A low-loss CRGO grade is important, but it does not automatically produce a low-loss transformer core.
Between the original CRGO coil and the finished transformer core, the material goes through slitting, cutting, handling, stacking, joint assembly and clamping. Every one of these processes can influence the final magnetic performance.
For transformer manufacturers, this creates an important distinction:
Material performance and finished core performance are not the same thing.
This is why transformer core loss testing should not stop at checking the CRGO mill certificate.
What Does a CRGO Mill Certificate Actually Tell You?
The mill test certificate, or MTC, provides important information about the original grain-oriented electrical steel.
Depending on the steel producer and specification, it may include:
- Material grade
- Sheet thickness
- Specific core loss
- Magnetic induction
- Coil dimensions
- Coating information
- Mechanical properties
This information is essential for incoming material verification.
However, it mainly describes the magnetic properties of the steel before it becomes a finished transformer core.
It does not show what happens after the material is slit, cut and assembled.
That difference matters.
Why Can Core Loss Increase During Processing?
CRGO is highly sensitive to manufacturing quality.
Its magnetic properties depend on grain orientation, coating integrity and controlled magnetic flux direction. Poor processing can partially reduce the advantage of using high-grade material.
Several manufacturing factors deserve particular attention.
1. Cutting Stress
Mechanical cutting introduces stress near the cut edges of CRGO laminations.
If cutting conditions are unstable, the affected area can become larger and magnetic performance may deteriorate.
Tool condition, blade clearance and cutting accuracy therefore matter even when the same CRGO grade is used.
2. Excessive Burr Height
Burrs are raised edges created during slitting or cutting.
When burr height becomes excessive, adjacent laminations may contact each other more easily, increasing the risk of interlaminar electrical conduction.
This can contribute to additional eddy-current loss and local heating.
For this reason, Chenfan Electric controls lamination burr height to below 0.02 mm.
The number itself is small, but the effect of poor burr control can become significant across thousands of stacked laminations.
3. Joint Geometry
The joints between the core legs and yokes are critical magnetic regions.
Poor mitre accuracy, incorrect overlap or inconsistent step-lap positioning can increase the effective magnetic reluctance around the joint.
A properly manufactured multi-step lap structure helps distribute magnetic flux more smoothly through the joint area.
But step-lap design alone is not enough.
The individual laminations must also be cut and positioned accurately.
4. Stacking Quality
A transformer core is not simply a pile of CRGO sheets.
Lamination alignment, stack thickness, joint positioning and mechanical stability all affect the final structure.
Low stacking density may indicate unnecessary gaps or inconsistent assembly.
At Chenfan Electric, the stacking factor is controlled at above 97% for applicable transformer core designs.
5. Mechanical Stress During Assembly
Excessive clamping force or improper handling can introduce additional mechanical stress into the core.
This is particularly important for large transformer cores.
A core that performs correctly during production should also maintain its geometry and mechanical condition during lifting, packing and transportation.
Why Epstein Testing Is Useful
The Epstein test is widely used to evaluate the magnetic properties of electrical steel samples under controlled conditions.
For transformer core manufacturing, it can serve an important quality-control function.
Instead of relying only on the original steel mill data, samples from processed material can be evaluated to determine whether slitting or cutting has caused abnormal deterioration.
This creates another inspection point between raw material and finished core.
A practical quality-control chain may therefore include:
CRGO mill data → incoming verification → processed material testing → finished core verification
Each stage answers a different question.
The mill certificate tells you what material was purchased.
Processed material testing helps determine whether manufacturing has damaged that material.
Finished core testing shows what the complete manufacturing process has actually produced.
Why Finished Transformer Core Testing Matters Most
For a transformer manufacturer, the final concern is not the theoretical performance of an individual CRGO sheet.
The concern is the behavior of the assembled magnetic circuit.
This is where finished transformer core testing becomes valuable.
Depending on the core design and agreed testing procedure, an assembled core can be evaluated before shipment for its actual magnetic performance.
This can reveal problems that material certificates alone cannot identify, including:
- Unexpected building loss
- Joint-related magnetic loss
- Assembly deviations
- Abnormal excitation behavior
- Manufacturing damage
- Inconsistent stacking or clamping
At Chenfan Electric, finished transformer cores undergo magnetic performance verification according to the applicable project and agreed test requirements before shipment.
For critical projects, testing at multiple excitation levels can also provide additional information about performance stability.
What Is the Transformer Core Building Factor?
Transformer engineers often evaluate the difference between the theoretical material loss and the actual loss of the assembled core.
This relationship is commonly discussed using the building factor.
In simple terms, it reflects the additional loss introduced when CRGO laminations are converted into a complete transformer core.
A good CRGO grade can still result in an unsatisfactory building factor if manufacturing quality is poor.
Factors influencing the building factor include:
- Lamination cutting quality
- Joint design
- Step-lap accuracy
- Burr height
- Mechanical stress
- Stacking accuracy
- Core geometry
- Material handling
This is why comparing suppliers only by CRGO grade can be misleading.
Two manufacturers can use nominally similar electrical steel and still produce transformer cores with different finished performance.
What Should Transformer Manufacturers Check Before Accepting a Core?
When sourcing a CRGO transformer core, several inspection points should be considered together.
Material Traceability
The supplied CRGO grade should correspond to the project requirement and supporting material documentation.
Lamination Accuracy
Dimensions, mitre angles and step-lap geometry should match the approved transformer core drawing.
Burr Control
Burr height should remain within an agreed manufacturing limit to reduce the risk of interlaminar problems.
Stacking Factor
A consistent stacking factor provides useful information about assembly quality and effective core cross-section.
Finished Dimensions
Window dimensions, leg centres, stack thickness and overall geometry should be checked after assembly.
Magnetic Verification
For projects where magnetic performance is critical, finished core testing provides much stronger evidence than raw-material documentation alone.
Material Quality Is the Starting Point, Not the Final Result
High-quality CRGO is essential for an efficient transformer.
But purchasing premium electrical steel does not remove manufacturing risk.
The final transformer core is the result of several interacting factors:
CRGO grade + cutting accuracy + burr control + joint geometry + stacking quality + mechanical control + final testing
Ignoring any one of these can reduce the benefit of using better magnetic steel.
This becomes increasingly important when manufacturing larger power transformer cores, low-loss distribution transformer cores or projects with strict no-load loss requirements.
From CRGO Coil to Verified Transformer Core
A reliable transformer core manufacturing process should create traceability from raw material to finished assembly.
Chenfan Electric manufactures customized CRGO transformer cores for oil-immersed and dry-type transformers based on customer drawings and technical requirements.
Our manufacturing control focuses on measurable parameters rather than material grade alone, including:
- Burr height below 0.02 mm
- Stacking factor above 97%
- Multi-step lap core construction
- Dimensional inspection after assembly
- Magnetic performance verification according to project requirements
For transformer manufacturers, the objective is straightforward:
Do not evaluate a transformer core only by the CRGO written on its material certificate.
Evaluate what the manufacturing process has done to that CRGO.
Because the transformer does not operate with a certificate.
It operates with the finished core.