Incorrect selection of insulating varnish is a leading cause of premature motor failure. Based on standards such as GB/T 11021 and IEC 60085, this article systematically outlines the three core criteria for selecting motor insulating varnish: thermal resistance, adhesion, and environmental compliance. It also provides a selection comparison chart and a guide to common pitfalls across different operating conditions. Designed for procurement and R&D professionals in home appliances, new energy vehicles, and industrial motors, this resource helps ensure precise material selection and mitigate quality risks.
I. Why Do Insulating Varnish Selections Keep Failing?
Why did a turn-to-turn short circuit occur in the motor after six months of operation, even though I purchased insulation varnish according to the manufacturer's recommended model?
Switched to water-based paint and passed environmental standards, but failed the heat resistance test. What's going on?
In motor manufacturing, selecting the wrong insulation varnish is a leading cause of early failure. Many purchasers focus solely on price, while engineers fixate on specifications—overlooking the core principle of "application compatibility."
As Guangzhou Feisi's technical team, we've distilled three key selection criteria—temperature resistance, adhesion strength, and environmental compliance—to help you build a scientific selection framework.
II. Criterion 1: Heat Resistance – The Lifeline of Insulating Varnish
Understand Thermal Classifications (GB/T 11021 / IEC 60085)
The thermal class of the varnish directly determines motor lifespan. Common classifications are listed below:
Y Class (≤90°C): Largely phased out; used only for micro-motors with minimal requirements.
Class A (105°C): Common grade for traditional phenolic enamels
E Class (120°C) – Minimum Requirements for Small General-Purpose Motors
Class B (130°C): Commonly used in home appliance motors; cost-effective.
F Class (155°C): Recommended rating for industrial inverter motors and servo motors
H Class (up to 180°C): Mandatory grade for new energy vehicle motors and compressor motors
Class C (> 180°C): For extreme high-temperature applications, such as aerospace motors
(Note: The temperature value represents the maximum continuous operating temperature permitted for the insulating material, not the short-term peak thermal tolerance.)
Pitfalls to Avoid in Selection
Mistake 1: Focusing only on temperature rating, not thermal aging life
The paint is rated for 180°C, but after 1000 hours of operation at 180°C, its bond strength dropped by 50%, making it effectively Class F.Myth 2: Ignoring hotspot temperatures
The internal temperature of motor windings is typically 10~20°C higher than the surface. When selecting insulation, account for this margin. Relying solely on nameplate temperature rise ratings is insufficient; winding hot spots must also be considered.
Guangzhou Feisi Suggestions
Appliances/General Motors: Select at least Class B (130°C)
Industrial Inverter Motor / Servo: F Class (155°C) recommended
New Energy Vehicle Motors/Compressors: Must be H-class (180°C) or higher
3. Standard 2: Adhesion – The Backbone of Insulating Varnish
Why is adhesion so critical?
Insulating varnish must not only insulate but also adhere strongly. Poor adhesion triggers a chain of failures:
Loose coil → Increased vibration noise → Enameled wire abrasion → Turn-to-turn breakdown
Thermal expansion and contraction → Paint film cracking → Moisture intrusion → Decreased insulation resistance
Long-term operation → Paint film peeling → Winding loosening → Motor failure
Comparison of Adhesion Test Methods
Cross-cut Test (GB/T 9286): A grid pattern is scored into the paint film using a cross-cut cutter, then evaluated by visual inspection after applying adhesive tape. Ratings of ≤1 indicate excellent performance (smooth cut edges with no peeling). Suitable for various substrates, this is the most widely used rapid evaluation method in the motor industry.
Pull-off method (GB/T 5210): A tensile tester is used to detach a glued metal stud from the paint film, quantifying adhesion strength in MPa. While highly accurate, sample preparation is labor-intensive; this method is typically employed for type testing during new product development.
