Introduction

Fiberglass covered wire, as an important high-temperature insulated conductor, plays a critical role in transformers, motors, reactors, and other electrical equipment. Its insulation performance depends not only on the physical and chemical properties of fiberglass itself, but more importantly on the type of impregnating insulating varnish used. Different impregnating varnish types give fiberglass covered wire different thermal classes, electrical performance, mechanical properties, and environmental resistance.

In practical applications, correctly selecting and understanding the insulation types of fiberglass covered wire is crucial for ensuring the long-term reliable operation of electrical equipment. This article systematically elaborates on the insulation types of fiberglass covered wire from the aspects of insulation system composition, main impregnating varnish types and their performance characteristics, technical comparison of different insulation types, application field selection, and quality inspection methods, providing comprehensive material selection references for engineering technicians.

1. Insulation System Composition of Fiberglass Covered Wire

1.1 Fiberglass Braided Layer

Fiberglass is the basic insulation material of fiberglass covered wire, with its main chemical component being silicon dioxide (SiO₂, typically 52%~62%), supplemented by aluminum oxide (Al₂O₃), calcium oxide (CaO), magnesium oxide (MgO), boron oxide (B₂O₃), and other oxides. Based on chemical composition differences, fiberglass can be classified into the following types:

E-Glass (Electrical Glass Fiber): The most commonly used fiberglass type, with good electrical insulation properties and mechanical strength at a relatively low cost, widely used in the magnet wire field. The SiO₂ content of E-glass is typically 52%~56%, and CaO content is 16%~25%.

S-Glass (High-Strength Glass Fiber): Has higher tensile strength and elastic modulus, suitable for application scenarios with higher mechanical strength requirements. The SiO₂ content of S-glass is typically 63%~66%, and Al₂O₃ content is 24%~28%.

C-Glass (Chemical-Resistant Glass Fiber): Has better chemical corrosion resistance, suitable for mildly corrosive chemical environments. The SiO₂ content of C-glass is typically 63%~67%, with higher B₂O₃ content.

The diameter of fiberglass filaments is typically between 5~13 micrometers, braided onto the conductor surface in double or triple layers using specialized braiding equipment, with braiding density typically 85%~95%.

1.2 Impregnating Insulating Varnish

The fiberglass braided layer itself has certain hygroscopicity, with insufficient electrical performance and moisture resistance, and requires impregnation with insulating varnish to improve. Impregnating insulating varnish is the key factor determining the final performance of fiberglass covered wire, with main functions including:

Filling Voids: Fully filling the gaps between fiberglass fibers, forming a dense insulation layer, improving breakdown voltage and insulation resistance.

Moisture Protection: Preventing moisture intrusion into the fiberglass layer, improving moisture resistance.

Enhancing Adhesion: Bonding fiberglass filaments into a whole, improving the mechanical strength and wear resistance of the insulation layer.

Assigning Thermal Class: The thermal resistance of the impregnating varnish determines the final thermal class of fiberglass covered wire.

1.3 Conductor Materials

The conductor materials of fiberglass covered wire are mainly copper or aluminum. Copper conductors use oxygen-free copper rods (OFC) with purity typically above 99.95% and conductivity of not less than 100% IACS. Aluminum conductors use high-purity aluminum (above 99.7%) with lightweight advantages.

2. Main Impregnating Varnish Types and Performance Characteristics

2.1 Polyester Resin Impregnating Varnish

Polyester resin impregnating varnish is one of the most commonly used impregnating varnish types in fiberglass covered wire, with the following characteristics:

Chemical Composition: With polyester resin as the main film-forming material, typically formed by polycondensation of polyols and polyacids. The molecular structure contains a large number of ester bonds (-COO-), endowing it with good mechanical and electrical properties.

Thermal Class: Typically Class B (130℃) or Class F (155℃). Through modification, the thermal class can be further improved.

Electrical Performance: High insulation resistance, good breakdown voltage, dielectric constant approximately 3.5~4.5, dielectric loss factor (tanδ) approximately 0.01~0.03.

Mechanical Properties: Has good flexibility and wear resistance, effectively protecting conductors during winding processes such as winding, embedding, and shaping.

Chemical Resistance: Has good resistance to mineral oil and transformer oil, but poor resistance to strong polar solvents (such as acetone, ethanol, etc.).

Cost: Relatively low, suitable for large-scale mass production of electrical equipment.

