Optimal Control Method Of Plasma Cutting Power Source Based On Plasma Arc Characteristics

As a new cutting technology,plasma cutting has gradually replaced the traditional thermal cutting method with its higher economy,practicability and excellent cutting performance,and has become more and more important in the market.At present,the domestic plasma cutting machine is mainly based on the inverted cutting power supply,whose Total Harmonic Distortion(THD)characteristics are poor.Using three-phase PFC as the input of cutting power supply can ensure good input THD characteristics.Improving switching frequency and using new digital control method can optimize the power density and efficiency.Most of the cutting power supply design and selection control methods follow the design principles of traditional power electronic converter,which is often difficult to meet the demand of load characteristics,resulting in poor cutting characteristics.In fact,the load of cutting power supply is a non-linear plasma arc which is affected by different factors.So it is necessary to optimize the control method according to its load characteristics.In this paper,different modes in the cutting process are analyzed and optimized,and the cutting performance is optimized.The arc initiation mode is to break down the air gap between the electrode and the nozzle by several kilovolts of high frequency oscillation voltage,forming the initial plasma channel.The key of this process is the high frequency arc-starting circuit which generates high voltage.Firstly,the state change of plasma is analyzed,and the mathematical model of plasma arc is established.The volt-ampere characteristic curve of plasma arc is obtained by simulation.On this basis,the working principle of high frequency arc-starting circuit is analyzed,and the detailed design principle of parameters is given.Arc transformation mode is the necessary mode of cutting machine from arc initiation mode to cutting.Its duration is very short and the load characteristics changes very fast.Under high pressure,plasma generated by arc initiation mode contacts the workpiece surface through a narrow nozzle.At this time,conductive channels forms between electrodes and workpiece,and cylindrical channels are formed under the constraints of high current speed and narrow channels.The plasma current rises rapidly and becomes arc cutting arc.In this process,the load characteristics of plasma vary greatly and the switching time is very short.The variation of plasma volt-ampere characteristics and the requirement for the output characteristics of cutting power supply are analyzed,and the control method adapting to the arc condition is given.After entering the cutting mode,the plasma arc is in the state of thermal arc.At this time,the volt-ampere characteristics of the arc are also affected by the cutting state,and the equivalent load time is changing.Due to the system is a two-stage converter,the sudden change of load may cause the fluctuation of bus voltage,affect the output characteristics of cutting power supply,and even cause system instability.The cutting power supply is regarded as a cascade system.Its cascade stability is analyzed,and its control method is optimized to reduce the fluctuation of bus voltage due to the change of load characteristics.