Teflon Fluorothenes
Maximum cable operating voltage a.c. (kV)
3.0 0.6 0.6 5.0 28.0
0.6 15.0 5.0 5.0
Range of operating temperature (0C)
- 40 to 70 - 40 to 75 - 30 to 90 - 40 to 150 - 40 to 80
- 30 to 105 - 60 to 80 - 54 to 250 - 54 to 150
5.6 APPLICATIONS IN POWER CAPACITORS
It is well-recognized that power capacitors are indispensable for power system administration and are used for voltage regulation of power transmission systems and for the improvement of power factor of power distribution networks.
In most of the industrial applications, the power requirements are reactive in nature and a lagging current is drawn from the power lines. This requires additional generat- ing capacity. This can be compensated by using capacitors which take a leading current in a.c. circuits. Hence the greatest use of power capacitors is with the power frequency systems. Capacitors are made in simple units with voltage ratings for 220 to 13800 V with kVAR ratings varying from 0.5 to 25 kVAR. Power capacitors are normally made using impregnated paper dielectric. Power capacitors are also used for high frequency applications such as power factor correction in high frequency heaters and induction furnaces. At high frequencies the dielectric losses increase very rapidly, and the capacitors have to be cooled externally using water cooling. Capacitors are also used in d.c, applications such as impulse voltage generators, energy storage, welding and high intensity flash x-ray and light photography.
Generally, power capacitors are made of several layers of insulation paper of adequate thickness and aluminium foil of 6 microns thickness as electrodes inter- leaved and wound. Single units are connected in parallel depending on the rating of the capacitor unit to be manufactured. These are placed in containers hermetically sealed, thoroughly dried, and then impregnated with insulating oil. The impregnating oils used are either mineral oil or chlorinated diphenyl oil. Capacitors made with mineral oil are quite expensive, and hence capacitors made with chlorinated diphenyl are preferred for power factor correction applications because of their low cost and non-inflammability.
Properties required for the insulation paper for capacitor applications are high dielectric strength, low dielectric loss, high dielectric constant, uniform thickness, and minimum conducting particles. The recent discovery of polypropylene film has considerable power dielectric loss and higher operating voltage. However, paper is still widely used partly, mainly due to the reason that paper after impregnation offers many desirable properties required for use at high voltages in addition to economy.
The impregnant for power capacitors should provide high dielectric strength, dielectric constant equal to that of paper, high permeability to paper, and sufficient viscosity to enter the voids in paper. Its flash and solidifying points should be above 12O0C and below -1O0C respectively. The impregnants used are broadly classified into mineral oil and synthetic oil (askarels). The dielectric properties of the tissue paper and the capacitor impregnants are given in Tables 5.4 and 5.5.
Table 5.4 Characteristics of Tissue Paper (a) Fibre Composition
Unbalanced sulphate Ash
Moisture (b) Water Extract
Conductivity pH value Chloride content Standard thickness Conducting particles Density
Dielectric strength
100%ASTM less than 0.3%
4 to 8%
less than 4 p. Siemens/cm 6.0-7.5
less than 5 mg/kg
10, 12, 13, 14, 15, and 18 micron nil
1 or 1.2 g/cm3
30-40 V (d.c.)/micron Table 5.5 Properties of Impregnating Oils
Property Mineral oil Trichlorodiphenyl (Askarel) Specific gravity 0.90 1.378
Boiling point (0C) 270-300 325-360
Flash point (0C) 152 None
Solidifying point (0C) -40 -19
Dielectric constant 2.25 5-8 Minimum resistivity 1012 ohms 1012 ohms
at 10O0C at 10O0C
tan S at 5O0C 5 x IQ"4 2 x IQ"3
The electrode material extensively used in aluminium foil of 6 microns thickness because of its high tensile strength, low specific resistance, high melting point, low specific gravity, low cost and easy availability.
As already mentioned, the latest trend in capacitor manufacture is to replace paper by polypropylene plastic films. This results in a drastic reduction in size. Its use results in cheaper capacitors for high voltagfe ratings because of its high working stress. As regards impregnants, askarels are harmful to the environment and hence are being banned. The latest trend is to develop uther types of materials. With this in view, research is being directed towards the use of vege able oils like castor oil.
