An actuating drive for a rotor blade in a wind power installation is provided with an electric motor and a frequency converter which has a rectifier that is to be fed from a power supply system, a link connected to the rectifier, and a d.c.-to-a.c. inverter for feeding the motor, the link carrying a specified nominal operating voltage in a network-fed normal operation of the frequency converter. Furthermore, an auxiliary energy supply device is provided that is connected to the link. The auxiliary energy supply device includes and energy store which makes available a support voltage that amounts to less than 80% of the nominal operating voltage of the link. A diode is connected between the energy store and the link, and is arranged, with regard to its forward direction, in such a way that it feeds a support voltage from the energy store into the link if the link voltage falls below the support voltage. The diode prevents a current flow between the energy store and the link if the link voltage is greater than the support voltage. A charging device for the energy store has a lower charging voltage than the nominal operating voltage of the link.