Motor control circuits and ladder logic
CERT 3 · Commercial & Industrial Systems · ~12 min
Industrial control separates the power circuit (the fat conductors feeding the motor through the contactor) from the control circuit (small conductors, often 120 V, that energize the contactor coil). A ladder diagram draws the control circuit as rungs between two vertical rails: L1 on the left, neutral on the right, each rung a path of contacts ending in a coil.
The three-wire start/stop station is the pattern to know cold. The stop button is normally closed and wired in series; the start button is normally open. Pressing start energizes the coil M, which closes an auxiliary contact wired in parallel with the start button — the holding (or seal-in) contact. Release the start button and the seal-in keeps the coil energized. Press stop and the series path breaks, dropping the coil and the seal-in with it.
That design is deliberately fail-safe: any loss of control voltage drops the contactor, and the motor will not restart by itself when power returns. A two-wire control scheme using a maintained switch does the opposite — it restarts automatically — which is why three-wire control is mandatory anywhere an unexpected restart could injure someone.
Exam focus: identify the seal-in contact and explain its function, know that stop is normally closed and start normally open, and recognize the low-voltage-release behavior that distinguishes three-wire from two-wire control.
Quiz — 3 questions
1. In a three-wire start/stop circuit, the seal-in contact is wired:
2. The stop pushbutton in a standard control circuit is:
3. Three-wire control is preferred over two-wire because on power restoration it: