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Insulating materials used in modern Electrical Machines

Insulating materials are non metallic materials that do not allow electric current to pass through them at normal temperatures. They may be organic or inorganic, uniform or heterogeneous in composition, natural or synthetic. There are number of insulating materials used in Electrical apparatus and machinery.




Mica is a widely used insulating material. It is used in the form of sheets. Hence it is difficult to work over mica. Therefore it is used in the form of sheets of splitting with shellac, bitumen or synthetic or polymer binding.

Mica folium

It consists of mica splitting which is to be paper and air dried wrapping. It may be moulded directly on conductors, then rolled and compressed between heated plates to solidify the material and to expel air.

Fibrous glass insulated wires


Fibrous glass

It is made from material which is free from alkali metal oxides which may form a surface coating that may attack the glass silicates. Tapes and clothes woven from continuous filament yarns of glass have a high resistivity, thermal conductivity and tensile strength and form a good insulation with temperature withstand capacity of 130oC. Fibrous glass insulation are prone to abrasion. Glass does not absorb moisture but capillary action between fine filaments makes may attract moisture. These insulation are widely used in induction motors.


This material is mechanically weak and poorer insulating material when compared with fiber glass. Laminates of asbestos with synthetic resin have good mechanical strength and thermal resistivity. Asbestos in the form of wire has a good abrasion resistance.

Cotton fiber insulated wires


Cotton fiber

Cotton fiber woven from acetylates cotton has very good heat resistance and are much less hygroscopic than ordinary cotton.


Polyamides in the form of nylon tapes have high mechanical strength. They have a very good space factor. Nylon film is one of the few plastic films having adequate resistance to temperature. It could withstand tearing.

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Synthetic resin enamels

Vinyl- acetate or nylon when covered with synthetic resin enamels have an excellent smooth finish and have been used in most of the windings. They give good binding to the windings.


Wood is a form of synthetic resin impregnates. It has proven itself to be a robust and accurate material packing blocks, coil supports and spacers. However presence of humidity considerably reduces the insulating property of wood.

Silicone insulated conductor



Silicones are semi organic materials with a basic structure of alternate silicon and oxygen atoms. They are extremely resistant to heat. They act as binders and permit their continuous operations at temperature even more than 180 oC. Even when they disintegrate at excessive temperatures silica, which is also an insulator, is formed. Silicones are water repellent and anti corrosive. They are used in dry transformers, traction motors, mill motors, and miniature aircraft machines operating over a winding temperature range of over 200 oC. They have a high thermal conductivity, improved heat transfer coefficient, which facilitates heat dissipation from the conductors.


Epoxide thermosetting resins.

These materials have assumed importance in casting potting, laminating adhesive and varnishing applications and in the encapsulation of small transformers.

Synthetic resin

Bonded paper, cotton and glass fiber with synthetic resin laminates have good electrical and mechanical properties as sheets in large cylinders and tubes.

Transformer oil


Petroleum based mineral oils

They are extensively used in the cooling and insulation of oil immersed transformers. The important characteristics of Petroleum based mineral oils are chemical stability, expansion coefficient, resistance to sludging by oxidation and viscosity. Their have very good electrical characteristics when they are clean. Their properties diminish with increase in the level of impurities and moisture.

Physical constants of Transformer oil

PropertyRecommended value



Thermal conductivity

0.12 W/m deg C

Specific Heat

2.06 kJ/kg deg C

coefficient of Expansion

0.00078/ deg C

Mean density factor

0.00065/ deg C


These are non inflammable insulating liquid. When these liquids are stuck by an electric arc evolves non explosive gases and hence they are safe. The commonest askarel is a 3/2 mixture of hexachlorodiphenyl trichlorovenzine giving a low pour point and a satisfactory viscosity/temperature characteristics.

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