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.

Reading a ladder: buttons, a relay and four lights

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

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Five rungs, read top to bottom. Every symbol is drawn at rest; the bridge and the copper show what is happening now.

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

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

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