Specifications

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1. WINDING INSULATING MATERIALS
1.1 INTRODUCTION
The 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. This is evident when we touch an electric machine when it is under operation.
We don’t receive any electric shocks, because of the insulation. Breakdown of insulation results
in short circuiting of the coils, causing electric currents to flow in unintended paths. This may
also cause, electric shocks to humans operating the machinery and also damage the machines.
Requirements of a good insulating materials involve physical properties, reliability, cost,
availability, adaptability to machining operations etc. Electrical insulation and dielectric materials
includes various forms of materials that surround and protect electrical conductors and prevent
unwanted current flow, leakage. Electrical specifications include electrical resistivity, dielectric
strength, and dielectric constant.
1.2 ELECTRICAL PROPERTIES
Electrical Resistivity : It is the electrical resistance (ohm-cm) to the flow of current through it.
Its value should be very high. Resistivity is the inverse of conductivity.
Dielectric Strength : Dielectric strength is the maximum voltage gradient that the material can
withstand before electrical breakdown occurs. This value specified as ‘kV/mm’ should be very
high even for very thin films.
1.3 CLASSIFICATION OF INSULATING MATERIALS : The insulating materials are classified
in the following two ways : 1. Classification according to substances and materials.
2. Classification according to temperature.
Classification according to substances and materials :
(i) Solid Insulating Materials [Inorganic and organic]
Mica, wood, slate, glass, porcelain, rubber, cotton, silk, rayon, terylene, paper and
cellulose materials etc.
(ii) Liquid Insulating Materials [Oils and Varnishes]
Refined hydrocarbon minerals oils, Linseed oil, spirit and synthetic varnishes, etc.
(iii) Gaseous Insulating Materials
Dry air, carbon dioxide, argon, nitrogen, etc.
Classification according to temperature : The insulating materials are classified mainly based
on the thermal limit. The performance of the insulation depends on its operating temperature.
The higher the temperature, the higher will be the rate of its chemical degrading, and hence the
lower will be its useful life as shown in fig.1.1. If a reasonably long life of insulation is expected,
its operating temperature must be maintained low. Therefore, it is necessary to determine the
limits of temperature for the insulation, which will ensure safe operation over its expected life.