Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 60626 Combined Flexible Insulation Materials: Complete Guide to Composite Laminates for Electrical Applications ⚡
IEC 60626 is the definitive international standard governing combined flexible materials used in electrical insulation systems. These composite laminates — typically consisting of plastic film bonded between layers of non-woven fabric — serve as critical insulating components in rotating machines, transformers, and other electrical equipment. The standard specifies dimensional, mechanical, electrical, and thermal performance requirements that ensure reliability across diverse operating conditions. Understanding IEC 60626 is essential for design engineers, quality assurance professionals, and anyone involved in specifying insulation materials for electrical apparatus.
1. Understanding Combined Flexible Insulation: Material Construction and Types 🔬
Combined flexible materials under IEC 60626 are multi-layer laminates that strategically combine different material properties into a single sheet. The fundamental construction involves a central plastic film layer that provides the primary dielectric barrier, sandwiched between two layers of non-woven fabric that contribute mechanical strength, tear resistance, and compatibility with impregnating processes.
The most widely used material combinations include:
- DMD (Dacron-Mylar-Dacron): A three-layer construction with polyester non-woven fabric (Dacron) on both faces and polyester film (Mylar/PET) in the center. DMD is the workhorse material for Class B (130°C) and Class F (155°C) applications, offering an excellent balance of cost, dielectric performance, and mechanical durability.
- NMN (Nomex-Mylar-Nomex): Replaces the polyester non-woven with meta-aramid paper (Nomex), elevating the thermal capability to Class H (180°C). NMN provides superior thermal endurance and maintains good dielectric properties at elevated temperatures.
- DM (Dacron-Mylar): A two-layer construction used where only one face requires non-woven fabric properties, often employed in phase insulation and barrier applications.
- NHN (Nomex-Polyimide-Nomex): Uses polyimide film (Kapton) as the dielectric core for applications requiring Class N (200°C) or Class R (220°C) thermal ratings. The polyimide film offers exceptional thermal stability and dielectric strength.
- Glass fabric combinations: Substituting glass fabric for organic non-wovens provides maximum thermal endurance and dimensional stability, suitable for the most demanding high-temperature applications.
The bonding between layers is achieved through adhesive systems that must themselves meet the thermal class requirements of the composite. The standard specifies requirements for delamination resistance, ensuring the layers remain bonded throughout the service life.
2. Key Performance Parameters in IEC 60626 🏭
IEC 60626 establishes a comprehensive framework for evaluating combined flexible insulation materials. The standard prescribes test methods and minimum performance thresholds across multiple critical parameters that directly influence insulation system reliability.
Dielectric Strength
Dielectric strength — the ability to withstand voltage without breakdown — is the primary electrical performance metric. IEC 60626 specifies breakdown voltage requirements that scale with material thickness. Typical DMD materials in the 0.20–0.35 mm thickness range achieve breakdown voltages of 6–12 kV when tested in oil per IEC 60243. The plastic film core is the dominant contributor to dielectric performance, and the standard requires testing both at room temperature and after thermal aging to verify performance retention throughout the insulation’s service life.
Tear Resistance and Mechanical Properties
Mechanical integrity is crucial because combined flexible materials must withstand the rigors of manufacturing processes — including slitting, stamping, forming, and automated insertion into motor slots. Tear resistance, tensile strength, and elongation at break are specified parameters. The non-woven fabric layers provide the tear resistance that pure plastic films lack, preventing catastrophic tear propagation that could compromise insulation integrity during assembly or service.
Thermal Endurance and Classification
IEC 60626 references IEC 60216 for thermal endurance evaluation methodology. Materials are subjected to accelerated thermal aging at multiple elevated temperatures, and key properties are monitored until they fall below 50% of initial values. The time-temperature data establishes the thermal class — the temperature at which the material provides 20,000 hours of useful life. This systematic approach allows engineers to select materials with appropriate thermal margins for their specific application.
