PLC Motor Control in Structured Text (ST). A Practical Guide

This practical guide focuses on one of the core tasks of industrial automation – controlling electric motors with programmable logic controllers (PLCs). Written for practicing engineers, students, and automation specialists, it goes far beyond simple “on/off” switching, covering safe, efficient, and reliable motor management.

The book systematically covers:

physical principles and motor parameters;

typical wiring diagrams and protection methods (thermal, overcurrent, temperature);

programming techniques in Structured Text (ST) – from basic logic to state machines;

monitoring of current, temperature, vibration, and speed;

starting strategies (DOL, soft starter, current‑limited start);

common logic errors and how to avoid them;

complete examples for pumps, compressors, fans, and integrated systems.

All code examples include comments in Russian and are adapted for CoDeSys and TwinCAT environments. This guide will help you prevent typical mistakes, design fault‑tolerant algorithms, and gain confidence when working with real industrial equipment.

Introduction to Motor Control – motor types, key parameters (power, current, torque, efficiency).

Schematics and Protections – typical wiring, protection methods (current, temperature, voltage, phase imbalance), I/O signals.

Variables and Basic Program – data declaration, input reading, simple start/stop logic.

Simple On/Off Control – edge detection, runtime counting, time‑limiting.

Current and Temperature Monitoring – signal filtering, threshold alarms, automatic shutdown on overload/overtemperature.

Interlocking and Safety Logic – mutual exclusion, emergency stop (E‑stop), dependent shutdowns.

Start‑up and Acceleration Strategies – Direct‑on‑Line (DOL), soft starter, current‑limited start.

Common Motor Logic Errors – 8 most frequent mistakes: simultaneous forward/reverse, ignoring acceleration phase, missing timeouts, improper flag usage, E‑stop reset issues, sensor miscalibration, etc.

Complete Control System – state machine for motor control, integrated monitoring and diagnostics.

Practical Examples – pressure‑controlled pump, unloaded compressor, temperature‑controlled fan; tips and recommendations.