Abstract: A multi-rotor A.C. electric drive device comprises a stator capable of producing a rotating or an alternating magnetic field and at least two rotors capable of producing an electromagenetic torque at asynchronous speed. The rotors are respectively called the main rotor and the auxiliary rotors. In low speed operating mode of said device, the electromagnetic torque of the auxiliary rotor is amplified through a transmission portion and drives an output element together with the main rotor; after the auxiliary rotor is accelerated to the synchronous speed, an automatic unloading device releases the auxiliary rotor from the driving engagement with the output element. Examples illustrating several types of electromagnetic portion, transmission portion and the automatic unloading device are given.