Zhiqiang Yu

When an urban rail vehicle moves, its receptor obtains the current from the contact rail (CR) through the sliding friction. Thus, the receptor could be subject to localized overheating, which causes increased wear and even damage. Based on the dynamic equilibrium equations of the heat transfer and dissipation between the receptor and the CR (referred to as the boot and rail, BAR), and the dynamic electric contact theory, a three-dimensional thermal field model for the BAR system during the vehicle motion is established. The effects of the current, contact force, and motion speed on the temperature rise of the BAR are discussed. The results show that the current is the main factor affecting the temperature rise of the BAR, which exhibits a positive correlation. The temperature decreases with the increase of the contact force. Especially the temperature rise decreases the fastest when the contact force varies from 100 to 120 N, 2.86 times that when it varies from 80 to 100 N, 9.1 times that when it varies from 120 to 140 N. Also the temperature increases with the speed. And the temperature difference from 80 to 100 km/h is 1.5 times that from 60 to 80 km/h. The innovation of this paper lies in comprehensively deriving the formula for the total dynamic contact heat source power based on electrical contact theory and friction theory. A new three-dimensional dynamic thermal field finite element model for the BAR is established. And the trail effect near the electrical contact zone (ECZ) of the CR is researched. The contribution is that the research results are of the reference for the local thermal protection of the receptor and its structural optimization.

Currently, most urban rail vehicles obtain the electrical energy from the contact rail (CR) via the current collector shoe (CCS). Due to the vibration and turning of the vehicle during its motion, the CCS and CR (referred to as shoe and rail, SAR) are prone to the offline discharge. The discharge arc generates the harmonics to affect the current collection quality of the vehicle. Meanwhile, the high heat produced worsens the damage to the surface of the SAR, resulting in large economic losses. This paper establishes a SAR offline discharge model based on the magnetohydrodynamics (MHD). Combining the dynamic mesh technology and traction network circuit, the electrical behaviors of the SAR during the rise or fall of the CCS under the working conditions are studied. The results show that under a DC supply voltage of 1500 V, when the CCS rises, its temperature can reach a maximum of 2900 K, which is 1.6 times higher than that when it falls. As the air gap increases, the gas flow velocity decreases from 0.169 to 0.0472 m/s. Furthermore, considering the effects of Darcy damping force, arc pressure, Marangoni effect and recoil force, this paper also uses the melting-solidification model and evaporation-sublimation model to investigate the dynamic changes in the melting pool caused by the arc and the impact on the material of the SAR. The innovation of this paper lies in studying the electrical behaviors during the SAR discharge caused by the rise or fall of the CCS and the SAR erosion by the discharge. The results of this paper provide a reference for improving the stability of the power supply system of the urban rail transit, reducing arc damage to electrodes and extending the service life of the SAR.

Arc melting pools are generated when the pantograph and catenary (PAC) are offline during the rise and fall of the pantograph. They can lead to the material damage, accelerated wear, increased splashing, and exacerbated PAC fatigue through the thermal cycling, shortening its service life. In this paper, the voltage and current are measured by a high-speed PAC experimental platform when the PAC occur the offline discharge. And the arc erosion images are obtained using a scanning electron microscope. Based on the principle of the magnetohydrodynamics, a simulation model of the PAC arc is established and its correctness is verified by comparing the experimental data. The distribution of the heat flow input density of the arc is obtained by adopting the moving mesh technique and coupling the actual auto-transformer (AT) traction grid circuit. Meanwhile, the melting solidification model and evaporation sublimation model are introduced to study the behavior of the material erosion by the arc when the pantograph rises and lowers. When the PAC offline discharge occurs during the rise of the pantograph, the heat flow density is concentrated and the temperatures of the contact line and pantograph are 3550 K and 4290 K, respectively. The melting pool is of a deep and narrow type. Its depth is 1.71 mm, which is 80% more than that during the fall of the pantograph. While its width decreases by 17.48% compared with that during the fall of the pantograph, which increases the risk of the localized overheating and thermal stresses. As the pantograph falls, the density of the heat flow decreases and the temperature of the contact point is lowered. The melting pool is shallow and wide, making it more susceptible to splashing and accelerating the wear. The increased temperature in the melting pool triggers the Marangoni effect, which leads to the eddy currents. They increase the instability and spattering on the surface of the pantograph, accelerating the material damage. The innovation of this paper lies in the integration of the arc model, erosion model, and AT traction circuits, and the introduction of the Marangoni force, evaporation-sublimation behaviors, and moving mesh technology, to systematically study the PAC erosion behaviors during the rise and fall of the pantograph. Its purpose is to provide theoretical support for the selection of the PAC materials, their structural design, and preventive maintenance.

