With the rapid development of modern power electronics, electric transmission control technologies, and communication technologies, rail locomotives use more high-power frequency conversion inverters, rectifiers, and various network control devices, communication signal devices, and other electronic devices. . Due to the centralized application of a large number of electric and electronic equipments and the significant increase in their signal processing speed and bandwidth, as well as entertainment and communication devices carried by drivers and passengers, numerous electronic and electrical devices in the orbital locomotives constitute a dynamic system filled with complex electromagnetic energy or signals. This makes it more difficult to ensure that the electrical and electronic devices in the vehicle are compatible and reliable.
As a public vehicle, the vehicle's electrical and electronic systems must ensure that they can operate safely and reliably under any abnormal electromagnetic conditions. No external or internal electromagnetic interference problems may cause safety problems in the system. These are all on the electrical and electronic equipment of railway vehicles. The electromagnetic compatibility design puts forward higher requirements. Correct electrical bonding is an important means of ensuring that all electronic systems and devices are electromagnetically compatible.
1 The basic theory of lap joints 1.1 The importance of electrical laps A lap is an electrical unit that connects two metal structures or objects, providing a low-impedance path for the flow of current between the structures or objects. Thus, overlap refers to the process of establishing a certain degree of electrical continuity between the connected conductive surfaces. The structure or object involved may be a device housing, a component, a component or device, a backplane for an electronic component, or a rack for an electrical device.
Electrical laps are often the first factor to consider when creating electromagnetic compatibility problems. Degradation of system performance also tends to be traced back to poor laps of circuit loops and signal networks or gradually degraded lap designs, such as performance degradation due to aging, corrosion, or other environmental stress.
Lapping is very important for other interference control measures. For example, good overlap between the connector housing and the equipment housing is very important for the integrity of the cable shielding; and the overlap of the gap or joint is a high level of shielding effectiveness of the electromagnetic shielding. Effective means. As shown in the filter, the interference current at the source may reach the load end due to the high lap impedance. Conversely, the interference current at the load end may also reach the source end.
The filter performance is affected by the overlapping impedance ruler B. The commonly used lapping mode for 1.2-track locomotives involves a mechanical interface between two contact surfaces, and may also include components for interconnecting two separate structures. There are two ways: direct overlap and indirect overlap.
1.2.1 Direct Overlaps Direct Overlaps are lap joints that directly connect two metal components that need to be overlapped without using an intermediate transition conductor, providing the desired electrical path between the two interconnecting components. , as shown.
Direct laps may be permanent or semi-permanent, and permanent laps should take certain measures to avoid performance degradation caused by galvanic corrosion.
Proper combination of direct laps has a low DC resistance and is sufficiently low impedance RF to allow for overlap, so direct laps are preferred, but direct laps are only available for 2 parts that can be directly connected together, and Will not move relative to one another.
At present, the main direct lap joint technologies include the following: In terms of electrical performance, fusion welding is the best lap joint method.
The ultra-high temperature can remove the impurity film and the peripheral substances, form a continuous metal bridge at the junction, and its conductivity is close to that of the overlapped object. The net resistance of the overlap is substantially equal to zero.
Brazing (including silver brazing) is another metal flow process that is permanently lapped. It uses a filler metal and the welding temperature is lower than the melting point of the wickers. Brazed lap resistors are also close to zero and are ideal for splicing different materials, but they therefore need to consider overlapping corrosion protection.
Solder is an easy-to-use metal flow bonding process with a low process temperature and is mainly used for the lap joint of several highly conductive metals (eg, copper, tin, etc.), but can also be used with aluminum or with suitable fluxes. Other metal laps. Its lap resistance is basically close to the first two methods, but its melting point is low and cannot be used in occasions where large currents may occur, such as lightning protection.
In addition, the mechanical strength of soldering is low, and it is easy to appear microscopic embrittlement under mechanical stress, and it is difficult to observe visually, and it is not suitable for a field that may be subject to mechanical stress.
Bolt laps are the most common semi-permanent lap joint method. This lap joint method has great flexibility and convenience. The bolts or screws only play a fastening role and provide the necessary pressure between the lap joints. The fastener itself does not necessarily have to be conductive, but conductive fasteners are a better choice. In general, tapping screws cannot be used for lap joint processing. The size, number and spacing of the fasteners should meet the contact pressure requirements on the lapped surfaces.
Riveting is not as popular as the previous lap joint methods. It has poor flexibility and insufficient corrosion protection. The biggest advantage is that it can be automated and installed quickly and consistently.
