Closed-loop Speed Control for a Three-Phase Alternating-Current Motor using a Modbus Network

This paper presents a closed-loop controller for speed regulation of a three-phase induction motor by means of a Modbus network, which is compound only by industrial-purpose hardware, namely two programmable logic controllers, a human-machine interface and a variable-frequency drive. This represents a step beyond the state of the art because the results in similar works reported in the literature were obtained using personal computers, academic-purpose hardware or a combination of both academic-industrial technologies. The speed regulation was achieved through a proportionalintegral controller executed in a programmable logic controller, whose feedback was supplied by another similar device completely dedicated to read and decode an industrial optical encoder; furthermore, the control signals were commanded to the alternating current motor using a variable-frequency drive. The proposed control network was tested through diverse regulation experiments, under different conditions, in order to assess its effectiveness and robustness. The obtained results were successful in terms of accuracy of the speed set-point tracking in comparison to the most similar works of the state of the art, namely zero steady-state error was achieved, 12 seconds earlier for 75% larger references and 22% less RMSE than other works, which are the best results reported so far. Such successful achievements encourage the later implementation of more advanced control techniques on three-phase-motor based industrial machinery using only industrial-purpose hardware.