Motor control · lesson 5 of 12

More than one station

To control one motor from several places, put every Stop in series and every Start in parallel with the seal-in. Series means any one Stop can break the rung; parallel means any one Start can complete it.

Dos estaciones: Paros en serie, Arranques en paralelo

Active JavaScript para presionar los botones y hacer funcionar este circuito.

Haga esto

Un motor, dos estaciones de botones — por ejemplo, una en cada extremo de un transportador.

Why this works

Current has to pass through every contact in a series string, so opening any one of them stops it — that is the Stop buttons. Current needs only one path through a parallel group, so closing any one of them is enough — that is the Start buttons and the seal-in. It is the same series and parallel you learned with lamps; the series / parallel practice page drills the idea.

In the field

Adding a station to an existing starter means breaking into the Stop wire to add the new Stop in series, and landing the new Start across the existing Start terminals. The seal-in contact does not change. On a long conveyor you will often see a station at each end and a pull-cord or e-stop in series with the Stops along the length.

En la obra

Adding a station to a running machine is one of the most common small jobs in a plant, and the one wiring mistake in it is invisible when you test the buttons one at a time.

Common mistakes

A Stop wired in parallel. It only works if the other Stop is pressed at the same moment, because the other one still carries the rung. Press each Stop by itself and it looks fine — that is why this one gets missed.
A Start wired in series. Now both Starts have to be pressed together, from two ends of a conveyor. Everyone assumes the button is faulty.
Breaking into the seal-in instead of the Stop wire when adding a station. The seal-in does not change; the new Stop goes into the series string and the new Start lands across the existing Start terminals.
Using the new station's terminals as a splice for the run onward. Pull that station for service and the whole string goes with it.
Leaving the new station's enclosure unbonded or entering it without the fitting the location needs.

On a real panel

On a long conveyor you will usually find a station at each end and a pull-cord or e-stop in series with the Stops along its length. Same string, same rules.
A long control run is worth checking for voltage drop on a big coil — a coil that pulls in at the panel and chatters at the far station is telling you about the wire, not the button.
Wire numbers are your friend here. Every Stop in the string shares a number pattern; a wire that does not fit it is the one somebody added wrong.

Prove it before you leave

Start from every station, one at a time. Each Start alone must start it.
Stop from every station, one at a time. Each Stop alone must stop it, and it must stay stopped.
Hold one Stop in and press every Start. Nothing may run.
Label the new station and add it to the panel schedule. The next person needs to know there are three places this thing starts from.

Preguntas frecuentes

How do you wire two start/stop stations?

All the Stop buttons in series with each other, all the Start buttons in parallel with each other and with the seal-in contact. Any Stop then breaks the circuit, and any Start completes it.

What happens if a Stop is wired in parallel by mistake?

That Stop only works if the other Stop is pressed at the same moment — on its own it does nothing, because the other one still carries the current. Each button tests fine by itself, which is why it is a hard fault to spot.

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