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.