Bend Test: Bend the coated copper or aluminum sheet around a mandrel with a diameter of 180°, then inspect the coating for cracks. Simple, intuitive, and ideal for rapid on-site assessment.
Pitfalls to Avoid in Selection
Misconception: Higher viscosity means better adhesion.
Fact: Viscosity affects only penetration and workability during impregnation. Adhesion depends on the strength of chemical bonds between resin molecules and the metal substrate (silicon steel, copper wire). Some low-viscosity coatings actually achieve better adhesion due to more thorough penetration.
Guangzhou Feisi Suggestions
Silicon steel sheets and copper wires must be clean and dry (moisture content < 0.1%); oil and rust are the primary enemies of adhesion.
Waterborne epoxy systems generally offer superior adhesion compared to traditional oil-based paints, as epoxy molecular chains contain active functional groups such as hydroxyl and epoxide groups that form chemical bonds with metal surfaces.
4. Standard 3: Environmental Compliance — The Green Pass for Insulating Varnish
Quick Overview of Global Environmental Regulations
China: GB 30981-2020 "Limits of Harmful Substances in Industrial Protective Coatings" — Waterborne coatings VOC limit ≤100g/L, Solvent-based coatings ≤420g/L
EU: RoHS 2.0 (2011/65/EU) restricts up to 6 hazardous substances including lead, mercury, and cadmium; REACH regulates Substances of Very High Concern (SVHC).
North America: EPA NESHAP imposes strict total plant emission limits on VOCs from coatings.
Trend: China's "14th Five-Year Plan for Green Industrial Development" explicitly promotes the substitution of low-VOC coatings, with multiple regions listing solvent-based insulating varnishes on their "restricted use" lists.
Water-Based Paint vs. Solvent-Based Paint Comparison
Water-based Epoxy Insulating Varnish: VOC content ≤ 100g/L (near zero), non-flammable liquid (no hazardous material restrictions for transport), low odor (improves working environment), compliant with RoHS/REACH (barrier-free export).
Solvent-based insulating varnish: VOC content 300~500 g/L. Flammable liquid (requires explosion-proof storage and transport). Has a pungent odor; export to the EU requires an exemption certificate.
Pitfalls to Avoid in Selection
Myth: Water-based paints are not heat-resistant and have poor adhesion.
Fact: Modern waterborne epoxy/polyurethane technologies have overcome temperature resistance limits (up to H-class 180°C). Adhesion has been significantly enhanced through chemical modification (e.g., adding silane coupling agents), outperforming traditional solvent-based systems across multiple metrics.
Guangzhou Feisi Suggestions
Exporting to the EU or North America: You must request RoHS and REACH compliance declarations and third-party test reports from your supplier.
Regions with strict environmental impact assessments: Prioritize water-based paints to reduce environmental compliance risks and minimize investment in exhaust treatment facilities.
5. Scenario Matching: Quick-Reference Motor Selection Guide
Washing machine motor: Grade B (130°C), compatible with both water-based and solvent-based coatings; prioritize moisture resistance.
Air Conditioning/Fan Motor: Class B-F (130-155°C), compatible with water-based or solvent-based formulations; noise reduction is a key requirement.
Range hood motor: F class (155°C); oil-resistant epoxy system recommended, with emphasis on stain resistance.
Electric Vehicle Drive Motor: Class H (180°C). Waterborne epoxy systems are recommended. Focus on heat resistance and thermal shock resistance.
Gasoline Generator: Class F (155°C); waterborne epoxy system recommended (excellent adhesion); prioritize vibration resistance.
Refrigerator/Air Conditioner Compressors: F-H class (155-180°C); must have high dielectric strength, with emphasis on high-voltage resistance and refrigerant corrosion resistance.
High-Frequency Transformer: Class H (180°C). Recommended to use a low dielectric loss system, with a focus on electrical stability under high-frequency conditions.
Power Tools (Universal Motors): Class F (155°C). Recommended anti-tracking compound; prioritize spark resistance.
