Case Study - PLC Start/Stop Motor Logic

Project Overview

This project demonstrates a basic industrial motor control system using Ladder Logic. The goal was to create a Start/Stop motor circuit with a latching (seal-in) function so the motor remains ON after pressing the Start button and turns OFF safely using the Stop button.

Problem

A motor should not require the operator to continuously hold the Start button. In real industrial systems, once started, the motor should stay running until a Stop signal is triggered.

Solution

Implemented Logic:

Start_Button → Normally Open (NO)
Stop_Button → Normally Closed (NC)
Motor_Output → Output coil
Seal-in Contact → Motor_Output feedback contact

The seal-in contact creates a holding circuit. After pressing Start, the motor output energizes and its own contact keeps the circuit active even after the Start button is released.

Project Screenshots

PLC Project Screenshot 1 — Initial Ladder Logic Setup

PLC Project Screenshot 2 — Start/Stop Latching Circuit

PLC Project Screenshot 3 — Debugger Testing & Simulation

Variables Used

VAR_INPUT
    Start_Button : BOOL := FALSE;
    Stop_Button  : BOOL := TRUE;
END_VAR

VAR_OUTPUT
    Motor_Output : BOOL := FALSE;
END_VAR

Testing & Debugging

I used the OpenPLC simulator and debugger to manually force variable states between TRUE and FALSE to verify the behavior.

Pressing Start_Button turns the motor ON
The motor remains ON after releasing Start
Pressing Stop_Button breaks the circuit and stops the motor

Challenges Faced

The OpenPLC interface was initially confusing, especially when creating variables and assigning input/output classes individually. I also had to understand how the debugger works and how to manually test boolean states.

This project helped me better understand visual PLC programming compared to traditional software engineering because the logic flow is visible immediately.

Key Learning Outcomes

Ladder Logic fundamentals
Normally Open vs Normally Closed contacts
Output coils and seal-in circuits
PLC simulation and debugging
Industrial motor control basics

Tools Used

OpenPLC Editor
Autonomy Edge
Ladder Diagram (LD)
PLC Simulator / Debugger

Case Study: PLC Start/Stop Motor Latching Circuit