Currently, many urban rail or subway vehicles obtain electrical energy through their receptors in contact with the contact rail linked to the traction network. As urban rail vehicles arrive at the station frequently or stop temporarily on the rail line, the contact rail still needs to provide the electric power for the on-board equipments. At this point, the receptor is in a static state, where the local overheating happens. This could lead to the damage of the receptor, even causing the vehicle to stop running. Based on the electrical contact theory and Fourier's law, this paper establishes a three-dimensional thermal field finite element model for the receptor and contact rail (referred to as the boot and rail, BAR). Using the model, the change of the contact resistance under the working conditions is investigated. And the effects of the contact force, transmitted (or power supply) current and receptor wear on the temperature behaviors of the receptor are discussed. The results show that the temperature is negatively related to the contact force but closely related to the contact resistance. When the contact force is less than 100 N, the contact resistance is larger and the temperature is obviously enhanced, with the rise rate of 0.05 ℃/s. The temperature of the receptor is positively related to the transmitted current. The larger the current, the higher the temperature, more intense the temperature rise; the smaller the current, the more obvious the saturation phenomenon of the temperature, i.e. the rapid attainment of the thermal equilibrium. The larger the wear area of the BAR, the larger the convective heat flux of the receptor, leading to a lower temperature in its steady state. The innovation of this paper is to establish a three-dimensional thermal field model that can more accurately characterize the BAR system under the static electrical contact, by applying the electrical contact theory to the finite element method. Its contribution is to provide a methodology for the study of the multi-parameter three-dimensional temperature behaviors of the BAR system. And its research results present an important reference for the optimization of the receptor structure and the safe operation of urban rail vehicles.

A bulk high-temperature superconductor (HTS) employed in the high-temperature superconducting levitation device is susceptible to cracks or defects during its fabrication and utilization. This may cause changes in the electromagnetic (EM) and vibrational behaviors of the HTS, even affecting the normal operation of the device. To this end, based on the magnetic, thermal and motion field equations for the transversely isotropic superconducting material, we establish a 3D coupled model of a levitation system containing an HTS with a slot defect and permanent magnet (PM), to investigate the vibrational and EM behaviors of the HTS with the defect above the PM, and discuss the effects of the depth of the defect, frequency of the external excitation, and combined defect on the behaviors in the time or frequency domains. The results show that the intrinsic frequencies of the HTS with defects of different depths are in the ranges of 6–8 Hz, 13–16 Hz and 19–20 Hz. With the increase of the defect depth, the intrinsic frequencies and EM losses increase but the vibration amplitude decreases. Under the action of the simple harmonic excitation with the same amplitude, the larger the defect depth, the lower the vibration amplitude and the better the stability of the phase trajectory. With the increase of the excitation frequency, the vibration amplitude decreases and the losses increase. However, when the excitation frequency is close to the intrinsic frequency, the HTS resonates and its internal fluxes undergo the violent motion. When the HTS contains a composite defect consisting of two single defects, its intrinsic frequencies are higher than those for the HTS with either defect included. And its EM losses are greater than the sum of their losses. The modeling method presented here can provide an idea and a reference for the research of EM-vibration behaviors of superconducting levitation systems containing defects.

Presently, with the increase in the operating speed of a train, the off-line phenomenon of its pantograph and the contact wire occurs intermittently, which affects the quality of its receiving current. In this paper, a distributed parameter-based Habedank arc model for a fully parallel AT power supply network is developed to examine the electrical characteristics under the typical operating conditions, including the normal travel and entry/exit stations. The incorporation of the train acceleration parameter facilitates further enhancement of the Habedank arc model and the equivalent dynamic resistance, also validating them. The results show that the off-line discharge arc voltage of the train pantograph and the locomotive voltage waveform during the overcurrent phase separation interval show a decreasing and increasing trend respectively when entering and leaving the station. Due to the nonlinear dynamic characteristics of the arc, the overall waveform contains many harmonics. The advantage of this model is that it is a simple, rapid, and highly accurate method for constructing an off-line discharge circuit model of the pantograph and catenary (PAC) under the working conditions. This paper introduces a novel approach to studying the electrical characteristics of the PAC discharge model with distributed parameters in a fully parallel AT power supply network. Also, it considers the effects of the operational conditions of the train, such as the entering and leaving stations, on the discharge characteristics. The research results of this paper can provide a reference for the identification of the electrical characteristics and protection of a PAC equipment during the operation of the train under the working conditions.

