Coil Winding Varnish Guide: 3 Options by Coil Design, Thermal Class and Curing Process
Selecting a coil winding varnish involves more than checking its thermal class. Inductive coils, relay coils, small electromagnetic coils and reactor windings can differ in wire shape, winding structure, impregnation conditions and curing equipment. You therefore need to evaluate electrical properties, bonding requirements, impregnation process compatibility and production cycle time together.
This guide compares three coil insulation options from Changxian based on the technical data sheets for E962, E57A and E57-89H. E962 is designed for Class B applications such as inductive coils that require an air-drying process. E57A is a Class B–F option for small and medium-sized electrical products where baking facilities are available. E57-89H is intended for Class H–C, solventless impregnation and round- or flat-wire windings. Final suitability should still be confirmed through testing with your actual coil design and production process.
1. What 5 Conditions Should You Confirm Before Selection?
When selecting a coil winding varnish, define the following five inputs before comparing individual grades:
| Selection Input | What to Confirm | How It Affects Selection |
| Coil type | Inductive coil, electromagnetic coil, relay coil, reactor winding or another winding design | Helps you check whether the TDS explicitly covers the intended application |
| Wire and winding structure | Round or flat wire, coil dimensions and impregnation difficulty | Determines whether penetration and impregnation compatibility require particular attention |
| Thermal class | Class B, F, H or a higher thermal class | Eliminates options that do not meet your design temperature requirements |
| Curing conditions | On-site air drying, low-temperature baking, oven curing or vacuum impregnation | Eliminates processes that do not match your available equipment or production cycle |
| Verification requirements | Dielectric strength, bonding strength, thermal conductivity and other reliability requirements | Defines the priorities for sample testing and production validation |
The thermal class should be confirmed as part of your electrical insulation system design. The short-term temperature resistance of a cured varnish film should not be treated as equivalent to the long-term operating temperature of the complete coil insulation system. IEC 60085 provides an external reference for the thermal evaluation and designation of electrical insulation: IEC 60085 official page.
2. Which Applications Do the 3 Coil Insulation Options Fit?
These three coil winding varnish options differ in resin chemistry, thermal class, curing route and explicitly documented applications.
2.1 E962: A Class B, Air-Drying Option for Inductive Coils
Resin system: Modified polyester resin.
Thermal class: Class B, 130°C.
Curing characteristics: Air-drying, allowing you to evaluate a process that does not depend on high-temperature baking.
Applications explicitly listed in the TDS: Circuit board protection, inductive coils, transformers, small and medium-sized motors, and other electrical products.
Key specifications: Dielectric strength ≥40 kV/mm, volume resistivity ≥1×10¹² Ω·cm, rotational viscosity approximately 145–230 mPa·s, and SG 0.95.
Additional feature: The TDS states that it does not interfere with soldering.
For inductive coils or small coils processed under repair conditions, E962 provides a clearly defined air-drying selection path. However, air drying does not mean that the coil can be energized before complete curing. Actual drying time should be determined according to the applied coating, environmental conditions and process trials.
2.2 E57A: A Class B–F Option for Production with Baking Facilities
Resin system: Alkyd-amino resin system.
Thermal class: Class B–F, 130–155°C.
Curing characteristics: Baking type. The TDS states that it provides good securing performance and fast through-drying.
Applications explicitly listed in the TDS: Small and medium-sized transformers, small and medium-sized electric motors, and other electrical products.
Key specifications: Dielectric strength ≥60 kV/mm, solid content 45–55%, rotational viscosity approximately 150–235 mPa·s, and SG 0.98.
Process consideration: The dilution ratio and baking time should be determined through testing according to the product’s shape, dimensions and requirements.
If you produce small electromagnetic coils or relay coils with access to baking facilities and require Class B–F insulation, you can include E57A in your sample evaluation. Its suitability for a specific relay coil should be confirmed through impregnation, curing and electrical tests on the actual winding.
2.3 E57-89H: A Class H–C Solventless Impregnating Option
Resin system: Solventless epoxy-polyester, supplied as a one-component impregnating varnish.
Thermal class: Class H–C, ≥180°C.