The conductive adhesive is a silver-filled two-component epoxy that cures to form a conductive material that can be used between contact surfaces to create a low-resistance bond. It does not require the application of heat to form a direct overlap and is suitable for applications where fire or explosion hazards are averted. In combination with bolts, bridges have high corrosion resistance and mechanical strength, but resistance increases with time. Another disadvantage is that it is difficult to disassemble.
1.2.2 Indirect overlap In some situations or where the equipment is located, it is not appropriate to choose a direct overlap. For example, parts requiring relative movement must be connected to the grounding point of the structure by means of auxiliary overlap conductors. The straps are also used to bypass certain structural components, such as the hinges of the power distribution box, to eliminate their influence by strong electromagnetic fields. Common indirect lap designs are shown.
A good indirect overlap should maintain a sufficiently low impedance over the entire operating spectrum range and life cycle, usually done by straps or tabs (jumpers). The jumper method is only used in the low frequency range below 10 MHz, preferably solid Metal strips, usually copper or aluminum. Indirect splicing is an alternative when direct splicing is not feasible. Indirect overlap use is shown as an example.
2 Overlap Impedance and Its Effect 2.1 Impedance Requirements for Electrical Overlapping The first requirement for lap bonding is to form a low impedance connection between two connected objects, so the lap joint impedance is an important parameter for verifying the quality of the lap joint. In general, the overlap impedance is defined by “resistanceâ€, but this is a DC overlap resistance, and it is not an indicator that the quality of the overlap can be measured at high frequencies. Therefore, the impedance is a decisive factor, including the inductance of the intrinsic conductor, the distributed capacitance, and the standing wave. Effect and path resonance and other factors.
The lap impedance should maintain a low state over time and the use of the device. In product design, due to operability, it is only possible to bridge resistors (ie, DC link impedance). For the same type of lap joint, the DC resistance and its high frequency impedance have a certain corresponding relationship, so the DC resistance can be compared to reflect the lap joint performance. Defining the DC lap resistance is beneficial to encourage designers to improve process measures, ensure the implementation of process specifications, and improve the reliability of lap joints.
If a lower lap resistance is required, it must be ensured that the contact surface is cleaned, sufficient lap area and contact force are ensured, and other factors such as corrosion that cause the increase in lap resistance are minimized.
In the product design, the general structural parts are connected by M5 threads to ensure a lap area with a diameter of more than 15mm, and it is easier to achieve a lap resistance of less than 100mO.
For electric/electronic equipment in railway locomotives, it is recommended that the lap resistance be less than 100m0, which can satisfy the application in most cases, and the technical implementation is also feasible.
The impedance of the direct overlap can be kept low enough to meet the application requirements when properly implemented. However, indirect overlap involves the introduction of straps (lines), which involves more complex impedance calculations.
2.2 Lapder impedance equivalent circuit and impedance characteristics A reasonable length of patch cable can be used at low frequencies, but at high frequencies, the RF impedance of the patch cable becomes a critical design consideration. Straps may exhibit self-resonance or parasitic resonances with additional parasitic devices. In either case, the impedance of the overlap path will increase significantly.
The basic frequency of the straps can be approximated as follows.
(1): A resonant frequency of the landing strip; A self-inductance; distributed capacitance between the cC landing strip and the overlapped part.
An approximate representation of an equivalent circuit is shown provided that the strap length is shorter than the wavelength.
The physical size of the strap. Inductive reactance and capacitive reactance are also affected by factors such as the device housing being connected in the overlap path. The circuit U) shown does not consider the influence of the device housing or other overlapped components, and (b) more fully considers the ride. Connect the system. The intrinsic inductance of the device housing is the capacitance between the device housing and the plane.
In practical applications, /, then the impedance of the equivalent circuit at high frequencies can be expressed as: 2) 2.3 The lap resistor The DC resistance of the straps per unit length can be calculated according to the following equation.
At high frequencies, the AC resistance is greater than the DC resistance due to the skin effect. Assuming that all currents are concentrated in the first skin depth, for the round conductors, the AC resistance can be expressed as direct type I direct type I tin, lead and tin-lead alloys directly type I direct type I or type copper and copper alloy plating Tin or chrome gasket Type I or Type Direct Type I or Type Nickel and nickel alloy Tin or Chrome Plating Gasket Type I or Type Direct Type I or Type Stainless Steel Tin or Chrome Plating Gasket Type I or Type Direct Type I or Type Silver, Gold and Precious Metal Tinned or Chrome Plated Gasket Type I or Type Direct Type I or Type 25 Lapping Performance Lap Performance Used to illustrate the effect of the voltage reduction achieved by the lap strips, which is a 24cm long lap joint lap measurement A positive value indicates that the strap has a lower induced voltage, and a negative value indicates that the strap increases the induced voltage at the measurement frequency.
