TL;DR:
- Slot liner selection is decided by three factors working together: system voltage, cooling method and the temperature class of the winding.
- A real specification — 690 V, oil-cooled, 105 °C winding temperature, from a customer inquiry by Lim Chee Kian — maps cleanly onto Class F (155 °C) insulation with margin to spare.
- NMN and AMA laminates are the recommended Class F slot liner materials; DMD suits lower thermal stress, while NHN and Nomex serve Class H (180 °C) and above.
- Oil-cooled windings demand laminates whose films and adhesives resist oil saturation without losing dielectric strength or delaminating over time.
- A complete slot liner RFQ states thickness, width, thermal class and withstand voltage — four lines that turn an inquiry into a quotable order.
Slot liner selection is the decision that separates a winding that survives its design life from one that fails at the slot wall. The slot liner is the sheet of flexible insulating laminate that lines the stator or rotor slot, standing between the copper winding and the grounded steel core, and its material must match the electrical, thermal and chemical conditions of the machine. This article explores the three factors that drive the choice, compares the common material options from DMD to Nomex, and explains what oil cooling changes, providing an RFQ checklist you can send straight to your supplier.
The Three Factors: Voltage, Cooling & Temperature Class
Voltage and Cooling Define the Working Stress
Every slot liner specification starts with two numbers: the system voltage the insulation must withstand, and the cooling method that decides its chemical environment. A recent inquiry from a customer, Lim Chee Kian, specified a 690 V winding that is oil-cooled with a 105 °C operating temperature — a compact description that already eliminates most unsuitable materials. The 690 V level sets the dielectric requirement, the oil sets the compatibility requirement, and the cooling directly limits how hot the slot liner actually runs. State these three parameters first, and material selection becomes an engineering exercise rather than guesswork.
Temperature Class: Class F (155 °C) vs Class H (180 °C)
The temperature class of the insulation system, defined by IEC 60085, must exceed the hottest temperature the liner will see in service, including hot-spot allowance. A winding designed for a 105 °C operating temperature sits comfortably inside Class F, which permits a 155 °C hotspot rating, leaving substantial thermal margin for overloads and aging. Class H at 180 °C serves machines with higher hot-spot temperatures or tighter cooling margins. Choosing a class is not about buying the highest number available — it is about matching the class to the winding’s real thermal profile with margin, as covered in our NMN insulation paper specifications.
Matching the Three Factors to a Material
The three factors combine into a filter. Voltage and thickness together give the required dielectric strength; temperature class filters the polymer system; cooling method filters the film and adhesive chemistry. For the 690 V oil-cooled case above, a 0.18–0.25 mm Class F laminate satisfies the dielectric requirement with margin, and oil immersion rules out any laminate whose adhesive or film swells or degrades in mineral or ester oil. This is why experienced buyers send all three factors in the first email: a supplier can confirm suitability immediately instead of trading clarifications for a week.
Slot Liner Material Options Compared
NMN and AMA: The Class F Recommendation
For Class F slot liners, SIDA’s standard recommendation is NMN — a three-ply laminate of Nomex aramid paper, polyester film and Nomex — or AMA, its aramid-polyester-aramid counterpart. Both combine the conformability and tear resistance of aramid paper surfaces with the high dielectric strength of a polyester film core, and both tolerate oil-cooled service without delamination. NMN is the more widely stocked construction and usually the first quote; AMA serves where a slightly different surface character or supply chain preference applies. Either handles 690 V oil-cooled duty at 105 °C with ample margin.
DMD: The Economical Option
DMD — polyester nonwoven, polyester film, polyester nonwoven — is the economical laminate for slot insulation where thermal stress is moderate. Standard DMD is Class B (130 °C), and F-grade DMD extends the same construction to Class F, which covers many standard industrial motors at a lower price point than aramid-based laminates. Its limitation is surface toughness: the polyester fleece layers scuff more easily than aramid paper during coil insertion, so for tight slots, long coils or automated winding, the extra durability of NMN usually pays for itself in reduced scrap and rework.
