Motor Control Ladder Logic (Start/Stop Diagram)
CircuitDiagramMaker
6,238 views
▶ Try it free in your browser: https://circuitdiagrammaker.app/try?u...
Motor control ladder logic, built one contact at a time — the classic start/stop diagram with the seal-in holding contact, explained from scratch. Draw it free in your browser: https://circuitdiagrammaker.app/try?u...
This is the single most important circuit in industrial control: a motor that starts when you tap a button, keeps running when you let go, and stops the instant you tell it to. We rewind a running rung and rebuild it piece by piece so every contact makes sense.
First the skeleton: a normally open START button and a normally closed STOP button in series, feeding the contactor coil M. Because they are in series, that is an AND — STOP must be closed and START pressed for the coil to pull in. But release START and the coil drops out instantly, which is useless on a real machine.
The fix is the seal-in. We add a normally open auxiliary contact of the coil, wired in parallel with START. Tap START, coil M energises, and its own seal-in contact snaps closed — a second path around the button. Let go, and current keeps flowing through the seal-in: the rung holds itself on, hands-free. Press STOP and the NC opens, the coil drops, and the seal-in springs open, so the motor stays off until START is pressed again. That is three-wire, momentary control — and it is fail-safe, because a power loss will not restart the motor on its own.
Then we round it out like a real starter: an overload (OL) NC contact in series for motor protection, RUN and OFF indicator lamps driven by the same auxiliary read two ways, a jog variation that cuts the seal-in out so the motor follows the button, and a two-wire vs three-wire contrast. Finally we map every symbol back to the physical direct-on-line (DOL) starter it represents — contactor coil, auxiliary block, push-button station, thermal overload relay, and the motor's main contacts.
Sources & further reading:
https://circuitdiagrammaker.app/tools...
https://circuitdiagrammaker.app/blog/...
https://circuitdiagrammaker.app/diagr...
https://circuitdiagrammaker.app/symbo...
https://plcsimulationsoftware.com/?ut... — sister tool: run the ladder program scan by scan
— Chapters —
0:00 Motor control ladder logic: the start/stop rung
0:33 The four parts of three-wire control
1:14 START and STOP in series feed the coil
1:54 The problem: release START, motor drops out
2:30 The seal-in: latching the coil on
3:13 Press STOP: collapsing the seal-in
3:57 Why it is called three-wire control
4:42 Overload (OL) contact for motor protection
5:25 RUN and OFF indicator lamps
6:07 The jog / inch variation
6:52 Two-wire vs three-wire control
7:38 The physical DOL starter it represents
8:22 Recap, the map, and CTA
#plc #plcprogramming #ladderlogic
4 Comments
One thing worth pinning: pressing STOP and an overload tripping do the exact same thing to the rung — they break the series path, the coil drops, and the seal-in springs open. Same collapse, two causes.
Build the DOL start/stop rung yourself, free, no signup: https://circuitdiagrammaker.app/try?template=dol-motor-starter&utm_source=youtube&utm_medium=pinned_comment&utm_content=yt-119
Motor Control Ladder Logic (Start/Stop Diagram)
CircuitDiagramMaker
6,238 views
▶ Try it free in your browser: https://circuitdiagrammaker.app/try?u...
Motor control ladder logic, built one contact at a time — the classic start/stop diagram with the seal-in holding contact, explained from scratch. Draw it free in your browser: https://circuitdiagrammaker.app/try?u...
This is the single most important circuit in industrial control: a motor that starts when you tap a button, keeps running when you let go, and stops the instant you tell it to. We rewind a running rung and rebuild it piece by piece so every contact makes sense.
First the skeleton: a normally open START button and a normally closed STOP button in series, feeding the contactor coil M. Because they are in series, that is an AND — STOP must be closed and START pressed for the coil to pull in. But release START and the coil drops out instantly, which is useless on a real machine.
The fix is the seal-in. We add a normally open auxiliary contact of the coil, wired in parallel with START. Tap START, coil M energises, and its own seal-in contact snaps closed — a second path around the button. Let go, and current keeps flowing through the seal-in: the rung holds itself on, hands-free. Press STOP and the NC opens, the coil drops, and the seal-in springs open, so the motor stays off until START is pressed again. That is three-wire, momentary control — and it is fail-safe, because a power loss will not restart the motor on its own.
Then we round it out like a real starter: an overload (OL) NC contact in series for motor protection, RUN and OFF indicator lamps driven by the same auxiliary read two ways, a jog variation that cuts the seal-in out so the motor follows the button, and a two-wire vs three-wire contrast. Finally we map every symbol back to the physical direct-on-line (DOL) starter it represents — contactor coil, auxiliary block, push-button station, thermal overload relay, and the motor's main contacts.
Sources & further reading:
https://circuitdiagrammaker.app/tools...
https://circuitdiagrammaker.app/blog/...
https://circuitdiagrammaker.app/diagr...
https://circuitdiagrammaker.app/symbo...
https://plcsimulationsoftware.com/?ut... — sister tool: run the ladder program scan by scan
— Chapters —
0:00 Motor control ladder logic: the start/stop rung
0:33 The four parts of three-wire control
1:14 START and STOP in series feed the coil
1:54 The problem: release START, motor drops out
2:30 The seal-in: latching the coil on
3:13 Press STOP: collapsing the seal-in
3:57 Why it is called three-wire control
4:42 Overload (OL) contact for motor protection
5:25 RUN and OFF indicator lamps
6:07 The jog / inch variation
6:52 Two-wire vs three-wire control
7:38 The physical DOL starter it represents
8:22 Recap, the map, and CTA
#plc #plcprogramming #ladderlogic