Robert Boschgmbh

Martin Gerschner

Friedrich Hornung

Fritz Schadlich

Kurt Paule

Klaus Gunther

Thermal overload of a motor is avoided by measuring temperature and, when a limit temperature is exceeded, reducing the phase 'window' within each power cycle during which the motor is energized. The limit temperature is sensed by a temperature sensor which is warmed either by heat conduction through a cooling body or by an air stream from the motor. The overload protection apparatus requires no measuring resistance in the load circuit.

Phase control circuit for protection of a motor against thermal overload

To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to the tool bit, and current flow to the tool bit is detected by a current measurement stage (10; 31, 32) to disconnect energy to the drive motor (5), for example by disabling firing of a thyristor (15), or closing a valve of a compressed-air tool. The coupling capacitor should have a capacity small enough to provide high impedance to network power frequency, and to insure a substantial change in current being supplied to the tool bit holder, typically a chuck, if the tool bit meets a metallic object in the wall. The a-c supply circuit can be formed as part of a trigger circuit for the thyristor, by pulse-energizing a trigger diode (28) upon repetitive charge of a capacitor (26) through a charging network (22, 23, 24, 25).

Safety circuit for hand tools, and method for safe operation thereof

Electronic control and regulating system

A transformer having a secondary winding that delivers a regulating magnitude to a speed regulator circuit that approaches zero as the speed approaches the speed setting, is provided with two primary windings wound and connected so that they oppose each other in operation. The first is the conventional primary winding in series with the universal motor of which the speed is being controlled (as by a conduction phase angle control of the input voltage) while the second primary winding is in series with a resistance and, in some cases, also with a capacitor, with this series combination being connected in parallel with the armature winding of the motor for the best regulation, or in parallel with the motor as a whole if it is important to avoid additional internal connections to the motor. The capacitor is used to compensate for the inductive reactance of the second primary winding in cases in which that reactance is not negligible. A second capacitor across the secondary filters out commution 'hash.'

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Leinfelden-Echterdingen, Germany

Martin Gerschner