Abstract: A safety device for a tool powered by an electric motor. It has a main switch, electronic controls, and a tachometer-generator. In order to provide a device for switching off a motor as a function of motor speed that is simple and economical and that will retain the same nominal switch-off value for the total life of the device, a relay is wired in parallel with the motor and controls the supply of power to the motor through a relay contact.

Abstract: An automatic speed controller for a universal, series-wound, electrical motor, is part of a system whereby the motor is energized by a A.C. supply from control means including a triac, in turn, controlled by the speed controller. Within the controller signals are derived comprising solutions of the equation: ##EQU1## or a derivative thereof, or an approximation thereof, where N is the armature speed, R.sub.a is the resistance of the armature, K is a constant dependent upon the motor design, (V.sub.a -I.sub.a R.sub.a) is the true average of the alternating quantity (v.sub.a -i.sub.a R.sub.a), where v.sub.a and i.sub.a are, respectively, the instantaneous armature voltage and current, and .0..sub.AV is the true average flux in the magnetic circuit of the motor, both true averages being obtainable when the motor is operating under steady state conditions.