DMD vs NMN vs NHN vs AHA vs PMP: Which Flexible Composite Paper for Your Application?

TL;DR

• Class B/F motors, general slot insulation — DMD (polyester nonwoven + PET film) is the cost-performance default.
• Class F with better tear and varnish behavior — NMN (Nomex + PET film + Nomex) for motors and generators.
• Class H machines and dry-type transformers — NHN (Nomex + polyimide film + Nomex); step up to AHA for 180–220°C hot spots.
• Oil-immersed transformer layer insulation — PMP (presspaper + PET film + presspaper), class A/E, not a dry-application material.

Open any motor slot or transformer winding and you will find a flexible composite insulation paper — but which one? DMD, NMN, NHN, AHA, and PMP look similar on a roll, cost very differently, and fail in completely different ways when mis-applied. The wrong choice usually isn’t discovered at incoming inspection; it’s discovered at the warranty claim. This guide decodes each material’s three-layer construction, puts all five in one comparison table, and gives you a two-step decision path — by thermal class, then by equipment type — plus the three mis-selections we see most often.

Flexible Composites in One Paragraph

All five materials share the same idea: a three-layer sandwich. The center film (polyester or polyimide) provides the dielectric backbone and dimensional stability; the two outer fibrous layers (polyester nonwoven, aramid paper like Nomex, or kraft presspaper) provide mechanical toughness, varnish absorption, and a conformable surface that seats tightly against conductors. Change the film or the fiber and you change the thermal class, the price, and the correct application. That is the entire logic of the family — the letter codes just tell you what’s inside: D = Dacron (polyester), M = Mylar (PET film), N = Nomex (aramid), H = polyimide film, P = presspaper, A = aramid. Once you can read the code, you can predict any composite’s thermal class and rough price before you ever see a datasheet.

Each Material Decoded

DMD — Class B/F, the Volume Workhorse

DMD (polyester nonwoven + PET film + polyester nonwoven) is the default slot and phase insulation for class B (130°C) and class F (155°C) motors. Typical thicknesses run 0.15–0.45 mm with breakdown strength around 5–12 kV depending on build. It absorbs varnish well, punches and folds cleanly, and costs the least of the aramid-free composites. Two construction details are worth checking on any quote: the film share of the total thickness (a 0.25 mm DMD built on a 0.19 mm film insulates far better than one built on 0.10 mm film with thick fuzzy faces) and the nonwoven’s bond quality (delaminated faces jam insertion tooling). Its honest limit: above 155°C the polyester layers degrade, so DMD simply does not belong in class H machines. SIDA F-grade DMD covers the standard thickness range in roll, strip, and formed slot liners.

NMN — Class F with Aramid Toughness

NMN (Nomex + PET film + Nomex) keeps the polyester film core but replaces the nonwoven with aramid paper. The result at the same 155°C class: far better tear resistance (important on automatic insertion machines), superior abrasion behavior against magnet wire, and excellent varnish uptake. The aramid faces also tolerate short thermal excursions — a 180°C varnish bake that would embrittle DMD leaves NMN unaffected — which is why many class F designs standardize on NMN even when DMD would technically qualify. NMN is the standard upgrade when DMD tears in production or when a class F motor needs longer thermal life. See SIDA NMN insulation paper.

NHN — Class H for Dry-Type Transformers and H-Class Machines

NHN (Nomex + polyimide film + Nomex) swaps the PET core for polyimide film, lifting the system to class H (180°C) continuous. This is the default flexible composite for dry-type transformer layer insulation, traction motor slots, and any class H winding. Breakdown strength is comparable to NMN at equal thickness; what you are buying is thermal endurance — the polyimide core retains its dielectric properties after thousands of hours at 180°C where a PET core would have failed within weeks. SIDA NHN is stocked in the common 0.13–0.35 mm range.

AHA and PMP — The Specialists

AHA (aramid + polyimide + aramid, with high-temperature adhesive or adhesive-free constructions) extends the NHN concept toward class 180–220°C duty: severe hot spots in dry-type transformers, high-speed traction machines, aerospace actuators. It is the most expensive material in this family — specify it where NHN’s margin is genuinely insufficient, not as a blanket upgrade. For bonded-barrier and coil-consolidation work, prepreg variants such as AMA prepreg and AHA prepreg add a B-stage resin that cures during your bake cycle.

PMP (presspaper + PET film + presspaper) is the odd one out: a class A/E (105–120°C) composite designed for oil-immersed transformers, where its kraft presspaper faces impregnate fully with transformer oil. It serves as layer and turn insulation at a fraction of aramid cost. What it is not: a dry-application material. In air at 130°C+, its presspaper faces are simply out-classed. See SIDA PMP insulation paper.

Master Comparison Table

All values are typical market ranges for standard constructions; your supplier’s datasheet should confirm the exact build (film share, adhesive type, calendering) for the grade you order.

