I want to make a transformer with a core of laminated electrical steel sheets. I have some EI core sheets, 0.5mm thick. My design is for a gapped core transformer, and the shape of the gap is unusual, so I need to cut the sheets to get the gap I want. I am considering using water jet cutting as the way to cut the sheets. The water jet cutting shop has told me that the process will make the steel sheets a little rusty.

I will stack the sheets first and bond them with a non water soluble adhesive, so the rust will not be on every sheet, but on the surface of the stack of sheets. How much of a problem would rust on the surface of the sheets be? What effect would it have on the efficiency/core loss of the transformer?

Water jet cutting involves no heat from friction so there is no danger of the heat affecting the magnetic properties of the steel. Conventional cutting does involve some heat from friction, but whether this is enough to affect the steel, I am not sure. It seems to be a case of No Heat but Rust VS No Rust, but Friction/Heat. Any thoughts on which is the best option?

PS. The summary of the study at the link below says “The experimental results show that the iron loss of the specimen by water jet cutting is the lowest, but the magnetic induction under the low magnetic field is the highest” https://link.springer.com/article/10.1007/s11595-014-1076-3

However, this study says ”As a result of the test measurements, losses occurred in electrical steels cut with abrasive water jet are found to be higher than the other cutting methods. http://journals.sagepub.com/doi/abs/10.1177/0954405416666899

Hard to know which to follow, so for now I am just trying to think about the rust!

  • \$\begingroup\$ Personally I would go for the method that produces the best mechanical results (accuracy, surface finish). I doubt the difference in losses is huge. If you really need to push efficiency, then you're best off buying those papers and having a read through. For what it's worth, a Trumpf engineer at CWIEME told me that the edge effects of laser cutting were similar to those produced through stamping which makes the method well suited for prototypes if you plan to mass produce later. \$\endgroup\$
    – Jon
    Nov 16, 2017 at 17:14
  • \$\begingroup\$ Is there any way to include a post-cutting treatment to get rid of any surface rust? \$\endgroup\$
    – Joren Vaes
    Nov 16, 2017 at 17:21

1 Answer 1


All Insulators are dielectrics and visa versa. Transformer Steel uses silicate coating as an insulation between layers.

From what I have found in Distribution Transformers (DT's) < 10MVA and > 10kV is that Dielectric Breakdown Voltage (DBV) is preceded by Partial Discharge (PD) at a lower voltage and all PD is caused by contaminants in gas or liquids and air voids in solids.

Rust is a contaminant. So rust and any contaminants especially steel burrs and attached silicate dust from the edges is a severe cause of PD. Invisible [ppm] amounts of contaminants are the chief cause of BDV variability in oil during new oil samples.

I suggest any additive to prevent rust is necessary or an air knife.

"Further on, cutting increases the iron loss due to two phenomena. On the one hand by modifying the magnetization profile and on the other hand due to the increase of hysteresis loss through modifications in the micro-structure and stress state of the material. "

An Application-Oriented Approach for Consideration of Material Degradation Effects Due to Cutting on Iron Losses and Magnetizability (PDF Download Available). Available from: https://www.researchgate.net/publication/270282285_An_Application-Oriented_Approach_for_Consideration_of_Material_Degradation_Effects_Due_to_Cutting_on_Iron_Losses_and_Magnetizability [accessed Nov 16 2017]."

  • \$\begingroup\$ Thanks for your answer Tony. I think it will be hard for me to get rid of the rust, but if I recoat the rusty sheets with a dielectric coating in spray form, would that negate the danger of rust causing PD or DBV? \$\endgroup\$
    – EddieP
    Nov 17, 2017 at 15:22
  • \$\begingroup\$ Possibly. But if this low voltage, no problem for BDV. It depends on your specs and process quality, why can't they cut and clean to prevent rust with WD40 edge coating? Fill factor, fringe effects on magnetic losses, and E field hot spots. \$\endgroup\$ Nov 17, 2017 at 15:47

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