A resonant motor unit that has a resonant motor, a control unit, a measurement unit, and an evaluation unit is described. The control unit drives the resonant motor at a driving frequency, short-circuits the resonant motor during at least a first short-circuiting phase in successive driving cycles and concludes the first short-circuiting phases by switching off the current flow through the resonant motor at least at a predetermined first time instant within the driving cycles. The measurement unit successively measures at least a first voltage signal provided by the resonant motor at the predetermined first time instant, and the evaluation unit for determines whether the first voltage signal has changed between successive measurements.
