Research On Frequency Conversion Adjustable-speed Of Brushless Doubly-fed Motors

The brushless doubly-fed machine (BDFM), with simple structure and favorable reliability, has gained renewed attention in adjustable-speed drive and variable-speed constant-frequency generator applications in recent years for its features adjustable power factor and high efficiency. In BDFM, since both power winding and control winding are mounted in stator, current is doubly fed into the machine without electric brush and slip ring hence improving robustness and security can be achieved in this configuration. BDFM allows the brushless extraction of slip power via control winding, so this leads to a smaller and less expensive inverter, and less harmonics at the network. Up to now, the domestic study on BDFM is just confined to laboratory and there is no practical system used in industrial applications, so it is necessary to go deep into its research.In this paper, systematic research on variable-frequency adjustable-speed drive based on BDFM is carried out. Beginning with its historical development, this paper presents the BDFM's calculating model and operating principle. And a simulation is carried out to verify its open-loop performance. The BDFM has been constructed using a cage rotor together with a standard induction machine stator which is rewound with two separate windings, and a series of tests is carried out on the machine. And then, the.research is focused on the control strategy of the BDFM. On the fundament of analyzing BDFM's principle and the direct torque control method, a field orientation control scheme of BDFM is presented. With the help of different coordinates, a simple method of calculating flux linkages & torques is proposed. A system controller block diagram is proposed in this paper according to the rotor reference frame d-q model. The simulation to the torque and frequency characteristics illustrates the effectiveness of the proposed method, and shows that fast response and precision of control are achieved. The system is reasonable at under synchronization electromotor condition and super synchronization electromotor condition.The research in this paper has a reference value for the practical application of BDFM, and will promote the BDFM's designing, modeling and controlling.