Contactor wiring diagram

Motor: on

Open the control and watch the power contact spring open with the coil — a milliamp of control current drops tens of amps of load. That separation is the whole reason a contactor exists. The copper highlight is the live path, not a wire colour.

A contactor lets a small control current switch a large load: the coil is an electromagnet, and energizing it pulls the power contacts closed. Here a maintained control switch feeds the coil; the coil pulls the power contact shut and the motor runs. Open the control and the contact springs open with the coil.

Trace it yourself

Two rungs between two rails, so trace them one at a time — and then find the letter that ties them together.

Find the rails. L1 on the left is the hot side, L2 on the right the return. Every rung is fed from L1 and ends at L2.
Top rung, the control. Finger on L1, across to the control switch. It is closed, so go through. Along to the coil, marked K. Cross the coil — it is a load, the only place the two sides meet on this rung — and on to L2. Rung complete. K is energised.
Now go hunting for the letter. There is one other K on the drawing: the power contact on the lower rung. It is a normally open contact moved by the coil you just energised, so it is closed.
Bottom rung, the power. Finger on L1, through the closed power contact, into the motor. Cross the motor. On to L2. Motor runs.
Open the control switch. The top rung breaks at the switch; the coil drops; the spring throws the power contact open; the bottom rung breaks there and the motor stops. Notice the two rungs never touch electrically. The only thing that crossed from one to the other was the pull of the coil.

How this circuit works

There are two separate circuits sharing one device. The control circuit is the small one: a switch feeding the coil. The power circuit is the big one: the supply, the power contact, and the motor. They are electrically apart — the only thing linking them is the mechanical pull of the coil on the contact. Close the control switch and the coil energizes; the moment it does, it drags the power contact closed and the motor gets its supply.

Take the control away and it all lets go at once — coil de-energizes, spring opens the contact, motor stops. That clean separation between a light control signal and a heavy load is the entire reason contactors exist. A real one gangs three power poles for a three-phase motor; this is that same coil-and-contact trick, shown once.

What goes wrong

1. The control switch wired into the power rung

Symptom: it runs — and the little switch gets hot, arcs, and eventually welds or burns out.

Move the switch in your head from the top rung to the bottom one, in series with the motor. The motor still starts and stops with it. But now every amp the motor draws goes through a switch built to carry the coil's trickle. The whole reason a contactor exists is to keep the big current off the small switch; wire it this way and you have paid for a contactor and used none of it.

2. A coil fed the wrong voltage

Symptom: the contactor hums and chatters without pulling in — or pulls in, gets hot, and the coil goes open a week later.

The coil's pull comes from the current through it, and that current is set by the voltage it is built for. Feed it less than its rating and the magnet is too weak to seat the armature: it lifts, falls back, and buzzes on the edge. Feed it more and it seats hard and cooks. The circuit topology is perfect either way. Read the coil's rating on the device and the voltage between the two rails it is wired across, and make them match; where they do not, that is what a control transformer is for.

3. A welded power contact

Symptom: you open the control, the coil drops, and the motor keeps running.

Trace it with the coil dead: the spring should have thrown the power contact open, so the bottom rung should be broken. If the motor is still running, the rung is not broken — the contact face has welded itself closed, which heavy arcing can do over time. No spring opens a weld. The lesson for reading: when a symptom contradicts the drawing, one of the devices is no longer doing what its symbol says, and that is where to look. The motor stops when the upstream disconnect opens, and not before.

En la obra

A contactor is two circuits in one lump of plastic, and most of the trouble comes from forgetting which one you are holding. The control side is small, cheap and easy to ring out with the power still locked off — so ring it out, every time, before anything turns.

Common mistakes

Control conductors on power terminals. The coil terminals and the power poles are different sizes and usually different ends of the device. If a conductor fits badly, you are probably on the wrong terminal.
A coil fed from a source you did not check. Read the coil's rating off the coil, then measure across the two points you are about to feed it from. A coil that hums instead of clunking is telling you those two numbers disagree.
Calling it a starter. A starter is a contactor with running overload protection attached. If the thing in your hand has no overload, the motor's running protection has to be somewhere else — know where before you energise.
Leaving the auxiliary contact off when the control scheme expects a seal-in. The motor then runs only while somebody holds the button, and everyone blames the button.
Mounting it in an orientation the device does not allow. Gravity acts on the armature; the manufacturer's sheet says which way up it may sit.
Clamping insulation in a power terminal. A big terminal will happily tighten onto a jacket and read fine until it carries current.

What the inspector looks for

Running overload protection for the motor — on this device or demonstrably elsewhere in the circuit.
The control circuit's own protection and supply, including the control transformer where one is used and the grounding arrangement the book asks for on it.
Conductor sizes and device ratings worked from the motor's nameplate figures and the book's rules. The plate is the source; nothing here is guessed.
The enclosure rated for the location, closed, and bonded — along with the motor frame.
Entries, glands and supports for both the power and the control runs.

Check it before you call it done

Prove both circuits dead. Control and power often come from different places, and locking one does nothing to the other.
With everything dead, push the armature in by hand. The power poles should make together and the auxiliaries should change over. A contactor that is stiff or gritty by hand will not behave electrically either.
Ring the control rung out end to end with a continuity tester before you energise anything.
Energise the control first, with the motor disconnect still open. The coil should pull in with one clean clunk. A buzz or a chatter means the coil is not getting what it wants.
Now close the motor disconnect and run it. Check rotation before anything is coupled to the shaft.
While it runs, measure across each closed power pole. A good contact drops next to nothing; a measurable drop across a contact that is supposed to be closed is a face on its way out.

Preguntas frecuentes

What is the difference between a contactor and a relay?

Same idea, different scale. Both use a coil to pull contacts closed. A relay switches small control or signal loads; a contactor is built for the heavy power contacts and arc suppression that switching a motor or a bank of heaters demands. When people say contactor they mean the big one.

Why switch a big load with a small coil?

So the thing you press, or the thermostat that closes, only has to handle a trickle of coil current, while the tens of amps go through robust power contacts nearby. It keeps the control wiring small and safe, and it lets a low-voltage or remote signal command a large load.

What happens when the coil loses power?

A spring pulls the contacts open again the instant the coil de-energizes. Open the control switch in the diagram and the power contact springs open with the coil, and the motor stops. There is no memory here — no power to the coil means open contacts.

Ver también

Motor start/stop station — add a seal-in and pushbuttons.
Occupancy sensor wiring — another small signal commanding a load.
All wiring diagrams

¿Ve algo incorrecto en esta página? Repórtelo

Gracias — su reporte fue enviado.

No se pudo enviar. Vuelva a intentarlo más tarde.