The invention relates to an apparatus for regulating the exciter current for a rotary-current generator. It has a voltage source, a rectifier, three phase windings, an evaluation unit, and a voltage regulator. The evaluation unit has three input terminals, and each of these input terminals is connected to one of the phase windings. The evaluation unit is intended for evaluating the phase voltages derived from the three phase windings, in order to detect error functions of the rectifier and/or of the phase windings. If such unauthorized work states are detected, then via the voltage regulator, the exciter current flowing through the exciter winding is reduced.