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Magnetic Field Still Present in Solenoid When Powered Off?

01/23/2015 6:14 PM

I'd made a simple solenoid with carbon steel casing and plunger. When the plunger was fully inside the solenoid, I'd a close magnetic field circuit and very strong pulling resistance force with the coil energized. When I switched off the current, I was surprise that there was quite a significant retention force left required to pull out the plunger. But I went put the plunger back inside the solenoid also without current, the high retention force disappeared (or very negligible). I could repeat this behavior many times.

I wonder if this phenomenon was more likely be due to energy stored as magnetic field from self-inductance of the solenoid OR from the remnant magnetic field of the carbon steel? The former could explain why after the plunger was pulled out from the solenoid the retention force disappeared. I'm not sure about latter.

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#1

Re: Magnetic field still present in solenoid when powered off?

01/23/2015 7:14 PM

Check out magnetizing/demagnetizing hysteresis curves for various materials. Dead-soft iron gains/loses magnetism the best; carbon steel gains/loses less well.

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#2

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/23/2015 9:39 PM

The concept is called "residual magnetism". Look it up.

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#3

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 1:04 AM

Thank you for your suggestions about residual magnetism and hysteresis curves of various materials. However, it is to my understanding to demagnetize the materials I need to reverse the H field. In my experiment I only removed the plunger from the solenoid body and put it back in again without any current. With that single movement the remnant force disappeared significantly. Only a small residual magnetic force was left as I would expect. By moving the plunger out of the solenoid, could anyone explain how the materials got degaussed?

Furthermore, as the solenoid acts like an inductor what happens to the energy stored in the system when I cut off the current?

Thanks for your help.

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#5
In reply to #3

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 2:20 PM

As to your second question, the energy stored in the coil is dissipated as heat as in burning the contacts of the switch or I2R as heat in the diode you should have used across the coil to ground the negative going potential transient. (assuming a DC coil)

Now, to your first question, I've never thought about this before, but you are moving a magnetic field (the residual magnetism of the plunger) through a coil, which will induce a current in the coil and an opposing magnetic field.

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#6
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Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 2:31 PM

this is totally off the topic, but I just noticed that the super-script and sub script function in the editors do not seem to work. When I try to write "I squared R" as I2R it looks fine in the editor but when I publish it it comes out as you see it

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#17
In reply to #6

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/26/2015 2:00 AM

I2R better than I used to be...

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#18
In reply to #6

Re: Magnetic Field Still Present in Solenoid When Powered Off?

10/03/2019 6:58 AM

Off topic,but many users may find this link beneficial.

Use the ALT ASCII character set.Turn on Num Lock.Hold down ALT key,and enter the ALT ASCII CODE for square,which is ALT 0178.(For example ²)

Here is a link to ALT ASCII CODES that you may find useful.

I find ALT 255 very useful to create folders that are hard to find at first glance.It is the code for space,but a file cannot be named space by using the space bar,but it can be by using the ALT code.

https://usefulshortcuts.com/alt-codes/maths-alt-codes.php

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#13
In reply to #3

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 10:46 AM

It lost some of the field because you changed the amount of iron available as a path for the field by removing it from the solenoid body.

I would suggest you take a look at a simple experiment by Ed Leedskalnin called the perpetual motion holder. Anyone can do it, and I'm not suggesting that Ed's description of what is happening is scientifically correct. However, doing it is very informative!

here is one such demonstration:

https://www.youtube.com/watch?v=Oa4oBUBSEzs

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#4

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 7:58 AM
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#7

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 9:22 PM

The effect will be caused by both residual magnetism and commutation of the coil upon collapse of the magnetic field which induces a field in the core and plunger such as to aid the original field and thus hold the plunger in place.

If the plunger was able to make physical contact with the insides of the core, then residual magnetism in both the core and plunger would be the likely contender for the difficulty in removal of the plunger after switch off.

If a return spring does not produce a fast enough release (obviously the spring has to be weak enough that it does not restrict the pull in power of the coil), a small button magnet placed at the inner end of the plunger's travel and with its magnetic poles in opposition to that of the induced magnetic field will assist in the release of the plunger as will a small winding wound under the main coil and with its turns in the opposite direction and its ends connected together.

Whilst a commutating diode across the coil will protect the switch from arc damage, it will also increase the hold in effect and therefore release time of the coil.

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#10
In reply to #7

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 5:07 AM

I forgot to mention that I'd power off the coil by simply disconnecting the terminal. I don't know if this relevant to our discussion. I could follow the effect of the residual magnetism up to the point when the power was removed. However, I would also expect this effect remains more or less unchanged when after removing the parts and put them together again without re-powering the coil. But I see significant immediate drop in retention force (example 3N to <<1 N).

I think the answer regarding store energy theory from UNIQUE48 (#8) is an interesting one. For this it would imply contradictorily materials with lower coercivity should exhibit higher retention force after removing the current. The close loop magnet field formed was able to self-maintain to some degree better. And when the parts were removed, there was only insignificant residual magnetism left in the parts that could explain a sudden drop in retention force when the parts were put together.

However, I can also imagine it may be a combination of materials and stored energy theory. Not sure how to prove the stored energy theory though.

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#8

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/24/2015 9:42 PM

once you take off the supply the energy retained in inductive circuit as magnetic energy and the effect to take as electromagnet till the discharge occurs

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#9

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 4:05 AM

Check out Ed Leedskalnin's 'perpetual motion magnetic holder', an interesting instance of a 'solenoid core' held across a horseshoe with great holding force, and once removed, the force is gone.

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#12
In reply to #9

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 7:40 AM

Great input. Just saw a video of Ed Leedskalnin's 'perpetual motion https://www.youtube.com/watch?v=V14uuh92cz4. Tthis effect confirms the stored energy theory to be the reason as I've described previously. Therefore it's not due to residual magnetism in the classical sense as I also initially thought. Thanks to this forum for helping to resolve this problem!

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#11

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 7:34 AM

All from memory! So here goes:-

If you make a magnetic "short circuit", that will usually slow/stop the armature (depending upon the contact area and the type of metal.... returning to its original position. What used to be done, was to place a tiny nipple, usually made of copper if I remember correctly, to stop a magnetic short circuit. It holds the armature a tiny bit open and I mean tiny!

You can test for this by placing a piece of paper between the armature and the plunger, but as you did not post a picture, I leave it up to you to decide where to place it to stop the magnetic short circuit....if it then quickly disengages, you know where the problem lies!!

Best of luck.

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#14

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 2:09 PM

You may try reversing the polarity, for a fraction of a second, before disconnecting the terminal (by using a two pole toggle or so?). If this demagnetizes the armature, then you get some clarity, right?

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#15

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 4:45 PM

I am unsure of the veracity of my comments and thoughts. I could not find a definite relationship online, but I hope that the OP can test my idea using a piece of paper and report back here asap....

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#16
In reply to #15

Re: Magnetic Field Still Present in Solenoid When Powered Off?

01/25/2015 6:28 PM

I can confirm I had put a very thin plastic foil 0.05mm between the core and plunger to prevent metal-metal contact and indeed the retention force went significantly down to less < 1N. I also noted that although significant force was needed to pull the plunger out when the parts were in direct contact but if I disturbed the unit with my finger slightly, the retention force dropped suddenly. I think the self sustained magnet field is in an unstable state and can be destroyed by small mechanical disturbance.

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