NHN and Nomex: Class H and Above
When the winding design pushes into Class H, the material system changes. NHN replaces NMN’s polyester core with polyimide film, lifting the laminate to 180 °C, while pure Nomex aramid paper constructions serve the most severe thermal and chemical environments. These materials cost significantly more than Class F laminates, so they should be specified only when the thermal design genuinely requires them — overspecifying temperature class is one of the most common ways buyers overpay for slot insulation. The table below summarises the practical differences.
| Material | Construction | Thermal Class | Best For |
|---|---|---|---|
| DMD | Polyester fleece / PET film / fleece | B (F-grade available) | Standard motors, cost-sensitive |
| NMN | Nomex / PET film / Nomex | F (155 °C) | General slot liners, oil-cooled |
| AMA | Aramid / PET film / aramid | F (155 °C) | Class F alternative to NMN |
| NHN | Nomex / polyimide film / Nomex | H (180 °C) | High hot-spot machines |
| Nomex paper | Pure aramid paper | H (180 °C)+ | Severe thermal and chemical duty |
Why Oil-Cooled Windings Need Different Insulation
Oil Compatibility and Saturation
In an oil-cooled machine the slot liner lives immersed in oil for its entire service life, so every component of the laminate must be oil-stable. The polymer film must not swell or plasticise, and the adhesive system bonding the plies must resist long-term oil exposure without softening — a laminate that delaminates inside a slot loses both its dielectric and mechanical function at once. Aramid-polyester laminates such as NMN have a long service record in oil-filled equipment precisely because their chemistry is stable in both mineral and ester fluids. Always tell your supplier the oil type; it is part of the material decision, not an afterthought.
Dielectric Strength in Oil
Dielectric strength figures quoted on datasheets are measured in air, but the slot liner works in oil, and the two environments are not interchangeable. Oil impregnation generally improves the dielectric behaviour of the porous paper layers by displacing air, while the film core carries most of the electrical stress either way. What matters for the buyer is that the laminate’s rated breakdown voltage is confirmed for the intended thickness, and that the construction has a track record in oil-immersed service. For a 690 V winding, even a 0.18 mm NMN liner provides dielectric margin far beyond the requirement.
Thermal Aging Under Oil Immersion
Thermal aging in oil follows different rules than aging in air: the oil excludes oxygen, which slows oxidative degradation of the cellulose and polymer components, but the combination of heat, oil and mechanical vibration stresses the adhesive bonds over decades. Qualification data for laminates in oil — thermal endurance testing under IEC 60216 principles — is therefore more meaningful than air-only ratings. A Class F laminate properly qualified for oil-immersed duty will outlast a nominally higher-class material whose adhesive system was never validated in oil. Ask for the aging basis when the application is oil-cooled.
How to Specify a Slot Liner RFQ
Thickness, Width and Form
A quotable RFQ starts with the physical specification: thickness in millimetres, width in millimetres, and the supply form — roll, slit strip or pre-cut slot liner pieces. Thickness drives dielectric strength and slot fill: typical slot liners run 0.13–0.35 mm, with thicker liners giving higher breakdown voltage at the cost of slot space. Width must match the slot depth plus any fold or cuff the winding shop uses. If liners are supplied pre-cut, include the piece length and any shaping. The Lim Chee Kian inquiry mentioned earlier could be quoted within a day precisely because these dimensions arrived in the first message.
Thermal Class and Withstand Voltage
Next, state the performance requirements: the thermal class of the insulation system (Class F or Class H), and the withstand or breakdown voltage the liner must meet. For withstand voltage, give the test level your design standard requires rather than a generic “high voltage” note — for example, the hipot level the finished winding must pass. This lets the supplier confirm that the proposed thickness and construction meet the requirement with documented test data. Mentioning the cooling medium — mineral oil, ester oil or air — belongs on the same line, because it constrains the adhesive system of the laminate.
Quantity, Processing and Documentation
Close the RFQ with commercial details: quantity in kilograms, square metres or pieces, target delivery, and any processing such as slitting, cutting or creping. If your quality system requires it, request a batch test report covering thickness, breakdown voltage and tensile strength, and state whether third-party inspection applies before shipment. A complete slot liner RFQ reads like the checklist below — send it once, and the quotation that comes back is one you can actually compare across suppliers.
- Material and construction (NMN / AMA / DMD / NHN), or the duty for a recommendation
- Thickness (mm), width (mm), supply form (roll / strip / cut pieces)
- Thermal class (F / H) and required withstand voltage
- Cooling medium: mineral oil / ester oil / air
- Quantity, packaging, documentation and inspection requirements
Motor vs Transformer Insulation: What’s Different
Mechanical Stress and Vibration
Although motors and transformers share the same material families, the slot liner’s working conditions differ fundamentally. A motor winding lives with continuous mechanical vibration, electromagnetic force cycling at line frequency, and the abrasion of coil insertion — stresses that punish weak surfaces and brittle films. Transformer insulation, by contrast, sits largely static once clamped, with short-circuit forces as the rare mechanical event. This is why slot liner selection weights tear resistance, abrasion and conformability so heavily, and why aramid-surfaced laminates dominate motor slots while cellulosic papers dominate transformer windings.
