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Thursday, 3 October 2019

Common Insulating Materials used in Electrical Apparatus

Some common insulating materials used in electrical apparatus are explained below.





4. Insulating Textiles:

Textiles are woven from fibred materials like cotton, jute and hemp. These are from natural fibres. Sometimes silk and animal origin is also used. Nylon, teflon, rayon, etc., are synthetic fibres. They are cheap and can be easily processed.

Types of Insulating Textiles

No:
Name

Properties
Applications
1
Cotton
Flexible, mechanically strong, hygroscopic
Cotton covered wires are used for transformers, rotor, stator windings
2
Silk
Thin, strong and hygroscopic. Better space factor than cotton, low thermal conductivity.
Same as above
3
Vulcanised paper treated with zinc chloride
Tough, properties, depend upon water absorption, it does not absorb oil hence baked and treated with Wax to improve electric characteristics.
Bushings, washers and as low voltage insulation

They have a high mechanical strength and flexibility. As they are hygroscopic, their electrical properties can be improved by impregnation. Inorganic fibres like asbestos and glass have high heat resistance but they are not elastic and flexible like organic fibrous materials. Properties and application of some insulating textiles are given in Table.


5. Varnished or Impregnated Textiles:

Cotton or silk textiles can be varnished by oil varnishes. The normal thickness of varnished textiles vary between 0.08 mm to 0.25 mm. This material belongs to Class A insulating materials. It has a good mechanical and dielectric strength with low hygroscopicity. It offers low resistance to organic solvents. The limiting temperature upto which it can be used is 105° C. It is normally used in cables as wrappers and liners, for windings in electrical machines of low and medium range. It is also used in the form of adhesive tapes.

6. Asphalt and Bitumens :

Asphalt is obtained underground near oil fields and is black or brc .vn in colour. A freshly mined asphalt contains mineral impurities. It is hydrocarbon in composition and is insoluble in water and chemically inert. Its softening temperature is about 60°C and its melting point 100°C. In its natural form is a hard substance.

Bitumen is the oldest known plastic and was employed about 3000 BC ago. In modern days it is used chiefly in electrical industry and they are made from bituminous products by mixing with a suitable filler such as slate dust or asbestos in powder form. It is solid or semi solid material obtained by refining crude petroleum. Pitch asphalt, naphtha tars etc., all are known as bitumen. They are hydrocarbons in composition and are good insulating materials. Though they are moisture resistant they are inflammable materials and readily catch fire. Asphalt is a natural product and bitumen is a refined product. Its softening temperature varies between 30°C to 140°C depending on the variety of bitumen and its specific gravity is one. It is acid and alkali resistant and is easily oxidised. It possesses great chemical stability but is affected by oil. It is highly soluble in mineral and synthetic oils. They are sometimes call sealing compounds.

They are used in underground cables for protection of lead and steel armour against corrosion. Bitumen compounded paper hessian and cotton tape are widely used in the manufacture of underground cables to provide bedding and serving for steel armour and in cable joint boxes

7. Waxes:

It is naturally obtained insulating material having high resistant to water. Its chemical composition is oxygen, carbon, hydrogen, and eastern. Depending on the variety its melting point varies upto 130°C. It is a satisfactory dielectrics and dielectric constant varies between 2 to 3. When it changes the state, it undergoes considerable shrinkages and expansion due to which its usage is limited to low voltages only. At room temperature it is solid and water resistant. It is used as a constituent of sealing compound and when mixed with insulating oils improves viscosity and pour point to form non-draining cable compound.

Another form of wax is called Paraffin and Microcrystalline wax. These are hydrocarbons in composition and are obtained by the process of distillation of mineral petroleum oils. They are easily soluble in mineral and synthetic oils and are mechanically weak. It gets oxidised when heated beyond its melting point. The melting point of micro-crystalline waxes is higher than that of paraffin waxes and when they change the state they show remarkable change in volume. The uses are similar to that of ordinary wax. General properties of these waxes are shown in Table.

General Properties of Paraffin and Microcrystalline Waxes

Property

Paraffin wax
Microcrystalline wax
Flash point (°C)
200 - 245
235 - 300
Fire point (°C)
200 - 280
280 - 350
Melting point (°C)
50 - 65
67 - 87
Specific gravity
0.907
0.930
Resistivity at 25°C (ohm - cm x 10-12)
100 - 200
100 - 200
Dielectric Constant (25°C)
2.20 - 2.25
2.20 - 2.30
Power factor (x 103) at 25°C
0.2 - 0.4
0.2 - 0.5
Power factor (x 103) at 100°C
0.5 - 1.8
0.7 - 3.0

8. Glass Insulating Materials :

Glass in an inorganic material made by the fusion of different metallic oxides. Different oxides involved in the manufacture of glass are those of silicon, baron, phosphorous, sodium, potassium, magnesium, calcium, zinc, lead etc. Silicon oxide invariably used in the manufacture of glass. Addition of other oxides in different proportions and combinations give different properties to glass. Glass is normally transparent, brittle and hard. They are resistant to most of the chemicals. They are fragile, dense and heavy. They have low dielectric loss, high dielectric strength of 25 kV / mm to 50 kV / mm and their resistivity is 106 to 1015 ohm - m. The relative permittivity is 3.8.


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