power contact below it. Motor off.
Press Start. The upper branch closes. Through it, along to coil M, cross the coil, on to L2. Rung complete. M pulls in.
Find every M. The holding contact under Start is an M — it closes. The power contact on the bottom rung is an M — it closes too. Bottom rung: L1, through the contact, through the motor, to L2. Motor runs.
Let go of Start. The upper branch opens. But the lower branch, the holding contact, is closed, so the rung is still complete through it. The coil keeps itself in. That is the seal-in.
Press Stop. The rung breaks before the fork, so it does not matter that the holding contact is closed — nothing reaches it. The coil drops, both M contacts open, the motor stops. Release Stop: the rung is closed again at Stop but open at the fork. It stays off until Start.

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.

What goes wrong

Three classic ones, and each is diagnosable from the symptom without a meter once you can read the rung.

1. The holding contact landed across Stop instead of Start

Symptom: it starts fine — and Stop does nothing. It runs until the power is killed.

Draw the holding contact from L1 to the fork instead of across Start. Press Start: the coil pulls in and the holding contact closes. Now press Stop. Stop opens — but the holding contact bypasses it, straight from L1 to the fork, and the coil stays fed. The one contact that was supposed to keep the coil in after Start is now keeping it in after Stop as well. The holding contact goes in parallel with Start and nothing else; Stop must sit ahead of the fork so that opening it breaks everything.

2. Holding contact missing or not landed

Symptom: it runs while your finger is on Start and stops the moment you let go.

With no lower branch, the fork has only one path — Start. Press it and the coil pulls in; release it and the rung opens right there, the coil drops, the motor stops. The circuit is behaving exactly as a momentary switch should. The seal-in is the only thing that turns a momentary press into a latched motor, and a loose wire on that one contact takes the whole idea away. First place to look when someone says "I have to hold the button".

3. A normally closed button used for Start

Symptom: the motor starts on its own the instant power is applied, and Start does nothing.

Swap the Start symbol for a normally closed one and trace at rest: L1, through Stop, through the closed "Start", to the coil. The rung is complete with nobody touching anything. The coil pulls in, seals itself through the holding contact, and the motor runs. Press Start now and it opens for a moment — but the holding contact is already carrying the rung, so nothing changes. The button types are not interchangeable: Stop is the closed one, Start is the open one, and the symbols on the drawing tell you which is which before you touch a wire.

En la obra

There are more of these in a plant than any other control circuit, and the one thing every person around the machine assumes is that the red button works. Prove it does, with the motor disconnect still open, before anything turns.

Common mistakes

A normally open button fitted as Stop. Stop breaks a path that is already carrying; it has to be the closed one. A closed button also fails safe — a broken wire to Stop stops the motor instead of making it unstoppable.
The seal-in landed across Stop instead of across Start. It starts beautifully and never stops. Across Start, and nothing else.
No overload contact in the rung. The overload's job is to break this rung. If its contact is not in it, tripping the overload does nothing to the coil.
Using the station's terminals as a splice for a multi-station run. Pull one station and the rest of the circuit goes with it.
Long control runs bundled with power conductors. A long parallel run can couple enough voltage to hold a sensitive coil in after Stop is pressed. If a circuit will not drop out and the drawing says it should, that is worth remembering.
Mounting Stop where you cannot reach it from where the machine is actually operated from.

What the inspector looks for

A stop that is reachable from the operating position and that stays stopped once pressed.
The control circuit protected, and — where the book requires it — arranged so that a single ground fault cannot energise the coil on its own.
The overload's reset arrangement, and whether the motor is allowed to restart by itself after a trip. The book has a view on that, and it depends on what the motor drives.
Enclosure rating for the location, closed and bonded, with proper entries for the control conductors.
Conductor sizes and device ratings worked from the motor's own nameplate.

Check it before you call it done

Prove both the control supply and the motor supply dead, and lock them.
Ring the rung out cold: Stop reads through at rest, Start reads open, the seal-in contact reads open at rest and reads through when you push the armature in by hand.
Energise the control only, with the motor disconnect still open. Press Start: the starter pulls in and seals. Let go: it holds. Press Stop: it drops out and stays out.
Hold Stop in and press Start. Nothing may happen. If it starts, Stop is not ahead of the fork.
Trip the overload by hand. The starter must drop out, and it must not pick up again on its own when you reset it.
Only now close the motor disconnect. Bump it and check rotation before anything is coupled, then run it and stop it from every station on the circuit.

Preguntas frecuentes

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.

Ver también

Motor control course — twelve interactive lessons, from reading a ladder to fault finding with a meter.
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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