5.7 APPLICATIONS IN ELECTRONIC EQUIPMENT
The progress in electronic industry depends on the availability of dielectric materials.
Modern electronic components and instruments are very complex, and their perfor- mance will depend on the nature and reliability of dielectric Materials used in their construction. The electronic equipments have to operate with d.c. and a.c. voltages, under varying humidity and temperature conditions. No single material can meet these requirements, and different materials are used under different conditions.
The general properties of insulating materials for electronic use are the same as those in electrical industries, but the relative importance is different Properties such as electrical and mechanical strength, and thermal and electrochemical endurance are important. In addition, size, reliability, and failure rates are important factors in selecting materials for electronic industries. There is an ever increasing demand from the electronics industries to make components which are smaller, more reliable, more stable, and capable of operating under adverse electrical environmental conditions.
Dielectric materials are divided into two different groups as far as applications in the electronic industry are concerned based on the frequency range over which they operate, namely polar dielectrics and the nonpolar dielectrics. The polar dielectrics are normally used for d.c. and power frequency (50 Hz) applications, while the non- polar dielectrics are used at very high frequencies. Typical non-polar or low loss materials are polystyrene, polyethylene, teflon, quartz, etc. and typical polar substances are phenolic plastics, nylon, plasticised cellulose acetate, castor oil, etc.
5.7.1 Materials for Low Frequency Applications (Polar Materials) Large quantities of wires and cables are used for connecting various components in electronic equipments. All these, except the high frequency cables, utilize polar dielectrics. Early polar dielectrics were natural rubber and textile fibre. These were highly susceptible to moisture absorption. They were replaced later on, by synthetic resins of thermoplastic type such as polyvinylchloride. The presence of plasticizer limits the use of these wires to high temperature work. Therefore, nylon jacketed P. V.C. wires were considered. But the susceptibility of nylon at high temperatures and the resultant brittleness made this unsuitable for high temperature applications. This problem was overcome by the use of teflon for the insulation of cables. Teflon insulated cables are very compact, possess very high resistance, and can be operated up to 25O0C. The only disadvantage is their very high cost.
5.7.2 Materials for High Frequency Applications
As already mentioned, the examples of materials of this group are teflon, polyethylene and polystyrene which possess the required ideal properties for high frequency applications, such as low dielectric constant, high dielectric strength, and low dielectric loss. The properties of all these materials were already discussed in the previous chapter.
5.7.3 Materials for Resistors
Resistors are of two types, namely fixed and variable. The necessity to select proper insulating materials is more important in the case of fixed resistors to achieve stability and compactness. A resistor will have two insulating components, the core and the encapsulant. The core is normally made out of high grade non-porous electro-ceramic material. The core material may have to withstand about 1000'C, when a carbon film is deposited on it. Epoxy resin is a very popular encapsulant. It has high insulation resistance, is impervious to moisture, and has good heat resistance and thermal conductivity. For wire wound resistors, again high grade ceramic is used for the core, and vitreous enamel is used as a protective coating. The power handling capacity of this type of capacitors is high, and the protective covering has to withstand much higher temperatures.
5.7.4 Materials of Electronic Capacitors
Capacitors were also made of both fixed and variable types. Electrolytic capacitors are used for d.c. applications, while capacitors made of paper, plastic film, mica, ceramic, and semiconductor materials are used for a.c. applications.
As already discussed, paper capacitors are widely used for power and high voltage applications. In the field of electronics and communication engineering other types of capacitors are popular. Polystyrene film, polyester, and polyprophylene are the popular dielectrics for these applications. Polystyrene film capacitors are extremely stable in capacitance value and can be used up to several mega-cycles. The main disadvantage is their poor heat resistance. They can be used only in the temperature range between - 4O0C and + 7O0C. Except the limitations on the operating tempera- ture range, polystyrene film capacitors are very popular and have replaced mica capacitors for many applications.
Polyester capacitors, on the other hand, have a wide operating temperature range, low water absorption, and low dielectric losses and are fast replacing paper capacitors. Polyester film capacitors coated with epoxy resin are extensively used in radio frequency circuits for by-pass, inter-stage coupling, etc. They can be used up to very high frequencies. Metallized polyester film, developed in recent years, is a big boon to the capacitor industry. Use of this film drastically reduces the size of the capacitors.
