Electric tool driven by universal motor - has electronic speed preselector and thyristor for automatic changeover to mains voltage

摘要

The system relates to speed control for electric tools driven by a universal motor. The speed to be achieved is pre-selected to suit the worked material via a potentiometer (5) which controls the maximum firing voltage applied to the thyristor (4). Operation of the main switch (22) thus provides progressive speed control from zero to the pre-selected speed, speed control in the selected range is achieved by a potentiometer (7) ganged to the main switch. The resulting half-wave output from the thyristor is applied to the rotor and the field windings (2 and 3). If selected speed is equal to maximum tool speed, travel of the main switch (22) to its full extent results in the closing of limit switches (23 and 24) which disconnect the thyristor and apply the mains voltage directly to the motor.