Reseacrh On Control Technique Of Digitally Controlled VVVF Signal Source

VVVF signal source can output a sinusoidal voltage of any frequency and any amplitude within a certain range.It is widely used in impedance measurements of new energy equipment,electrical equipment tests,medical equipment power supply.However,there are still many deficiencies in the existing digital VVVF signal source: when the amplitude of output voltage is low,the waveform quality will be significantly deteriorated;digitaly controlled VVVF signal source is difficult to obtain modulation waves of different frequencies through a sine table;when VVVF signal source uses PI controller,it will have obvious steady-state error on voltage amplitude.This thesis conducts research and analysis on control techniques of VVVF signal source.The main work and researchment of this thesis including:Firstly,this thesis analyzes the relation betwwen SPWM modulation ratio and deadtime effect,and designs main circuit including a DC/DC converter and a PWM inverter.When the output voltage amplitude is low,VVVF signal source reduces the DC bus voltage of the PWM inverter to obtain a larger modulation ratio.In this way,the waveform quality of the output voltage is effectively improved.Secondly,this thesis designs a simple lookup algorithm which is suitable for digital control.This algorithm can generate modulation waves of different frequencies by using a high-resolution phase pointer.In this way,VVVF signal source can output voltage of any frequency.It also reduces the memory space by replacing the full cycle sine table with a quarter cycle sine table.Thirdly,this thesis designs dual-loop control strategy with outer loop of capacitor voltage and inner loop of inductor current.Considering that PR controller only works for a single frequency and is not suitable for occasions where the frequency of output voltage can be adjusted,PI controller is used in dual-loop control.Because PI controller cannot accurately track AC reference,this thesis proposes feedforward voltage references control,which basically eliminates the steady-state error of the output voltage.Delay of digital control is also taken into account,which makes the system model more accurate and the design more reasonable.Lastly,control techniques proposed in this thesis is simulated and analyzed on MATLAB platform,and a 20 k VA digital VVVF signal source prototype is set up for experimental verification.Simulation and experiment results show that the VVVF signal source designed in this thesis can output voltage with adjustable frequency within10Hz～400Hz,and adjustable amplitude within 100V～220V(RMS).The output voltage has basically no frequency deviation and steady state error,and has good waveform quality.