Motor control system for sewing machine

Abstract

A motor control system for sewing a machine is disclosed in which motor drive thyristors, a trigger phase control circuit for the thyristors to control motor speed, and a dynamic brake control circuit means are employed. The dynamic brake control circuit means includes a shunt thyristor connected in parallel with the armature winding of the motor, and a brake command circuit which triggers the shunt thyristor so as to stop the motor by dynamic braking force. The brake command circuit is energized and triggers the shunt thyristor when a stop command signal and a deceleration detection signal are supplied. Thus dynamic braking of the motor of a sewing machine can be accomplished at a predetermined lower speed. To use the motor as a dynamo for dynamic braking stop control of the motor, a trigger phase clamp circuit is employed. To stop the motor at predetermined halt positions, a timer circuit, a predetermined constant trigger phase command signal generator circuit, and needle position detectors are employed. The timer circuit is energized by the stop command signal and generates a signal in a predetermined time interval, by which the generator circuit is energized and controls the trigger phase of motor drive thyristors to decelerate the motor. The brake command circuit triggers the shunt thyristor when motor speed is within a predetermined threshold level and the needle position detectors detect the lowered halt position or the raised halt position of the sewing needle or thread take-up lever.