Insulating materials 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 increase in load the temperature of winding also increases due to increase in current flows.So based on the temperature factor insulating materials are classified into various categories.
The classes of insulating materials given here.
Classes of Insulating Materials:
Insulating materials are classified into 7 classes.
Class Y – 90°C
Cotton, silk,paper and similar organic materials.
Class A – 105°C
Impregnated paper, silk, cotton, polymide,resins.
Class E – 120°C
Enameled wire insulations on base of molded polyvinyle epoxy resins, powder plastics.
Class B – 130°C
Inorganic material (Mica, Fibre, Glass, Asbestos) impregnated with varnish and other compounds.
Class F – 155°C
Mica, polyster epoxide varnished and other varnish and in the high heat resistance.
Class H – 180°C
Composite materials on mica, fibre glass and other asbestos bases, impregnated in the silicon rubber except other rubber compounds.
Class C – >180°C
Mica, Ceramics, Glass, Teflon and Quartz.
Class-F Insulation for Induction Motors:
Induction motors of high capacity usually provided with F-class insulation. We can see from the below figure which is the name plate of the Induction motor.
The stator winding has MICALASTIC insulation. It mainly consists of mica paper and 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 healthiness of 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 Ohms
If the measured value of IR is less than the above, there is every reason to suspect insulation deterioration due to presence of moisture, contamination or damage.
How to increase the life of insulating materials?
The life of the insulation 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 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 raise of temperature like ONAN, ONAF, OFAF and OFWF by using water, air and oil as coolants.
For large capacity turbo generators hydrogen is used as 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 increase in efficiency of the machine at full load.
- 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 causes deterioration of the life of insulating material.
The enclosed construction must have a proper ventilation passage that keeps dirt and moisture out from windings.