Compatibility with Impregnating Varnishes
The non-woven outer layers serve a critical secondary function: they provide a capillary structure that absorbs and bonds with impregnating varnishes and resins. During VPI (Vacuum Pressure Impregnation) or trickle impregnation, the varnish penetrates the fibrous structure, filling voids and creating a monolithic insulation system after curing. IEC 60626 includes provisions for evaluating varnish absorption characteristics, bond strength after impregnation, and the retention of dielectric properties in the impregnated state.
| Material Type | Construction | Thermal Class | Typical Thickness (mm) | Breakdown Voltage (kV) | Key Applications |
|---|---|---|---|---|---|
| DMD | Dacron/PET/Dacron | 130°C (B) / 155°C (F) | 0.15 – 0.50 | 5 – 12 | Motor slot liners, phase barriers, low-voltage transformer layer insulation |
| NMN | Nomex/PET/Nomex | 155°C (F) / 180°C (H) | 0.13 – 0.51 | 4 – 11 | High-temperature motor insulation, traction motors, aerospace applications |
| NHN | Nomex/PI/Nomex | 200°C (N) / 220°C (R) | 0.15 – 0.35 | 5 – 10 | Severe duty motors, nuclear applications, high-reliability transformers |
| DM | Dacron/PET (2-layer) | 130°C (B) / 155°C (F) | 0.10 – 0.35 | 4 – 9 | Interphase insulation, wrapper insulation, barrier applications |
| Glass/PI/Glass | Glass fabric/PI/Glass fabric | 220°C (R) and above | 0.18 – 0.40 | 4 – 9 | Extreme high-temperature motors, specialty transformers, military applications |
3. Engineering Design Insights: Material Selection and Application Tradeoffs 📊
Thermal Class Selection Strategy
Selecting the appropriate thermal class is more nuanced than simply matching the material rating to the expected operating temperature. Engineers must account for hot-spot temperatures within windings — which can exceed average winding temperatures by 10–30°C — and build in safety margins. A motor designed for Class F (155°C) operation typically requires insulation materials rated to at least Class H (180°C) to accommodate hot spots and provide service life margin. IEC 60626 materials are available across the full thermal spectrum, enabling engineers to select the most cost-effective option that meets all safety and reliability requirements.
The Thickness Tradeoff Dilemma
One of the most challenging engineering decisions when specifying IEC 60626 materials is selecting the optimal thickness. This decision involves competing priorities that directly impact motor performance, reliability, and cost:
Thinner laminates (0.15–0.25 mm) offer significant advantages in thermal management. The reduced thermal resistance improves heat transfer from copper windings to the stator core, lowering winding temperatures and potentially enabling higher current density or extended service life. Thinner materials also maximize the available slot area for copper conductors, improving motor efficiency and power density. However, the tradeoffs include lower dielectric breakdown margins, reduced mechanical robustness during automated insertion, and potentially higher manufacturing defect rates from handling damage.
Thicker laminates (0.30–0.50 mm) provide superior electrical protection. The increased dielectric thickness provides greater voltage withstand capability and better resistance to partial discharge in higher-voltage machines. Mechanical durability is substantially improved, reducing scrap rates during high-speed automated slot cell insertion. The primary penalty is thermal: thicker insulation impedes heat flow, leading to higher winding temperatures that may necessitate derating the machine or accepting reduced efficiency and shorter insulation life.
The optimum selection process typically involves thermal modeling of the specific machine design, analysis of manufacturing process capabilities, and a thorough risk assessment of dielectric failure consequences.
Moisture Absorption Considerations
Moisture absorption is a critical but often underappreciated factor in combined flexible insulation performance. Non-woven fabric layers, particularly those based on aramid fibers (Nomex), are hygroscopic and can absorb significant moisture from humid environments. Absorbed moisture degrades dielectric strength, increases dielectric losses, and can cause dimensional changes that stress laminate bonds. In applications where high humidity exposure is expected — such as tropical environments or machines subject to condensation — engineers should consider sealed impregnation systems, moisture-resistant variants, or additional environmental protection measures. The standard includes conditioning procedures that account for moisture effects during testing.
