To escape any electrical action between components at totally different potentials, insulation is employed. A perfect insulating material ought to possess the subsequent properties.
- High dielectric strength.
- It should with stand high temperature.
- Good thermal conductivity.
- It should not undergo thermal oxidation.
- It should not deteriorate due to higher temperature and repeated heat cycle.
- Must have high value of resistivity (like 1018 Ωcm).
- Less power consumption.
- Low dielectric loss angle.
- It should withstand stresses due to centrifugal forces, electro dynamic or mechanical forces.
- It should withstand vibration, abrasion, bending.
- Not to absorb moisture.
- Must be flexible and cheap.
Generally insulating materials can be categorized into Solid, Liquid and Gas, and vacuum. The word insulating material is at times used in a broader sense to entitle also insulating liquids, gas and vacuum.
Solid Insulating Material is used with field winding, armature winding, transformer cores etc.
The examples are:
- Fibrous or inorganic animal or plant origin, natural or synthetic paper, wood, card board, cotton, jute, silk, rayon, nylon, asbestos, fiber glass etc.,
- Plastic or resins. Natural resins-lac, amber, shellac etc.,
- Synthetic resins-phenol formaldehyde, melamine, polyesters, epoxy, silicon resins, bake lite, Teflon, PVC etc.
- Rubber: natural rubber, synthetic rubber-butadiene, silicone rubber, hypalon, etc.,
- Mineral: mica, marble, slate, talc chloride etc.,
- Ceramic: porcelain, steatite, alumina etc.,
Glass: soda lime glass, silica glass, lead glass, borosilicate glass.
- Non-resinous: mineral waxes, asphalt, bitumen, chlorinated naphthalene, enamel etc.,
Liquid: Used in transformers, circuit breakers, reactors, rheostats, cables, capacitors etc., & for impregnation.
The examples are:
- Mineral oil (petroleum by product).
- Synthetic oil askarels, pyranols etc.,
- Varnish, French polish, lacquer epoxy resin etc.,
Gaseous: The examples are:
- Air used in switches, air condensers, transmission and distribution lines etc.,
- Hydrogen though not used as a dielectric, generally used as a coolant.
- Nitrogen use in capacitors, HV gas pressure cables etc.,
- Inert gases neon, argon, mercury and sodium vapors generally used for neon sign lamps.
- Halogens like fluorine, used under high pressure in cables.
- No insulating material in practice satisfies all the desirable properties. Therefore a material which satisfies most of the desirable properties must be selected.
The insulation system (also referred to as insulation class) for wires utilized in generators, motors transformers and different wire-wound electrical elements is split into completely different categories according the temperature that they will safely withstand. The utmost operative temperature is that the temperature the insulation will reach throughout operation and is that the summation of standardized ambience temperature i.e. 40 degree Celsius, tolerated temperature rise and allowance tolerance for warm spot in winding. As an example, the utmost temperature of sophistication B insulation is (ambient temperature 40 + allowable temperature rise 80 + hot spot tolerance 10) = 130oC.
Insulation is that the weakest part against heat and may be an essential think about deciding the lifetime of electrical instrumentality. The utmost operative temperatures prescribed for various category of insulation are for a healthy life of 20,000 hours. The peak temperature permissible for the machine elements is sometimes concerning 2000C at the utmost. The utmost operative temperature can have an effect on the lifetime of the insulation. As a rule of thumb, the lives of the winding insulation are going to be reduced by 0.5 for each 10ºC rise in temperature. The current day trend is to design the machine using category F insulation for sophistication B temperature rise.