Energy conversion apparatus with a frequency converter comprising a rectifier unit and a converter unit and with a magnetically geared generator for converting rotational energy having a varying rotational frequency (Ω P) into electrical energy having a fixed output frequency, (ω grid) comprising: a stationary inner winding unit having a first number of pole pairs P R, a stationary outer winding unit arranged coaxially around the inner winding unit and having a second number of pole pairs P S; a central axis is a symmetry axis of the inner and outer winding unit (201, 203), wherein the number N P of pole pieces corresponds to N P = P R ± P S, with P R > P S,; a rotatable magnetic flux modulator operated with variable mechanical input speeds (Ω P) via a main shaft and comprising a plurality of pole pieces from iron inserts, wherein the rotatable magnetic flux modulator is arranged coaxially between the inner winding unit and the outer winding unit and rotatable about the central axis, wherein the inner winding unit, the outer winding unit and the rotatable magnetic flux modulator are adapted to convert rotational mechanical input energy provided at the rotatable magnetic flux modulator into electrical output energy provided as an alternating output current at one of the inner winding unit and the outer winding unit, characterized in that the inner winding unit, the outer winding unit and the rotatable magnetic flux modulator are adapted to control an output frequency (ω grid) of the alternating output current by feeding output energy as an alternating feedback current from one of the inner winding unit and the outer winding unit to the other one of the inner winding unit and the outer winding unit via the rectifier unit and the converter unit and by adjusting a feedback frequency (ω R) of the alternating feedback current with respect to the rotational input frequency (Ω P) into the magnetically geared generator by means of the converter unit (305, wherein ω R =(P R -P S).Ω P +ω grid. a stationary inner winding unit having a first number of pole pairs P R, a stationary outer winding unit arranged coaxially around the inner winding unit and having a second number of pole pairs P S; a central axis is a symmetry axis of the inner and outer winding unit (201, 203), wherein the number N P of pole pieces corresponds to N P = P R ± P S, with P R > P S,; a rotatable magnetic flux modulator operated with variable mechanical input speeds (Ω P) via a main shaft and comprising a plurality of pole pieces from iron inserts, wherein the rotatable magnetic flux modulator is arranged coaxially between the inner winding unit and the outer winding unit and rotatable about the central axis, wherein the inner winding unit, the outer winding unit and the rotatable magnetic flux modulator are adapted to convert rotational mechanical input energy provided at the rotatable magnetic flux modulator into electrical output energy provided as an alternating output current at one of the inner winding unit and the outer winding unit, characterized in that the inner winding unit, the outer winding unit and the rotatable magnetic flux modulator are adapted to control an output frequency (ω grid) of the alternating output current by feeding output energy as an alternating feedback current from one of the inner winding unit and the outer winding unit to the other one of the inner winding unit and the outer winding unit via the rectifier unit and the converter unit and by adjusting a feedback frequency (ω R) of the alternating feedback current with respect to the rotational input frequency (Ω P) into the magnetically geared generator by means of the converter unit (305, wherein ω R =(P R -P S).Ω P +ω grid.