Which materials are used in the construction of the core and coils of your Dry Type Transformer?

Dry-type transformers are a popular choice in environments where fire safety, reduced maintenance, and eco-friendly design are critical. A key aspect of their performance and reliability lies in the materials used for the core and coils. This guide outlines the primary materials selected for these components, highlighting their properties, functions, and role in transformer durability and efficiency.

What Materials Are Used in the Core of a Dry-Type Transformer?

The performance, efficiency, and reliability of a dry-type transformer heavily depend on the quality and composition of its magnetic core. This core is responsible for transferring energy between primary and secondary windings through magnetic induction. If the core material is poorly selected, the transformer suffers from high core losses, excessive heating, acoustic noise, and reduced lifespan. Selecting the right core material isn’t just a design choice—it’s an economic and operational decision with long-term impact on efficiency and environmental compliance.

The core of a dry-type transformer is primarily made from laminated grain-oriented silicon steel or, in high-efficiency models, from amorphous metal alloys or nanocrystalline materials. These materials are selected based on their magnetic permeability, low hysteresis loss, low eddy current loss, and thermal stability. Grain-oriented silicon steel is the most common due to its cost-effectiveness and reliable performance, while amorphous metal and nanocrystalline alloys are used in premium designs where high efficiency and low no-load losses are critical.

Material choice significantly influences transformer size, efficiency, temperature rise, and sound level.

1. Grain-Oriented Silicon Steel (CRGO): The Industry Standard

Typical Use: Standard power and distribution dry-type transformers (100 kVA – 10 MVA range)

Construction:

• Laminated core sheets stacked in step-lap or mitered configurations
• Oriented to minimize magnetic reluctance and losses

2. Cold-Rolled Non-Grain-Oriented (CRNGO) Steel: Less Efficient Alternative

Specific losses: ~2.0 W/kg or higher at 1.5 T
📌 Rarely used for modern dry-type transformers due to energy efficiency mandates

3. Amorphous Metal Alloys: For High-Efficiency Designs

Core Loss Reduction:
Up to 70% less no-load loss compared to silicon steel

Applications:

• Energy-efficient distribution transformers (especially Tier 2 / DOE compliant)
• Eco-design certified transformers
• Renewable power plants

Challenges:

• Brittle material → Careful handling
• Higher cost
• Larger volume for equivalent power due to lower saturation flux

4. Nanocrystalline Alloys: Premium Core Performance

Used In:

• High-frequency dry-type transformers
• Specialized reactors, UPS systems, smart grid devices

Drawbacks:

• Cost-intensive
• Limited availability for large core sizes

5. Comparative Table of Core Materials

6. Core Assembly and Insulation Integration

In dry-type transformers:

• Core limbs are tightly wound and clamped
• Core is vacuum impregnated or cast in epoxy to minimize vibration and noise
• Magnetic shielding may be applied to prevent flux leakage

Insulation must be:

• Thermally compatible (Class F or H)
• Mechanically stable
• Resistant to moisture and UV (especially for outdoor dry-types)

📌 The core’s magnetic and thermal behavior must be balanced with winding insulation and cooling design.

7. Environmental and Efficiency Considerations

EU Ecodesign, U.S. DOE 2016, and India’s IS 1180 standards all drive the use of low-loss core materials in distribution applications.

Why Is CRGO Silicon Steel Commonly Used?

In transformer design, achieving a balance between cost, efficiency, thermal performance, and manufacturability is key. Cold-Rolled Grain-Oriented (CRGO) silicon steel has become the dominant core material in power and distribution transformers because it delivers precisely that balance. It combines excellent magnetic properties in the rolling direction, low hysteresis losses, and high saturation flux density—all critical for efficient magnetic flux conduction in transformers. If inferior materials are used instead, the result is excessive core loss, increased operating temperature, larger physical size, and reduced energy efficiency.

