Applications of Insulating Materials | Electrical Engineering

In this article we will discuss about insulating materials and their application in electrical engineering.

1. Solid Insulators:

The various materials which can be used as solid insulators are given below:

i. Asbestos:

(a) Chemically it is a hydrated magnesium silicate. It is very useful inorganic material. Chrysotile 3MgO, 2SiO₂, 2H₂O belongs to the group of fibers in nature.

(b) The fibrous structure of asbestos combined with its non-inflammability, makes it particularly suited for thermal as well as electrical insulation.

(c) When properly impregnated and formed into suitable shapes, it is useful for many structural purposes.

(d) It has a relatively low value of dielectric strength and provides good insulation at low voltages but its property of resistance against heat makes it very useful in high temperature windings such as those in railway, crane, mill or compressor motors where wide temperature changes may be encountered. It is applied in the form of tape, sheet or pulp. One commonly finds asbestos in welding machines, electric ovens, electric bottles and other table appliances.

ii. Bakelite:

(a) It is a synthetic resin obtained by condensation of formaldehyde with phenol or cresol.

(b) It is heat proof, acid proof and mechanically strong.

(c) It is commonly used for manufacture of all kinds of small electrical fittings— lamp-holders, switches, plugs etc.

iii. Cotton or Silk:

(a) Cotton is hygroscopic and has low electric strength, so it must be impregnated with varnish or wax after winding.

(b) Cotton covered wire is extensively used for winding of small magnet coils, armature winding of small and medium sized machines, chokes and small transformer coils etc.

(c) Silk is more expensive than cotton but takes up less space and is therefore used for windings in fractional horse power machines.

(d) Silk is less hygroscopic and has a higher dielectric strength than cotton, but like cotton it requires impregnation.

(e) The operating temperature of cotton and silk is 100°C and the material carbonise above this temperature.

iv. Mica:

(a) It is a mineral composed of silicate of aluminum with silicates of soda potash and magnesia. It occurs in the form of crystals which can easily be split into lamina.

(b) It is affected by oils.

(c) Its dielectric strength varies from 40 to 150 kV/mm.

(d) It is least affected by heat but dehydrates at high temperature.

(e) It has high dielectric strength and low power loss.

(f) It is rigid, tough and strong.

(g) Its electrical properties are deteriorated with the presence of quartz and feldspar.

(h) Mica sheets are used for the insulation leaves between commutator segments. These are however not available in large thickness.

(i) Thick mica tape is largely used for taping armature and field coils of traction motors (in case where glass tape is not employed) and for taping the starter coils of H.V. alternators.

v. Micanite:

(a) It is formed from mica fragments bonded by shellac varnish into large sheets.

(b) Its dielectric strength is 30 kV/mm

(c) It is mechanically weak at high temperatures.

(d) As mica sheets are not available in large size to be used in commutators of considerable dimensions. ‘Micanite’ has been developed for this purpose. This is obtained by switching mica slitting on sheets of paper by means of a drying varnish such as shellac. These sheets are pressed between heated plates and then cooled.

(e) It is used for slot lining of H.V. machines.

It is also employed for making bushes (Bush is an insulator which enables a live conductor to pass through an earthed wall or tank)

vi. Paper:

(a) It is prepared from wood pulp and manila fibers beaten and rolled into sheets.

(b) Its dielectric strength is 4 to 10 kV/mm.

(c) It is hygroscopic and absorbent and so is particularly suitable for impregnation.

(d) Electrical properties are adequately good.

(e) It is rarely used in un-impregnated condition but can be used successfully under oil.

(f) It carbonizes at 125°C so that the temperature of any paper insulated apparatus is limited to about 100°C.

(g) The paper is used for winding and cable conductor insulation, primary dielectrics in capacitors, backing for mica insulation, slot insulation of electrical machines, transformer insulation.

(h) The low density paper (0.7-0.8 gm/cm³) is preferred in high frequency capacitors and cables as the electric loss and the discharge current will be lower. Medium density paper (1-1.25 gm/cm³) is used in power capacitors.

High density papers are preferred in DC and energy storage capacitors and other DC insulation.

vii. Waxes:

Because of their high electrical resistance and exceptional water repellence, waxes are used as impregnating compounds in condensers and in the radio coils used in the tropics. The common variety used is paraffin wax.

