Electrical lap analysis and design technology of electrical equipment for rail locomotive

With the rapid development of modern power electronics technology, electric drive control technology and communication technology, rail locomotives use high-power variable-frequency transformer inverters, rectifier equipment, various network control devices, communication signal devices and other electronic devices. . Due to the concentrated application of a large number of electronic and electrical equipment and its signal processing rate and bandwidth, and the entertainment communication devices carried by the passengers, many electronic and electrical devices in the rail locomotive constitute a dynamic system full of complex electromagnetic energy or signals. It makes it more difficult to ensure the compatible and reliable operation of the electrical and electronic devices inside the car.

As a public transportation vehicle, the electronic and electrical systems inside the vehicle must be able to operate safely and reliably under any abnormal electromagnetic conditions. The safety problems of the system cannot be caused by external or internal electromagnetic interference problems. These are all electrical and electronic equipment for rail locomotives. The electromagnetic compatibility design presents higher requirements. Proper electrical bonding is an important means of ensuring electromagnetic compatibility for all electronic systems and equipment.

1 The basic theory of lap joints 1.1 Electrical lap joint importance lap joint is an electrical unit connecting two metal structures or objects to provide a low impedance path for the flow of current between the structure or object. Thus, lap joint refers to the process of establishing a desired degree of electrical continuity between the connected conductive surfaces. The structure or object involved may be a device enclosure, component, component or device, a backplane of an electronic component, or a rack of electrical equipment.

Electrical overlap is often the first consideration when creating electromagnetic compatibility problems. System performance degradation can also be traced back to poor overlap or gradual degradation of the circuit loop and signal network, such as performance degradation due to aging, corrosion or other environmental stresses.

Lap is important for other interference control measures. For example, a good overlap between the connector housing and the device housing is important to the integrity of the cable shield; the gap or joint overlap is electromagnetic shielding for high level of shielding effectiveness. Effective means. As shown in the filter, the interference current at the source may reach the load due to the high lap impedance. Conversely, the interference current at the load may reach the source.

Filter performance is affected by the lap joint impedance B. 1.2 Rail locomotives commonly used lap joints involve mechanical interfaces between two contact faces, and may also include components for interconnecting two separate structures. There are two ways of direct lap and indirect lap.

1.2.1 Direct lap Direct lap is a lap joint that directly connects two metal members that need to be joined together without using an intermediate transition conductor, providing a desired electrical path between the two interconnected components. As shown.

Direct lap joints may be permanent or semi-permanent, and permanent lap joints should take certain measures to avoid performance degradation caused by galvanic corrosion.

The proper combination of direct laps has a very low DC resistance and has a sufficiently low RF impedance allowed by the lap, so direct laps are preferred, but direct laps are only suitable for 2 components that can be directly connected together, and Will not move relative to each other.

At present, the main direct lap joint technologies include the following: In terms of electrical performance, fusion welding is the most ideal lap joint method.

The ultra-high temperature removes the impurity film and the surrounding material, forming a continuous metallic bridge at the joint, its conductivity is close to that of the object being overlapped, and the net resistance of the overlap is substantially equal to zero.

Brazing (including silver soldering) is another metal flow process for permanent lap joints that uses filler metal and has a soldering temperature below the melting point of the lap joint. The brazing lap resistance is also close to zero, which is ideal for lap joints of different materials, but it is therefore necessary to consider the corrosion protection of the lap joint.

Soldering is an easy-to-use metal flow lap process with low process temperatures and is mainly used for the bonding of several highly conductive metals (eg copper, tin, etc.), but with suitable fluxes, it can also be used for aluminum or Bonding of other metals. The lap resistance is basically close to the first two methods, but the melting point is low and cannot be used in applications where large currents may occur, such as lightning protection.

In addition, the mechanical strength of soldering is low, it is easy to appear microscopic embrittlement under mechanical stress, and it is difficult to observe visually, and it is not suitable for fields that may be subject to mechanical stress.

Bolt lap joints are the most common semi-permanent lap joint method. This lap joint has great flexibility and convenience. The bolts or screws only serve to tighten and provide the necessary pressure between the lap joints. The fastener itself does not have to be electrically conductive, but conductive fasteners are a better choice. Generally, self-tapping screws cannot be used for lap joint processing. The size, number and spacing of the fasteners shall meet the contact pressure requirements on the lap joint.

Riveting is relatively less popular than the previous lap joint methods. It has poor flexibility and insufficient corrosion protection. The biggest advantage is that it can be automated and fast and consistently installed.

