Tianjin Factory In China Provides High-Quality Iron Core With Size Customization Service
Transformer Iron Core: Complete Technical Guide
1. Definition and Core Function of Transformer Iron Core
Transformer iron core is the key magnetic circuit component of all electromagnetic transformers. Assembled by stacked insulated electrical steel sheets or special magnetic alloys, it provides a low-reluctance closed path for alternating magnetic flux generated by winding current. Based on electromagnetic induction theory, the iron core enables efficient energy transmission between primary and secondary windings.
Without a qualified iron core, most magnetic flux will leak into the air, resulting in extremely low energy conversion efficiency and serious heating. Iron core performance directly determines transformer no-load loss, magnetizing current, operating noise and overall service life. Cooperated with enamelled purple copper wire windings and epoxy resin bushings, the iron core constitutes three core components of medium and high voltage dry-type and oil-immersed transformers.
Both IEC 60076 and Chinese GB standards clearly specify material selection, lamination requirements and loss limits for transformer iron cores. With the growth of global demand for high-efficiency power equipment, customized-size laminated iron cores have become mainstream supporting parts for transformer manufacturers worldwide.
2. Main Materials for Transformer Iron Core
Three material systems dominate commercial transformer iron core production:
2.1 Cold-Rolled Grain-Oriented Silicon Steel (CRGO)
The most widely adopted material for power and distribution transformers. Internal crystal grains are arranged along the rolling direction, delivering ultra-low hysteresis loss when magnetic flux runs parallel to the grain orientation. Conventional thickness options include 0.23 mm, 0.27 mm, 0.30 mm and 0.35 mm. CRGO laminations are the primary choice for oil-immersed transformers and dry-type distribution transformers.
2.2 Cold-Rolled Non-Oriented Silicon Steel (CRNGO)
Magnetic properties are uniform in all directions. Mainly applied to rotating equipment, reactors, instrument transformers and small low-frequency transformers. Not recommended for large power transformers due to relatively higher iron loss.
2.3 Amorphous Alloy Strip
Formed by ultra-fast quenching of molten alloy without crystal structure. Its no-load loss can be reduced to 1/4 of ordinary CRGO silicon steel. It is widely used for high-efficiency amorphous alloy distribution transformers to meet strict energy-saving policies. Limited by mechanical brittleness, amorphous materials have higher processing requirements.
2.4 Nanocrystalline Alloy
Mostly used for high-frequency small transformers, current transformers and electronic inductors, rarely used for 50Hz power grid main transformers.
3. Classification of Transformer Iron Core by Structure
Core-Type Iron Core
Composed of vertical core legs and horizontal magnetic yokes. Windings are sleeved outside the core columns. Simple structure, convenient winding assembly, suitable for medium, high voltage and large-capacity power transformers, including most UHV and distribution transformers.
Shell-Type Iron Core
The iron core surrounds the outside of windings. Good magnetic shielding effect, low noise, generally used for small-capacity and special electronic transformers. Rarely adopted for large grid transformers.
Wound Toroidal Iron Core
Formed by continuous winding of silicon steel strips. Minimal air gap, low magnetizing current, commonly used for instrument transformers and small isolation transformers.
Stacked Laminated Iron Core
Adopts E-I, U-I or multi-step lap silicon steel sheets for staggered stacking. Flexible dimension adjustment, supports size customization, which is the mainstream structure for mass-produced distribution transformers.
4. Material Performance Comparison Table
| Material Type | Typical Thickness | Core Loss (1.5T,50Hz) | Saturation Flux Density | Noise Level | Application Scenario |
|---|
| CRGO Silicon Steel | 0.23–0.35 mm | 0.8–1.3 W/kg | 1.85 T | Medium | Dry-type transformer, oil immersed distribution transformer, power transformer |
| CRNGO Silicon Steel | 0.35–0.50 mm | 2.4–4.7 W/kg | 1.75 T | Medium-High | Reactor, motor, small control transformer |
| Amorphous Alloy | 0.025–0.03 mm | 0.2–0.35 W/kg | 1.56 T | Low | Energy-saving amorphous distribution transformer |
5. Key Technical Parameters of Laminated Iron Core
- Stacking Factor: Normally controlled between 0.95 and 0.98. Higher stacking factor means higher effective magnetic cross-section; affected by sheet flatness, insulation coating and burr size.
- Air Gap at Joints: Step-lap staggered stacking is adopted to reduce effective air gap and cut magnetizing current. Uneven gaps will increase no-load loss and vibration noise.
- Burr Height: Strictly limited after shearing. Excessive burrs may cause inter-lamination short circuit and local overheating.
- Residual Stress: Shearing and punching produce internal stress, which deteriorates magnetic performance. High-performance iron cores require stress relief annealing treatment.
Insulation Coating: Each silicon steel surface is covered with inorganic insulation film to suppress eddy current loss inside the laminated core.
