Motor control system for sewing machine

Abstract

A motor control system for a sewing machine is disclosed in which motor drive thyristors, a trigger phase control circuit for the thyristors, a comparator circuit for generating a trigger phase indication signal to accelerate or decelerate the motor to an indicated speed which corresponds to force downward on a foot controller, needle position detector means, a brake command circuit, and a predetermined constant trigger phase command circuit are employed. The predetermined constant trigger phase command circuit, which is energized by a stop signal from the foot controller, supplies a predetermined trigger phase indication signal to the trigger phase control circuit to lower the motor speed. The brake command circuit generates, at the time when the needle position detector means detects the sewing needle at a predetermined halt position, a brake command signal. The brake command signal is supplied to a brake means, e.g., dynamic brake means, mechanical brake means, etc., to stop the sewing needle or thread take-up lever at a predetermined 'halt' or stop position. A timer circuit is preferably connected between the foot controller and the predetermined constant trigger phase command circuit to deenergize the predetermined constant trigger phase command circuit and stop the motor after a predetermined time interval. This timing operation of the timer circuit prevents overheating of the motor when the sewing needle stops accidentally before reaching the predetermined halt position.