Need help identifying optimum material for electromagnetic core

[michaelpc]

Hi all

I'm not well versed in electromagnetics but I'm learning as I'm prototyping this product.

Essentially developing a C shaped, DC, electromagnet to hold down a steel door on the order of about 8 sq. inches, and to release it when power is pulled. My current steel core is getting magnetized so it's not releasing. I'm following the trail to soft magnetic materials and I think I've identified a couple candidate alloys, namely Permendur, Invar, products with about 50% iron, but I want to get your feedback.

Specs needed:
High strength (the door will impact the core itself
Very high flux density
Minimum residual magnetism
Lowest cost

Is there an alloy I haven't considered that might meet these goals?

Thanks much!

 

[djhurayt]

What material are you using now? Has it been properly annealed after cold working.

I would think for something like that any of the low carbo steel 1006, 1008, even something like a 1213 or 1215 as long as it is properly anneald after machining. The other thing you might thing about since you say the door will impact it would be some of the SS. With the low carbo steel you will have to plate them to keep them from rusting almost instantly. With the SS (ferritic or martensitic, not the 200 or 300 series austenitic stainless steels) you would not have to plate these which would likely wear/destroy the plating during repeted contact with this door.

Why does the door have to contact it?

 

[dgallup]

I've attached a good paper on the selection of soft magnetic alloys. I's a bit old but the principals are still completely valid.

Generally speaking, for DC applications where cost is a main consideration, most people just use low carbon steel like 1008 or even 12L14. It will have decent saturation flux density, the most important characteristic for high force. Now for your demagnetizing release characteristic, you want the lowest carbon content possible which puts you in the electrical iron family. These however, are quite soft, more expensive and hard to machine. The best way to get good release characteristics is to have a magnetic gap which will greatly reduce the effect of remenance. This does not have to be air, it can be a non-magnetic plating. Hard chrome plate and electroless nickel plating can both be used to create the magnetic gap and vastly improve wear resistance.

If you can't get enough flux density out of pure iron then you have to look to the iron cobalts but these are very expensive.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[EdStainless]

If it is small you might be able to find 1002, they use it for deep draw applications.
Machine it and then anneal it.
To handle the rusting and release you can pot it in epoxy, and make sure that the faces are covered with a thin layer.

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P.E. Metallurgy, Plymouth Tube

 

[michaelpc]

Thanks for all the ideas guys. So is it the carbon that retains the residual magnetism?

For annealing, does that have to be in an oven or can I torch anneal it?

djhurayt: to answer your question - I think it does given the application. I need a really fast pick up of the door as it comes in at a high velocity and the impact will ricochet the door away before it can catch without the dense flux right at connection. That's my experimental understanding anyway.

 

[djhurayt]

It will need to be a controlled vacuum furnace as you will need to hold the entire part at a temperature for a peroid of time, and then cool it at a specific rate until it reach a particular temp.

As a general recipe we use this with low carbon steels (1005, 12L14, etc.)
•anneal at 800 – 850°C for 1 hour in a dry hydrogen atmosphere
•cool at a rate of 100° per hour (max) rate to 500°C
•any cooling rate thereafter


As far as the impact, I suspect these things will get destroyed quit regularly. Can you not put some sort of damper cylinder at the end of the door travel? Or maybe some bumper cushions of some sort?

 

[djhurayt]

you may also want to try this

http://www.eng-tips.com/viewthread.cfm?qid=170413

 

[michaelpc]

djhurayt:

Thanks for the annealing - I'll list that as an experiment down the road if I must.

Regarding the referenced thread, thanks!! I'll try the non-polarized cap today!

 

[EdStainless]

We used to soak parts for days in dry hydrogen anneals just to lower the C levels.
If you allow impact against the core then it will have increased field retention.
It sounds like you need a polymer cover on the pole piece, to absorb some of the impact energy, protect the core, and provide an air gap for cleaner release. A high density urethane maybe?

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

 

[dgallup]

What is your design life? We once tried a polyester film as an air gap but it only lasted a few million cycles. That wasn't enough for us but it might be for you.

Carbon content and grain structure both affect residual magnetism. As EdStainless mentioned, long dry hydrogen anneals can de-carb steels. Cold work from your impact will undo all the "good" done by the anneal. A small air gap will go a long way toward reducing the magnetic sticking. A lot of our devices are built with just a 20 micron air gap, you don't even need that much but it's hard to control much less than that if the gap is truly air. That's why I suggested plating as you can just do 5 microns and get good release and wear resistance as well.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[michaelpc]

dgallup: Well, I think your air gap solved it for me! I'm using 100 microns (1x sheet of paper) and I'm getting everything I've been searching for! And I don't feel like I've sacrificed anything.

I'd still like to test the LC resonance djhurayt recommended, however I got lost in the instructions:

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measure the coil inductance L and select
a C value which resonates with L at no more than 100Hz
so when you turn off the current it generates an exponentially decreasing oscillation.
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How can I measure the coil inductance L? Is that a job for the multimeter or is a special tool needed for this?

Thanks again everyone!

 

[djhurayt]

You need a special meter called an LCR, you cannot do it with a stand VOM.

as EdSS said:
Measure inductance? We always just tried caps and keep going bigger until it worked.

 

[MagBen]

using low carbon steel, doing heat treatment etc, all make sense, however, for this simple application, I would go with a "dirty" approach, using whatever material you have in your hand (obviously it must be magnetic). The gap idea is a good one i would keep to decrease residual. keep in mind the force is also proportional to "contact" area, so if your B is lower, simply increase your mass. if the material is not annealed, simply increase your turns of winding. The gap will decrease the effective permeability drastically of your magnetic circuit, so the benefit from anneal for softness is very limited in this case.