Abstract: An apparatus (1) consists of an AC motor (2), a controller (5) and a power driver (6). In this case, the controller (5) and the power driver (6) are supplied with electric power firstly by an operating voltage (9) and secondly by a generator voltage (10) from the AC motor (2). When the operating voltage (9) is absent but the generator voltage (10) is present, the magnetization of the AC motor (2) is influenced such that the generator voltage (10) is kept between two limit values in order firstly to generate sufficient power for the operation of the controller (5) and of the power driver (6) and secondly to observe permitted voltage limits for the power driver (6).