Research On Characteristics Of Wound-Rotor Brushless Doubly-Fed Machine

The Brushless Doubly-Fed Machine (BDFM) is a new type machine which has a good prospect. As a generator, the BDFM can realize variable speed constant voltage and frequency, it can be used as shaft generator, wind turbines and so on; as a motor, it can realize Variable Voltage Variable Frequency speed regulation. Its advantage is that the associated converter only requires a fractional rating compared to that of the machine. BDFM as a motor has a hard mechanical characteristic. Because of this researchers all over the world have done extensive research on it.The BDFM with a wound-rotor is a induction machine with a new rotor structure. It is different from the BDFMs with'nested-loop'design rotors or reluctance rotors. By using a new rotor winding with multiple pitches and turns, it can effectively decrease the harmonic of rotor winding and increase the winding factors of the two fundamental waves. It can also reduce the loss of the machine caused by the harmonic and enhance the efficiency of the machine. Therefore the BDFMs introduced in this dissertation are all using wound-rotor.There are two aspects of this dissertation's research purposes. One is calculating the machine's performances and parameters accurately. That is to say, the calculated values should be consistent with the experimental results. The other is investigating the torque characteristic of BDFM in depth and gaining the stability criterion when the BDFM running as a motor.In this dissertation, the main works are as follows:First, the basic principles of BDFM with a wound-rotor are analyzed. The laws of power distribution in the two stator windings are derived. And the state equation of BDFM in the ABC coordinate system is derived and established. The transformation between the machine's parameters in state equation and that in equivalent circuit is also given.Secondly, one finite-element model of a wound-rotor BDFM is established. When the machine operated as a generator, the dissertation compares the calculated results of analytic method and finite-element method with the experimental results. Based on finite-element model, the method for calculating the active power and power factor of control winding is introduced. The estimation method for the machine efficiency is also introduced. When the machine operated as a motor, the dissertation compares the simulation result of finite-element model with that of MATLAB. Based on a simplified finite-element model for BDFM, taking advantage of the flux-MMF calibration curve, a method that amended the two excitation inductances of BDFM is presented. Through this method, the analytic and numerical methods have carried on effective combination. It can reduce computation time and increase calculation precision.Thirdly, the torque characteristic for BDFM with a wound-rotor is investigated. The torque components of BDFM in different operating modes are discussed. Using the machine's equivalent circuit, the methods for calculating asynchronous torque, alternating torque and synchronizing torque of BDFM are introduced. Under open-loop voltage control, the stability criterion when the BDFM operated as a motor is given.Finally, a prototype machine of wound-rotor BDFM with 4/2 pole pairs is designed. On load and no-load conditions, the voltage, frequency and THD of the machine are measured. At different speeds, the efficiency curves of this machine are also given. And combining with the finite-element model of BDFM, the harmonic analysis of air-gap field for the machine is carried out.