Motor control · lesson 2 of 12

Two-wire vs three-wire control

There are two basic ways to tell a motor starter what to do. Two-wire control follows a switch that stays where it is put. Three-wire control uses momentary buttons and latches itself. The difference only really shows up when the power blinks — so both labs below have a switch to kill the control power.

Two-wire: the motor follows the switch

A float switch, a pressure switch, a thermostat: devices that stay closed as long as the condition lasts. Put one in series with the starter coil and the motor runs exactly as long as the switch is closed. There is nothing to latch, so there is no seal-in.

Commande à deux fils : un flotteur fait tourner une pompe

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À faire

Un contact maintenu — l’interrupteur à flotteur — constitue toute la commande. Pas de contact de maintien, pas de bouton Marche.

Three-wire: the motor latches

Replace the maintained switch with a momentary Start button, a momentary Stop button, and a contact off the starter itself wired across Start. That contact — the seal-in — is what keeps the coil in after you let go. It is also what makes the circuit forget everything the moment the power goes.

Marche/arrêt à trois fils avec maintien

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À faire

Le même démarreur, commandé maintenant par des boutons à action momentanée et un contact de maintien.

Which one where?

Two-wire suits equipment that should carry on unattended — a sump pump, an exhaust fan on a thermostat, a compressor on a pressure switch. After an outage you want it back.

Three-wire suits anything a person works at — saws, mixers, conveyors, machine tools. If the power drops and comes back while someone is clearing a jam, a machine that restarts by itself can kill them. With three-wire control, nothing moves until someone presses Start on purpose. Whether a particular machine must have that protection is a code and safety-standard question for the job; the circuit behaviour is what this page is teaching.

Sur le chantier

The difference between these two circuits is what happens when the power comes back, and that is a safety decision before it is a wiring one.

Common mistakes

Fitting two-wire control to a machine somebody stands next to. Two-wire restarts by itself the moment supply returns. On a pump in a pit that is the point; on a saw it is how people get hurt.
Assuming a maintained switch makes it two-wire. What makes it two-wire is that the pilot device holds the coil in directly, with no seal-in. Look for the seal-in contact, not the switch type.
Adding a seal-in to a two-wire circuit "to make it hold" — that turns it into three-wire and the automatic device can no longer stop it.
Forgetting the overload contact when converting from one to the other. It belongs in the rung either way.

On a real panel

Two-wire is what you find on a float, a thermostat, a pressure switch — anything where the process is supposed to be in charge.
Three-wire is what you find wherever a person presses a button, precisely because it drops out on a power loss and stays out.
Whether a machine is allowed to restart by itself after a supply interruption is a question for the book and for the machine's own standard, not for convenience.
A Hand-Off-Auto selector is often how one panel gets both behaviours.

Prove it before you leave

Run the motor, then open the control supply and close it again. Watch what the motor does. That one test tells you which circuit you actually have.
If it restarts by itself, confirm that is what the machine is meant to do — and that nobody works inside its reach.
Trip the overload by hand and make sure the coil drops in both modes.

Questions fréquentes

What is the difference between two-wire and three-wire control?

Two-wire control uses a maintained contact — a float switch, pressure switch, thermostat or toggle — and the motor simply follows it. Three-wire control uses momentary Start and Stop buttons with a seal-in contact, so the motor latches on and drops out on a power loss.

What is low-voltage release and low-voltage protection?

Low-voltage release (two-wire) means the starter drops out when the voltage fails and picks back up by itself when it returns. Low-voltage protection (three-wire) means it drops out and STAYS out until someone presses Start again.

Why is it called three-wire?

Because of the wires between the pushbutton station and the starter: one from the supply to Stop, one from Stop to Start, and one from Start back to the coil (which is also where the seal-in contact lands). A two-wire device needs only two.

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