back EMF? 1

[Hol]

I have built an electromagnet that is capable of attracting ferromagnets, from a small distance.
When this electromagnet attract's small pieces of iron or steel, I assume the magnetic field of the electromagnet "changes" it increases due to the piece of iron. But what is the iron's B = to?

 

[waross]

I don't see any back EMF in this question but the field strength may actually decrease due to the increased length of the flux path with an extra piece of ferrous material in the path. With a large air gap in the flux path, the effect of the iron in the flux path may be ignored in some instances. An exception may be an arrangement with a very long ferrous path combined with a very short air gap.
With a straight bar type electromagnet, if the ferrous path is doubled in a straight line so that the return path through the air is also doubled, the field strength will be reduced to one half by the added length of the air path. the extra length of the iron will reduce the field strength by a factor of about 0.9999
That is if you double the length of both the iron path and the air path:
If you neglect the iron in your calculations the field strength will be 50%
If you include the iron in your calculations the field strength will be 49.9995%
This is a simplified example that ignores fringing of the flux among other affects.
I doubt that the added magnetizing effect of your ferro-magnet will add enough magnetizing force to overcome the added length of the air path.
It could happen though if the ferro-magnet is stronger than the electromagnet.
Google electromagnets and permeability.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hol]

@waross ,
How so? Why can't -V(emf) be induced to resist the change of an additional field generated by the alignment of the ferromagnet's domains?
And if there is a decrease in field strength that "change" induces -V to resist the change, wouldn't it?

I assumed that -V can be induced only in the presence of a magnet, or another functioning electromagnet. However, a ferromagnetic material that is "attracted" is only attracted due to it creating it's own magnetic field from the electromagnet that causes "change" in some way so that -V(emf) is induced to resist it.

I am terribly confused, I might return back to my electrodynamics book to review + research this online.
Your input is greatly helpful please contribute more...

 

[Skogsgurra]

Have a look at

http://gke.org/pub2/files/DC relay switch-off demo 1.png

It shows what happens when a relay switches off and the back EMF is influenced by the armature's movement. It is a bit like the situation you asked about. Only the other way round.

It is difficult to measure the voltage when the coil is activated because the voltage source is stiff and you don't see much influence from the moving iron. Much easier to observe when the voltage source is switched off. Like it is in this case.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

That's a nice graph Gunnar. Thanks.
Quaestion:

It is difficult to measure the voltage when the coil is activated because the voltage source is stiff

Will the effect show up if you graph the current rather than the voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes. Definitely. But I hadn't done that when I did this measurement.
Will be off for a few days. I may do that when I come back. I have such a nice little DC current transducer - why didn't I think of that?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I vaguely remember an issue with the inrush current of a solenoid. As the core moved into position the inductive reactance increased dropping the current.
Another case of a similar effect in reverse of the effect the OP is concerned with.
In the past I have been concerned with the change in Magneto Motive Force as affected by a change in the Amp turns per inch when the length of the flux path changes. I hadn't considered the effect on the back EMF.
Now that I think about it it is clear that the reduction of current in an AC solenoid is a reflection of the increased back EMF due to the shortened flux path. Inductive reactance is frequency dependent and is zero on DC but the back EMF change will still be there.
Thanks for the verification of my thoughts.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter