1、The selection of plastics for plastic electric insulation products
There are many kinds of electrical insulation materials, such as mica, various electrical properties are better than plastic materials, its dielectric loss is small, corona resistance and arc resistance are good. When to choose plastic as insulation materials generally follow the following principles.
(1) The following cases are suitable for the use of plastic
The product requires light quality.
Complex shape of the product.
High vibration load at work.
The use of temperature is not too high.
(2) The following cases should not be used plastic
Working environment load is too large.
The use temperature is too high.
working voltage is too high.
Generally choose mica, ceramic, glass and asbestos, etc.
2、The general principle of choosing plastics for plastic electric insulation products
The higher the voltage of the application of electrical insulation materials, the greater the heat loss of the insulation material, the greater the possibility of thermal breakdown. For this reason, especially in high-voltage occasions, in the selection of materials, to use plastic materials with small heat loss, such as small dielectric loss, small dielectric constant plastic varieties, in order to reduce heat loss as much as possible, improve the performance of voltage resistance.
With reference to the above principle, the following points should be noted when selecting the specific.
(1) Different uses of electrical insulation materials, the size of the dielectric constant requirements are different
In the capacitor, in order to reduce the volume, increase the capacity, often choose the dielectric constant value of plastic dielectric larger; and in electrical appliances and cables and other non-capacitor applications, in order to prevent excessive charging current and heat loss, often choose the dielectric constant of plastic dielectric smaller.
（2) in the same motor and electrical appliances, should use the same dielectric constant or close to the plastic dielectric
If the dielectric constant of plastic dielectric is different, it will affect the voltage distribution of the whole insulation system, so that the dielectric constant of small plastic dielectric to withstand the larger voltage, dielectric constant of large plastic dielectric to withstand the smaller voltage, thus reducing the insulation capacity of the whole system.
(3) in high-frequency applications, dielectric loss caused by the proportion of heat loss increases, electromagnetic induction, capacitive resistance and eddy current problems are more prominent.
For this reason, the dielectric loss angle tangent (tanδ) and dielectric constant (ε) should be selected as small as possible and does not increase significantly with the increase in frequency, high dielectric strength of plastic materials, commonly used PE, XLPE, F4, PP and PS, etc.
(4) In high-voltage applications, we should pay attention to the following three points.
① In high-voltage applications, heat loss is significantly increased, the corona resistance is particularly important, we must use plastic varieties with good corona resistance. Most of the plastic corona resistance is not good, only PE, XLPE, PI and F4 corona resistance can meet the requirements of high-voltage resistance.
② In high-voltage applications, we should also pay attention to the selected plastic’s pressure strength and volume resistivity should be high.
③ In high-voltage applications, the selected plastic materials should be absolutely pure and free of impurities, dust and moisture to prevent electrical breakdown.
(5) for often open occasions of insulation materials, to have excellent arc resistance performance
In the plastic materials, AF, F4 and PSF arc resistance is better, EP, PF and HDPE arc resistance is still possible.
(6) in wet applications, to use the electrical properties of the plastic species with the rise in humidity does not decline significantly, commonly used F4, PP and chlorinated polyether.
(7) in the strong corrosive environment, to use good corrosion resistance of fluorine plastics and chlorinated polyether.
3、Select plastic raw materials according to the type of plastic insulation products
(1) industrial frequency low-voltage electrical equipment
The requirements for the dielectric properties of plastic are not high, with general insulation properties can be, almost all plastic varieties can be used, but pay attention to its heat resistance and mechanical strength.
(2) High-frequency high-voltage electrical equipment
Requires good corona resistance of insulating materials and small dielectric loss, commonly used PE and XLPE, etc.
With resistors, diodes and transistors, capacitors are also commonly used electronic components.
Capacitors in the insulation medium commonly used plastic materials, mica and insulating paper, of which plastic accounts for the majority.
Capacitors with plastic materials dielectric performance requirements are: dielectric constant to be slightly higher; dielectric strength to be high; dielectric loss to be low; electrical properties with temperature, humidity and frequency changes to be small.
Specific requirements for plastic films for capacitors.
Film thickness is generally 0.002~0. 04mm range, requiring small thickness tolerance; film surface should be clean, preferably with appropriate roughness.
Capacitor plastic materials are often biaxially oriented plastic film, specific varieties of PP, commonly used in power capacitors, film thickness of 0.015 ~ 0.02mm, compared with insulating paper can improve the capacity of 25%; PET, film thickness of 0.006 ~ 0.01mm; PS, film thickness of 0.02mm; ceramic-filled F4, film thickness of 0.02 ~ 0.04mm, can be used in the 60 0.04mm, can be used between 60~250°C; others are PC, PE and PI, etc.
In the capacitor with insulating plastic materials, PP dosage can account for about 48%, PET dosage can account for 36%, the rest are PS, PC, PE and F4, etc.
(4) Open type insulation products
Open class insulation products including gate switches, automatic switches, circuit breakers, fuses, relays, travel switches, contactors, plugs and sockets, etc.. Its characteristics are often changing the open and closed state, easy to generate sparks and arcs.
The performance requirements of the plastic materials used in open and closed insulating products are high dielectric strength, good arc resistance and high heat resistance.
The commonly used plastic materials for open-break insulation products are thermosetting plastics such as amino plastics, epoxy plastics and phenolic plastics.
Low-voltage open break class insulation products are often used phenolic plastic, amino plastic, glass fiber reinforced silicone plastic and PSF, etc.
High-voltage open type insulation products often use polyester fiber reinforced epoxy plastic, inorganic filled epoxy plastic, alicyclic epoxy plastic, F4 and F46, etc.
(5) Printed circuit boards
Commonly used phenolic glass cloth laminate, epoxy glass fiber laminate and cyanate resin, etc.
(6) Insulation tape
SPVC film is commonly used.
(7) Encapsulation materials
Most commonly used epoxy resin, sometimes with phenolic resin and unsaturated polyester, etc.