Active ingredient content: 100%.
Applications explicitly listed in the TDS: Small and medium-sized coils and windings made with round or flat wire, and small and medium-sized reactor windings made with round or flat wire.
Key specifications: Dielectric strength ≥24 MV/m; bonding strength ≥140 N at room temperature and ≥50 N at 180°C; thermal conductivity ≥0.4 W/m·K; rotational viscosity approximately 165–355 mPa·s; and SG 1.075.
Applicable processes: The TDS provides procedures for vacuum impregnation and trickle impregnation.
When your coil design requires a higher thermal class, or when you need to evaluate bonding strength at elevated temperatures and thermal conductivity, E57-89H provides a clear technical distinction from E962 and E57A. Its TDS explicitly covers round- and flat-wire windings, making it a priority candidate for validation in these structures.
3. Coil Winding Varnish Specification Comparison
The following coil winding varnish comparison includes only items confirmed in the respective TDS documents. Where a test item is not listed for a particular grade, it is marked “—” rather than estimated or supplemented with an assumed value.
| Comparison Item | E962 | E57A | E57-89H |
| Resin system | Modified polyester | Alkyd-amino | Solventless epoxy-polyester |
| Thermal class | Class B | Class B–F | Class H–C |
| Primary curing route | Air drying | Baking | Curing after vacuum or trickle impregnation |
| Dielectric strength | ≥40 kV/mm | ≥60 kV/mm | ≥24 MV/m |
| Solid content / active ingredients | — | 45–55% | 100% active ingredients |
| Rotational viscosity | Approx. 145–230 mPa·s | Approx. 150–235 mPa·s | Approx. 165–355 mPa·s |
| SG | 0.95 | 0.98 | 1.075 |
| Volume resistivity | ≥1×10¹² Ω·cm | — | — |
| Bonding strength at room temperature | — | — | ≥140 N |
| Bonding strength at 180°C | — | — | ≥50 N |
| Thermal conductivity | — | — | ≥0.4 W/m·K |
| Coil applications explicitly listed in the TDS | Inductive coils | Other electrical products; validation required for the actual coil | Small and medium-sized round-/flat-wire coils and reactor windings |
Unit note: 1 kV/mm is numerically equivalent to 1 MV/m. However, the test conditions and specimen conditions in different TDS documents should still be reviewed separately. The volume resistivity unit for E962 is Ω·cm and must not be written as Ω·m.
4. How to Select a Coil Winding Varnish by Application
Your coil winding varnish validation route should follow the actual coil application and production conditions rather than a single laboratory specification.
Inductive Coils, Production Without an Oven, or Repair Conditions
Evaluate E962 first. Its TDS explicitly lists inductive coils and provides an air-drying route. Validation should cover complete drying, dielectric performance and actual application suitability. A surface that feels dry to the touch should not be used as the basis for energizing the coil.
Small Electromagnetic or Relay Coils with Baking Facilities
Consider E57A as a Class B–F candidate. Because its TDS does not specifically list relay coils, the phrase “other electrical products” should not be interpreted as confirmation that every relay design has already been validated. Test the varnish with your actual coil through impregnation, curing and electrical evaluation.
Round-/Flat-Wire Coils, Reactor Windings and Higher Thermal Requirements
Evaluate E57-89H first. Its TDS explicitly covers these winding structures and provides data for bonding strength at elevated temperatures and thermal conductivity. If your production line uses vacuum or trickle impregnation, you can determine the production conditions through trials based on the TDS and your actual workpieces.
5. Why Sample Validation Should Go Beyond Liquid Varnish Specifications
TDS specifications help you narrow the selection, but they cannot replace validation on the actual coil. Keep at least the following records during sample evaluation:
Coil information: Application type, wire shape, winding dimensions and target thermal class.
Varnish status: Grade, batch and application viscosity.
Process records: Impregnation method, draining condition, curing method and actual curing conditions.
Appearance and coverage: Check for visibly uncoated areas or abnormal varnish accumulation after impregnation.
Performance verification: Test electrical properties according to your design requirements. Add bonding, temperature-rise or other relevant reliability tests where required.