The conductive adhesive is a silver-filled two-component epoxy that cures to form a conductive material that can be used between the contact faces to create a low resistance bond. It does not require the application of heat to form a direct overlap, suitable for applications where fire or explosion hazards are avoided. Combined with bolt lap joints, the connecting bridge has high corrosion resistance and a certain mechanical strength, but the resistance increases with time. Another disadvantage is that it is difficult to disassemble.

1.2.2 Indirect lapping In some cases or where the equipment is not suitable for direct lap joints, such as parts that require relative motion, an auxiliary lap conductor must be used to connect it to the ground point of the structure. The lap strips are also used to bypass certain structural members, such as the hinges of the power distribution box, to eliminate the effects of strong electromagnetic fields. Common indirect lap designs are shown.

Good indirect laps should maintain a sufficiently low impedance over the entire operating spectrum range and lifetime, usually done by straps or straps (jumpers), and jumper mode is only used for low frequencies below 10MHz, preferably solid Metal strip, the material is generally copper or aluminum. Indirect lap joints are an alternative to direct lap joints that are not feasible. An example of indirect lap use is shown.

2 lap impedance and its effect 2.1 Impedance of electrical lap The first requirement of lap joint is to form a low impedance connection between two connected objects, so the lap impedance is an important parameter to verify the lap quality. Usually, the lap impedance is defined by "resistance", but this is a DC lap resistor. It is not an indicator that the lap quality can be measured at high frequencies, so impedance is a decisive factor - including intrinsic conductor inductance, distributed capacitance, standing wave Factors such as effects and path resonance.

The lap impedance should be maintained at a low state over time and equipment usage. In product design, limited to operability, only the resistance (ie, DC lap impedance) can be lapped. For the same lap type, the DC resistance has a certain correspondence with its high-frequency impedance, so the DC resistance can be compared to reflect the performance of the lap joint. The specified DC lap impedance is beneficial to the designer to improve the process measures, ensure the implementation of the process specification, and improve the reliability of the lap joint.

If a lower lap impedance is required, it is necessary to ensure that the contact surface is cleaned, that sufficient lap area and contact force are ensured, and other factors that cause increased lap resistance, such as corrosion, are minimized.

In the product design, the general structural parts are connected by M5 thread to ensure the overlapping area of ​​diameter greater than 15mm, and it is easier to achieve the lap resistance of less than 100mO.

For electrical/electronic equipment in rail locomotives, it is recommended that the lap resistance be less than 100m0, which is suitable for most applications and technically feasible.

The impedance of the direct lap can be kept low enough to meet the requirements of use when properly implemented. However, indirect lap joints involve more complex impedance calculations due to the introduction of laps (lines).

2.2 When the lapped impedance equivalent circuit and the impedance characteristic are low frequency, a reasonable length of the lap line can be used, but at high frequencies, the RF impedance of the lap line becomes a key design consideration. The lap strip may self-resonate or resonate with the parasitic reactance attached to the device, which in either case will result in a significant increase in the impedance of the lap path.

The fundamental frequency of the lap strip can be approximated as follows.

(1): Resonant frequency of the A lap; A self-inductance; distributed capacitance between the cC lap and the overlapped component.

An approximate representation of the equivalent circuit is shown, provided that the length of the strap is shorter than the wavelength.

The physical dimensions of the lap strip. The inductive reactance and capacitive reactance are also affected by factors such as the connected device casing in the lap path. The circuit U) shown does not take into account the influence of the device casing or other lapped components, and (b) is more fully considered. The impact of the system. It is the intrinsic inductance of the device's case, which is the capacitance between the device's case 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 resistance The DC resistance of the lap strip 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. Assume that all currents are concentrated in the first skin depth. For circular conductors, the AC resistance can be expressed as direct type I direct type I tin, lead and tin-lead alloy direct type I direct type I or type copper and copper alloy plating. Tin or chrome washers Type I or Type Direct Type I or Type Nickel and Nickel Alloy Tin or Chrome Plated Washers Type I or Type Direct Type I or Type Stainless Steel Tin or Chrome Plated Washers Type I or Type Direct Type I or Type Silver, Gold and Precious metal tin-plated or chrome-plated washer type I or type direct type I or type 25 lap joint performance lap joint effect is used to illustrate the voltage reduction effect obtained by the lap strip, which is a 24cm long lap strip lap joint performance measurement result A positive value indicates that the lap bar induced voltage is lowered, and a negative value indicates that the lap bar increases the induced voltage at the measurement frequency point.

