What is Nomex insulation paper?

In 1960, DuPont of the United States developed and produced a high-temperature resistant material belonging to H-Class insulation – Nomex fiber paper (Nomex insulation paper). In 1967, Nomex insulation paper truly formed an industrialized production scale. The emergence of Nomex insulation paper has caused a revolution in the field of high-temperature-resistant insulation materials for electrical engineering. Nomex insulating paper uses poly(m-phenylene isophthalamide) short fiber and precipitated fiber (also known as pulp) as raw materials, adopting a wet papermaking process of inclined mesh forming, and then drying and hot rolling. Poly(m-phenylene isophthalamide) is a linear macromolecule composed of aromatic rings between amide bonds. Its short fibers provide mechanical strength; precipitated fibers provide dielectric strength and act as fillers and adhesives. Fiber precipitates are an important raw material for Nomex insulation paper and a major difficulty in the production of Nomex insulation paper.

Performance of Nomex insulation paper

Nomex insulation paper is widely used in F-Class and H-Class motors, and has good thermal stability, electrical properties, mechanical strength, chemical compatibility and adaptability.

1.Thermal stability

The most important characteristic of Nomex insulation paper is long-term thermal stability. Even if the temperature exceeds 220°C, it can still maintain good stability and will not crack when running at high temperature in liquid cooling medium. Under normal working conditions, the insulation paper can withstand a temperature of 180°C (ordinary insulation paper in transformers is A-Class insulation, with a temperature tolerance of 105°C). At 180°C, Nomex insulation paper can be used for more than 10 years, and its service life is higher than that of industrial organic high-temperature resistant paper. Nomex insulation paper is placed in a dry heat state of 200°C for 1000 hours, and its mechanical strength retention rate is 75%; placed in a wet heat state of 120°C for 1000 hours, the mechanical strength retention rate is still more than 60%; a small amount of CO₂, CO and N₂ gas will be decomposed only at above 370°C.

2.Flame retardant properties

Nomex insulation paper has excellent flame retardancy, with a limiting oxygen index (LOI) of ≥28%. It will not burn, melt or produce droplets in the air, and will only begin to decompose at extremely high temperatures (>370°C). The flame retardant properties of Nomex are determined by its own chemical structure and will not weaken or disappear during use. From the perspective of molecular structure, since the main chain of the polymer contains a large number of aromatic groups, it can condense into aromatic carbon during combustion, producing less gaseous combustible products, and the carbon itself not only has a LOI value of up to 65%, but also the carbon layer formed can cover the surface of the burning polymer to extinguish the flame.

3.Electrical properties

Nomex insulation paper has excellent electrical properties. Compared with some commonly used insulating materials, the dielectric constant of Nomex products is 1.5-2.5, which is close to the dielectric constant of air. When the cooling medium is air, the electric field distribution between Nomex insulation paper and the cooling medium is relatively uniform, so high-voltage motors and transformers using Nomex products as insulation systems have high resistance to atmospheric overvoltage and internal overvoltage. The dense Nomex insulation paper can withstand an electrical stress of 18 to 40 kV/mm in a short time, depending on its model and thickness. Under power frequency voltage, Nomex insulation paper has excellent dielectric strength and low dielectric loss. Therefore, when used as an insulation material, the motor consumes little electricity and generates less heat, which can reduce the breakdown of the material due to overheating and is not easy to age.

4.Mechanical properties

Dense Nomex insulation paper has very high strength and good elasticity, while thinner grades are soft. Nomex insulation paper has good tear resistance and wear resistance.

5.Chemical stability

The molecules of Nomex insulation paper are linear macromolecules composed of aromatic groups connected to each other by amide bridge bonds. In its crystals, hydrogen bonds are arranged in two planes to form a three-dimensional structure of hydrogen bonds. Due to the strong hydrogen bonding, its structure is stable and has excellent chemical resistance, and it can withstand most high-concentration inorganic acids, organic solvents and chemical reagents.

6.Radiation resistance

Nomex insulation paper has excellent resistance to β, α and X-ray radiation. For example, after irradiation for 100 hours under 50kV X-rays, its strength retention rate is 73%, while polyester and polyamide fiber products have turned into powder under the same conditions; when the β-ray radiation accumulates to 1000Mrad, its strength remains basically unchanged.

Nomex insulation paper has many excellent properties, but it also has some shortcomings. For example, it has poor light resistance. Under artificial ultraviolet rays or sunlight, the fiber appearance will turn into a dark bronze color (the inherent color is white); the heat shrinkage of Nomex insulation paper is also a problem that cannot be ignored. In practical applications, the heat shrinkage of Nomex insulation paper may cause corresponding equipment to malfunction.

Application in motors and generators

Nomex insulation paper is widely used in rotating equipment such as generators and motors. Its specific applications include wire wrapping paper, slot insulation lining, stays in the middle and top of wedges, phase insulation, pole pieces and coil supports, commutator V-rings, lead insulation and ground insulation. Nomex insulation paper with a thickness of 0.18-0.38mm is soft and easy to shape, so it can be easily inserted as slot insulation lining; Nomex insulation paper with a thickness of 0.51-0.76mm is particularly suitable for slot wedges, top rods and middle rods.

When Nomex insulation paper is used in generators and motors as ground insulation, it can prevent premature damage to the motor and equipment shutdown. The excellent heat resistance of Nomex insulation paper can keep the motor running well when it encounters temperatures much higher than its set rated value in some cases. And Nomex insulation paper is tear-resistant, which can prevent the slot insulation lining of irregular winding motors from being damaged during coil winding and molding.

Application in transformers

Nomex insulation paper is widely used in various transformers and can be used as conductor wrapping paper, layer insulation, phase insulation, core insulation, line end insulation, static rings, stays and spacers, etc. Due to the excellent performance of Nomex insulation paper, transformers using Nomex insulation paper have many advantages over ordinary transformers:

1.Nomex is C-Classinsulation material, which can reduce the cooling space of the transformer, make the transformer structure more compact, and reduce the mass accordingly, which not only reduces the manufacturing cost, but also reduces the no-load loss;

2.Ordinary transformers compensate for the hot spot temperature during operation by reducing the increase in base temperature, thereby increasing the size of the transformer, while transformers using Nomex insulation paper have a large temperature reserve capacity to withstand hot spots, enhancing the transformer’s ability to withstand harmonic loads;

3.Transformers using Nomex insulation paper have strong operating capabilities beyond the nameplate rated capacity and do not crack;

4.Nomex’s excellent moisture resistance allows it to be used in very humid environments without changing its performance;

5.It has good environmental performance. Dry-type transformers using Nomex insulation paper will not emit toxic or corrosive gases when burned, and its insulation material can be processed and degraded after the transformer’s life ends.