The
insulation utilized should withstand the voltage fluctuations of the motor
under varying operating conditions. Depending on the load and its surrounding
conditions, there could be a rise in the temperature of the motor. The
insulation should withstand such temperature rises also.
The hot-spot
temperature in any part of the motor should not exceed the permissible limit of
the insulation used. In case of insulating materials, their thermal
characteristics are more sensitive than their dielectric characteristics, i.e.,
the failure of an insulating material is more due to thermal limitations than
due to voltage limitations.
In most
cases, the temperature rise or the rise in load does not produce a fault in the
winding of the conductor itself. The rise of load current or greater fault
current, when it is excessive, causes a thermal breakdown in the insulation
covering the conductor. This is what creates a fault in the winding.
Thus, the
maximum permissible temperature rise, in electrical motors, must be in tune with
the type of insulation used and the type of motor.
The main
characteristics, of insulating materials used in electrical machines are:
• Dielectric
strength
• Thermal
strength.
The
insulating material used for the electrical machines should satisfy the
following requirements:
• High
dielectric strength, high specific resistance, and minimum loss in alternating
electric field
• High
mechanical strength and elasticity of material
• Thermal
strength of insulation; the insulating material should preserve its insulation
and mechanical properties when subjected to the operating temperatures of the
windings for a long time
• The
material should remain unaffected by chemical influences.
The
temperature rise permissible can be determined, by deducting the ambient temperature,
from the maximum permissible temperature.
For
electrical machines, the following, are the types of insulating material that
have been classified and standardized as follows:
• Class A
insulation: Cotton, silk, paper, and similar organic materials, impregnated or
immersed in oil, and enamel applied on enameled wires. The limiting hot-spot
temperature for Class A insulation is 105 °C.
• Class E
insulation: An intermediate class of insulating materials between Class A and
Class B insulation materials.
• Class B
insulation: Mica, asbestos, glass fiber, and similar inorganic materials, in
built-up form with organic binding substances. The limiting hotspot temperature
for Class B Insulation is 130 °C.
• Class F
insulation: Includes insulation having mica, asbestos, or glass fiber base with
a silicone or a similar high-temperature-resistant binding material. The
limiting hot-spot temperature for Class F insulation is 155 °C.
• Class H
insulation: Includes insulation having mica, asbestos, or glass fiber base with
a silicone or a similar high-temperature-resistant binding material. The
limiting hot-spot temperature for Class H insulation is 180 °C.
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