| 摘要 |
The invention relates to a switching device for a drive motor, for example a single-phase induction motor, having an operating capacitor which is utilised, for example, for driving a circular saw and which has a switching contactor which, after connection, is latched via a latching circuit, and has a DC braking circuit which becomes effective via a phase-controlled thyristor for a predetermined time interval after disconnection, the switching contactor alternately connecting the motor to the mains and the windings of the same to the braking circuit. In order to achieve reliable removal of power from the braking circuit once the braking interval has elapsed even in the event of defective components of the control loop or in the event of a defective thyristor, and hence in order to protect the windings of the motor both against overloads and to protect said windings against renewed starting, a contact of a relay which can be energised via the switching contactor is provided in the DC braking circuit, which contact drops out with a delay once the predetermined time interval for driving the thyristor has been completed, so that the braking interval itself is not adversely affected. The delay can be achieved easily in this case by connecting the winding of the relay in series with that of the switching contactor and a diode and by the relay winding being connected in parallel with a charging capacitor which can also be used as the power supply for the drive.
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