Material	Construction	Thermal class	Thickness range	Relative cost	Typical applications
DMD	Polyester nonwoven / PET film	B–F (130–155°C)	0.15–0.45 mm	1×	Motor slot & phase insulation, general layer insulation
NMN	Nomex / PET film / Nomex	F (155°C)	0.13–0.40 mm	2–3×	Class F motors & generators, auto-insertion lines
NHN	Nomex / polyimide film / Nomex	H (180°C)	0.13–0.35 mm	4–6×	Dry-type transformers, class H machines, traction
AHA	Aramid / polyimide / aramid	H–220 (180–220°C)	0.13–0.30 mm	6–8×	Severe hot spots, high-end dry-type, aerospace
PMP	Presspaper / PET film / presspaper	A–E (105–120°C)	0.10–0.35 mm	~1×	Oil-immersed transformer layer & turn insulation

Decision Tree: Thermal Class First, Equipment Second

Step 1 — your insulation system’s thermal class fixes the shortlist:

• Class A/E (105–120°C), oil-immersed → PMP (or plain pressboard/paper; see our insulation paper guide)
• Class B/F (130–155°C) → DMD by default; NMN if insertion tearing or abrasion is a problem
• Class H (180°C) → NHN
• Class 200–220°C hot spots → AHA

Step 2 — the equipment type refines the choice:

• Low-voltage motor slots → DMD 0.25–0.30 mm is the industry default
• Automatic slot insertion (high-speed lines) → NMN; its tear strength survives the inserter
• Dry-type transformer layer insulation → NHN at full winding temperature; AHA only at proven hot spots
• Oil-immersed transformer layers → PMP, calendered grades for tight coil windows
• Coil consolidation before varnish/VPI → prepreg grades (AMA/AHA prepreg) that bond during your bake

For the full product range with datasheets, see the flexible composite insulation overview.

One final check before ordering: if your insulation system carries UL recognition (common for North American motor programs), the composite must appear in that system’s listed materials — swapping DMD for NMN changes the system’s chemistry and may require re-qualification. Ask for the material’s UL RTI and system listings at RFQ stage.

Common Mis-Selections

1. DMD in a class H machine. The motor passes its initial tests — then fails insulation resistance at 18 months. Polyester faces at 180°C age roughly 8× faster than at 155°C. If the nameplate says class H, the composite must be NHN or better. Field check: if failed insulation comes out brittle and brown at the slot bottom, you are looking at thermal aging of a polyester-based material, not a winding fault.
2. NHN everywhere “for safety.” At 4–6× DMD’s cost, blanket NHN specification in a class F motor buys nothing measurable. Upgrade only the components that see the hot-spot temperature — typically the slot liner in the hottest winding section — and keep DMD or NMN for phase and wedge insulation elsewhere.
3. PMP in dry applications. PMP’s presspaper faces are designed to drink transformer oil; in air at class B temperatures they dry out, shrink, and crack. Dry-type layer insulation means NHN, full stop. The reverse error also exists: aramid composites in small oil transformers work electrically but waste 4–6× the material cost where PMP meets every requirement.

Frequently Asked Questions

Q: What’s the real difference between NMN and NHN?
Only the center film: NMN uses polyester film (class F, 155°C), NHN uses polyimide film (class H, 180°C). Mechanically they behave similarly; thermally they are a full class apart. For a deeper dive see our NMN vs NHN comparison.

Q: Can DMD replace presspaper in an oil transformer?
It can serve, but PMP or plain kraft is usually the better tool: presspaper faces impregnate faster and cost less at class A/E duty. DMD’s polyester nonwoven is optimized for air/varnish systems, not oil.

Q: How do I specify thickness?
Match the composite thickness to your slot fill or layer voltage: 0.13–0.18 mm for tight slots and turn insulation, 0.25–0.35 mm for slot liners and layer barriers, heavier builds for phase insulation. Breakdown scales with thickness — a 0.25 mm NMN typically exceeds 8 kV.

Q: Do you supply formed/cut pieces or only rolls?
SIDA supplies rolls, slit strips, and die-cut or formed slot liners to drawing, with free samples for insertion-line trials before bulk orders.

Q: How should composite papers be stored before use?
Dry, flat, and sealed: below ~70% relative humidity, away from direct sunlight, in the original moisture-barrier packing. Aramid-faced composites (NMN, NHN, AHA) are hygroscopic — a roll left open in a humid shop can pick up enough moisture to cause voids in varnish or oil impregnation. If storage was uncontrolled, a 4-hour pre-dry at 105–120°C restores processing condition.

Q: What width and MOQ should I expect?
Standard master rolls are 1000 mm wide; slit widths from about 5 mm up are routine. Volume-focused plants quote ton-level MOQs per width, while direct manufacturers like SIDA slit to order from carton quantities — practical when a motor line runs six widths of the same material and none of them justifies a full jumbo roll.

Summary

The five flexible composites form a clean ladder: PMP for oil-immersed class A/E, DMD for class B/F motors, NMN when aramid toughness is worth 2–3× at class F, NHN for class H dry-type duty, and AHA for 180–220°C hot spots. Choose by thermal class first and equipment type second, avoid the three classic mis-selections, verify the film share on the datasheet — and the material will outlive the machine.

Send your thermal class, thickness, and application for free samples across the DMD/NMN/NHN/AHA/PMP range — matched to your production line.

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References

1. IEC Webstore — IEC 60626, Combined Flexible Materials for Electrical Insulation
2. NEMA — Insulation Materials and Thermal Classification Standards
3. UL Solutions — Insulation Systems (OBJY2) Component Recognition
4. SIDA — Flexible Composite Insulation: DMD, NMN, NHN Overview
5. SIDA — What Is DMD Insulation Paper? Properties, Uses, Applications
6. SIDA — NMN and AMA Insulation Paper in Electrical Insulation Systems