| At present,a good effect of speed regulation of three phase motor are stator-side frequency control and rotor-side cascade control,which are belong to modern AC speed regulation technology.Frequency control and cascade control are integrated use of the AC motor,power electronics,control theory,computer,power measurement technology and other disciplines.Frequency control is accessed from the motor stator side.All power output of the motor needs flow through the inverter,and which is called full power control.Because of high current or voltage of high power motor,power electronics technology in parallel or in series of technology needs to be resolved well.Cascade control devices is accessed from the rotor side,which control power is called motor power slip.The maximum motor power slip is only 14.815% of rated power of the motor.Motor speed is at 50% of rated speed,rotor voltage is only 50% of the rotor open circuit voltage.When rotor speed increases,the rotor voltage is reduced,the rotor voltage is typically less than 1k V.Compared to 6k V or 10 k V voltage level of the inverter,it belongs to the low-voltage category.Cascade control equipment is low cost,and loss of their own is less than 1%of the motor rated power.Cascade control's operating environment is low,which can only be placed in the general plant.Chopper cascade speed regulation system also has advantages of a simple structure,safety devices,high system reliability.Even cascade control device is fails,the induction motor can be completely out of the chopper cascade speed regulation and run at full speed.The inverter is only stopped and checked when fault occurs.Currently research on chopper cascade speed regulation system is mainly focuses on theory,simulation modeling,parametric design calculation,and combining engineering applications is small.The paper is in-depth research and analysis for chopper cascade speed regulation system,and combined with engineering practice,mainly research work in the following:1.Chopper cascade speed regulation system normal startup is analyzed.Wound rotor induction motor starting characteristics has a direct relationship of series resistor.According to a simplified motor mechanical characteristics formula,different resistance corresponding motor torque-speed curves are drew.For fans,pumps square torque load,the calculation method of wound rotor induction motor starting resistor and motor start-up time is derived.For transient analysis of normal speed regulation process of chopper cascade speed control system,the input and output relations between the main electric parameters of the main circuit of the chopper cascade speed regulation system are derived.Because of inductor current and capacitor voltage are a ripple,ripple of the inductor current and capacitor voltage are analyzed and deduced.2.According to the asynchronous motor circuit,the parameter conversion and the chopper cascade speed regulation system,the equivalent DC circuit is established.According to the established equivalent DC circuit,the dynamic relation of the chopper cascade speed regulation system is the three order nonlinear differential equations with two input and two output parameters.According to the equation of state,the structure block diagram of the transient characteristic of the system is established.The variation of speed and current of chopped wave cascade speed control system with load disturbance and its anti disturbance performance such as amplitude and recovery time are analyzed.The simulation is carried out,and the steady state data,the peak value and the recovery time of before and after the load disturbance are compared under different steady conditions.The operation characteristics of chopper cascade speed control system under the condition of light load and overload are analyzed,and the influence of the system is analyzed.The transient analysis of chopper cascade speed control system is carried out when voltage drop,and the measures are put forward when voltage drop.3.Fault of rectifier bridge is analyzed.According to the diode is burnt,respectively,of the single burnt out,and on the same side of the two diodes is burned and different side different leg two diodes are burnt in a variety of situations.IGBT and reverse diode open-circuit and short-circuit fault conditions and after the failure of the waveform are analyzed,and the effect on other devices after fault is analyzed.Thyristors faults in the inverter of chopper cascade speed control system are analyzed.Single thyristor open circuit and short circuit fault waveform are analyzed.For high-voltage power failure,transient process of chopper cascade speed control system is analyzed and theoretical calculation is carried out.4.Characteristics of fast recovery diode in chopper cascade speed control system's key devices is deeply analyzed.A new modeling method for fast recovery diode is proposed,and the accurate mathematical model is established.The basic structure and working principle of IGBT device are analyzed comprehensively.a precise IGBT model of turn on and turn off process data is established.The establishment of IGBT model can completely characterize the actual switching transient process of dynamic change of voltage,current and device characteristics,and can reflect the influence of the switching characteristic of the system in a certain extent.5.The advantages and disadvantages of the series and parallel connection between chopper cascade speed control system and water resistance starting unit are analyzed.For parallel mode and series mode,starting control logic of chopper cascade speed control system is designed respectively.On the basis of the seriesconnection mode,the control and fault control logic of the direct feeding speed control is designed,and solved the problem of chopper cascade speed control system can not be directly into speed regulation state.Based on the design principle of control logic of chopper cascade speed control system,the control logic of chopper cascade speed control system is designed,and mainly include: full speed control logic,speed to full speed control logic,speed to stop control logic.Design control logic is tested on test prototype,and control logic design correctness is verified according to recorded waveform.Then protection measures of traditional chopper cascade speed control system are analyzed: contactor action protection and fast-cut protection.In view of the disadvantages of contactor action protection and fast-cut protection,electronic protection circuit is designed.Theoretical analysis and calculation of electronic protection circuit are carried out when AC inverter subversion and DC inverter subversion,and the selection basis of electronic protection circuit device is given.Then switching process transient analysis of electronic protection circuit during power failure and momentary power failure is analyzed.Finally,according to characteristics of main circuit of chopper cascade speed control system on steady state and system speed characteristic and mechanical characteristic,differential equation of chopper cascade speed control system by state average method is established.Speed characteristics of chopper cascade speed control system is introduced.On the basis of two order and three order engineering method,speed double closed loop control and engineering calculation method of controller parameters of chopper cascade speed control system are given. |
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