If you specify insulation for transformers or motors, you have almost certainly met the abbreviations NMN and DMD. They look similar on a datasheet — both are three-layer flexible composites, both used for slot liners and layer insulation — but choosing the wrong one quietly costs you either margin or reliability. This guide explains what each material actually is, where they differ, and how to specify the right one without over- or under-buying.
What Is Flexible Composite Insulation?
Flexible composite insulation (sometimes called a flexible laminate) is a multi-layer material that bonds a thin dielectric film between two layers of non-woven or aramid paper. No single layer does the job alone: the film carries the dielectric strength, while the outer layers add mechanical toughness, absorb impregnating varnish, and survive winding stress. The three-letter codes describe the stack — and the stack decides the temperature class.
DMD: The Cost-Effective Workhorse
DMD = Dacron – Mylar – Dacron.
Two outer layers of polyester non-woven fabric (Dacron) bonded to a central polyester film (Mylar / PET).
– Thermal class: Class B (130°C) up to Class F (155°C) for F-grade versions
– Breakdown voltage: ~8–15 kV/mm
– Cost: lowest of the three
– Best for: general-purpose motors, oil-immersed and dry-type transformer layer/phase insulation, standard industrial equipment where temperature is moderate
DMD is the default choice when the application sits comfortably below 155°C and budget matters. It handles impregnation well and is widely stocked, which keeps lead times short.
NMN: The Thermal Upgrade
NMN = Nomex – Mylar – Nomex.
Two outer layers of Nomex® aramid paper around the same polyester film core.
– Thermal class: Class F (155°C) up to Class H (180°C)
– Breakdown voltage: ~12–20 kV/mm
– Cost: moderate (typically 20–30% above DMD)
– Best for: dry-type (cast resin) transformers, traction and wind generators, higher-temperature motors, anywhere edge-tear and chemical resistance matter
The aramid outer layers are what you are paying for: superior thermal aging, better puncture and tear resistance, and stable performance under thermal cycling. When a design pushes toward or past 155°C, NMN is the natural step up from DMD.
NMN vs DMD: Side-by-Side
| Property | DMD | NMN |
|---|---|---|
| Outer layers | Polyester non-woven (Dacron) | Nomex aramid paper |
| Core film | Polyester (Mylar / PET) | Polyester (Mylar / PET) |
| Thermal class | B (130°C) – F (155°C) | F (155°C) – H (180°C) |
| Breakdown voltage | ~8–15 kV/mm | ~12–20 kV/mm |
| Tear / puncture resistance | Good | Superior |
| Relative cost | Low | Medium |
| Typical use | Standard motors, general transformers | Dry-type transformers, traction, high-temp motors |
The third common grade, NHN (Nomex – polyimide – Nomex), pushes to Class H/C 180–220°C for EV traction and aerospace duty. See our NHN product page for that top tier.
How to Choose: Three Rules
1. Start from the operating temperature. If the hottest spot stays under 155°C, DMD is usually sufficient. Cross 155°C and NMN becomes the safe specification.
2. Match the duty, not the catalog. A standard LV motor does not need NMN; a dry-type transformer running warm does not want to save a few cents on DMD. Specify to the actual thermal and mechanical demand.
3. Watch edge-tear and impregnation. Automated winding lines and VPI processes punish weak outer layers. Where insertion and resin uptake are aggressive, the aramid toughness of NMN earns its premium.
Common Specification Mistakes
– Over-specifying: putting NMN in a 120°C appliance motor just because it is “better.” You pay for temperature margin you never use.
– Under-specifying: running DMD in a design that regularly hits 160°C. The composite ages early and you trade a small saving for field failures.
– Ignoring thickness: both grades come in 0.13–0.50 mm ranges. Slot-fill and creepage distance still have to be met — grade and thickness are separate decisions.
Conclusion
DMD and NMN are not competitors so much as rungs on the same ladder: DMD for standard Class B/F duty at the best price, NMN for thermal and mechanical margins up to Class H. Pick by operating temperature and real duty, and you get reliability without paying for unused headroom.
Need help specifying the right composite for your transformer or motor? Contact our team with your temperature class and application — we will match the grade and thickness to your build.
Related reading: Insulation Materials for Dry-Type Transformers: A Complete Selection Guide
Browse all flexible composites: Flexible Composite Insulation category