The measurement results show that: 1 low frequency, the lap bar reactance is low, the lap bar can effectively achieve the lap target; 2 when parallel resonance occurs, the lap bar will enhance the reception of the unwanted signal; 3 after the parallel resonant frequency is exceeded, The lap strip has essentially no effect on the radiation signal.

3 electrical lap design method and practice 3.1 galvanic corrosion control lap design often involves different metal contacts. In air with a certain humidity, the direct contact of different metals will cause corrosion, which will damage the electrical joint integrity, reduce the lap joint effect, and even cause electromagnetic interference due to the nonlinearity of the corrosion zone, such as the mutual incidence of incident RF signals. Tune.

In order to avoid corrosion, the metal to be directly contacted should be as close as possible to the metal material with a relatively close potential sequence.

3.2 The choice of lap joint material is generally lapped using copper or aluminum. The electrode potential of the lap joint material should be close to the electrode potential of the lapped metal. If the phase difference is far apart, appropriate plating should be applied, or an auxiliary material such as a gasket should be added (the conductivity should be ensured).

3.3 Electrical lap design of rail locomotive products In order to facilitate the design, the electrical and electronic equipment of rail locomotive products are lapped into three categories: lap joint of outer casing (chassis), lap joint of shielded cable, and lap joint of other metal parts.

3.3.1 Casing of the outer casing (chassis) At present, the chassis of most products are made of metal materials, and the lap joint design of the slits or joints is an important part of the overall shielding. In the shielding design of the metal casing, screws, gaskets, and reed connections are mainly used.

If the screws are used, attention should be paid to the spacing of the screws, the cleanliness of the lap joints, and the corrosion protection. The design examples are as shown in 0. If necessary, conductive gaskets or reeds can be used to improve the contact.

m shielded chassis lap joints using conductive reeds in non-metallic chassis shields need to be surface-conducting process, such as conductive spraying, coating, etc., as shown in Figure 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 lap joint of the shielded cable mainly includes: the lap joint of the cable shield and the connector, and the lap joint of the cable connector and the equipment casing; in the case of not using the connector outlet, it is necessary to consider the crimping Connection method. Regardless of the type of overlap, the conductive contact between the shield and the connector, the connector and the outer casing, the shield and the crimp is required to be achieved 360.

Most of the products of the track locomotive are shielded cables terminated by the method shown in Figure 2. The shield is pressed close to the vehicle body.

The use of cable flanges is also an effective way to lap the cable shield to the shielded enclosure, as shown in Figure 4.

For the overlap of the shielded cable and the metal casing connector, the conductive 360 ​​connection between the stripped shield and the metal shell wall of the connector shall be ensured, as shown in Figs.

The overlap between the connector and the device housing may depend on direct contact between the two, or with conductive rubber, gaskets, or other components that improve connector contact, as shown in Figures 7.

3.3.3 Other metal parts of the lap track locomotive products In addition to the above two types of lap joint design, there are other lap joint requirements between the metal parts and the car body or the chassis (housing), such as wire binding frames, cables Pallets, wiring tubes, etc. Regardless of the metal component, the basic requirements and design methods of the lap joint design should be followed in the vehicle body or in the equipment cabinet.

3.3.4 Typical bad lap joint forms and hazard Track locomotive products involve many parts and components. Poor lap joint design may affect the stable operation of equipment or system, and even endanger equipment and personal safety. Connection design and hazard: There is an unreliable connection point on the current path, or the contact point is loose due to vibration. An electric spark at the junction may also generate a disturbance signal with a frequency of several hundred megahertz.

In a lightning protection network, when a lightning strike current passes through a bad junction, a voltage drop of several thousand volts is generated at the junction, and the resulting arc discharge may cause a fire or other hazard.

In the ground system, poor grounding between the various devices can make the grounding measures useless. Poor lap joints increase the lap impedance and create an interference voltage drop at the lap joint, damaging the ground equipotential requirements.

It should be used in subsequent product design to avoid similar designs. 9, 0 is a typical example of error overlap.

4 Conclusion Electromagnetic compatibility of electrical and electronic equipment in the locomotive affects the stable and reliable operation of the system, and electrical bonding 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 a more detailed and feasible design method and ideas for the erection design of electrical and electronic equipment for rail locomotives, which provides for improving the electromagnetic compatibility of products. However, due to the complexity and diversity of the system, some specific design methods still need further research and analysis. For example, the overlap of the overall shielded multi-core shielded cable is still a problem.

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