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# Short solenoid/electromagnet cores - permeability, annealing, sources

## Short solenoid/electromagnet cores - permeability, annealing, sources

(OP)
I'd really appreciate anybody's help on this.

Can anybody give me an idea of how big a deal not having hydrogen annealing is on effecting the magnetic permeability of a short solenoid core cut from steel 1018 rod? Core length is 7-10mm, core diameter is 10mm.

Also, does anyone happen to know an equation for the B of the above short solenoid, or alternatively have a recommendation for an FEA software trial? I'm assuming it's significantly different than that of a long solenoid: B=u0*urel * I * turns/length. I've looked for hours and can't find one.

Lastly, if I end up needing an annealed or some NiFe alloy core, can anybody recommend a source for small prototyping quantities?

Thanks so much, I haven't done any magnetics design before.

Scott

### RE: Short solenoid/electromagnet cores - permeability, annealing, sources

How big of a deal the anneal is really depends on two factors, the operating frequency and the current anneal condition.
If the frequency is low (or DC) the initial perm is not a big deal, and the actual perm over a wider range will likely not be very different.
And if the material was reasonably well annealed after being rolled then it will have decent properties.
If one or both of these conditions is not met then I would look for a better material and a better heat treatment.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Short solenoid/electromagnet cores - permeability, annealing, sources

"Core length is 7-10mm, core diameter is 10mm". This is a too short core length to core Outside Diameter ratio. It will create mechanical stuck and erratic behavior of the core movement. The length has to be at least 1.25 times the OD (1.5 is better). In such cases a special design is needed where the core has an extended thin long concentric stem driving into a hole at the center in the solenoid.

### RE: Short solenoid/electromagnet cores - permeability, annealing, sources

(OP)
Thanks israelkk for the book recommendation, can't wait to check it out. There is no moving armature for the solenoid, it's being used to create a magnetic field along the lines of an electromagnet. But because of the short length, I haven't found the math to calculate B when there's a core. I have found math though for calculating L of the short solenoid with an air core written by Grover, but it did not yield close to the measured results with the steel core. The online calculators I've found have been significantly off compared to measured results for calculating both B and L with the core.

Thanks EdStainless on the annealing info, very helpful!

### RE: Short solenoid/electromagnet cores - permeability, annealing, sources

As far as the importance of the anneal, it all depends on what you are doing with this solenoid. Since you are using rather high carbon core material it can make a difference, particularly on remanance/coercive force. You could just use a better quality material. You can also do a vacuum anneal, it's nearly as good as a hydrogen anneal but won't pull the carbon out the way hydrogen will.

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