CRGO silicon steel is commonly used in transformer cores because it provides high magnetic permeability and low core losses along the grain direction due to its grain-oriented crystalline structure. This reduces hysteresis and eddy current losses, making transformers more energy efficient, compact, and thermally stable. Its laminated construction minimizes eddy currents, and its ability to operate at high flux densities allows for reduced core size and lower total cost of ownership.

This makes CRGO steel the ideal material for the majority of power and distribution transformers globally.

1. What Is CRGO Silicon Steel?

Cold-Rolled Grain-Oriented (CRGO) silicon steel is a specialized magnetic steel that undergoes a precise rolling and annealing process to align its crystal grain structure along one direction—typically the rolling direction.

The result is a material that conducts magnetic flux efficiently, resists magnetization losses, and minimizes internal heating.

2. Magnetic Benefits of Grain Orientation

Grain orientation helps minimize energy wasted as heat during magnetic field reversals—critical for transformers that run continuously.

3. Loss Reduction Mechanism

Total core losses consist of:

• Hysteresis loss (due to magnetic domain switching)
• Eddy current loss (due to circulating currents in the steel)

CRGO steel minimizes these via:

• Thin laminations (0.23–0.30 mm) to reduce eddy paths
• High resistivity from silicon content to limit current flow
• Insulating coatings between laminations

✅ CRGO delivers excellent performance while remaining cost-effective and mechanically durable.

4. High Flux Density Capability

CRGO steel supports saturation flux densities of up to 1.8 Tesla, which enables:

• Smaller core cross-sections
• Lower copper winding volume
• Reduced transformer size and weight

This is particularly important in applications where space and efficiency are at a premium.

5. CRGO in Laminated Core Construction

Transformers use CRGO sheets in laminated stacks, commonly arranged in:

Each lamination is coated with insulating varnish to prevent eddy currents between layers.

6. Noise and Vibration Reduction

CRGO steel’s smooth magnetic transitions reduce magnetostriction—a cause of transformer hum.

📌 Especially valuable for urban substations, hospitals, and noise-sensitive zones.

7. Manufacturing and Standardization

CRGO grades are standardized by IEC, ASTM, and JIS:

Manufacturers like Nippon Steel, POSCO, Baowu, and AK Steel produce high-grade CRGO materials globally.

8. Applications of CRGO Steel in Transformers

What Are the Typical Coil Conductors Made Of?

The conductor material in a transformer’s coil windings is a key factor determining efficiency, heat dissipation, size, cost, and longevity. A poorly selected conductor—either in material or geometry—can result in excessive power losses, overheating, or failure under fault conditions. Therefore, selecting the proper conductor is not just a design formality—it's central to reliable electrical performance and transformer lifespan.

Transformer coil conductors are typically made of copper or aluminum, selected based on electrical conductivity, thermal performance, mechanical strength, cost, and application. Copper is preferred for high-performance, compact transformers due to its superior conductivity and mechanical robustness, while aluminum offers a lighter and cost-effective alternative, often used in distribution transformers. Both materials are shaped as round wires, rectangular strips, or foil, and are coated with enamel or wrapped in insulating paper or fiberglass for dielectric protection.

The choice between copper and aluminum must consider the electrical current, physical constraints, cost targets, and mechanical durability required in service.

1. Copper: The Premium Choice for Performance

Copper is the most widely used conductor material in high-performance transformers because of its:

• High electrical conductivity (~58 MS/m)
• Excellent thermal conductivity (~385 W/m·K)
• Superior mechanical strength (useful during short circuits)
• Long-term oxidation resistance

Application Areas:

• Power transformers
• Compact dry-type transformers
• High-reliability installations
• Windings subject to high short-circuit stresses

2. Aluminum: A Lightweight, Cost-Effective Option

Aluminum is about 60% the conductivity of copper but also significantly lighter and less expensive.

Advantages:

• Lower cost
• Reduced transformer weight
• Easy to extrude into foil or strip
• Widely used in pad-mounted and pole-mounted distribution transformers

Drawbacks:

• Lower mechanical strength
• Higher losses (larger cross-section needed)
• Requires stronger connections to mitigate galvanic corrosion

3. Conductor Types by Shape and Arrangement

📌 Winding geometry directly affects leakage reactance, heat distribution, and insulation design.

