Motor control · lesson 1 of 12

Reading a ladder diagram

A ladder diagram is how every motor control circuit is drawn, and you read it the same way every time: start at the left rail, follow the rung to the right, and ask at each symbol "is this closed right now?" If you can get all the way to the output, the output is on. That is the whole skill. The lab below lets you practise it — press the buttons on the station (or on the drawing itself) and watch the power go.

Leer un diagrama de escalera: botones, relevador y cuatro luces

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

Haga esto

Cinco peldaños, que se leen de arriba abajo. Cada símbolo está dibujado en reposo; el puente y el color cobre muestran lo que pasa ahora.

The rails

The two vertical lines are the control supply. On a print they are usually marked X1 and X2 when the control power comes from a small control transformer, or L1 and L2 when it is taken straight off the line. The left rail is the "hot" side; the right rail is the return. Nothing happens on a rung unless there is a closed path all the way from one to the other.

The rungs

Each horizontal line is a rung, and they are numbered down the left side. A rung is read left to right: first the conditions (contacts — buttons, switches, relay contacts), then the output at the right-hand end (a coil, a pilot light, a horn). The outputs all line up in one column against the right rail, so you can see at a glance what the circuit controls.

Normally open and normally closed

Every contact is drawn in its normal state — the way it sits when nothing is acting on it. Two plain plates is normally open (NO): no path until something closes it. Two plates with a slash through them is normally closed (NC): a path until something opens it. Pushbuttons have their own symbol — a bar on a stem — and the same idea: a bar sitting above the terminals is NO, a bar resting on them is NC.

The words describe the device, not what it is doing right now. A normally open contact on a running machine may well be closed. That is why the lab bridges a contact when it is actually closed, and paints copper where the power actually reaches — the printed symbol never changes.

Coils and their contacts

A coil is an electromagnet inside a relay or a motor starter. It does not do any work on the rung it sits on — it moves contacts, and those contacts can be anywhere on the drawing. The link between them is the name: every contact marked CR belongs to coil CR and moves when CR pulls in. Energize the coil and all its NO contacts close and all its NC contacts open, at the same instant.

Under each coil the lab prints the rung numbers where its contacts are used — the cross-reference. An underlined number is a normally closed contact. On a real print with forty rungs and six relays, that is how you find everything a coil moves without reading the whole page.

Reading any rung in four questions

Where is the output? (Right-hand end of the rung.)
What has to be closed to reach it? (Every contact in series on the way.)
Where there is a fork — a parallel branch — does any branch make it through? One is enough.
For each contact: what device owns it, is it NO or NC, and is that device actuated right now?

En la obra

Reading a print is a skill you use standing in front of an open panel with somebody waiting, so the habits matter as much as the symbols.

Common mistakes

Reading a symbol as "what it is doing". Every contact is drawn in its rest state. A normally open contact on a running machine is closed, and the print still shows it open. The symbol names the device, not the moment.
Skipping the cross-reference. The numbers under a coil are the rungs its contacts live on. Change what a coil does without following those numbers and you will move something on rung 19 you never looked at.
Assuming the rung numbers are wire numbers. They are not. Wire numbers change at every device; rung numbers count lines down the page.
Ignoring which rail is which. The left rail is the supply side. If a print has one rail grounded, a single ground fault on the wrong side of a coil can energise it — worth knowing before you start jumpering.

On a real panel

The print in the door pocket is often not the panel in front of you. Mark the print as you find changes, and date it.
Control power may come from a small transformer inside the panel, so the disconnect for the motor does not kill the control rungs. Two sources, two proofs.
Find the outputs column first. Everything on the page exists to feed one of those, and naming the misbehaving output picks your rung in seconds.
Terminal strips carry the wire numbers the print uses. That is the bridge between the drawing and the copper.

Prove it before you leave

Pick one coil on the print and find every contact it owns from the cross-reference. Then find them in the panel.
Operate one device and predict out loud which outputs change before you look.
Anything you changed: redline the print, in the panel, before the door shuts.

Preguntas frecuentes

Why is it called a ladder diagram?

Because it looks like one: two vertical rails — the control supply — with horizontal rungs between them. Every rung is one complete path from one side of the supply to the other, with the switching on the left and the thing being switched on the right.

Is a contact drawn open or closed?

In its normal state — the state it is in sitting on the shelf, with its coil de-energized and nobody pushing it. A normally open contact is drawn open even if, on a running machine, it is closed right now.

What do the numbers under a coil mean?

They are the rung numbers where that coil’s contacts appear, so you can find every contact a coil moves. An underlined number is a normally closed contact. On a big print this cross-reference is how you chase a relay across several pages.

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