Motor control apparatus for electrically-driven sewing machine

摘要

A motor speed control apparatus for an electrically-driven sewing machine capable of effecting single stitch comprises a main motor circuit for supplying a motor current to a motor from an AC power supply through a main switch and a semiconductor element with a controlled electrode, a first control means for controlling the semiconductor element in order to effect normal operation, a controller adapted to change between its first and second state so as to control the speed of the motor continuously when the first control means is energized, a second control means for controlling the semiconductor element for effecting single stitch, and a change-over means for changing over the operation between the first and second control means. The second control means includes means for setting the trigger voltage for conducting the semiconductor element, a switching element coupled with the trigger-voltage setting means, a bias means for energizing the switching element, and a charge-discharge circuit. The bias means has a needle position detector means for detecting the position of a needle bar. The bias means supplies the switching element with a first bias sufficient to turn the switching element on when the needle position detector means detects the absence of the needle bar at the predetermined position, and supplies the switching element with a second bias insufficient to turn the switching element on when the needle position detector means detects the fact that the needle bar is at the predetermined position. The charge-discharge circuit renders the bias means in such a state as to supply an insufficient bias to turn the switching element on for a predetermined period of time after the time point at which the controller is set in the states between the second states and the predetermined state between the first and second states, which the conduction of the switching means renders the trigger voltage setting means ineffective.