The present invention relates to a safety device for a tool powered by an electric motor and having a main switch, electronic controls, and a tachometer-generator.
Known electric power tools are switched on and off through a main switch. The motor is controlled through electronic controls that receive an actual value from a tachometer-generator. When the controls fail, it can happen that the speed of the motor increases impermissibly. This means that, in right-angle grinders for example, the speed will increase so much that the grinding disk cracks. It would accordingly be desirable for the motor to shut off when the controls fail.
One solution is a centrifugal switch of the type described in German Pat. No. 638,928. This method of switching off a motor as a function of motor speed is however very complicated because of the mechanical parts involved, and hence expensive. The system is also very sensitive to contamination, which affects operating precision etc.
USPTO Backgrounds
USPTO Backgrounds