When a transformer’s measured no-load loss (NLL) exceeds design values, the root cause is rarely the raw material itself. It is the invisible mechanical degradation during the shearing process. If your supplier only provides a mill MTC, you are effectively buying a “blind box.”
The Physics of Magnetic Degradation
CRGO (Cold-Rolled Grain-Oriented) silicon steel is high-performance because of its precise grain alignment. Mechanical stress—specifically from shearing and slitting—disrupts this alignment.
The Failure Chain:
- Mechanical Stress → Plastic deformation along the cutting edge destroys the magnetic domain structure, causing immediate spikes in hysteresis loss.
- Excessive Burrs (> 0.02mm) → High clamping pressure forces burrs to penetrate the C-5 insulation coating, creating inter-laminar short circuits.
- Circulating Eddy Currents → These shorts allow large eddy current loops to form, leading to localized overheating and a catastrophic increase in total no-load loss.
Engineering Constraints for Loss Control
To ensure the magnetic performance of the raw material is preserved in the final core, we enforce the following physical parameters:
- Burr Height Control: Strictly < 0.02mm. Maintained through high-precision carbide tooling and monitored every 500 cuts.
- Stacking Factor: > 97%. Minimizing air gaps through optimized pressure and flatness control, ensuring maximum magnetic flux density.
- Step-Lap (MSL) Joints: Implementation of 5-step or 7-step lap technology to eliminate flux congestion and reduce noise levels by 3-5 dB.
Beyond the MTC: Dual-Verification Testing
A mill’s Material Test Certificate (MTC) validates the coil, not the core. A reliable engineering report must include:
- Epstein Frame Test: Comparing sheared sample performance against raw coil data to quantify shearing loss (Exciting Power).
- Final NLL Report: Real-world measured no-load loss of the fully assembled core before it leaves the factory.
Technical Discussion
For design engineers struggling with abnormal excitation currents or NLL variances that don’t match the design math, auditing the shearing quality and joint geometry is the first step toward optimization.