Engineering · PLC · Project 1 of 15 · Motor control · Level 1

Motor start / stop (seal-in)

The most important rung in industry: press START, the motor keeps running by itself until STOP or an overload.

Learning model only. This page teaches the program logic. A real machine needs a proper control panel with an isolator, fuses, motor protection and a hard-wired emergency stop. Mains and motor wiring is for qualified electricians. A PLC output never drives a motor directly.

You will learn

The seal-in (latching) contact: MOTOR in parallel with START remembers that the motor is running after you let go of START. Stop buttons are wired normally closed, so the PLC input is ON when everything is healthy. A broken wire then looks like a stop press, which is the safe way to fail.

How it works

Press START. MOTOR turns ON and its own contact in parallel holds it.
Release START. The motor keeps running because MOTOR feeds itself.
Press STOP. The STOP contact opens, the rung loses power and MOTOR drops out.
Flip the Overload relay. The motor stops and will not restart until the relay is reset.

In real life

Every pump, fan, compressor and conveyor motor in a factory starts with this rung. Real panels add an emergency-stop and a motor-protection breaker in the same series chain.

Inputs, outputs and addresses

The same signals on seven PLC brands. Inputs and outputs are numbered in order; check your own hardware and software, because addresses depend on the model and module position.

Name	What it is	Type	Siemens S7-1200	Allen-Bradley CompactLogix	Mitsubishi iQ-F FX5U	Delta DVP / AS	Omron CP2E	Schneider M241	Inovance H5U
START	START push-button (N.O.)	Input	`%I0.0`	`Local:1:I.Data.0`	`X0`	`X0`	`0.00`	`%IX0.0`	`X0`
STOP	STOP push-button (N.C.)	Input	`%I0.1`	`Local:1:I.Data.1`	`X1`	`X1`	`0.01`	`%IX0.1`	`X1`
OL	Thermal overload contact (N.C.)	Input	`%I0.2`	`Local:1:I.Data.2`	`X2`	`X2`	`0.02`	`%IX0.2`	`X2`
MOTOR	Motor contactor KM1 and RUN lamp	Output	`%Q0.0`	`Local:2:O.Data.0`	`Y0`	`Y0`	`100.00`	`%QX0.0`	`Y0`

Ladder diagram (shown for Siemens, live state)

Green wires carry power in the moment captured below. Switch the brand and the language in the simulator.

Structured text, Siemens SCL

(* Siemens S7-1200, CPU 1214C DC/DC/DC - TIA Portal - SCL *)
(* Rung 1: Start / stop with seal-in and overload *)
"MOTOR" := ("START" OR "MOTOR") AND "STOP" AND "OL";

Structured text, Allen-Bradley

(* Allen-Bradley CompactLogix 5069 (L306ER) - Studio 5000 Logix Designer - Structured Text *)
(* Rung 1: Start / stop with seal-in and overload *)
MOTOR := (START OR MOTOR) AND STOP AND OL;

Try this

Watch the green power flow in the ladder while you press START and release it.
Hold STOP and press START. What happens? (Nothing: stop wins.)
Switch the brand and compare how the same rung looks in each language.

Parts to buy

The links open a search on each store, so you see what is in stock today. Check the model, voltage and price before you order.

#	Item	Buy / compare
1	PLC with 3 inputs and 1 output	Amazon.in · IndiaMART · Robu.in
2	24 V DC supply	Amazon.in · IndiaMART · Robu.in
3	Green N.O. and red N.C. push-buttons	Amazon.in · IndiaMART · Robu.in
4	Contactor with 24 V DC coil	Amazon.in · IndiaMART · Robu.in
5	Thermal overload relay	Amazon.in · IndiaMART · Robu.in

All projects 2. Forward / reverse with interlock →

This is a learning tool. It teaches the logic, not the exact keystrokes of each vendor’s software. Check the manual of your own PLC.