Research On Switching Power Amplifier For Active Magnetic Bearing

In recent years,the active electromagnetic bearing technology has been paid more and more attention in the application of high speed mechanical equipment,and more and more practical applications are applied.As a part of the actuator in the active electromagnetic bearing system,the performance of the switching power amplifier(SPA)directly affects the performance of the electromagnetic bearing system.The maximum speed of the rotor in the rotating machinery depends on the bandwidth of the SPA,and it can be broaden by increasing the bus voltage of the SPA.And the focus of this article is to achieve a SPA that can work at a high bus voltage.Firstly,the power amplifier is classified in this article.And the basic performance of the current mode SPA is analyzed in detail,such as bandwidth,output current ripple,power and loss.The theoretical analysis and experiment of the two-level and three-level PWM modulation is presented in this article as this method is widely used in current mode SPA.And the three level modulation method is generally used in the past high bus voltage condition because the output current ripple of two-level SPA is directly proportional to the bus voltage.And the digital controllers such as DSP and FPGA are widely used in this condition.So the design of the system is relatively complex,the cost is large,and there will be a delay which may have a great impact on the performance of the SPA system.So the simple two-level control circuit is presented in this article,and the higher switching frequency is used in order to reducing current ripple.After increasing the switching frequency,the traditional silicon based power devices may not meet the requirements.The appearance of wide band gap silicon carbide(SiC)devices can solve the contradiction between the voltage level and the switching frequency of the power device,and it is suitable for SPA with high bus voltage and high switching frequency.However,the third generation of silicon carbide MOSFET used in this article is relatively small in the range of driving voltage,and the parasitic inductance of the driver circuit may cause the voltage oscillation and overshoot.So the theoretical analysis of the voltage in the driver circuit is presented and the value of the voltage overshoot has been reduced by optimizing the design of PCB circuit to reduce the parasitic inductance.As the parasiticparameters of the SiC device is too small,the parasitic inductance in power circuit may cause the voltage and current oscillation easier during the switching transient.And the current oscillation isthe focus of attention in this article.So the transient circuit is modeled.And the current oscillation is suppressed by the method of power device parallel capacitance and the experiment is used to verify the circuit.Then,this article also pointed out that the output ripple current may affect the stability and steady state error of the current modeSPA.According to the actual working principle of the switching power amplifier and the outputripple current,the condition of the stable duty of the power amplifier is given.What's more,the steady-state error of the output current is calculated,and a new method which can reduce the steady-state errorand ensure the fast response of the output current is proposed in this article.And it is verified by simulation and experiment.At last,the current mode SPA of 200V/5A is designed in detail by the method of modular design.The performance of single circuit SPA is verified by simulation and experiment.And the eight groupSPAsare tested on the high motor platform of 75kW.