Study On Basic Parameters And Control Methods Of A Bearingless Switched Reluctance Motor

In recent years, technologies of bearingless motors have become one of the research focuses on electrical drives. Applying the bearingless technologies to a switched reluctance motor can combine characteristics of switched reluctance motors with advantages of bearingless technologies, which is of benefit to switched reluctance motors to break through development bottleneck and enter the domains of micro-motor and super-high speed motor. Studying on bearingless switched reluctance motors is becoming more and more popular.In this paper, systemic study on basic parameters and control methods of a bearingless switched reluctance motor (BSRM) are developed. The basic operation theory of a BSRM is described and the structure parameters of a sample BSRM are designed. Design principles of main parameters are fixed on based on characteristics and operation demands of BSRM. Formulas used to calculate main structure parameters are proposed according to design method of switched reluctance motors considering demands of radial force. Main structure parameters are finally calculated considering rating demands of the sample BSRM. On the basis, a finite-element (FE) analytical model of the sample BSRM is established and systemic simulations for characteristics of radial force and torque are made, including variation trends analysis of radial force and torque along with rotational angle, winding current and radial displacement of rotor. Besides, coupling relation of radial force on two freedom-degrees and complex nonlinear coupling relation between radial force and torque are analyzed.In view of limitations of traditional mathematical model of BSRM on conduction region, an extension mathematical model to full angle of one phase cycle is derived in detail. Coefficient of magnetic path is first educed from FE analysis, then the permeances of air gap are derived according to magnetic path separation and the inductance matrix was constructed based on the magnetic equivalent circuits. Accordingly, new mathematical formulas of radial force and magnetic torque were derived based on the theory of electromechanical energy conversion. The validity of the model is proved through FE numerical calculations.In view of complex nonlinear coupling relations between radial force and torque of BSRM under traditional single-phase-conduction control strategy, an idea of taking torque adjustment and force disturbance as uncertain factors of suspending system is presented. A variable structure robust control law which is not sensitive to system parameter's variation and disturbance is designed for radial displacement control of suspending system. Inverse system method is first applied to decouple radial force on two freedom-degree to acquire two independent linear subsystems with uncertain terms of torque winding's current variation and force disturbance, then a slide mode is constructed through solving a Riccati equation corresponding to the decoupled uncertain linear subsystem. Accordingly, a variable structure robust control law is designed using index reaching law. Finally, simulations on radial force decoupling and radial displacement control are carried out and the results have proved the validity of designs.A decoupling control strategy based on two-phase respectively conduction is proposed in view of the characteristic of little mutual inductance between phases of BSRM. Torque-angle characteristic is analyzed and a suitable two-phase conduction region is found. In the conduction region, one phase products radial force, called suspending phase, and another phase products average torque, called torque phase. The suspending phase can products max radial force and little coupling relation exists on two radial freedom-degrees. Accordingly, independently control of average torque and radial force can be realized. Mathematical model of two-phase respectively conduction mode is derived using traditional model and full angle extension model. Good decoupling performance of two-phase respectively conduction control strategy is vilified through FE comparison analysis with traditional single-phase conduction mode. Fuzzy controllers used to control radial displacement and speed respectively are designed under two-phase respectively conduction mode. Simulation results show that designed fuzzy controllers have good performance.Take the designed sample BSRM of 12/8 structure as control object, hardware of digital control system is preliminary designed, including power converter and control system. In the power converter design part, half-bridge asymmetric structure is used as main circuit of torque winding which is commonly used in switched reluctance motors and three-phase half-bridge inverter is used as main circuit of suspending winding considering reversibility of current direction of suspending winding. Structure designs of driving and buffer circuits of main switches are implemented. DSP+CPLD structure is used to design control system and their control tasks are reasonably arranged. Outside-chip analog PID design programme is used in radial displacement control of rotor and inside-chip digital control programme is used in speed control. Systemic structure designs of detection circuits of rotor location and radial displacement, outside-chip analog PID controller, current hysteresis controller, protection circuits and so on are implemented. Some design ideas are applied to a six-phase 12/10 structure switched reluctance motor to develop some experiments such as acquiring square wave current of torque winding. Good effects are acquired.The basic structure and operation principle of an axial-radial hybrid magnetic bearing used to BSRM are discussed and the basic mathematical model is derived.