Speed Control for Doubly Fed Induction Motors With and Without Current Feedback

This paper presents a method to control the speed of doubly fed induction motors. The motivation lies in the possible use of such motors for hybrid electric propulsion. The stator of the motor is assumed to be supplied by three-phase voltages with frequency and magnitude that may vary. The rotor is supplied by a bidirectional converter. Two versions of the algorithm are presented. The first version does not rely on inner current loops, but instead commands directly the voltages applied to the rotor. These voltages are computed using an open-loop controller for torque and for stator reactive power, which is augmented to follow a speed reference profile. The second version uses stator and rotor current measurements to ensure tracking and limiting of the rotor currents. All the parameters of the control systems can be computed based on estimates of the machine parameters together with a desired time constant of the speed response. The alignment of an incremental encoder is integrated in the initialization of the algorithm. Practical implementation and testing are performed easily and rapidly. Experiments performed on a small-scale laboratory testbed show very good tracking performance of a speed profile in tests involving one motor, as well as two motors on a single three-phase supply. Improved current limiting is observed with the version incorporating current feedback.