Motor Speed Control with a Variable Frequency Drive and Siemens LOGO PLC

Goal:
To move beyond fixed-speed motor starters and learn how industry controls induction motors at variable speed, by combining a variable frequency drive with a programmable logic controller (PLC) in a working control system.

What I built:
A motor control system in which a Siemens LOGO PLC commands a VFD driving a three-phase induction motor. The PLC program handles start and stop, forward and reverse direction, and speed selection, while the VFD controls the motor's speed by varying the supply frequency, with configured acceleration and deceleration ramps for smooth starts and stops. Push buttons and selector switches on the control panel act as operator inputs to the PLC.

My role:
Working with my project team at DUET, I designed the control wiring between the PLC, the VFD and the motor, wrote the LOGO PLC program, configured the VFD's parameters, and tested the system under different speed and direction settings.

Challenges / results:
The main challenges were matching the PLC's outputs to the VFD's control inputs, and setting the drive's parameters, such as motor ratings, frequency limits and ramp times, so the motor ran smoothly and safely. The finished system started, stopped, reversed and changed speed reliably from the control panel. It showed clearly how variable speed control cuts starting current and mechanical stress compared with direct-on-line and star-delta starting.

What came after:
The project built on my star-delta starter and conveyor control projects, and gave me practical experience of the drives and PLCs found throughout industrial plants. That experience proved useful in my later work as a shift engineer at Summit Power Limited's power plants.