Intelligent management and control of electrical loads using microcontroller-based embedded systems

Abstract

The design and implementation of an intelligent 3-phase, programmable soft-start controller, for low voltage induction motors is presented. The controller is implemented with a low-cost microcontroller and is based on a timer-driven, fuzzy-logic closed loop. A TRIAC-based switching circuit in series with the line voltage is used for current flow control during start-up. A timer-based embedded application adjusts the TRIAC triggering angle by shifting the TRIAC ignition pulse within the voltage period at a constant rate. Multitasking logic controls all three phases simultaneously. As a result, the voltage is gradually increased in the three-phase system, until the motor reaches full speed. An external user-defined setting can adjust the time frame from start to full speed. Certain improvements are introduced with regard to former similar designs, the most important being a closed-loop, voltage sensing, three-rule fuzzy system that effectively balances the inductive behavior and restores a predictable time-ramp at start-up. Because of the inductive character of three-phase asynchronous motors, the residual current after switch-off leads to higher nominal voltage across the motor coils, depending on the actual load. Therefore, it is found that an open-loop timer-driven switching logic is not adequate for a reliable soft start controller, since it can result in start-up time being unpredictably shorter than the one corresponding to user settings. In order to adjust torque to load behavior and produce a constant ramp depending only on user-defined time settings, a closed-loop fuzzy-logic system is introduced. A voltage sensor acquires the residual voltage in one of the system’s phase lines at adequate sampling rate and computes an input to a three-rule fuzzy system, in the form of an error function. The output of the system infers the time interval to the next ignition pulse. Both the timer logic and the fuzzy system logic are embedded in a lowcost PIC18F252 microcontroller. The behavior of the soft-start controller is smooth and predictable. The voltage ramp from zero to maximum voltage is very close to that corresponding to an equivalent ohmic load.