Research Of Control Strategy Of Novel Five-Phase Six-leg Switching Power Amplifier Applied In Magnetic Levitating Bearing System

In order to improve the integration level, decrease the cost and power loss of the magneticlevitating bearing system, the paper focus on a five-phase six-leg topology and research the controlstrategies in depth. This topology can take place of five H-bridge switching power amplifiers to drivefive DOF magnetic bearing system. As a result, it can reduce the number of power switch and logicinterfaces. For magnetic levitating bearing system, compared with voltage-mode switching poweramplifier, current-mode switching power amplifier can simplify the model of control system and morelikely to make the system stable. As a result, current-mode switching power amplifier is the focus ofstudy.The paper introduced a non-linear current control method, which also called the maximum currenterror with sample/hold control. This method can be applied to any multi-phase switch power amplifiersystem. In the method, control of neutral leg and phase leg was independent. Control method ofneutral leg was the improved maximum current error modulator, which could reduce the couple offive output currents to the largest extent by adding a current error polarity judgment module, andcontrol of other legs was improved sample-hold modulator.Based on researches above, three control strategies used for five-phase six-leg topology have beenproposed, which contain one linear control method and tow non-linear control methods. Theadvantages and disadvantages of these methods were discussed. The linear control method was callednode potential control, which was based on current PID regulating. The non-linear control methodswere maximum current error hysteretic control and variable duty ratio sample/hold control. In themethod of node potential control, phase currents were controlled by node potentials of each bridge.The advantages were not complicated and small current ripple, disadvantages were PID parametersaffected by the load parameters which need a large amount of time and extensive experience ofengineer to fix out the right PID parameters. The maximum current error hysteretic control kept theneutral leg controlled by maximum current error control strategy, at the same time, five other legswere regulated by tow-state hysteretic comparator. In such case, the control strategy of six-bridgepower system was simplified by not using any D filp-flops comparing with the sample/hold control.On the other hand, it also had problems like switching frequency in phase legs were affected by busvoltage, current loop width and inductance. The variable duty ratio sample/hold control method hadbenefits of both linear and non-linear control methods. Duty ratio of neutral leg can change from1to0according to the need of load current, which decrease current ripple and increase control accuracy.Based on analysis and simulation, the experiments were carried out with a prototype using CPLDas the core controller. Simulation and experimental results verified the feasibility and effectiveness ofthese control methods. Five-phase six-leg power system can output five independent current and trackthe current reference successfully with advantages like small current ripple, fast dynamic response and little impact on load parameters.