Insulation Materials

A transformer consists of a core (made of a ferromagnetic material), insulation material, and primary and secondary coil windings. The insulation material in a transformer must be reliable and durable to prevent failures, enhance energy efficiency, and ensure electrical safety. By resisting electrical, mechanical, thermal, and chemical stresses, quality insulation can help transformers meet challenging performance requirements over a long service life.

Transformer insulation systems are categorized by classes according to their maximum possible operating temperature limit. These classes define the insulation material’s ability to withstand heat without degrading and its thermal endurance capabilities. The UL 1446 recognized insulation systems are identified by the following letters:

Class A: Maximum operating temperature of 105 ºC
Class E: Maximum operating temperature of 120 ºC
Class B: Maximum operating temperature of 130 ºC
Class F: Maximum operating temperature of 155 ºC
Class H: Maximum operating temperature of 180 ºC
Class C: Maximum operating temperature of 220 ºC

At Triad Magnetics, our experienced engineering team utilizes the most advanced materials to design electrical systems that are cost-effective, safe, and long-lasting. On this page, you’ll learn more about the types of transformer insulation materials and methods to help you select the right one for your application.

Modern Insulation Material Types
Varnishing Impregnation for Transformer Insulation
Types of Varnishes
Contact Triad Magnetics for Quality Transformers

Modern Insulation Material Types

1. Kapton^® (Polyimide) Tape
1. Kapton^® (Polyimide) Tape
Extreme temperature resistance (up to 400°C / 752°F)

Excellent dielectric strength

Used for formed coils, motor slot liners, and high-temperature masking

Typically features silicone adhesive for strong bonding
2. Teflon^® (PTFE) Tape
2. Teflon^® (PTFE) Tape
High temperature range (-180°C to 260°C)

Exceptional chemical resistance

Non-stick surface and low friction

Reinforced versions offer higher strength
3. Nomex^® Aramid Paper Tapes
3. Nomex^® Aramid Paper Tapes
Extreme temperature resistance (up to 400°C / 752°F)

Excellent dielectric strength

Used for formed coils, motor slot liners, and high-temperature masking

Typically features silicone adhesive for strong bonding
4. Glass Cloth Tapes
4. Glass Cloth Tapes
Extreme temperature resistance (up to 400°C / 752°F)

Excellent dielectric strength

Used for formed coils, motor slot liners, and high-temperature masking

Typically features silicone adhesive for strong bonding
5. Polyester (Mylar^®) Film Tape
5. Polyester (Mylar^®) Film Tape
Extreme temperature resistance (up to 400°C / 752°F)

Excellent dielectric strength

Used for formed coils, motor slot liners, and high-temperature masking

Typically features silicone adhesive for strong bonding

Key Differences: Kapton vs. Teflon (PTFE)

Adhesion

Kapton adheres better to metals and plastics, making it ideal for securing components.

Flexibility

Kapton is more conformable to complex shapes. Teflon is more rigid but offers superior non-stick properties.

Application

Kapton is preferred for electronics and high-temperature masking.
Teflon is ideal for chemical resistance and release/lubricating applications.

Common Use Case

Kapton → Transformer winding
Teflon (reinforced) → Specialized insulation requiring slip/release properties

Transformer Tape Applications

These tapes are critical for:

Layer insulation (separating wire layers)
Lead anchoring
Coil wrapping/banding
High-temperature protection during varnish impregnation and operation

Performance Characteristics

Thermal Class

Many materials rated Class H
(180°C) or higher

Chemical/Moisture Resistance

Especially strong with PTFE

Flame Retardancy

Kapton often carries UL-94 V-0 rating

Key Transformer Bobbin Materials

Thermoplastics: Widely used, cost-effective materials. Often glass-reinforced to improve strength and heat resistance.
Nylon (PA66): Offers good toughness and temperature resistance (approximately 115°C–260°C when reinforced).
PBT (Polybutylene Terephthalate): Known for strong electrical insulation properties and reliable heat resistance.
PET (Polyethylene Terephthalate): High strength and rigidity with good resistance to deformation.
Polycarbonate: Used where specific insulation and impact-resistance properties are required

Varnishing Impregnation for Transformer Insulation

Applying a liquid varnish, known as varnishing impregnation, helps to fill existing internal voids in the transformer itself. As the liquid resin hardens, it forms a protective film that enhances the insulation properties of the transformer components, offering the following benefits:

Thermosetting resins, such as phenolics or epoxies, are types of impregnating resins and insulating varnishes that ensure transformers—as well as generators, motors, sensors, and other electrical devices that function by electromagnetic induction—have the required structural integrity and electrical insulation for operation.

When applied, these resin materials crosslink, making them inherently durable, strong, and environmentally stable. Also called functional insulation or wire enamel, the insulating varnish prevents electrical shorting in transformer coil windings.

The completed transformer consists of insulating paper and an assembly of coil windings sealed with a thermosetting resin to protect against environmental factors like dust and moisture. This protection enhances strength and prevents air from entering the transformer, as air can conduct heat and electricity.

Types of Varnishes

Potting Varnish
Potting Varnish
Potting materials are compounds used in electronics manufacturing to seal and fill cavities in assemblies. These compounds protect the electronic components from the following external forces that could lead to electrical failure or corrosion:

Dust

Moisture

Shock

Vibration

The potting technique utilizes a pot (or case) to load the device and fill it with liquid potting. Once completed, the pot becomes part of the unit. It is a technique commonly used in high-volume manufacturing.

Depending on your device’s operating environment, you can choose from a broad range of potting varnishes, including those that provide radiation protection, flame retardance, or low outgassing properties.
Vacuum Varnish
Vacuum Varnish
Vacuum pressure impregnation (VPI) involves fully submerging the transformer’s winding in a resin. With the help of a pressure cycle and a wet/dry vacuum, the resin gets absorbed into the winding. Once the components are thermally prepared, the impregnated windings form a monolithic, homogenous structure.

These windings exhibit desirable characteristics, such as minimal maintenance requirements, corrosion resistance, and longevity. The vacuum pressure impregnation process uses negative pressure for optimal productivity and better varnish penetration in the transformer coil.

Varnish used on transformers can shield the windings from moisture, impurities, and other environmental issues. It can also limit the vibrations experienced by the windings and significantly decrease noise levels, both of which can effectively prolong the transformer’s lifespan.

Contact Triad Magnetics for Quality Transformers

Triad Magnetics is a trusted source for standard and custom magnetic solutions. With thousands of standard designs, custom capabilities, and global distribution, we are committed to delivering the magnetic products you need for your specific application. Our transformers feature UL-recognized insulation systems for different classes and temperature ranges.

Contact us to learn more about insulation materials in transformers and how they impact device performance and longevity. You can also request a quote to get started on your project.