PURPOSE: To reduce the time needed for stopping a drum to be braked under adjusting braking conditions in a rotary roller, for example, an apparatus for braking a drum, which is placed in a preparative machine, e.g. an opener, a clearer, a card or a carding machine.

CONSTITUTION: This rotary clearer is characterized in that an asynchronous motor is used as the driving motor for a drum, the fixed minor winding of the asynchronous motor is connected to an alternating current source and a direct current generating apparatus via a switching device, and the switching device performs the switching from the alternating current source to the direct current generating apparatus, or vice versa.