Insulating wax is used as an impregnating material for paper and cloth insulation, as a dipping medium, and as a directly applied coating on conductors.

viii. Rubber:

The materials used in cable insulation are – (i) pure rubber, (ii) impregnated paper, and (iii) vulcanised bitumen. Raw rubber cannot be used as an insulating material directly because it is brittle and hard when cold but soft and stiff when hot. In addition, it perishes under the action of oil and air. Sulphur addition makes the product harder. If the sulphur is added to an extent of 30%, vulcanite or ebonite is obtained which is hard material with excellent insulating properties. The operating temperature of rubber is 50°C.

ix. Ceramics:

(a) Ceramic materials contain phases that are compounds of metallic and non- metallic elements. Ceramics are mechanically hard possess good electrical properties. The ceramics used as dielectric may be broadly described as clay containing porcelains, alumina ceramics, talc containing steatites and the titanates (mostly barium titanate which have high dielectric constant).

(b) Ceramics with dielectric constant below 12 are more advantageously used in bulk as insulator, bushing, housing and the like. Materials with _r < 12 are porcelains, steatite, torsterit, wollastonite, alumina ceramics etc. Other applications of low permittivity ceramics include suspension insulators for high voltage lines and pin insulators for low voltage lines, station insulators and bushing for transformers, oil breakers, disconnecting switches. At high temperature they are used for support of heaters and furnace windings, wire wound power resistor, insulators for thermocouples, cathode heaters, insulation in valves and sparking plugs.

(c) Material with high dielectric constant (i.e. ԑ_r > 12) are based primarily on mineral rutile (TiO₂) and on titanates. Titanate ceramics (BaTiO₃, SrTiO₃, and CaTiO₃) have high dielectric constant of about 3000-7000 which renders them especially useful for some capacitor applications.

x. Resins or Polymers:

(a) Resins are organic substances with very high molecular weight. They are available in nature as well as artificially made. Synthetic resins are synthetic polymers known as plastics and form an important group of insulating materials.

(b) They are bad conductors of electricity and unsuitable for high temperature service.

(c) They have high dielectric strength.

xi. Glass:

(a) Ordinary glass is a good insulator but it is too brittle to be used for anything; scientific instrument parts, containers etc.

(b) Toughened glass is employed for insulation in extra H.V. lines having voltage above 100 kV. It is produced by processing ordinary glass to desired shape at about 100°C and then cooling quickly in compressed air current.

(c) The electric strength of toughened glass under lighting conditions is 3-4 times, higher than that of porcelain.

(d) Glass is used in the production of glass fiber formatting tapes similar to cotton or silk tapes.

(e) It is widely used as an insulating material to form the envelop and for internal supports in electric light bulbs, electronic valves, X-ray tubes etc.

(f) Glass having high dielectric constant, very low temperature coefficient is used as a dielectric material in capacitors.

(g) Potash, Potassium, Sodium Pyrex and silica glass are used for making insulating supports such as antennas, line insulators and bushings.

2. Liquid Insulators:

Liquid insulators are used in transformers, regulators and oil circuit breakers where they help to dissipate the heat generated by convection. They are also used for impregnation of solid insulators like paper (in cables).

A low viscosity oil is required for easy flow. Dielectric strength which is important in all cases is affected greatly by the presence of moisture and other impurities such as dissolved gases. They are mineral oils consisting of complex series of hydrocarbons obtained from refining of crude oils.

As the principal use was in transformer in the early stage of development, they are given the name transformer oil. The oil before being used is made free from moisture and filtered to remove sludge etc.

Synthetic liquid dielectrics, e.g., Pyranol, Inerteen etc. have replaced mineral oils because they, in addition to insulating properties, possess the property of being non-inflammable and do not emulsify water.

3. Gaseous Insulators:

Gaseous insulators are used both as an insulants and heat transfer medium. The major problems associated with them are the abnormalities in dielectric behaviours at high pressure, temperature instabilities and fire hazards.

Although, all known gases are dielectric gases when pure, but from the engineering point of view, taking into consideration the dielectric strength, dielectric loss, chemical stability and corrosion etc. they may be classified as:

i. Simple Gases:

This group includes air, nitrogen, neon, hydrogen, helium, argon etc.

ii. Oxide Gases:

Carbon dioxide, sulphur dioxide etc.

iii. Electronegative Gases:

Sulphur hexafluoride (SF₆), CH₂Cl₂ etc.

iv. Hydrocarbon Gases:

Methane, ethane, propane, Freon etc.