The measurement results show that: 1 At low frequencies, the straps have lower reactance, and the straps can effectively achieve the overlap target; 2 When parallel resonance occurs, the straps will enhance the reception of unwanted signals; 3 After the parallel resonant frequency is exceeded, Lap bars have little effect on the radiation signal.
3 electrical lap design methods and practices 3.1 galvanic corrosion control lap design, often involves different metal contacts. In the air with a certain humidity, direct contact of different metals will cause corrosion, which will damage the integrity of the electrical joint, reduce the overlap effect, and even cause electromagnetic interference due to the non-linearity of the corrosion area, such as the mutual interference of the incident RF signals. Tone.
In order to avoid corrosion, metals that are in direct contact should be selected as close to the metal material as possible.
3.2 The choice of lap material is generally lapped using copper or aluminum. The electrode potential of the lap material should be close to the potential of the electrode to be lapped. If there is a difference between them, suitable plating should be applied or additional accessories such as gaskets (to ensure conductivity) should be applied.
3.3 Electric locomotive design of rail locomotive products To facilitate the design, the electrical and electronic equipment of track locomotive products is divided into three categories: lap joints of housings (chassis), shielded cable overlaps, and laps of other metal components.
3.3.1 Overlapping of Enclosures (Chasses) At present, the chassis of most products are made of metal materials. The lap joint design of gaps or seams is an important part of the overall shielding. In the shielding design of the metal shell, screws, gaskets, and reeds are mainly used for connection.
When screws are used, attention must be paid to the spacing of the screws, the cleanliness of the overlapping surfaces, corrosion protection, and other restrictions. Design examples are shown as 0. If necessary, use conductive gaskets or reeds to improve contact.
m shielded chassis lap joints The use of conductive reeds for non-metallic chassis shield overlaps requires the use of surface conductive processes, such as conductive spray coating, coating, etc., as shown in 1. Such designs need to be used in conjunction with pads or other ways to improve conductive contact.
3.3.2 Shielded cable overlap The overlap of the shielded cable mainly includes: the overlap of the cable shielding layer and the connector, and the splicing of the cable connector and the device shell; in the case of not using the connector, the crimped layer must be considered. Connection method. Regardless of the overlap, 360 conductive contact between the shield and the connector, the connector and the housing, the shield and the crimp is required.
The products of rail locomotives are mostly shielded cables that are terminated in the manner shown in Fig. 2. The shielding layer is crimped to the vehicle body in the nearest vicinity.
The use of a cable flange is also an effective way of splicing the cable shield to the shield case as shown in 4.
For the lap joint between the shielded cable and the metal shell connector, the conductive 360 ​​connection between the peeled shield and the metal shell wall of the connector should be ensured, as shown in 5 and 6.
The overlap between the connector and the device housing may depend on the direct contact between the two, or use conductive rubber, gaskets, or other components that improve the connector's contact effect, as shown in Figs.
3.3.3 Overlapping Rails of Other Metal Parts In addition to the above two types of lap joint designs, there are other requirements for lap joints between some metal parts and the vehicle body or the case (casing), such as wire racks and cables. Pallets, wiring ducts, etc. Regardless of the type of metal components, the basic requirements and design methods of the lap joint design should be followed in the vehicle body or in the equipment enclosure.
3.3.4 Typical Bad Lapping Forms and Hazards The track locomotive products involve many parts and components. Bad lap design may affect the stable operation of the equipment or system, and even endanger the safety of equipment and personnel. Connection design and hazards: There are loose connection points in the current path, or the connection point is loose due to vibration. The generation of spark at the connection point may also generate a disturbance signal with a frequency of several hundred megahertz.
In a lightning protection network, when a lightning strike discharge current passes through a bad overlap point, a voltage drop of several thousand volts will occur at the overlap, and the resulting arc discharge may cause fire or other hazards.
In ground systems, poor bonding between devices will make grounding measures virtually useless. A poor lap increases the lap impedance, creating a voltage drop at the lap and destroying the ground potential.
In the follow-up product design should be cited as a guide to avoid similar designs. 9,0 shows a typical example of a false connection.
4 Conclusion The electromagnetic compatibility of the electrical and electronic equipment in the locomotive affects the stable and reliable operation of the system, and electrical lapping is an important design link to achieve this goal. This paper discusses in detail the basic technical requirements and design methods of electrical lap joints, and provides detailed and feasible design methods and ideas for the design of lap joints of electric locomotive equipment for rail locomotives. It provides improved electromagnetic compatibility of products. However, due to the complexity and diversity of the system, some specific design methods have yet to be further studied and analyzed. For example, the splicing of shielded multicore shielded cables is still a problem.
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