A system comprising: a constant-speed driver configured for rotating at a substantially constant rotational speed; a rotating machine configured to be driven into rotation by the constant-speed driver at a variable rotation speed; a controller , for controllably changing a rotation speed of the rotating machine ; a variable speed transmission , arranged between the constant-speed driver and the rotating machine ; wherein the variable speed transmission comprises a speed summing gear arrangement with a first input shaft , a second input shaft and an output shaft ; wherein the output shaft is drivingly coupled to the rotating machine ; wherein the first input shaft is drivingly coupled to the constant-speed driver ; wherein the speed of the output shaft is a function of the speeds of the first input shaft and of the second input shaft ; wherein a continuous variable transmission device (43; 243; 343) is mechanically coupled to the constant-speed driver and configured to modify the speed of the second input shaft of the speed summing gear arrangement (21 wherein the continuous variable transmission device (43; 243; 343) is functionally coupled to the controller , such that the rotation speed of the rotating machine drivingly coupled to the output shaft can be modulated, while the rotation speed of the constant-speed driver remains constant by the controller controlling a fraction of power transmitted from the constant-speed driver through the continuous variable transmission device (43; 243; 343) to the rotating machine , characterized in that the continuously variable transmission device comprises a magnetic coupling or a hydro viscous driver . a constant-speed driver configured for rotating at a substantially constant rotational speed; a rotating machine configured to be driven into rotation by the constant-speed driver at a variable rotation speed; a controller , for controllably changing a rotation speed of the rotating machine ; a variable speed transmission , arranged between the constant-speed driver and the rotating machine ; wherein the variable speed transmission comprises a speed summing gear arrangement with a first input shaft , a second input shaft and an output shaft ; wherein the output shaft is drivingly coupled to the rotating machine ; wherein the first input shaft is drivingly coupled to the constant-speed driver ; wherein the speed of the output shaft is a function of the speeds of the first input shaft and of the second input shaft ; wherein a continuous variable transmission device (43; 243; 343) is mechanically coupled to the constant-speed driver and configured to modify the speed of the second input shaft of the speed summing gear arrangement (21 wherein the continuous variable transmission device (43; 243; 343) is functionally coupled to the controller , such that the rotation speed of the rotating machine drivingly coupled to the output shaft can be modulated, while the rotation speed of the constant-speed driver remains constant by the controller controlling a fraction of power transmitted from the constant-speed driver through the continuous variable transmission device (43; 243; 343) to the rotating machine , characterized in that the continuously variable transmission device comprises a magnetic coupling or a hydro viscous driver . <EOS>