Insulating materials are divided into several classes based on the temperature withstanding limits. Insulating materials used in machines like Transformers, Generators, and Motors based on their thermal heat withstanding capacity.
As we know that with an increase in load the temperature of winding also increases due to an increase in current flows. So based on the temperature factor insulating materials are classified into various categories.
The classes of insulating materials are given here.
Classification of Insulating Materials:
Insulating materials are classified into 7 classes.
1. Class Y Insulating Materials- 90°C
Cotton, silk, paper, and similar organic materials.
2. Class A Insulating Materials – 105°C
Impregnated paper, silk, cotton, polyimide, resins.
3. Class E Insulating Materials – 120°C
Enameled wire insulations on the base of molded polyvinyl epoxy resins, powder plastics.
4. Class B Insulating Materials – 130°C
Inorganic material (Mica, Fibre, Glass, Asbestos) impregnated with varnish and other compounds.
5. Class F Insulating Materials – 155°C
Mica, polyester epoxide varnished, and other varnish and in the high heat resistance.
6. Class H Insulating Materials – 180°C
Composite materials on mica, fiberglass, and other asbestos bases, impregnated in the silicon rubber except for other rubber compounds.
7. Class C Insulating Materials – >180°C
Mica, Ceramics, Glass, Teflon, and Quartz.
Induction Motors uses Class-F Insulating Material:
Induction motors of high capacity are usually provided with F-class insulation. We can see from the below figure which is the nameplate of the Induction motor.
The stator winding has MICALASTIC insulation. It mainly consists of mica paper and the polyester film carries bonded with synthetic resin. The conductor insulation is provided with mica paper tape. The coils are insulated with mica paper folium and epoxy resin. Proper care is taken to avoid voids in the insulation formation.
Healthiness of Insulation:
Checking of insulation resistance of windings is one of the most common methods used to ensure the healthiness of the windings of the machine. Megger is used to measure the insulation resistance of the windings. A healthy winding should have a minimum insulation value of 5 to 10 Mega ohms when checked with 5KV Megger under normal temperature conditions.
Minimum IR value= KV rating + 1 Mega Ohm
If the measured value of IR is less than the above, there is every reason to suspect insulation deterioration due to the presence of moisture, contamination, or damage.
How to increase the life of insulating materials?
The life of the insulation of electrical machines is affected by many factors, such as temperature, electrical and mechanical stresses, vibrations, and surrounding atmospheric conditions which involve chemicals, moisture, dirt, and radiation.
The following precautions need to take to increase the life of insulating material and hence the life of the machines.
- By Providing Effective Cooling Methods:
The use of chemically inert gases, or liquids, as cooling or protective media may increase the temperature capability of insulation.
Ensure that the cooling mechanism should always run without any problems.
For example, we know that different cooling methods are available for the transformer to limit the rise of temperatures like ONAN, ONAF, OFAF, and OFWF by using water, air, and transformer oil as coolants.
For large capacity, turbo generators hydrogen is used as a cooling medium. Compared to air hydrogen thermal conductivity is seven times higher. The lesser the density of the cooling medium, the less will be the losses. Also find that the use of hydrogen will result in an increase in efficiency of the machine at full load.
Various Generator Cooling Methods read here.
- By Proper Covering of Insulating Parts:
The insulating material parts should not expose freely to the industrial atmosphere. They should be properly covered always otherwise due to presence of oxygen, moisture, dirt, and chemicals in the surrounding atmosphere cause deterioration of the life of insulating material.
The enclosed construction must have a proper ventilation passage that keeps dirt and moisture out from windings.
