Integrated Control of a Motor/Generator Set Composed of Doubly Fed Induction Machines

This article considers the control problem for a motor/generator set, where a prime mover drives a generator and the electrical power produced drives a motor. Both the generator and the motor are assumed to be doubly fed induction machines with direct ac connection between their stators. The rotors are controlled by three-phase converters, so that operation is possible with motor and generator speeds that are different from each other, and not synchronized with the electrical frequency of the stator voltages. The strong couplings between the two machines motivate the design of an integrated controller. This article proposes a general framework for such a design based on a joint model of the two machines. A specific method is also developed for the control of the stator voltages and the motor velocity. In its simplest form, the proposed algorithm does not rely on current sensors. A current command option is also developed that ensures closer tracking and limiting of the rotor currents. The algorithm is relatively simple and all its parameters can be computed based on the estimates of the machine parameters. Practical implementation and testing can be performed rapidly. Experiments performed on a small-scale laboratory testbed show very good tracking performance of a speed reference profile.