4. Insulation Applied to Coil Conductors

Regardless of conductor material, insulation is critical to prevent turn-to-turn or layer-to-layer short circuits.

📌 Thermal class should match or exceed transformer operating limits to ensure insulation longevity.

5. Performance Comparison Table: Copper vs. Aluminum

6. Thermal and Electrical Impact on Sizing

To maintain the same resistive losses:

$$
A{\text{Al}} \approx 1.6 \times A{\text{Cu}}
$$

Where:

• $A_{\text{Al}}$ = cross-section of aluminum conductor
• $A_{\text{Cu}}$ = cross-section of copper conductor

However, larger aluminum conductors may reduce winding space, impacting the overall transformer design.

7. Industry Trends and Selection Criteria

Many modern transformers are engineered with hybrid designs, using copper in HV windings and aluminum in LV to balance performance and cost.

What Type of Insulation Is Applied on the Coils?

Coil insulation is one of the most critical elements in transformer design. While the conductor carries the electrical current, it is the insulation that protects against voltage breakdown, dielectric failure, and thermal degradation. A failure in insulation often leads to arc faults, fires, and catastrophic breakdowns. Therefore, selecting the proper insulation type—not just by material, but also by thermal class, dielectric strength, and mechanical resilience—is fundamental for safe and reliable transformer operation.

The types of insulation applied to transformer coils include enamel coatings, thermally upgraded paper, polyester films, Nomex® aramid paper, mica tape, fiberglass, and epoxy resins. These are chosen based on voltage class, temperature rating, winding geometry, and transformer type. Oil-immersed transformers typically use paper and enamel insulation, while dry-type transformers use a combination of Nomex, fiberglass, mica, and resin systems for higher thermal stability and moisture resistance. Each insulation type is classified according to its thermal endurance class (A, B, F, H, etc.) defined by IEC and IEEE standards.

Proper insulation ensures dielectric integrity, mechanical strength, and long service life under electrical and thermal stress.

1. Enamel-Coated Wire (Magnet Wire)

📌 Often the first insulation layer before wrapping or impregnating with other materials.

2. Kraft and Thermally Upgraded Paper (TUP)

🛢 Must be used in conjunction with insulating oil to function properly.

3. Polyester Film and Polyimide Films (e.g., Mylar®, Kapton®)

📌 Flexible, high-dielectric films used between layers or phases.

4. Nomex® (Aramid Paper)

🛡 Used as phase barriers, interlayer insulation, or wrap for LV/HV coils.

5. Fiberglass Insulation (Glass Yarn or Mat)

📌 Often impregnated with resin or varnish for rigidity and bonding.

6. Mica Tape (Phlogopite or Muscovite)

🧯 Used in dry-type transformers for partial discharge suppression and fireproof insulation.

7. Epoxy Resin and Cast Resin Systems

8. Thermal Class and Selection Chart

📌 IEC 60085 and IEEE C57.12.01 define thermal endurance and classification.

9. Layered Insulation Strategy in Transformer Coils

Transformers typically use multi-layer insulation systems:

• Primary turn insulation: Enamel or polyimide film
• Interlayer insulation: Nomex, polyester film, or paper
• Interphase insulation: Nomex barriers or mica-tape
• Overall coil insulation: Fiberglass + resin coating or epoxy casting

This layered approach ensures redundancy, mechanical protection, and thermal resilience.

10. Insulation Selection Depends on Transformer Type

How Are Core and Coils Protected from Moisture and Dust?

Moisture and dust are among the most destructive environmental contaminants for transformers. When water vapor or fine particulates infiltrate the core or coils, they degrade insulation, reduce dielectric strength, corrode conductors, and lead to tracking, arcing, or complete electrical failure. In dry-type transformers, which lack the protective oil bath found in oil-filled units, moisture and dust ingress must be proactively controlled through specialized design and material techniques. Without these protections, transformer reliability, safety, and service life are seriously compromised.

