maximum torque t max determined by the maximum torque determining means 23 is therefore reduced by the torque correction value u03b4t max determined by the torque shift 32 thus forming a modified maximum value t max in contrast when the classifier 31 finds that an underfrequency has occurred the inhibitor 33 is operated via the output signal that is correspondingly emitted by the classifier 31 a change in the value of the maximum torque t max is therefore blocked such that no adaptation takes place in the event of an underfrequency the torque of the generator converter system 14 15 is therefore restricted in the event of an overfrequency but the maximum permissible torque remains unchanged when an underfrequency occurs a frequency dependent rotation speed limiter 4 is also provided this comprises a lower branch 41 and an upper branch 42 once again the signals determined by the classifier 31 for positive frequency discrepancies and negative frequency discrepancies u03b4f and u2212 u03b4f are applied to its inputs the signal for positive frequency discrepancies is in this case applied to the lower limiter module 41 but not to the upper limiter 42 this means that the lower limit for the nominal rotation speed is increased when.