Research On The Static Load Capacity And Direct Torque Control Of Brushless Doubly Fed Machines

A brushless doubly fed machine(BDFM)is a new type of AC variable speed motor that has the characteristics of both asynchronous and synchronous motors.BDFM has the special structure,and it offers high reliability and low-maintenance requirements by cancelling the brush gear and slip ring.Therefore BDFM has good application prospects in AC driving systems,wind power generation.Furthermore in the variable speed system,the BDFM required variable frequency device capacity is small,so the input costs of speed regulation system is reduced.However the BDFM is a high-order,multivariable,and strong coupling nonlinear system.Compared with asynchronous and synchronous motors,BDFM has a complicated structure and a complicated running mechanism,thus the establishment of a mathematical model,the analysis of stability,and the design of a control strategy are difficult.Therefore,the in-depth theoretical study on BDFM has great realistic meanings and practical significance.In this article,the modeling method,the static load capacity and the direct torque control(DTC)of BDFM are studied.The main contents of this dissertation are summarized as follows:Firstly,this article summarizes literature at home and abroad,introduces the development history and application prospect of BDFM,generalizes the research status of structure,mathematical model and control strategy at home and abroad,and analyzes the structural characteristics and operating principle of BDFM.Secondly,according to the structural characteristics and operating principle of BDFM,the mathematical model of BDFM in the rotor reference frame is deduced.By taking the negative conjugation of the control motor(CM)variables in rotor reference frame,under the static state,the complex variables of the PM and the complex variables of the CM(after taking negative conjugation)rotate in the same direction and have the same speed relative to the rotor.After taking negative conjugation,the synchronous reference frame and CM static reference frame state-space model of BDFM is derived.All variables of the former are DC values under the static state,the algebraic equation of the static working point is easily obtained.The former is used to study the static load capacity of BDFM.The latter is used to study the DTC of BDFM.Therewith,follow the above ideas,on the basis of the synchronous reference frame state-space model,the static equations of BDFM are obtained.By solving the static equations,the theoretical output capacity upper and lower limits of BDFM are obtained,and then the relationships between the output capacity of the motor and the flux and speed are analyzed.These results have guiding significant meaning for the BDFM's design and development and also for the control strategy study.Based on the static solutions,the operating characteristics of BDFM under open-loop are analyzed by small signal model.And the operation efficiency of BDFM is discussed.Finally,on the basis of the CM static reference frame state-space model,the losing control problems of DTC for BDFM are studied and analyzed by the flux and the torque derivatives.For the losing control problems,the two improved control strategies are presented,the synthetic vector direct torque control by increasing the number of the switch sectors and voltage vectors simultaneously and the flux-angle-difference feedback control by using the flux-angle-difference feedback hysteresis comparator instead of the torque feedback hysteresis comparator,respectively.The two improved control strategies are compared with the DTC system of BDFM by simulation.The simulation results show that,the two improved control strategies can solve the losing problems of BDFM,improve the system operating performance,and have good dynamic and static characteristics,and keep the advantage of conventional DTC,such as less dependence on motor parameters,simple structure,and strong robustness.An experiment platform based on dSPACE is built.