Abstract: A multi-rotor electric motor having a wound main rotor mounted on a drive shaft which is rotatable within a cup-shaped stator, which is so wound, that when connected to a source of polyphase power, as to provide a rotating magnetic field. A second rotor equipped with permanent magnets is mounted for rotation about and exteriorly of the stator and which are effected by the rotating magnetic field so as to tend to rotate the second rotor at synchronous speeds in the same direction as the main rotor. The two rotors are normally connected for mutual rotation between starting speed and pre-determined speed less than synchronous speed by a spring-closed centrifugally-opened clutch assembly which disengages when the main rotor reaches said pre-determined speed so as to enable both rotors to rotate independently of each other and which re-engages both rotors for mutual rotation when the speed of the main rotor falls below said pre-determined speed.