Research On Dual-PWM Frequency Control

Because of many shortcomings of widely used AC-DC-AC variable frequency speed control system, this paper have studied a new dual-PWM variable frequency speed control system which uses PWM rectifier to take place of the uncontrolled diode in rectifier side and adopts vector frequency control method in inverter side. Not only the DC voltage has been adjusted in a certain range by the dual-PWM system, but also the current in AC side of the rectifier has become sinusoid, and make the system running in unity power factor and the power transmission reversible.In research of dual-PWM inverter, severa1 control methods on AC motor are introduced in this paper first, then the mathematic models in different coordinate system are analyzed, the focus is on the basic principles of vector frequency control and the rotor magnetic field oriented vector control methods Finally the mathematic model of the inverter side is discussed.The PWM rectifier part is the research focus in this paper. The basic topologized structure of PWM rectifier is studied first and the mathematical mode is analyzed by using the VF mode in inverter. After analyzing and comparing two kinds of current control methods of three-phase VSR: indirect current control and direct current control,the author used the latter as main control scheme which control the phase and amplitude of ac-side current directly, by constructing dual closed-loop which voltage is the outer loop and current is the inner loop, and sampling currents of phase a,b,c and converting them by 3s/2r method with"constant power"to measure active current and reactive current of ac-side current, and directly control the phase and amplitude of the ac-side.The SVPWM control technology is used both in the rectifier side and inverter side of dual-PWM. Because of the similar structure of the two sides, the inverter side is used as an example to analyze the theory and control method of SVPWM.Based on the above-all study, experiments are done by using two frequency converters to construct a dual-PWM system, then test waves are recorded and the results are analyzed which prove the accuracy and effectiveness of the design. It also has significant effect in the direct-digital control system for excavator.