Industrial Variable Frequency Drive (VFD) Motor: Class H (180°C), must use corona-resistant formulation, with emphasis on pulse voltage surge resistance.
Servo motor: Class F-H (155-180°C); waterborne epoxy system recommended; prioritize reliability of precision control.
Selection should be based on a comprehensive assessment of actual operating conditions, impregnation processes, and equipment capabilities.
VI. Five-Step Selection Guide for Guangzhou Feisi
Step 1: Define operating conditions
Specify the motor's actual operating temperature range, ambient humidity, vibration level, and whether it is exposed to chemical media (oil, grease, refrigerants, etc.).
Step 2: Determine the level
Select the appropriate insulation class (B/F/H) based on winding hotspot temperature plus a 20°C margin to ensure long-term lifespan and reliability margin.
Step 3: Match Processes
Select immersion method: Standard, Vacuum (for high fill), or Dip (for high-line efficiency). Each process requires specific resin viscosity and gel time; ensure product model compatibility.
Step 4: Verify Performance
After small-batch pilot production, conduct the full suite of type tests including insulation resistance, power frequency withstand voltage, and thermal aging (GB/T 11026 series). Skipping pilot production to switch directly to mass production is not recommended.
Step 5: Compliance Review
Verify the supplier's RoHS, REACH, and VOC test reports to ensure compliance with environmental regulations in the target market.
VII. Frequently Asked Questions (FAQ)
Q1: Can water-based insulation varnish completely replace solvent-based varnish?
Yes, in most application scenarios. Water-based epoxy coatings now outperform traditional solvent-based ones in electrical insulation filling, adhesion, and environmental performance, with H-class thermal resistance fully matured. The key consideration is the baking process: water-based coatings require a longer preheating phase (60-80°C) to remove moisture, while the curing temperature (130-150°C) remains comparable to that of solvent-based coatings.
Q2: How to quickly assess varnish quality?
Two simple tests: (1) After curing, bend the coated copper sheet at 180°; the coating is acceptable if no cracks appear. (2) Boil for 1 hours and inspect; the coating is acceptable if there are no blisters or peeling. These tests provide a quick screening of adhesion, flexibility, and wet-state insulation performance. Formal evaluation still requires full type testing according to standard methods.
Q3: What is the acceptable insulation resistance after soaking in water for 24 hours?
By industry standards, industrial-grade performance requires volume resistivity of ≥1×10^10 Ω·cm (normal conditions) and ≥1×10^8 Ω·cm after 24h water immersion. Guangzhou Feisi water-based epoxy paint achieves ≥1×10^14 Ω·cm under normal conditions and maintains ≥1×10^12 Ω·cm even after 24h in water—far exceeding requirements.
Q4: Can Guangzhou Feisi provide customized services?
A: Yes. Key parameters such as temperature rating (B/F/H), viscosity range (to suit various impregnation processes), and cure speed can be adjusted to meet customer requirements. For specialized performance needs (e.g., corona resistance, high thermal conductivity), our technical team will evaluate and provide a customized solution.
About Guangzhou Feisi
Guangzhou Feisi Synthesized Materials Co., Ltd. specializes in the R&D and production of high-performance water-based insulation materials. Our product portfolio includes water-based epoxy insulating varnishes, amorphous strip composite adhesives, and water-based phenoxy resins, serving customers across home appliances and motors, new energy vehicles, industrial motors, and high-frequency transformers.
Silicon Steel Self-Adhesive Coating / Waterborne Epoxy Insulating Paint — Choose Guangzhou Feisi Brand
Official hotline: 020-82793066
Water-based Paint Business: Engineer Liu 15920494556
Thiol Business: Engineer Zhang 13609004936
Overseas Helpline: +86 15920494556
For more information on Feis's full range of products in Guangzhou, to request samples, or to schedule a process trial, please contact us by phone or email.