Research On Modeling And Soft Switching Technology Of Switching Power Amplifier For Magnetic Bearing

This paper focus on three-level power amplifier modeling and its soft switching power amplifier technology in magnetic bearing. The switching power amplifier is a nonlinear system. The existing power amplifier model has some drawbacks and switching devices still work under the hard switching conditions. So switching power amplifier has some problems such as large switching loss and output current interference.Firstly, the several structure types and their working principles of power amplifier are analyzed. By using the method of circuit analysis and the method of inductance voltage-second balance principle, it points out that the use of the three-level switching power amplifier has some advantages in magnetic bearing system. Based on the analysis of the three-level control principle, the mathematical model of the three-level power amplifier is established by using equivalent switch model. The power circuit is divided into two sub-circuits and the whole model of three-level power amplifier can be derived by two sub-circuit models. This method can not analyze the characteristics of output current ripple. In order to slove this problem, the Fourier analysis method is proposed to deduce the three-level power amplifier mathematical model. The simulation results of this model are in excellent agreement with experimental results. These results show that the model using the equivalent model can effectively reflect the actual characteristics of power amplifier. The model based on the method of Fourier analysis can be analyzed not only the characteristics of power amplifier, but also the output current ripple. It can be guide to design the circuit parameters of power amplifier from the mathematical model. Based on the derived model, the effect of control system for three-level power amplifier by changing the circuit parameters is discussed.In order to reduce the switching losses and the output current interference in the power amplifier circuit, a new parallel resonant DC link three-level power amplifier of magnetic bearing is proposed. The auxiliary resonant circuit is added into the traditional three-level power amplifier and it makes all devices zero-voltage switching or zero current switching. The working principles of parallel resonant DC link three-level power amplifier are analyzed. Each of the circuit mode equation and the condition of soft-switching circuit parameters are derived. For this circuit needs more complex control circuit, an improved soft switching three-level power amplifier is proposed. The action mode of each principle and circuit mathematical equation are deduced, circuit parameters of meeting the conditions of soft-switching resonant are derived. Output voltage duty cycle and output current ripple equation are derived by mathematical analysis.Theoretical simulation and experimental results show that switching devices of parallel resonant DC link three-level power amplifier work under all soft-switching conditions. It effectively reduces the traditional three-level amplifier switching losses and output current interference. However, the improved soft-switching three-level power amplifier not only has the former advantages but also reduces the switch voltage stress. It has the same small output current ripple as the traditional three-level power amplifier.Based on this improved soft switching three-level power amplifier, the high speed magnetic bearing stably suspends and runs successfully at 30000r/min. Comparing with hard switching three-level power amplifier, the fundamental frequency current amplitude of soft switching three-level power amplifier reduces by forty percent.