In a series-wound motor speed control system comprising a solid state controlled rectifier connected in series with the motor and a source of pulsating rectified current, the rectifier is controlled by a synchronized unijunction transistor triggering pulse former including an RC timing network. The time position of the triggering pulses is subject to a threefold control, namely, a manual control, by varying the resistance of the network, to adjust the motor speed, a first automatic control in dependence upon variations of the motor load, and a second automatic control depending upon supply voltage variations, to maintain the adjusted motor speed constant independently of said variations.