Three-Wire Control: Start/Stop with Seal-In

This is the most important circuit in motor control. Once the seal-in clicks for you, you understand the trick at the heart of nearly every industrial panel.

The problem it solves

We want to start a motor by tapping a Start button and stop it by tapping a Stop button. But buttons are momentary — they spring back the instant you let go. So how does the motor keep running after you release Start? The circuit needs a way to remember that it was started. That memory is the seal-in (also called the holding contact or maintaining contact).

The three pieces

• Stop button — a normally-closed (NC) momentary button. Current passes through it until pressed. It sits first in line so it can break the whole circuit.
• Start button — a normally-open (NO) momentary button. Pressing it completes the path to the coil.
• Seal-in contact — a NO auxiliary contact controlled by the coil itself, wired in parallel with the Start button.

Walk through what happens

1. At rest: Stop is closed (NC), Start is open, the seal-in M is open. No path to the coil. Motor off.
2. Press Start: now there's a path — through Stop, through Start, to the coil. Coil M energizes and the contactor pulls in. The motor runs. At the same instant, coil M closes its seal-in contact.
3. Release Start: Start springs back open — but it no longer matters. Current now flows through the seal-in contact instead, which is held closed by the very coil it's feeding. The circuit holds itself in. The motor keeps running.
4. Press Stop: the NC Stop button opens and breaks the whole rung. Coil M drops out, the contactor releases, the motor stops — and the seal-in contact opens again. Release Stop and nothing restarts, because Start is open and the seal-in is open. You're back at rest.

The mental model

Think of the seal-in as the circuit "latching" itself. Start is a momentary nudge that gets the coil going; the coil then reaches back and holds its own door open through the seal-in contact. Stop is the one thing that can break the latch. This latch-and-break pattern shows up everywhere once you start looking — it's the foundation of relay logic.

What to take away

Three-wire control = NC Stop, NO Start, and a coil-driven seal-in contact in parallel with Start. The momentary Start kicks it off, the seal-in holds it, and Stop breaks it. It requires a deliberate restart after power loss, which is what makes it safe for machinery.