NOMEX TYPE 418
NOMEX Type 418 is designed for high-voltage applications, including motor conductor and coil wrap, transformer ground and layer insulation. It is a calendered product with high inherent dielectric strength (30 to 40 kV/mm), which can be readily impregnated with varnishes where this is desirable. This calendered blend of aramid and mica offers increased voltage endurance over NOMEX® Type 410 when subjected to corona attack.
The typical electrical property values for NOMEX® Type 418 papers are shown in Table I. The AC Rapid Rise dielectric strength data of Table I, representing voltage stress levels, withstood 10 to 20 seconds at a frequency of 60 Hz. These values differ from long-term strength potential. DuPont recommends that continuous stresses in transformers not exceed 3.2 kV/mm (80 V/mil) to minimize the risk of partial discharges (corona). The Full Wave Impulse dielectric strength data of Table I were generated on flat sheets, such as in layer and barrier applications. The geometry of the system has an effect on the actual impulse strength values of the material.
Since NOMEX Type 418 is composed 50% of inorganic mica, its properties are even more stable with temperature than Type 410. Dielectric constants of NOMEX Type 418 papers are essentially unchanged over the range from 23 to 250°C. The effects of temperature and frequency on the dissipation factor of dry NOMEX® Type 418 – 0.13 mm (5 mil) paper are shown in Figure 1. Surface and Volume Resistivities of dry NOMEX® Type 418 – 0.13 mm (5 mil) paper are shown in Figure 2 as functions of temperature. Corresponding values for other thicknesses are very similar. Like other organic insulating materials, NOMEX® paper is gradually eroded under attack by corona discharges. However, NOMEX Type 418 is specifically designed to provide voltage endurance (long times to failure under corona attack) at least equivalent to the best inorganic insulations, and greatly superior to other organic materials, These data were obtained at 360 Hz frequency; times to failure at 50-60 Hz are approximately 6–7 times as long as those indicated. Due to its superior corona resistance, NOMEX Type 418 paper has been used commercially for many years to insulate stator coils in AC motors up to 13.6 kV class.
NOMEX Type 418 paper retains at least 50% of its room temperature tensile strength and elongation at temperatures up to 250°C. Water is a plasticizing agent for NOMEX Type 418 paper. Dipping or soaking NOMEX Type 418 papers in water reduces their tensile strengths to 30-50% of the typical values shown in Table III, but also increases break elongation by about 3 times and makes the paper softer and more conformable. This effect can be used to advantage in some applications (similar to the common practice of dipping mica composite tapes in mixtures of water and alcohol to improve their handling characteristics during wrapping of motor coils). Due to the permeable nature of NOMEX® Type 418 paper, absorbed water can be readily removed during normal drying or baking procedures. This restores the paper properties to their normal values.
Arrhenius plots of thermal aging behavior for NOMEX are exemplified by Figures 7, 8 and 9 of the NOMEXType 410 data sheet. Similar aging of NOMEX® Type 418 papers at elevated temperatures has resulted in their recognition as 220°C insulating materials. The thermal conductivity of NOMEX® Type 418 – 0.25 mm (10 mil) paper is shown as a function of temperature. The total system construction may affect the overall thermal conductivity, therefore, care should be taken in applying individual sheet data to actual situations. For example, two sheet insulations with identical thermal conductivities may have quite different effects on heat transfer from a coil, due to differences in stiffness or winding tension (which affect the spacing between the insulation layers) or differences in the absorption of impregnating varnishes.
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