Abstract: A variable-speed three-phase drive is disclosed. The drive includes a switchable, undersynchronous converter cascade for driving two mechanically coupled asynchronous motors which have slip ring rotors and which can be connected separately to a three-phase network. The slip ring rotors of the two asynchronous motors are connected, respectively, with the three-phase terminals of different uncontrolled rectifiers. The latter two rectifiers, together with two inverters and two other rectifiers, are arranged in a ring circuit in such a manner that two rectifiers are always followed by an inverter. In accord with the invention, a switch is provided for connecting the three-phase terminals of the two rectifiers which are disposed on the respective cathode sides of the two inverters, and for also connecting the three-phase terminals of the two rectifiers which are disposed on the respective anode sides of the two inverters.

Abstract: A speed controllable three phase drive including an asynchronous motor having a slip ring rotor which can be connected to a three phase network and a d.c. motor mechanically coupled to the asynchronous motor with the d.c. motor fed through an uncontrolled rectifier connected to the slip ring of the asynchronous motor has a controlled static converter in series with the uncontrolled rectifier and d.c. motor. In an alternate embodiment the d.c. motor is replaced by a static converter motor fed by a controlled inverter connected to the uncontrolled rectifier. The controlled static converter is coupled to the three phase network to result in a three phase drive with a greater speed range and a lower reactive power requirement as compared to subsynchronous static converter cascades.