Core and coils in transformers are protected from moisture and dust using a combination of insulation materials, encapsulation methods (such as vacuum pressure impregnation or cast resin), enclosure design (IP-rated or sealed), anti-condensation systems, surface treatments, and environmental controls. These strategies ensure the electrical integrity, thermal performance, and mechanical stability of windings and cores by preventing ingress of water vapor and airborne contaminants.

Proper environmental protection is especially critical in humid, dusty, marine, and industrial installations.

1. Vacuum Pressure Impregnation (VPI): A Moisture Barrier Technique

Benefits:

• Seals insulation and windings against moisture ingress
• Improves thermal conductivity
• Increases mechanical rigidity and short-circuit strength
• Reduces corona discharge

Applications:

• Indoor dry-type transformers
• Medium-voltage class (up to 36 kV)
• General industrial environments

2. Epoxy Cast Resin Encapsulation: Total Environmental Shielding

Advantages:

• Totally enclosed coil protection
• Fire-retardant and self-extinguishing (Class F or H)
• High tracking and moisture resistance
• Used in marine, tunnel, mining, desert, and offshore installations

📌 Used extensively for outdoor and high-pollution environments.

3. Ingress Protection (IP) Enclosures

Transformers are installed in sheet metal enclosures with:

• Sealed gaskets
• Filter ventilation grilles
• Louvered air inlets
• Anti-condensation heaters (in cold climates)

📌 Enclosure choice must match site environmental classification (IEC 60529 or NEMA ratings).

4. Insulation Systems That Resist Moisture

These are used in multi-layer composite insulation systems, tested per IEC 60085 thermal classification.

5. Breathers, Heaters, and Desiccants

In some sealed dry-type or indoor transformers:

Especially useful in coastal, tropical, or high-altitude climates.

6. Surface Treatments and Core Protection

📌 Surface insulation integrity is regularly inspected per IEEE C57.12.01 or IEC 60076-11.

7. Site and Environment Adaptations

🧪 Routine maintenance includes surface cleaning, humidity checks, and enclosure integrity tests.

8. Testing for Moisture and Dust Resistance

📌 Results help verify dry-state reliability before field commissioning.

How Do These Materials Affect Transformer Lifespan and Efficiency?

A transformer’s performance and service longevity are heavily dictated by the quality and compatibility of materials used in its construction. Improper selection—such as low-grade insulation, subpar core steel, or incompatible conductor materials—can result in accelerated aging, energy losses, hot spots, mechanical deformation, and ultimately, transformer failure. Conversely, when premium materials are selected and engineered correctly, they ensure decades of high-efficiency, low-maintenance service, even in demanding environments.

Transformer materials such as copper or aluminum for windings, CRGO steel or amorphous alloys for cores, and high-grade insulation like Nomex, epoxy resin, and VPI varnish directly affect both lifespan and efficiency. High-conductivity copper minimizes load losses, CRGO steel reduces core losses, and thermal Class H insulation enables operation under elevated temperatures. These materials improve energy transfer, reduce heat buildup, and extend insulation life, ensuring long-term transformer durability and operational efficiency.

Material synergy is the key: the best designs pair materials that work together electrically, thermally, and mechanically.

1. Conductor Material (Copper vs. Aluminum)

🔍 Copper is preferred in high-performance, compact transformers, while aluminum is used where cost and weight are prioritized, with design adaptations.

Copper helps maintain efficiency across decades by reducing I²R losses, limiting thermal aging.

2. Core Material (CRGO Steel vs. Amorphous Metal)

Amorphous metal is used in premium designs where no-load loss is a key cost and emissions concern.

Benefits to lifespan:

• Less heat = less insulation stress
• Lower magnetostriction = quieter operation
• Reduces thermal cycling damage

💡 Especially beneficial in renewable power and 24/7 loaded networks.

