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Several large wind-tunnel drives recently built have involved a system of speed control which has seldom been used commercially, and a number of new problems had to be solved. The system consists of a wound-rotor induction motor whose slip rings are connected to a synchronous motor driving a variable-speed d-c generator which feeds a constant-speed d-c-a-c set putting the major part of the secondary power back into the line. This drive is found to be very efficient and particularly suited to very large fan or pump drives where a wide range of speed is required. The problems of steady-state and dynamic stability are discussed and some novel methods of analysis given.