Impregnation Process Compatibility
The compatibility between the laminate and the impregnation system is fundamental to achieving rated thermal class performance in the finished machine. The non-woven fabric layers must effectively absorb the varnish or resin during the impregnation cycle, and the laminate must maintain dimensional stability throughout the curing process. Incompatibilities can manifest as delamination during curing, incomplete void filling (leaving partial discharge sites), or chemical degradation of the laminate’s adhesive system by aggressive varnish solvents. Pre-qualification testing of the complete insulation system — laminate plus varnish — per IEC 60034-18-1 or equivalent standards is strongly recommended before production commitment.
Design Insights Summary
The engineering value of IEC 60626 combined flexible materials lies in their ability to decouple conflicting insulation requirements. The plastic film core provides the high dielectric strength needed for voltage withstand, while the non-woven fabric faces provide the mechanical toughness and varnish absorption that pure films cannot deliver. This synergy has made combined flexible laminates the dominant insulation technology for low-voltage rotating machine slot liners and phase insulation worldwide. When selecting materials for a specific application, engineers should consider the complete system — thermal class with hot-spot margins, thickness optimization balancing thermal and dielectric requirements, moisture management strategy, and verified compatibility with the intended impregnation process. A systematic approach to these factors, guided by IEC 60626 requirements, ensures reliable insulation system performance throughout the expected service life.
Frequently Asked Questions
What is IEC 60626 and which materials does it cover?
IEC 60626 is an international standard that specifies requirements for combined flexible materials used in electrical insulation. It covers composite laminates composed of plastic films (such as PET/Mylar or PEN) bonded to non-woven fabrics (such as polyester fiber, aramid paper like Nomex, or glass fabric). Common material types include DMD (Dacron-Mylar-Dacron), NMN (Nomex-Mylar-Nomex), and DM (Dacron-Mylar). These laminates combine the high dielectric strength of plastic films with the mechanical toughness and impregnation compatibility of fibrous layers.
What thermal classes are defined under IEC 60626?
IEC 60626 defines combined flexible materials across a wide range of thermal classes from 130°C (Class B) through 220°C+ (Class R/C). The thermal class depends primarily on the materials used in the laminate construction. DMD constructions using polyester film and polyester non-woven fabric typically achieve Class B (130°C) or Class F (155°C), while NMN constructions using Nomex aramid paper can reach Class H (180°C). Advanced combinations using polyimide film (Kapton) with Nomex or glass fabric can achieve Class N (200°C) or Class R (220°C). The standard specifies test methods for thermal endurance evaluation including weight loss, dielectric strength retention, and mechanical property changes after thermal aging.
How does thickness affect the performance of combined flexible insulation materials?
Thickness selection involves critical engineering tradeoffs in combined flexible insulation materials. Thinner laminates (0.15-0.25 mm) provide superior heat transfer from windings to the stator core, reducing hot-spot temperatures and improving motor efficiency. They also occupy less slot space, allowing for more copper fill. However, thinner materials have lower dielectric breakdown voltage and reduced mechanical tear resistance. Thicker laminates (0.30-0.50 mm) provide higher dielectric strength margins and better mechanical durability during automated insertion processes, but they impede heat dissipation and consume valuable slot area. The optimal thickness balances thermal performance, dielectric safety margins, and mechanical handling requirements for the specific application.
Are IEC 60626 combined flexible materials compatible with impregnating varnishes?
Yes, compatibility with impregnating varnishes is a key design feature of IEC 60626 combined flexible materials. The non-woven fabric outer layers (Dacron, Nomex, or glass fabric) are specifically chosen for their ability to absorb and bond with impregnating resins and varnishes. During VPI (Vacuum Pressure Impregnation) or trickle impregnation processes, the fibrous layers wick the varnish into the laminate structure, creating a void-free, mechanically robust insulation system after curing. The standard includes test methods for evaluating varnish absorption, bond strength after impregnation, and the effect of varnish compatibility on dielectric properties. Proper varnish compatibility ensures the insulation system achieves its rated thermal class in the final application.