Polypropylene capacitors are also becoming popular. They have similar properties as polyster capacitors, but are much cheaper in price. They can be used as low frequency a.c. capacitors, but they have to compete in price with paper impregnated capacitors.
Polyethylene film has excellent electrical properties but poor mechanical proper- ties and hence cannot be drawn into a very thin film. Therefore, it cannot be used for low voltage capacitors, but can be employed in high voltage capacitors.
Teflon also cannot be easily drawn into thin film, and hence it has not become very popular as a capacitor dielectric. Another factor is its high cost.
Mica capacitors are made using mica as the dielectric. Mica has got all the good properties of a dielectric. The encapsulant is again an epoxy resin to protect the capacitors from the environment. To limit the loss of properties of the mica element, to fully realize the stability and the operating temperature, it is housed in a metal can filled with an inert atmosphere, and the leads are brought through ceramic seals.
Ceramic capacitors are made of ceramic, and are encapsulated by a dip coating in phenolic resin; if higher operating temperatures are expected, an epoxy resin coating is given.
For electrolytic capacitors, the dielectric is the oxide layer formed on etched aluminium or tantalum foil. The foils are separated by electrolytic grade Kraft paper.
The dielectric layer is very thin; hence the insulation resistance is low, and the leakage current is high. The stability of the oxide layer and hence the stability of the capacitance value depends on the type and purity of the electrolyte. Tantalum capacitors give stable capacitance with wider operating temperatures and lower leakage currents.
5.7.5 Materials for Professional Grade Electronic Components
Resistors, capacitors and other electronic components used in defence equipment, computers, space applications, etc. should be highly reliable and must be capable of stable operation under severe environmental conditions. These are called professional grade components, and they should be superior in characteristics regarding reliability, stability, close tolerance, wider range of operating temperatures, superior electrical and electronic properties, and capacity to operate under severe environmental condi- tions. The materials to achieve these superior characteristics should be chosen very carefully.
Dielectric materials used in professional grade components should serve the dual purpose of providing electrical insulation and protection from adverse environmental conditions. We have already elaborated on the materials with superior insulation properties used in resistors and capacitors. The protection against atmospheric in- fluences has to be almost perfect, and this is achieved by placing the capacitors in a nonferrous metal can and bringing out the leads through glass to metal seals. The next method in order of preference is encasing in metal cans and using ceramic to metal seals. Further down in priority would be to use metal can with ceramic bushings, or an epoxy free moulded can with epoxy end sealing, or a ceramic can with epoxy end sealing, etc.
5.7.6 Materials for Electromechanical Components
There are a large number of electromechanical components used in electronic in- dustry such as relays, connectors, valve bases, terminals, terminal boards, bushes, plugs and sockets, etc. These are made from pressed parts or moulded parts.
One of the most commonly used material for pressed parts is industrial grade phenolic resin laminate. There are various grades in these laminates, and choice can be made based on the insulation properties and resistance to water absorption. This is commonly used for entertainment grade components used in radio and television sets.
Glass epoxy laminates are used for printed circuit boards and some terminal boards.
Phenolic resin bonded cellulose acetate paper laminates and polyester filled with traces of glass fibre and laminated with epoxy resin are becoming increasingly popular now-a-days. The latter have excellent electrical properties, low water absorp- tion, and good punchability. Insulating plastic parts subjected to wear and those that come into contact with metal parts are made from polypropylene sheets.
Moulded insulating parts for electromechanical components are made from a number of plastic materials. When higher operating temperatures are not required, high impact polystyrene is used. For operation with higher temperatures, polycar- bonate moulding is employed. When higher mechanical strength is also desired along with higher operating temperatures, nylon moulding is used. When higher mechanical strength, high insulation resistance, high operating temperatures, and dimensional stability are required, mica filled bakelite or glass-filled diallylphthalate are used, and parts are compression moulded. Diallyphthalate is used for professional grade com- ponents such as, valve bases, printed circuit board connectors, multipin connectors, etc. For professional grade components, where vibration and shock are not present, steatite is also used.