Typical Applications: General-purpose dry-type transformers, general motors, reactors, etc.

2.2 Silicone Resin Impregnating Varnish

Silicone resin impregnating varnish is the most important high-temperature impregnating varnish type in fiberglass covered wire, with the following characteristics:

Chemical Composition: With silicone resin as the main film-forming material, the main chain structure is silicon-oxygen bonds (Si-O), with bond energy up to 444 kJ/mol, far higher than carbon-carbon bonds (347 kJ/mol), thus providing extremely high thermal stability and oxidation resistance.

Thermal Class: Typically Class H (180℃) or Class C (above 200℃). High-quality silicone resin impregnating varnish can maintain its electrical and mechanical properties at over 80% of initial values after long-term operation at 200℃.

Electrical Performance: High insulation resistance, excellent breakdown voltage, dielectric constant approximately 3.0~4.0, dielectric loss factor (tanδ) approximately 0.005~0.02. Maintains good electrical performance under high-temperature conditions.

Mechanical Properties: Has good flexibility and scratch resistance, but may become brittle under low-temperature conditions.

Moisture Resistance: Silicone resin has excellent hydrophobicity, effectively preventing moisture intrusion, with moisture absorption rate typically not exceeding 2%.

UV Resistance: Silicone resin has good resistance to ultraviolet rays, suitable for outdoor environments.

Cost: Higher, but has advantages in life cycle cost.

Typical Applications: High-temperature motors, dry-type transformers, reactors, industrial heating equipment, etc.

2.3 Polyurethane Impregnating Varnish

Polyurethane impregnating varnish is an impregnating varnish type with excellent flexibility in fiberglass covered wire, with the following characteristics:

Chemical Composition: With polyurethane as the main film-forming material, the molecular structure contains a large number of urethane bonds (-NHCOO-), endowing it with excellent flexibility and wear resistance.

Thermal Class: Typically Class B (130℃) or Class F (155℃).

Electrical Performance: Good insulation resistance, high breakdown voltage, dielectric constant approximately 3.5~5.0, dielectric loss factor (tanδ) approximately 0.02~0.04.

Mechanical Properties: Has excellent flexibility and wear resistance, particularly suitable for applications requiring frequent bending. Maintains good flexibility under low-temperature conditions.

Chemical Resistance: General resistance to mineral oil and solvents, not recommended for use in strong chemical corrosion environments.

Cost: Medium.

Typical Applications: Motor windings requiring good flexibility, home appliance motors, etc.

2.4 Polyimide Impregnating Varnish

Polyimide impregnating varnish is the impregnating varnish type with the highest thermal class in fiberglass covered wire, with the following characteristics:

Chemical Composition: With polyimide as the main film-forming material, the molecular structure contains a large number of aromatic rings and imide rings, endowing it with outstanding thermal stability, chemical stability, and electrical performance.

Thermal Class: Class C (above 240℃), one of the organic impregnating varnish types with the highest thermal class currently available. After long-term operation at 240℃, its performance can still be maintained within acceptable ranges.

Electrical Performance: Extremely high insulation resistance, excellent breakdown voltage, dielectric constant approximately 3.0~3.5, dielectric loss factor (tanδ) approximately 0.001~0.01. Maintains stable electrical performance under extreme high-temperature conditions.

Mechanical Properties: Has good mechanical strength and wear resistance, but flexibility is relatively poor.

Chemical Resistance: Has excellent resistance to most chemical substances, including organic solvents, mineral oil, weak acids, and weak alkalis.

Cost: Very high, mainly used for special operating conditions.

Typical Applications: Motors in extreme high-temperature environments, aviation motors, racing motors, special transformers, etc.

2.5 Modified Epoxy Resin Impregnating Varnish

Modified epoxy resin impregnating varnish is an impregnating varnish type with excellent chemical resistance in fiberglass covered wire, with the following characteristics:

Chemical Composition: With epoxy resin as the main film-forming material, improving its flexibility and thermal resistance by adding modifiers (such as silicone, polyurethane, etc.). The molecular structure contains a large number of epoxy groups and hydroxyl groups, endowing it with excellent adhesion and chemical resistance.

Thermal Class: Typically Class F (155℃) or Class H (180℃).

Electrical Performance: High insulation resistance, excellent breakdown voltage, dielectric constant approximately 3.5~5.0, dielectric loss factor (tanδ) approximately 0.01~0.03.

Mechanical Properties: Has excellent adhesion and mechanical strength, but flexibility is relatively poor.