3. Insulation Material and Thermal Class

📌 Each 10°C drop in operating temperature doubles insulation life. Thus, better insulation materials prolong service time exponentially.

High-end materials like Nomex and epoxy-mica systems resist:

• Moisture
• Electrical tracking
• Thermal fatigue
• Fire and smoke propagation

4. Impregnation and Encapsulation Techniques

These techniques prevent hot spots, dielectric breakdown, and premature insulation aging.

📊 Studies show transformers with CRT or VPI systems have MTBF (mean time between failure) > 30 years under nominal load conditions.

5. Mechanical Hardware and Accessories

Weak structural materials can lead to coil displacement, arcing, or vibration failure—shortening transformer life.

6. Real-World Efficiency and Lifespan Model

Example: 1000 kVA dry-type transformer

Annual savings in power losses (assuming \$0.12/kWh and 70% load factor):

• Economy: $9,417
• Standard: $6,552
• Premium: $3,898

Over 30 years, premium materials can save over \$160,000 in losses and extend lifespan by 2–3x.

7. Environmental and Long-Term Impact

High-grade materials also retain value in secondary markets or for refurbishing.

Conclusion

The performance and safety of a dry-type transformer depend heavily on the materials used in its core and coils. From energy-efficient CRGO steel cores to high-conductivity copper windings and high-temperature insulation systems, each component is selected for optimal functionality under various operating conditions. Understanding these materials not only aids in better product selection but also ensures longer service life and operational reliability in demanding environments.

FAQ

Q1: What material is used for the core of a dry-type transformer?
A1: The core of a dry-type transformer is typically made from:

Cold-Rolled Grain-Oriented (CRGO) Silicon Steel
This material:

Has high magnetic permeability

Minimizes hysteresis and eddy current losses

Is laminated to reduce heat and energy loss
Some designs may also use amorphous steel cores for ultra-low-loss applications.

Q2: What materials are used for transformer windings (coils)?
A2: The coils in dry-type transformers are generally made from:

Electrolytic Grade Copper (preferred for its low resistance and high conductivity)

Aluminum (used in cost-sensitive applications or lighter designs)
Both materials are chosen based on application requirements, cost, and weight considerations.

Q3: What insulation materials are used around the coils?
A3: Dry-type transformers use solid insulation systems, such as:

Epoxy Resin (Cast Resin Transformers): Protects against moisture, dust, and corrosive gases

Nomex or Mylar Wrapping: Heat-resistant materials for insulation in VPI (Vacuum Pressure Impregnated) transformers

Polyester or Glass Fiber Tapes: Provide additional thermal and mechanical strength
These materials are flame-retardant and suitable for indoor and harsh environments.

Q4: Why is silicon steel preferred for transformer cores?
A4: Silicon steel is used because it:

Reduces core losses due to its high electrical resistivity

Has directional grain orientation for improved magnetic properties

Supports compact core design with efficient flux conduction
This enhances transformer performance and reduces operational energy losses.

Q5: How do these materials affect transformer performance?
A5: Copper windings: Provide low resistance, improving efficiency and thermal performance

High-grade steel cores: Minimize magnetic losses, ensuring lower no-load loss

Solid insulation: Extends life expectancy, improves fire safety, and allows maintenance-free operation
The combination ensures high mechanical strength, thermal stability, and longevity.

References

Electrical4U: Dry Type Transformer Construction
https://www.electrical4u.com/dry-type-transformer/

IEEE C57.12.01: Standard for Dry-Type Transformers
https://standards.ieee.org/standard/C57_12_01-2020.html

Doble: Materials in Transformer Insulation Systems
https://www.doble.com/resources/transformer-insulation-systems/

NREL: Transformer Core and Coil Optimization
https://www.nrel.gov/docs/fy21osti/dry-transformers.pdf

ScienceDirect: Study on Transformer Material Performance
https://www.sciencedirect.com/science/article/pii/S1359431119311219