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.
Two-wire control: a float switch runs a pump
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A maintained contact — the float switch — is the whole control. No seal-in, no Start button.
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.
Three-wire start/stop with seal-in
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The same starter, now run by momentary buttons and a seal-in contact.
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.
Questions people ask
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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