CRGO 0.30/0.27 mm M4 Electrical Silicon Steel Coil for Transformer Cores

What Makes CRGO Different

CRGO is cold rolled grain-oriented silicon steel: strip with about 3 % silicon that has been processed so that secondary recrystallisation produces a Goss texture, aligning the easy magnetisation axis of the crystals with the rolling direction. Along that axis permeability is high and hysteresis loss is low, so a transformer core built from CRGO strip dissipates far less energy than one built from non-oriented steel of the same mass. Across the strip the properties are deliberately worse, which is why laminations are always laid so that flux travels along the rolling direction.

The strip is finished with a forsterite glass film formed during the high temperature anneal and then a phosphate based insulation coating. That coating system puts the steel under controlled tensile stress, which narrows the magnetic domains and lowers loss further, so coating condition is part of the electrical specification.

Thickness Choice: 0.27 mm versus 0.30 mm

Eddy current loss rises with the square of thickness, so 0.27 mm strip has a lower eddy component than 0.30 mm of the same grade, at the price of a slightly lower stacking factor and more laminations to handle. Distribution transformers with tight no-load loss guarantees therefore move towards 0.23 mm and 0.27 mm, while 0.30 mm remains the workhorse for general power and distribution cores. High frequency reactors and instrument transformers go thinner still. The decision is made on total owning cost: the saved watts over the life of the transformer are compared with the extra material and labour cost.

Grade Families and Typical Values

Values are typical supplier data for the grade classes and are quoted at 1.7 T and 50 Hz, the standard reference point for grain-oriented material. European designations in EN 10107 encode the loss in the grade name, where the number is the maximum specific total loss at 1.7 T and 50 Hz multiplied by 100 and the suffix identifies conventional, high permeability or domain refined material. IEC 60404-8-7 gives the parallel international specification and ASTM A876/A876M covers flat rolled grain-oriented silicon iron in the fully processed condition, quoting grade limits for loss and induction at defined test conditions. Loss is checked on Epstein specimens to IEC 60404-2 or by single sheet tester to IEC 60404-3.

Cutting, Stacking and Core Building

CRGO strip is unforgiving of rough handling. Shearing creates a stressed band along the cut edge where loss rises sharply, punching produces a wider damaged zone, and bending at a corner disrupts the texture. Burr height must be kept low, both to limit loss and to avoid short circuits between laminations. Where the design allows, cut laminations for wound and C-type cores are stress relief annealed after forming, but the anneal schedule has to be compatible with the insulation coating. Mitered 45 degree joints, step-lap staggering and controlled clamping pressure then determine how much of the material advantage survives into the finished core.

Applications

The main uses are power and distribution transformer cores, reactors, instrument transformer cores, current transformers and magnetic shielding. Where the requirement is lowest possible no-load loss at moderate rating, domain refined material competes with amorphous strip; above a few megavolt-amperes, grain-oriented CRGO remains the standard choice because of its higher saturation induction and better stacking factor.

Frequently Asked Questions

Q: Is 0.27 mm CRGO better than 0.30 mm?
A: Loss is lower because the eddy current component scales with the square of thickness, but the stacking factor is slightly lower and more laminations are needed, so the choice is made on total owning cost.

Q: What does the M4 designation mean?
A: It is a classical grain-oriented grade name from the M series used in procurement alongside modern European and Japanese designations, and it should always be ordered together with an explicit thickness and a loss limit at a stated induction and frequency.

Q: Why must laminations be laid in the rolling direction?
A: The Goss texture gives low loss and high permeability only along the rolling direction, so any flux crossing the strip at an angle sees much higher loss.

Q: Can CRGO laminations be laser cut?
A: They can, but the heat affected zone adds local loss, and cores with a very tight loss guarantee usually specify shear cutting with controlled burr, followed by stress relief annealing where the design permits.

Q: What coating protects the strip?
A: A forsterite glass film formed in the high temperature anneal plus a phosphate based insulation layer, together providing inter-laminar insulation and the tensile stress that reduces domain width and loss.

Q: How is the delivered loss verified?
A: The strip is characterised on Epstein specimens to IEC 60404-2 or by single sheet tester to IEC 60404-3, and the finished core or transformer is verified by the no-load loss test specified in IEC 60076-1.