Control system for switching loads on zero crossing

Abstract

An AC load switching system predicts when a zero voltage cross-over condition will exist for purposes of switching an AC load to minimize inrush currents. Prediction is carried out by characterizing the switching devices, which preferably are electromechanical in nature, and determining closure and bounce delays associated with each type of switching device. The system includes a processor, which could be a programmed microprocessor. The processor stores characterization parameters for the load switching elements including closure delay time as well as bounce time intervals. The processor energizes a switching element to close same at a time during the AC line cycle such that when the delay interval has passed and one half of the bounce interval has passed, the line voltage will be at a cross-over or zero voltage condition thereby resulting in minimal inrush current when the contacts have completely closed. Multi-phased loads can be switched at respective zero voltage cross-over conditions by adjusting load switching element energizing times by the delay of each of the respective phases. In a 60 hertz 3 phase system the delays are adjusted by 5.55 and 11.11 milliseconds from a reference phase.