Chemical Resistance: Has excellent resistance to most chemical substances, including organic solvents, mineral oil, weak acids, and weak alkalis.

Cost: Medium to high.

Typical Applications: Electrical equipment in chemical corrosion environments, special transformers, reactors, etc.

3. Technical Comparison of Different Insulation Types

3.1 Thermal Class Comparison

3.2 Electrical Performance Comparison

3.3 Mechanical Property Comparison

3.4 Environmental Resistance Comparison

4. Selection Principles for Insulation Types

4.1 Selection by Operating Temperature

Operating temperature is the primary consideration when selecting the insulation type of fiberglass covered wire:

Below 130℃: Select polyester resin or polyurethane impregnating varnish (Class B).

130℃~155℃: Select modified polyester or polyurethane impregnating varnish (Class F).

155℃~180℃: Select silicone resin impregnating varnish (Class H).

180℃~200℃: Select special silicone resin impregnating varnish (Class C).

Above 200℃: Select polyimide impregnating varnish (Class C).

4.2 Selection by Operating Environment

High-Temperature Dry Environment: Silicone resin impregnating varnish fiberglass covered wire.

High-Temperature High-Humidity Environment: Moisture-resistant silicone resin impregnating varnish fiberglass covered wire.

Outdoor Environment: UV-resistant silicone resin impregnating varnish fiberglass covered wire.

Chemical Corrosion Environment: Modified epoxy resin or polyimide impregnating varnish fiberglass covered wire.

Requiring Frequent Bending: Polyurethane impregnating varnish fiberglass covered wire.

4.3 Selection by Electrical Performance Requirements

High-Voltage Windings (>10kV): Select polyimide or silicone resin impregnating varnish, ensuring high breakdown voltage and excellent partial discharge resistance.

Medium-Low Voltage Windings (<10kV): Polyester resin or silicone resin impregnating varnish can meet requirements.

High-Frequency Applications: Select impregnating varnish types with lower dielectric constant and dielectric loss factor, such as polyimide or silicone resin.

5. Insulation Quality Inspection Methods

5.1 Insulation Resistance Testing

Insulation resistance is an important indicator of the insulation performance of fiberglass covered wire. Testing is typically conducted at room temperature (20℃) and high temperature (such as 155℃ or 180℃). The room temperature insulation resistance of high-quality fiberglass covered wire should be not less than 1,000 MΩ·km, and high temperature insulation resistance should be not less than 50 MΩ·km.

5.2 Breakdown Voltage Testing

Breakdown voltage testing is a key method for evaluating the electrical strength of fiberglass covered wire. During testing, the sample is placed in insulating oil, and the voltage is increased at a certain rate until breakdown occurs, recording the breakdown voltage value.

5.3 Thermal Aging Test

Thermal aging test is an important method for evaluating the long-term thermal stability of fiberglass covered wire. The sample is placed in a constant temperature oven, operated continuously at rated temperature for a certain period (typically 2,000~20,000 hours), with its electrical and mechanical performance tested regularly to evaluate performance degradation.

5.4 Moisture Absorption Rate Testing

Moisture absorption rate testing is used to evaluate the moisture resistance of fiberglass covered wire. The sample is placed in a constant temperature and humidity environment (typically 40℃, 95% relative humidity), weighed after a certain period, and the moisture absorption rate is calculated. The moisture absorption rate of high-quality fiberglass covered wire should not exceed 2%.

6. Conclusion

The selection of insulation types for fiberglass covered wire is a critical link in ensuring the long-term reliable operation of electrical equipment. Different impregnating varnish types give fiberglass covered wire different thermal classes, electrical performance, mechanical properties, and environmental resistance. Polyester resin impregnating varnish has lower cost and is suitable for general purposes; silicone resin impregnating varnish has excellent thermal resistance and is the preferred choice for high-temperature applications; polyimide impregnating varnish has the highest thermal class and is suitable for extreme high-temperature environments; polyurethane impregnating varnish has excellent flexibility and is suitable for scenarios requiring frequent bending; modified epoxy resin impregnating varnish has excellent chemical resistance and is suitable for chemical corrosion environments.

When selecting the insulation type of fiberglass covered wire, engineering technicians should comprehensively consider factors such as operating temperature, operating environment, electrical performance requirements, and cost, to select the most suitable insulation type, ensuring the safe, reliable, and economical operation of electrical equipment.