Material Science: Electrical Engineering materials

By Mona Kumari|Updated : June 14th, 2021

INTRODUCTION TO INSULATING MATERIALS

Electrical insulating materials are defined as materials which offer a very large resistance to flow of current, and for that reason they are used to keep the current in its proper path along the conductor.

A large number of substances and materials may be classified as insulators, many of which have to be employed in practice, as no single substance or material can satisfy all the requirements involved in the numerous and varied applications of insulators in electronics & electrical engineering. Such requirements involve consideration of physical properties, reliability, cost, availability, adaptability to machining operations etc.

INTRODUCTION TO INSULATING MATERIALS

Electrical insulating materials are defined as materials which offer a very large resistance to flow of current, and for that reason they are used to keep the current in its proper path along the conductor.

A large number of substances and materials may be classified as insulators, many of which have to be employed in practice, as no single substance or material can satisfy all the requirements involved in the numerous and varied applications of insulators in electronics & electrical engineering. Such requirements involve consideration of physical properties, reliability, cost, availability, adaptability to machining operations etc.

Thus in some applications the insulating material in addition to its function as an insulator may have to act as a rigid mechanical support to the conductor and may be installed out of doors, in which case the insulating qualities must be retained under all atmospheric conditions, in other cases extreme flexibility is required.

Again, in electric heaters the insulating materials must maintain their insulating qualities over a wide range of temperatures extending in some cases to 1100°C, and for radio purposes the insulating qualities must be maintained upto very high frequencies.

In electrical machines and transformers, the insulating materials applied to the conductors are required to be flexible, to have high specific electric strength (to reduce thickness to minimum) and ability to withstand unlimited cycles of heating and cooling.

  1. CHARACTERISTICS OF A GOOD INSULATING MATERIAL

A good insulating material should possess the following characteristics:

  1. Large insulating resistance.
  2. Highdialectic strength.

iii.     Uniform viscosity—it gives uniform electrical and thermal properties.

  1. Should be uniform throughout—it keeps the electric losses as low as possible and electric stresses uniform under high voltage difference.
  2. Least thermal expansion.
  3. When exposed to arcing should be non-ignitable.

vii.    Should be resistance to oils or liquids, gas fumes, acids and alkalies.

viii.   Should have no deteriorating effect on the material, in contact with it.

  1. Low dissipation factor (loss tangent).
  2. High mechanical strength.
  3. High thermal conductivity.

xii.    Low permittivity.

xiii.   High thermal strength.

xiv.   Free from gaseous insulation to avoid discharges (for solids and gases).

  1. Should be homogeneous to avoid local stress concentration.

xvi.   Should be resistant to thermal and chemical deterioration.

  1. CLASSIFICATION OF INSULATING MATERIALS

The insulating materials can be classified in the following two ways:

  1. Classification according to substances and materials.
  2. Classification according to temperature.
  3. Classification According to Substances and Materials:
  4. Solids (Inorganic and Organic):

Mica, wood, slate, glass, porcelain, rubber, cotton, silk, rayon, terylene, paper and cellulose materials etc.

  1. Liquids (Oils and Varnishes):

Linseed oil, refined hydrocarbon mineral oils, spirit and synthetic varnishes etc.

iii. Gases:

Dry air, carbon dioxide, argon, nitrogen etc.

  1. Classification According to Temperature:

Class

Insulating materials Included

Assigned limiting Insulating temperature

Y

(Formerly O)

Cotton, silk, paper, cellulose, wood, etc., neither impregnated nor immersed in oil. Materials of Y class are unsuitable for electrical machines and apparatus as they deteriorate rapidly and are extremely hygroscopic.

90°C

A

Materials of class Y impregnated with natural resin, cellulose esters, insulating oils etc. Also included in this list are laminated wool, varnished paper.

105°C

E

Synthetic resin enamels, cotton and paper laminates with formaldehyde bounding etc.

120°C

B

Mica, glass fibres, asbestos with suitable bonding substance, built up mica, glass fibre and asbestos laminates.

130°C

F

Materials of class B with bonding materials of higher thermal stability.

155°C

H

Glass fibre and asbestos materials, and built up mica, with silicon resins.

180°C

C

Mica, ceramics, glass quartz without binders or with silicon resins of higher thermal stability.

above 180°C

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