EI Transformer Core in CRGO Silicon Steel Black Sheet Laminations

EI Lamination Sets Made from Grain-oriented Material

EI transformer cores built from grain-oriented silicon steel laminations are used where a small 50 Hz or 60 Hz transformer must show low no-load loss. The laminations are punched from thin, fully processed oriented strip, then annealed and finished in the black, oxide-coated condition that gives the product its characteristic dark surface. The set is assembled as an E stack closed by I laminations, with the coil wound on the centre leg before the I pieces are fitted.

Why Silicon Additions Raise Resistivity

The silicon in electrical steel is added for one physical reason: it raises electrical resistivity. Plain iron has a resistivity of roughly 10 micro-ohm-centimetre, while a 3% silicon iron used for oriented grades reaches approximately 45 to 50 micro-ohm-centimetre. Eddy-current loss falls in proportion to the resistivity increase, so silicon is the single most effective alloying addition for reducing core loss. Silicon also reduces magnetocrystalline anisotropy when the grain structure is controlled, and it lowers the magnetostriction that generates noise in transformer cores.

There is a limit: silicon above about 3.5% makes the strip brittle and difficult to cold roll and punch, which is why commercial oriented grades stay near 3.0% to 3.3% silicon.

Black Surface Finish and Inter-laminar Insulation

The term black sheet describes oriented strip annealed at high temperature without an applied coating. The dark surface is an oxide film formed in the annealing atmosphere; it gives a degree of inter-laminar resistance and acceptable appearance, but it is not a controlled insulation system. Where lower core loss is required, a C5 or C6 type coating is applied, providing a defined inter-laminar resistance that survives shearing, stacking and, for C5 products, stress-relief annealing. Coating thickness is kept small, typically a few micrometres, because it occupies space in the stack and reduces the lamination factor.

Loss Data and Stacking Parameters

Grade naming follows the IEC 60404-8-7 convention: the digits after the material letter give nominal thickness in hundredths of a millimetre, and the trailing digits give the guaranteed maximum specific loss at 1.7 T and 50 Hz in units of 0.01 W/kg. Typical lamination factor after stacking is 0.95 to 0.97 for coated strip. Product is routinely supplied to GB/T 2521.1-2016, EN 10107 or ASTM A876/A876M, with loss and induction measured by the Epstein method of IEC 60404-2.

Stamping, Deburring and Annealing Practice

Oriented strip is sheared and punched with sharp, correctly clearanced tooling; a blunt die produces burrs that short adjacent laminations and destroy the benefit of the coating. Burr height is normally limited to a few hundredths of a millimetre. Where the design calls for stress-relief annealing, the cycle must be matched to the coating class so that the insulation is not destroyed, and the parts must be supported flat to avoid warping. Because oriented steel loses magnetic quality when bent or stressed, laminations should not be re-flattened after punching, and assembly pressure on the core must be controlled.

Frequently Asked Questions

Q: What does the R in a grade name such as 23R090 indicate?
A: The letter denotes the material family, the digits before it give the nominal thickness in hundredths of a millimetre, and the trailing digits give the maximum specific total loss at 1.7 T and 50 Hz, so 23R090 is 0.23 mm material with a 0.90 W/kg limit.

Q: Is black annealed sheet the same as coated oriented steel?
A: No. Black sheet relies on the natural oxide from high-temperature annealing. Coated strip such as a C5 product carries an applied insulation layer with defined inter-laminar resistance and better annealing resistance.

Q: What lamination factor can be expected for EI sets?
A: Between about 0.95 and 0.97 for thin coated laminations. Uncoated or heavily burred parts reduce the factor and so reduce the effective magnetic cross-section.

Q: Why is silicon added to transformer steel?
A: It raises electrical resistivity from roughly 10 micro-ohm-centimetre in plain iron to about 45 to 50 micro-ohm-centimetre at 3% silicon, which reduces eddy-current loss, and it also lowers magnetostriction.

Q: Can punched laminations be stress-relief annealed?
A: Yes, provided the coating class is selected for it, typically a C5 type. Annealing temperature and atmosphere must be controlled, and the parts must be stacked flat to prevent warping.

Q: How should incoming EI laminations be inspected?
A: Check thickness and burr height, verify the coating type and adhesion, confirm dimensions and lamination factor, and cross-check the mill loss certificate measured to IEC 60404-2 at 1.7 T and 50 Hz.