Study On Control System Of Double-Fed Induction Motors

The double-fed control strategy for wound-rotor induction machine is investigated in this paper. The double-fed machine's mathematical model and matrix converter as well as three-phase high frequency PWM converter are also studied. Firstly, according to the basic principle of electrical machine, the double-fed machine's mathematical model under M ? T coordinate system is deduced, stator flux oriented vector control strategy as well as relational expression between stator current and rotor current are also investigated. At the same time, active power and reactive power seen from stator side are also been studied. All the researches provided theoretic base to carry out more research for double-fed control strategy. Secondly, double-fed machine's equivalence circuit is deduced through winding convert and frequency convert. According to the equivalence circuit and from the point of view of the law of energy conservative, the relational expression of the active power between stator side and rotor side under steady state is analyzed. Moreover, the energy flow direction and distribute process is also studied. Point out definitely that just due to the effect of frequency convert, double-fed control system can regulate power factor at stator side economically by means of control lesser reactive power at rotor side. In this paper, the AC/DC/AC main circuit topological structure is adopted to the control of double-fed machine. So, this kind of converter can be bifurcated as AC/DC converter block and DC/AC inverter block. This converter has two back-to-back 3-arm bridges with DC bus linked electrolytic capacitor, thus the AC/DC module and the DC/AC module can be controlled respectively. The control strategy can be change conveniently as needed by write different DSP program well and the whole hardware maintenance the same. The whole hardware structure can be designed very compactly. The AC/DC converter block is controlled by three-phase high frequency PWM vector control strategy with direct current hysteretic loop control method. Under this control strategy, the converter maintains a high power factor while the energy flows into or out of the converter. The slip energy can be regenerated thus increased the hole system efficiency and reduced harmonic pollution. Experiment results show that the control strategy is feasible The DC/AC module is controlled by stator-flux oriented vector control strategy. When M axis is attached to the stator flux, the active power and reactive power can be controlled respectively. According to stator flux oriented vector control strategy, dual close loop control structure is adopted. The rotor speed and the reactive power of stator side are defined as object function of outer loop. The current of the rotor is defined as object function of inner loop. The rotor speed and reactive power is controlled by the rotor current. In addition, the principle and structure of the stator flux observer is also introduced. This dual close loop strategy ensures the quick-response ability of system. It is concluded that the double-fed control strategy works well. It can be designed very compactly. The AC/DC/AC double-fed controller uses DSP as the key device thus make up the all-digital control system. The other elements are also introduced. The detailed power device hardware structure and control block structure is introduced too. The whole simulation module is made up by simulink blocks in Matlab environment. Dynamic simulations and experiment results show that the whole control strategy is effective and advantaged. Proposed a method of switches acting on current zero-crossing point in matrix converter to control double-fed machine in open loop and carried out experimental study. The bi-direction switch unit formation and main circuit hardware constitution of the matrix convert has introduced. The circuit structures which are using for created control signal and course are also analyzed. Switching element's current transformation course under every possible work mode and potential problems are analyzed in detail. Further more, corresponding improvement strategy is proposed. According to this strategy, a sample device is made to carry out experiment study. The experiment results show that this control method is correct and feasible. All of these provided useful experience to carry out open-loop control of double-fed machine in future.