Electromagnet: lamination vs. solid core 1

[Mech133]

Right now I am using a coil that is wound onto a bobbin that is .300x.300" footprint and .200" window height. the core area is about .100x.100. The coil is wound to 30ohms and has about 600 turns. I am pulsing 500-800ma upon activation. Currently I am using silicon steel laminations as a core material. From what I have read, laminations help mainly with eddy currents in AC operation. This coil is strictly DC. My question is will I see any increase in field strength with a solid soft iron core as opposied to the laminations?

 

[MagMike]

Yes, you will see an increase in field strength with a solid iron core, even more if you anneal the core.

 

[Mech133]

Thank you for the information, is there a specific type of iron that gives the best results and are there specific annealing temperatures that I would need to specify?

 

[MagMike]

Go with as pure an iron as your budget allows. Cold rolled steel is cheap (relatively speaking) but contains approximately 0.18% carbon. Steels with less carbon are harder to source and generally cost more if/when you can find them. You'll get maybe 5 to 10% improvement going from cold rolled steel to (nearly pure) iron. The price will go up more than 5 to 10% though.

The annealing recipe generally calls for baking in a vacuum or dry hydrogen atmosphere at 1650degF (900degC) for 1 hour minimum. You'll also get a 5 to 10% improvement with annealing. Again, economics will determine if that is cost effective for you.

 

[CarlPugh]

A pulse is not DC.
What is the width of your pulse?
For short pulse widths (Say under 8 millisecond) a laminated core is usually advisable.

 

[Mech133]

Electrical isn't my strong suit but why can't a pulse be DC? It is powered off batteries and the pulse width is 60ms, it is only energized about once every 5 minutes in operation.

 

[MagMike]

Mech133,

CarlPugh raised a good point (and kudos to him for catching that in the original post). A 60ms pulse is not DC. It takes a finite amount of time for iron to respond to a magnetizing pulse. I don't think you'll get maximum output from iron in that time-frame. You can experiment to find out, but you'll probably have to stay with the silicon steel laminations.

 

[Mech133]

I have experimented briefly with using iron and my results seem to agree with what you have posted. I understand what you mean now that a pulse is not DC operation. I did indeed notice a delay in response time, however that delay was within acceptable limits. What I also noticed that led to my initial posting was a solid steel core provided higher flux at the same current level and responded at much lower current levels than the laminations. I could not positively confirm this as my experiemnt lacked absolute controls to isolate the core material. Would it be fair to say the steel core would result in a higher flux but the silicon steel will reach max levels faster?

 

[MagMike]

Mech133:
That is a good way to put it.

 

[israelkk]

The process is neither AC nor DC. It is a transient electrical and mechanical behavior of the electromagnet until it reaches steady state current condition (DC).

In the last 30 years I have developed low and high force and low and high speed ON-OFF DC solenoid valves. In addition I developed many electromagnets using only solid core low carbon alloys. I have never needed pure iron (ingot iron) and never laminated core.

Valve speed ranged from 1 msec to 100 msec (mechanical plus electrical) response time and valve and electromagnet forces ranged from 100 gramf to 100 kgf.

The disadvantage of a low carbon alloy is the residual magnetism when the current is reduced to zero. However, the much higher saturation flux density than silicon steel lamination steel more than compensates this disadvantage.

http://israelkk.googlepages.com/home

 

[Lakey]

In my opinion, Isrealkk is spot on. A star for you mate.

Regards,

 

[EdStainless]

Just a few comments.
Another reason that a solid core will give you higher field is that there is more metal. Laminations have a packing factor, the air between them does not help you.
There are multiple reasons to anneal. It will raise saturation (slightly), it reduces residual field (important if you really need the device to turn off), and it results in more uniform behavior (Depending on how sensitive you unit will be to field stregnth variations). If you don't need it , don't do it.

There is low carbon steel available that is not special magnet steel. There are commercial uses for 1002 and 1005 out there. I don't have any contacts for you, it has been too long.

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http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Mech133]

I sincerely thank you all for your help and it validates what I have been suspecting for some time. The residual magnetism is not an issue in this application. I imagine the part will have to be annealed after it is cut to the proper core shape for maximum effectiveness? Any increase in flux would be desired as I am trying to squeeze as much out of this coil at as small current levels as possible.