| In order to ensure national energy security and diversified supply,in recent years,the country has successively issued a number of major strategic plans to support shale gas exploration and exploitation.In order to improve the production of a single well,the electric drive fracturing pump system has gradually replaced the traditional diesel fracturing system due to its good speed regulation performance,high efficiency,low pollution,and low vibration noise.However,when designing the motor of the electric drive fracturing system,there is often a problem of poor performance parameter matching at non-rated points,and due to limited space factors,compared with foreign advanced products,its power density is small,efficiency is low,and heat generation is serious.The 4200 HP electric drive fracturing pump is selected as the research object of the dissertation.Based on its working characteristics,the output power of the driving motor required for the fracturing pump under various typical working conditions is obtained,and a 2500 k W three-phase asynchronous motor is designed and corresponding research is conducted on it.Based on the inherent relationship of electromagnetic fields,starting from the mechanism of loss generation,and taking into account the influence of factors such as high-order harmonics,rotating magnetization,small hysteresis loops,skin effect,winding distribution,and temperature on loss,a time-stepping finite element field-circuit coupling method is proposed to solve the loss density and various loss calculation methods of electric motors under rated operating conditions.By weighted average processing,the average loss density is calculated,which can be used as the heat source mapping for each component of the motor.The prototype loss test platform is built to verify the accuracy of the method.Secondly,based on the calculation results of various losses of the motor,the power speed-curve of the motor is obtained.By using the rotational speed as the constraint condition,the "point" matching of the "curve family" of the fracturing pump and the power of the electric motor under power frequency conditions is carried out.It is found that there would be severe "big horse pulling small car" in the non-rated point region,and the working points on the "curve family" of the fracturing pump in this region can not match the torque balance of the electric motor under the rotational speed constraint.On the basis,a variable frequency speed regulation method is adopted to solve and obtain the power-speed curve of the asynchronous motor used in the pump in the full frequency domain.According to the principle of optimal efficiency matching,with the optimal system efficiency as the objective function and torque and speed as constraints,control the frequency of the motor,adjust the "line to point" matching of the working and mechanical characteristics of the fracturing pump and motor,and adjust the frequency of the motor through frequency conversion to match the working point of the pump with the rated power point on the T-s curve of the motor in that frequency band.Based on the comprehensive efficiency of the system.Based on the overall efficiency of the system,the matching effect between the pump and motor is evaluated,and the power line matching between the fracturing pump and the driving motor is achieved.Once again,taking into account the structural characteristics,operating conditions,cooling fluid flow path,and fluid flow parameters of the electric motor,the fluid network method is proposed to decouple the global field of the electric motor into multiple partitioned flow fields.Then,the three-dimensional modeling is established locally for each region decoupling,and the flow heat coordination mechanism is used to solve the fluid and temperature fields.By using the fluid network,the boundary conditions of the inlet and outlet of the internal ventilation path are corrected for data,and the fluid and temperature fields in each region are iterated repeatedly until the error of the boundary conditions meets the calculation requirements,the relevant calculation parameters at the connections of each region are solved,and the fluid flow and temperature of the global internal and external ventilation path of the final motor are obtained.The accuracy of the proposed method is verified by comparing with experimental data,the problem of global temperature rise and fluid solution for a large model motor under finite computational conditions is solved.Finally,the principle of minimum exergy destruction is introduced to evaluate the heat transfer characteristic of the asynchronous motor for fracturing pumps from the perspective of energy conversion availability based on the thermal network method.On the basis of the exergy destruction results,the cooling conditions of the internal and external ventilation path of the motor are strengthened and improved to obtain the influence of air volume changes,fluid flow and other factors on the temperature distribution.By optimizing the number of blades,fan outer diameter,and blade deflection angle,the efficiency of the external fan has been improved;By adding arc plates and improving the cooling of the cooler in the external ventilation path,the phenomenon of eddy currents is eliminated;And by increasing the number of wind deflectors in the external ventilation path and changing their spacing,the heat dissipation capacity of the cooler is strengthened.In order to solve the problem of low utilization of fluid in the internal ventilation path of electric motor,a flow field simulation model with different heights and number of air deflector is proposed.The internal heat dissipation resources of the motor are re integrated,and the fluid distribution in the internal ventilation path of electric motor is optimized.The influence of different numbers and heights of air deflector on the fluid and temperature fields of the motor is obtained.The optimization of the cooling structure of the motor demonstrates the rationality and necessity of introducing exergy destruction and its energy-saving effect. |
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