Production confirmation: Lock the application process window only after validation with your actual coils and production equipment.
The TDS documents for E57A and E962 require relevant application conditions to be determined through testing according to the product’s shape, dimensions and specific requirements. E57-89H also requires impregnation and curing times to be determined according to the actual workpiece. A single set of trial parameters should therefore not be presented as a fixed process suitable for every coil.
6. How This Guide Differs from Motor and Transformer Selection Guides
Motor winding selection generally focuses on stator construction, impregnation processes and operating temperature rise. Transformer selection also needs to account for the active-part structure, insulation system and corresponding operating conditions. This coil guide focuses instead on distinguishing inductive, electromagnetic, relay and reactor windings while checking round-/flat-wire structures against the application scope stated in each TDS.
You can continue with these related selection guides:
Motor Winding Insulating Varnish Selection Guide
Transformer Insulating Varnish Selection Guide
Fast Drying Insulating Varnish Guide
Clear Insulating Varnish Guide
Insulating Varnish Price Guide
7. Frequently Asked Questions
Q1: Can you select a coil winding varnish by dielectric strength alone?
No. Dielectric strength is only one selection criterion. You also need to check the thermal class, coil structure, curing conditions and application scope stated in the TDS. For round- or flat-wire windings, impregnation compatibility and other design-related properties should also be evaluated.
Q2: Which option can you evaluate for inductive coils without an oven?
You can evaluate E962 first. Its TDS explicitly lists inductive coils and states that it is air-drying. The actual complete-drying time and conditions for energizing the coil still need to be confirmed through process trials.
Q3: Can E57A be described directly as a dedicated relay coil varnish?
No. The E57A TDS lists small and medium-sized transformers, small and medium-sized electric motors, and other electrical products, but it does not specifically list relay coils. It can be evaluated as a Class B–F candidate where baking facilities are available, subject to validation on your actual coil.
Q4: Why should you check the TDS application scope for round- or flat-wire coils?
Not every TDS explicitly lists these winding structures. E57-89H specifically covers small and medium-sized round-/flat-wire coils and reactor windings, providing a more direct starting point for validation.
Q5: Can an air-dried coil be energized as soon as its surface feels dry?
No. Surface dryness alone should not be used as the basis for energization. Air drying does not eliminate the need for curing, and a dry surface does not necessarily indicate complete drying. Confirm complete curing according to the TDS and your actual process trials before energizing the coil.
8. Conclusion
Choosing the right coil winding varnish starts with your coil type, wire structure, thermal class and curing conditions. You can then use TDS specifications to narrow the selection. E962 provides a Class B air-drying route for applications including inductive coils. E57A offers a Class B–F baking option, while E57-89H covers Class H–C solventless impregnation and round-/flat-wire windings.
If you are evaluating inductive coils, relay coils or reactor windings, send our engineers your coil type, target thermal class, current impregnation method and curing conditions. We can help you screen suitable grades and provide TDS documents, selection comparisons and free samples for testing.
About Changxian
Zhuhai Changxian New Material Technology Co., Ltd. has specialized in electronic chemicals for more than 30 years. Changxian is listed on China’s National Equities Exchange and Quotations (stock code: 831323) and recognized as a national-level Specialized, Sophisticated, Distinctive and Innovative “Little Giant” enterprise. Our approximately 40,000 m² production base in Zhuhai’s Gaolan Port area includes in-house resin synthesis capabilities. We have achieved ISO 9001, ISO 14001 and ISO 45001 management system certification, together with IATF 16949 certification. Relevant insulating varnish and conformal coating products are covered by UL files; specific grade coverage is subject to the applicable certification.
Changxian’s position is straightforward: specifications benchmarked item by item against international brands, with factory-direct pricing — Equal Quality, Factory Price. If you need coil varnish TDS documents, grade comparisons or free samples for testing, contact our engineers. We will provide application-specific support based on your coil structure and production conditions.
Source basis for this article: Product TDS documents for E962, E57A and E57-89H; the official IEC 60085 page; and confirmed company qualification records. Test items not listed in the applicable TDS are marked “—” and have not been estimated.
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