The voltage applied to a power converter is detected by a voltage detector, a function generator produces a voltage proportional to this detected voltage, and a high-pass filter detects an AC component superimposed on the voltage. The inventive rewind mechanism permits a flexible member to be pulled outside its housing and remain outside the housing without a rewind force on the flexible member simply by pulling on the flexible member and releasing it. This device 34 may be a particle-particle separator or a jet mill, where compressed air or high-pressure steam is the energy source.