Motor starter with dual-slope integrator

Abstract

A motor starter includes separable contacts interconnected between an alternating current (AC) power source and an AC motor for switching an AC load current which flows from the power source and through the separable contacts; a dual-slope analog-to-digital (A/D) conversion circuit for sensing the load current and generating a current value; and a microcomputer with a coil drive circuit for selectively opening the separable contacts as a predetermined function of the current value. The dual-slope A/D conversion circuit includes current transformers for sensing the AC load current and for selectively providing a received current therefrom, a current reference for selectively providing a reference current, a multiplexer for multiplexing the currents, and a class B preamplifier for amplifying and rectifying the multiplexed AC current. The dual-slope A/D conversion circuit also includes a dual-slope integrator which generates an integration value by positively integrating the received current and by negatively integrating the reference current; a comparator which compares the integration value with a predetermined threshold value and generates a comparison signal for the microcomputer; and a bias amplifier for biasing the current transformers with a voltage which is about equal to a bias voltage of the preamplifier. The preamplifier, the dual-slope integrator, and the bias amplifier may each include a Norton amplifier.