6. Standard Manufacturing Process of Silicon Steel Iron Core
- Raw material slitting: Cut wide silicon steel coils into narrow strips according to designed dimensions.
- CNC precision shearing: Punch or shear laminations into required shapes such as E piece, I piece and yoke piece.
- Deburring and inspection: Remove edge burrs and visually check surface coating damage.
- Stress relief annealing (optional premium process): Eliminate mechanical stress generated during cutting.
- Staggered stacking: Assemble laminations by step lap method to form complete core columns and yokes.
- Fastening and binding: Use insulation strapping to fix the whole core to avoid vibration during operation.
- Dimension inspection and magnetic performance sampling test.
All processes support customized size requirements. Manufacturers can adjust core width, window size, leg height and stacking thickness according to transformer design drawings.
7. Matching with Transformer Windings and Accessories
The design of iron core directly coordinates with enamelled purple copper wire windings. The core window size determines the maximum space available for winding arrangement. Designers need to balance core cross-sectional area, winding turns and wire diameter to achieve optimal copper loss and iron loss matching.
For dry-type cast resin transformers, the iron core must maintain sufficient insulation distance from windings and epoxy resin bushings. Special insulating brackets are installed between the core and coils to prevent induced potential and partial discharge risks.
Oil-immersed transformers install the iron core inside the fuel tank. The core needs reliable grounding, and the insulation structure matches oil-filled high voltage bushings.
8. Customization Scope of Transformer Iron Core
Most laminated iron core suppliers provide comprehensive customization services:
- Custom outer dimensions, core leg width, window height and width
- Select silicon steel material grade and sheet thickness
- Choose ordinary stacking or step-lap stacking structure
- Adjust stacking thickness according to transformer capacity
- Support special non-standard shapes for special transformers of wind power, photovoltaic and energy storage
Custom iron cores need to provide detailed engineering drawings, rated capacity, operating frequency and target no-load loss indicators.
9. Common Quality Control Standards
International projects generally refer to IEC 60076-11 for transformer core testing. Chinese domestic grid projects follow GB/T standards. Main inspection items include:
- Appearance inspection: no rust, coating peeling, obvious burrs
- Overall dimension tolerance detection
- Stacking factor measurement
- Sampling magnetic loss test
- Vibration tightness test after binding
Finished iron cores can be sampled for trial assembly matching with transformer windings to verify assembly compatibility.
10. Industry Trends of Transformer Iron Core
Combined with the development trends of transformers in China, three obvious trends exist in the iron core industry:
First, demand for ultra-low loss iron cores keeps rising. Energy efficiency standards push transformer manufacturers to adopt HiB grade CRGO and amorphous alloy materials.
Second, precision customization becomes mainstream. New energy special transformers require non-standard iron core sizes to adapt compact box-type structure.
Third, low-noise iron core processing technology is widely promoted. Optimized step-lap structure and stress annealing effectively reduce transformer operating noise for urban indoor substations.
Meanwhile, Chinese iron core manufacturers with size customization services continuously expand export business, supplying supporting magnetic components for IEC standard transformers worldwide.
11. Frequently Asked Questions
Q1: Why are transformer iron cores made of multiple thin silicon steel sheets instead of solid iron blocks?
A: Alternating magnetic flux induces eddy current inside metal. Solid iron produces huge eddy current loss and severe heat. Stacked insulated thin laminations limit eddy current within single sheet and greatly reduce energy loss.
Q2: What core material is preferred for dry-type distribution transformers?
A: CRGO laminated silicon steel iron core is the mainstream choice, matched with enamelled purple copper wire windings and epoxy resin bushings.
Q3: Can amorphous alloy iron cores be used for UHV large power transformers?
A: At present, UHV transformers still adopt CRGO silicon steel cores. Restricted by mechanical performance and manufacturing difficulty, amorphous alloy cores are mainly applied to medium voltage distribution transformers below 36kV.
Q4: What factors will cause increased transformer no-load loss?
A: Inferior silicon steel grade, excessive cutting residual stress, poor stacking process, large air gaps at splicing joints and inter-lamination short circuits caused by burrs.
12. Conclusion
As the magnetic circuit foundation of power equipment, transformer iron core determines the energy efficiency and stability of the whole transformer. Different silicon steel and amorphous alloy materials, plus diversified laminated structures, provide flexible solutions for distribution transformers, power transformers and new energy special transformers.
Reasonable material selection, precise manufacturing and flexible size customization help transformer manufacturers optimize design schemes. When matched with qualified enamelled purple copper wire windings and standardized epoxy resin bushings, the iron core supports the manufacture of high-performance transformers complying with GB and IEC standards. Facing the global energy transition and the demand for efficient power grids, high-quality customized laminated iron cores will maintain stable market demand in the power transmission and distribution industry.