The off-line discharge between the pantograph and catenary (PAC) occurs frequently in the actual operation of an electric locomotive, which affects the quality of the received current and the service life of the pantograph. The current experiments on the off-line discharge between the PAC are mainly low-power ones and the models established are static. To study the electrical behaviors during the PAC relative motion under the actual working conditions, in this paper, the voltage and current of the contact wire (CW) during its motion are measured. And the electrical behaviors during the rise, fall and reciprocating motions of the pantograph are investigated by combining a finite element method (FEM) model based on the magnetohydrodynamics with the auto transformer (AT) traction network circuit and moving mesh method. It is found that the harmonic frequencies are mainly in the range of 0-30 MHz. The voltage distortion is more obvious during the fall of the pantograph. And the negative polarity spike pulse has a larger amplitude and occurs more frequently. The novelty of this paper lies in the experimental test with the high current and high power, as well as the systematic analysis of the electrical behaviors and multi-physical field evolution during the motion of the pantograph under the working conditions. Its contribution is to present an effective method to study the dynamic behaviors of the PAC. The research results can provide an important reference for the identification of the electrical characteristics of the PAC and the protection of the PAC equipment.

The surface micro-dimple arrays can effectively improve the friction and lubrication performance of moving parts and are widely used in cutting tools and mechanical transmissions. Through-mask electrochemical machining (TMECM) is a process that uses the anodic dissolution principle to remove materials at low temperatures. It is suitable for efficiently processing large-area micro-dimple arrays on metal surfaces. However, the previous TMECM method still suffers from the problem of low processing adaptability. In this paper, a new method for scanning-cathode TMECM by utilizing the fluid dynamic pressure effect was proposed, and the flow field and electric field models of various inner-jet cathodes were developed as well. Through numerical simulations of the flow field and electric field, the flow velocity, electrolyte pressure, and current density distributions for different inner-jet cathodes were investigated to determine an appropriate cathode. Contrasting experiments were conducted to investigate the variations in electrolyte flow states and machining currents for different inner-jet cathodes. Additionally, the changes in micro-dimple dimensions under different machining parameters were investigated. Moreover, the micro-dimple arrays of 385.7 μm in diameter and 111.8 μm in depth, 288.8 μm in diameter and 40.3 μm in depth, and micro-dimple arrays with an etching factor of 1.69 were fabricated, and the results demonstrated the good processing adaptability of the scanning-cathode TMECM.

Currently, copper-oxygen high-temperature superconducting materials have strongly anisotropic electromagnetic properties, which are difficult to describe in the equations. And in the rotating machine containing the permanent magnet (PM) rotor and bulk high-temperature superconductor (HTS) stator, when the HTSs have defects, their electromagnetic-stress behaviours may affect the mechanical stability of the equipment and even cause it to not work properly. In this paper, we proposed an anisotropic electromagnetic equation based on the H-formulation and established a three-dimensional coupled model with the magnetic, thermal and stress fields to study the electromagnetic-stress behaviours of a HTS with a slot defect during the swing of a PM, discussing the effects of the interior angle and inclination angle of the defect on the behaviours. The results show that the interior and inclination angles of the defect have a large influence on the electromagnetic-stress behaviours. For the electromagnetic characteristics, the main influence is on the rotational losses. Especially the losses at an inclination angle of 60° are 16.5 times those without damage. This is related to the ‘thin wall’ structure near the upper surface of the HTS. The stress concentration point appears on the defect boundary. The novelty of this paper is the proposal of the anisotropic electromagnetic equations based on the H-formulation and the study of the electromagnetic-stress behaviours of a superconducting swing system containing a PM and HTS with a slot defect from a three-dimensional perspective. The research results of this paper can be the references for the design and structural protection of superconducting rotating machines.