Lead wires are an important component of motor products, especially for high-power, high-voltage products, where the selection and protection of lead wires are even more crucial.
Broadly speaking, a lead wire consists of a conductor and insulation, making it a relatively simple item. However, considering the requirements and suitability for various operating conditions, meeting these needs is not a simple task. The most direct requirements are that the conductor material and diameter meet current-carrying requirements, and that the insulation layer meets requirements for dielectric strength, aging resistance, and corrosion resistance. Therefore, the selection of motor lead wires should be based on a comprehensive consideration of the product itself and the actual operating conditions.
For motor products, different voltage products should select lead wires with different withstand levels. The most intuitive comparison is the difference between the lead wires used in high-voltage and low-voltage motors. Obviously, the insulation layer of the lead wires of high-voltage motors is much thicker.
During actual production, processing, and use, motor leads may suffer varying degrees of damage. For example, the insulation may be damaged, the conductor may break under pressure, or some conductors may not be fully utilized during wiring. It is important to emphasize that any damage to the leads can potentially harm the motor, and leads should be a primary focus of protection for both motor manufacturers and users.
Analysis of common faults in wires and cables
● Insulation dampness. This is also a common problem, usually occurring at cable joints in direct burial or conduit installations. For example, substandard cable joint construction or jointing in humid climates can cause water or water vapor to enter the joint. Over time, under the influence of the electric field, water trees (described in detail in this article) can form, gradually damaging the cable's insulation strength and causing a fault.
● External damage. Analysis of recent years' operations, especially in the rapidly developing Pudong area, shows that a significant number of cable faults are caused by mechanical damage. For example, improper installation can easily lead to mechanical damage; civil engineering work on directly buried cables can also easily damage the cables in operation. Sometimes, if the damage is not severe, it may take months or even years for the damaged area to completely break down and cause a fault. Severe damage can sometimes lead to short circuits, directly impacting safe production.
●Long-term overload operation. Under overload operation, the conductor inevitably heats up due to the thermal effect of the current. Simultaneously, the skin effect of the charge, eddy current losses in the steel armor, and dielectric losses in the insulation also generate additional heat, causing the cable temperature to rise. During long-term overload operation, excessively high temperatures accelerate insulation aging, eventually leading to insulation breakdown. Especially in hot summers, the temperature rise in cables often causes weak points in the cable insulation to break down first, resulting in a particularly high number of cable faults during the summer.
●Chemical corrosion. When cables are buried directly in areas with acidic or alkaline conditions, the cable armor, lead sheath, or outer protective layer is often corroded. Long-term chemical or electrolytic corrosion can cause the protective layer to fail, reducing insulation and leading to cable failure.
● Environment and temperature. The external environment and heat sources around the cable can also cause the cable temperature to become too high, insulation to break down, or even explode and catch fire.
● Cable joint failure. Cable joints are the weakest link in a cable line, and failures caused by direct human error are frequent. If construction workers fail to properly crimp the joints or heat them insufficiently during the cable joint fabrication process, the cable head insulation will decrease, leading to accidents.
Why did the electric tree branches break down?
Dendritic breakdown occurs in a region of insulation under the influence of a high electric field. With the continuous action of the electric field, these dendritic microchannels extend throughout the insulation along the direction of the field. It is often the primary factor determining the insulation's lifespan. Lightning is the earliest known example of dendritic breakdown. Later, it was also found in solid insulation containing air gaps and oil-impregnated paper insulation. Dendritic breakdown always originates in areas of concentrated electric field within the insulation. In dry media, initiation and development are primarily driven by the electric field strength, a phenomenon known as electrical treeing.
Pulsed voltage and ground short circuits can also generate electrical trees. In a moist medium, under conditions of relatively low electric field strength, trees can also be generated after a long period of electric field application. Since no discharge is observed during the tree's propagation process, this is called a water tree. If the medium contains impurities, moisture, or chemical solutions, colored trees will form under the long-term influence of a low electric field; these are called electrochemical trees. Because the initiation mechanisms of water trees and electrochemical trees are similar, they are conventionally referred to collectively as water trees.
Electrical trees have hollow channels, approximately 10 micrometers in diameter, and their shape and length can be observed under a microscope in transparent solids, revealing clear outlines. The image shows an electrical tree formed by injecting high-energy electrons into plexiglass and short-circuiting it to ground. Water trees, on the other hand, are very blurry optically, lacking branching; they consist of tiny water droplets and connecting filaments. The trees disappear when water escapes and reappear after immersion in water. Electrochemical trees can exhibit various colors depending on the chemical composition of the medium. Some water trees grow on a single core; when the core contains iron, aluminum, and sulfur, the electrochemical trees appear brownish-brown, blue, and green, respectively.
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