Electromechanical Coupling Dynamics Of Cutting Transmission System Of Coal-mining Machine Under Variable Speed And Variable Load Conditions

Coal mining is developing in the direction of intellectualization and unmanned pattern.This trend benefits the reduction of labor intensity of workers and the improvement of the safety production,however places greater demands on reliability,intelligence and adaptability of working condition of the coal-mining machines.The cutting transmission system is the key part for the coal-mining machine to complete the action of coal breaking.It operates under the non-stationary conditions with variable loads and speeds;at the same time it is an electromechanical coupling system with high-power motor drive and multi-stage transmission,containing complex structures and a large number of components with complicated inside and outside excitations.All the factors mentioned above introduce extremely complicated dynamic behavior to the cutting transmission system,which will lead to frequent malfunctions and long downtime,thus constraining production efficiency and economic benefits of the fully mechanized mining face.This thesis is supported by the National Basic Research Program of China(973 Program,Grant No.2014CB046304).Considering the complicated operation conditions and configuration characteristics of the cutting transmission system of coal-mining machine,the dynamic behavior from the coupling excitations of dynamic parameters including time-varying mesh stiffnesses,time-varying bearing stiffnesses,torsional electromagnetic stiffness and etc.are investigated.The effects from the typical operation conditions and structural parameters on the system dynamic characteristics are studied and the electromechanical coupling mechanism of the gear transmission system with electrical motor drive is revealed.These results can provide theoretical support for the dynamics design,operating-state monitoring,and condition-adaptability improvement of the cutting transmission system.The main contents of this thesis are as follows:(1)A lumped-parameter dynamic model for the fixed-shaft gear and planetary gear transmission is proposed through improving the techniques of model building and model parameter calculation.This model is suitable for the non-stationary operation condition with varied loads and speeds.For the model building,the rigid-body rotations are integrated into traditional gear vibration models and the gear radial and tangential inertial forces related to rigid-body rotations are brought in,which make the model suitable for the work condition with wide range of variable-speed.For the model parameter calculation,the numerical computation methods for the gear mesh stiffnesses and bearing stiffnesses are improved by considering the nonlinear variation of the contact deformation and contact length along the change of loads,which makes the model suitable for the work condition with wide range of variable-load.Periodic excitations such as gear mesh stiffness,mesh error,and bearing stiffness are represented as angular-domain functions and the pre-set system operation speed is no longer necessary,which makes the model suitable for the work condition with random variable-speed.(2)The electromechanical coupling model of the motor-driven multistage gear transmission system of the cutting unit in a coal-mining machine is established by integrating the gear dynamic model with variable speeds and loads and the equivalent circuit model of the asynchronous motor.The influences from various factors such as magnetic field,operating speed,and acceleration of the motor on the natural vibration characteristics of the cutting transmission system are studied.The Campbell diagram,modal energy method and speed sweep analysis are used to recognize the resonance speeds and identify the unsafe components of the system over the wide range of variable-speed.The effects from the gear system torsional vibration on the motor currents are studied and compared under normal and resonance speeds.(3)The motor torsional electromagnetic stiffness,time-varying mesh stiffnesses of gear pairs and time-varying bearing stiffnesses with different loads are calculated.The influences of external load on the natural vibration characteristics of the multistage gear system are studied.Compared with the traditional method,in which the natural characteristics are supposed to be independent of operating conditions,this method for showing the natural characteristics of the gear system in operation is related more closely to engineering practice.The transient response characteristics of the system vibration,dynamic loads,and currents under the impact load excitation are studied and the weakness of the system structures is identified.The electromechanical dynamic response of the system under wide range of varied-loads are calculated and compared and the sensitivity of the vibration signal from currents on the loads is obtained.(4)For the short-term overload condition of the cutting transmission system caused by coal-rock impact,the sensitive structural parameters affecting system performance are identified by parameter correlation analysis.Then,an optimization model for vibration reduction and impact-resistance performance of the system is established.The NSGA-? multi-objective genetic algorithm is used to optimize the parameters,which effectively reduces the instantaneous dynamic load and improves the system dynamic performance.For the long-term overload condition of the cutting transmission system caused by coal hardening,four kinds of speed regulation strategies are developed to reduce system load according to the kinematic relationship between the cutting unit and haulage unit.Through comparative study,the optimal speed regulation strategy for different overload conditions is selected.The safe operation is ensured and the coal productivity is increased with the optimal strategy.The researches mentioned above improve the adaptability of the cutting transmission system under non-stationary conditions respectively from the aspects of mechanical design and active control.(5)An experimental rig of a three-phase asynchronous motor-driven planetary gear transmission system is built.Experiments are carried out to investigate the electromechanical dynamic characteristics with conditions of impact load,speed-sweep,and active speed-control.The variation characteristics of dynamic load of the transmission system and currents of the motor under different work conditions are obtained.The theoretical results of electromechanical coupling dynamic characteristics are verified through comparison with experimental results.