Cooling Medium and Thermal Cycling
Transformers are almost universally oil-immersed or dry-type with slow, steady thermal profiles, while motors span air-cooled, hydrogen-cooled and increasingly oil-cooled designs with rapid thermal cycling every time the load changes. Each start-stop cycle expands and contracts the slot liner against the core, so the laminate must flex without cracking for the machine’s whole life. An oil-cooled motor combines both worlds — oil immersion plus cycling — which is exactly the combination that makes laminate adhesive quality decisive. Specify the cycling duty, not just the peak temperature, when the machine cycles daily.
Material Selection Implications
The practical consequence is that a material excellent in a transformer may be wrong in a motor slot, and vice versa. Kraft-based papers excel in static oil-immersed duty but lack the tear resistance for coil insertion; polyester-film laminates resist slot abrasion but must be matched to the thermal class; aramid papers do both at a price. Buyers sourcing across both product families — as many transformer and motor repair shops do — benefit from a supplier that stocks the full range, from kraft paper to NMN, NHN and Nomex, and can advise across the boundary rather than within one catalogue.
Frequently Asked Questions
Is NMN or DMD better for slot liners?
It depends on the thermal class and the winding process. For Class F systems and any slot where coils are inserted tightly or by machine, NMN’s aramid surfaces resist tearing and abrasion far better than DMD’s polyester fleece. For Class B duty with gentle manual insertion, DMD is more economical and entirely adequate. F-grade DMD narrows the thermal gap but not the mechanical one. If the machine is oil-cooled or cycles thermally every day, NMN is the safer default.
Is Class F insulation enough for a 105 °C winding?
Yes, with substantial margin. Class F permits a 155 °C hottest-spot temperature under IEC 60085, so a winding designed for 105 °C operation carries roughly 50 K of thermal headroom for hot spots, overloads and aging. That margin is why Class F laminates such as NMN are the standard recommendation for this duty. Specify Class H only when the design hot spot genuinely approaches or exceeds the Class F limit, because the price step to NHN or Nomex constructions is significant.
What thickness of slot liner do I need for 690 V?
For a 690 V winding, a 0.18–0.25 mm laminate is the common range, providing breakdown strength far above the operating and test voltages of this class. The final choice depends on your design standard’s hipot level, the slot fill budget and the mechanical abuse expected during insertion. Rather than fixing thickness from voltage alone, state the required withstand voltage in the RFQ and let the supplier confirm the minimum thickness with test data — then add margin for mechanical, not electrical, reasons.
Does oil cooling change the temperature class I need?
Oil cooling does not raise the required class — effective cooling usually lowers the temperature the liner actually sees — but it changes which materials qualify within that class. The laminate’s film and adhesive must be validated for long-term oil immersion, and ester oils behave differently from mineral oils. So the class decision stays thermal, while the oil becomes a chemistry filter applied on top. Always state both the temperature class and the oil type in the RFQ, and ask for the material’s oil-immersion track record.
Summary: Match the Material to the Machine
Slot liner selection reduces to three questions: what voltage must it hold, what temperature class must it survive, and what medium will it live in. Answer those — as the 690 V oil-cooled, 105 °C inquiry from Lim Chee Kian did — and the material shortlist writes itself, with Class F NMN or AMA covering most oil-cooled motor duty. For NMN, AMA, DMD, NHN and Nomex slot liner materials supplied in rolls, strips or cut-to-size pieces with batch test reports, contact SIDA.
Contact SIDA for slot liner material quotations and technical support:
- 📞 +86-15958243831
- 📧 jessie.feng@sidanm.com
- 💬 WhatsApp: Chat on WhatsApp
- 🌐 sidanm.com
References
- IEC 60085, Electrical insulation — Thermal evaluation and designation. International Electrotechnical Commission.
- IEC 60626-1, Combined flexible materials for electrical insulation — Part 1: Definitions and general requirements. International Electrotechnical Commission.
- IEC 60626-2, Combined flexible materials for electrical insulation — Part 2: Methods of test. International Electrotechnical Commission.
- IEC 60034-1, Rotating electrical machines — Part 1: Rating and performance. International Electrotechnical Commission.
- IEC 60243-1, Electric strength of insulating materials — Test methods — Part 1: Tests at power frequencies. International Electrotechnical Commission.
- IEC 60216-1, Electrical insulating materials — Thermal endurance properties — Part 1: Ageing procedures and evaluation of test results. International Electrotechnical Commission.
