I have a question regarding the permeability for a material, if I have an electromagnet of two different materials. So also two different values for permeability, does the lowest value for permeability apply. Or does this not matter?
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With electromagnets usually the path of the magnetic flux is partly through the air, which is then the lowest permeability encountered.
You may be right that as a first approximation the lowest permeability in the magnetic path will count to compare different cases (although that does depend on the precise geometry, which you do not describe). But since this lowest permeability is that of the air, the permeability of the other materials might still not matter very much. At least as long as it is much higher than that of air (so $\mu_\text{r}=100$, or $1000$, or $10000$, might not give very much difference. Definitely not $10\times$ or $100\times$ difference...)
If, however, you pick up metal and it fits to the yoke without much airgap, then the magnetic materials will become dominant and in your example you will see the mix of the two different permeabilities. So in conclusion: for picking up material from some distance it will not matter too much, but after picking it up the holding force may depend more strongly on the materials. And then your assumption that the lowest permeability will give a pessimistic (but safe) estimate. If the lower permeability is only a small fraction of the material the result will of course be better.
answered May 28 at 7:39
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$\begingroup$ If you would have a standerd E core with a diameter of 100 mm and a height of 50 mm, but the tips of the E core are made from a hardere material (to prevent wear). Lets say that the core is made of carbon steel wich has a relative permeability of 100, and the tips of the core are made from hardend steel wich has a reletive permeability of 40. $\endgroup$– laurentCommented May 28 at 8:08
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$\begingroup$ See added remarks in my answer! My feeling would be: not much difference for picking up, but for the holding force at most in the order of 10 percent (if the tips are a reasonably thin layer of added material). To be sure you have to compute it with a numerical solver using the precise dimensions... (and still there will be two cases: with and without much air distance between the magnet and the object) $\endgroup$ Commented May 28 at 8:40