Magnetize and Demagnetize an Electromagnet

Hello BigEasy

It depends on the size of your electromagnet, and the type of installation you are using it on.

There is a simple way, and that is to incorporate an extra winding, through which you pass AC, after removing the DC supply, which will cancel the residual magnetism in the iron circuit.

The AC only needs to be on for a very short time, normally less than a second.

If you advise with type of installation, then it is easier to advise you.

Kind Regards....

The amount of reverse current depends on your material you used. try 1/10 to 1/30 by experience.

if you use -90dc, that is worst. this will make reverse magnetize.

the best bet is using a AC demagnetize coil. the operation is very simple, if you see by your eyes.

power depends on your magneize degree.

I suggest ou 'd buy one such coil on market.

If I remember correctly soft iron does not retain its magnatism.

QED?

Jack Jersawitz

Hello Jack Jersawitz

It would appear that BigEasy is not using pure soft iron for the complete magnetic circuit, and thus has the "residual magnetism" problem.

Perhaps if BigEasy replies as asked, with the details, better advice may be given.

Kind Regards....

Thank you all for the replies. So, according the Guest, I can reverse the polarity of my 90VDC and leave it on for, let say 1 or 2 seconds before it become a full -90VDC, right? By the way, the detail of this is I use a DC motor drive for this application, so the 90VDC is for forward that I use for the magnetization and the reverse is for demagnetization. My application is to lift a long steel bar and move it to where it suppose to be and drop it instantly. Please advice.

Ummm..

It would appear that Big Easy, despite his non de plume, is not here in the States or he would see such operations around the corner at every junk yard.

His response seems to make clear he is talking about a magnet suspended from a crane.

In that case residual magnatism, relative to the loads the crane is moving, is meaningless and not a worry.

One proviso! If big easy is talking about moving stuff like new structural steel, like in a yard I once worked at, he does not want to use a magnet at all; rather the proper cables and hooks, straps etc. In a junk yard where there is nothing below, like people who are going to work on that steel, a magnet is fine. In a location where other people are under, or near to under where the load is being handled, a magnet could be a disaster.

Am I right about your use Big Easy?

j.

This is the side view of the bundle.

That is the bundle of structure steel that I want to stack neatly. There are magnet heads in parrallel to hold a long layer of steel bars swing over by a hydraulic driven arm, then when the layer get to the top of the bundle, it need to be demagnetized cleanly before the arm swing back. Please advice. Thanks.

First in response to your previous comment - you clearly need to understand the basics much better. The reverse pulse will "ideally" reach 90V immediately and the current will increase somewhat linearly. Also the reverse pulse can not just be assumed to be 1 0r 2 seconds - in many applications it will be closer to 1 or 2 mS - this all depends on a variety of factors mentioned in my previous comment. The decaying AC suggestions will work well to demagnetize but all you are looking to do is drop the material.

Second and most important - stop all this before you hurt somebody at worst and at the least damage the material. Homemade electromagnetic lifts are not a good idea for what you are trying to do. They will magnetize your materials resulting in dust attraction that will hold moist causing rust - rot - mold - whatever - it is not good. ever try to remove iron filings from a magnet its not fun. More importantly that stuff is heavy and one bad solder joint, loose connection, overstressed wire or transistor, etc... and that stuff is falling and possibly injuring somebody or at the very least causing significant property or material damage.

Others have suggested you use cables and such to accomplish this task - take their advise.

I vote for guest for his detail description. and your act must be very trouble. and you cannt control it very well by hands.

the main procedure is not voltage control, the main content is depending on current.

you can use higher voltage or lower votlage, if you can control its voltage*second. you get your aim.

you seem to use electomagnetic crane to move the bars. if its AC crane(I dont know only guess), if you reduce the ac crurrent(voltage) the object will drop and during this time, they are demagnetized automaticlly.

Excuse me for taking up more space.

Nonetheless I note a number of comments from folks who seem not to have the slightest idea about electromagnetic principles or who thought the query was about something like a relay or magnetic switch.

I cannot stress too much that you are getting in over your head. With loads of steel that is very dangerous.

I don't know where you are but you should consult a load handling specialist of the sort I sent you a URL for.

j.

Thank you all for the advices. I really appreciated your effort. Again thanks. Good luck to all.

Hi, j,

If you just look at left you will see where I'm from. I'm waiting for your url, it may be helpful to me who is a nonprofessional at the field.

I say I haven't ever worked in such factory so far. I know only a little about the electromagnetic lift from my middle school textbooks. I know the crane has DC and AC model. when the electric current is on, the magnetic force will take up the iron material from ground and when it off, the object will fall down. that is the case. Correct? What dangerous are you meaning?

I note most of the suggestions here are useful and the participators are all familiar with the electromagnetics, although some of them don't know how to demagnetize and its principle. but the equipment has to be controlled by a switcher or relay. The key issue is how you control time and current and let it reduce gradually.

If you ever owned a tape recorder, you will understand what I'm saying.

Equipment is designed by man, we can design it according to our needing and prevent from dangerous.

CNPOWER,

The URL is above but here it is again:

http://productionmachining.com/articles/bar-handling-made-easy.aspca

Safety is first over all. The use of an electromagnet for lifting in a warehouse situation, all other issues aside, where there are usually persons working with the stock is that an electromagnet is not fail safe. Power fails, magnet ceases and releases, and load falls on whomever may be below or whatever may be below.

This opposed to strapping the load with cable or flat nylon straps, etc., power fails, load stops moving and stays suspended.

As for the AC/DC thing.

I hope you understand that electromagnets are DC operated because they require a steady magnetic field. When you see magnetic lifting equipment that may be rated for AC (Alternating current as opposed to Direct current) the AC is the input current to the entire device but would be rectified in order to provide DC for the magnet.

Where you are from, China, a great deal of technology has been developed at a much faster pace then it was originally developed in the West. Obviously, and I have seen pictures and articles of some horrendous things - for instance persons working what here would be a very crude foundry, in just a loin cloth and shoes, and pouring from hand carried ladles iron manhole covers destined for NY City - in large areas of China there has been very little penetration of that technology, or at least the theory behind it, and so when individuals seek to become rich in this new turn toward capitalist enrichment and industry, many are set to work with little knowledge and few tools and of course, being human beings, try to work out ways to solve those problems.

For anybody reading this in China, or any other of the countries now trying to rapidly develope industry, I would suggest that you use your access to the internet to seek out the tutorials that teach things like the basics of electricity, or of fluid mechanics, or any of the myriad things that you are being confronted with in trying to work and keep yourself together as opposed to agriculture.

Indeed, what I see is replication of the same horrors developing capitalist interests, in England for instance with very young children working the mines, that our industrial workers were put through here in the West.

I could say much more on these essentially social questions but I don't want to get anybody in trouble.

Jack Jersawitz

404-892-1238

bigjackjj@yahoo.com

A good day to you all. I am from South-Africa. I would like to know; how much weight of metal can be lifted per electrical charge with an electro magnet? In other words; how much energy is needed to lift how much kilogram/s of weight? Furthermore, is it easier to create a negative charge or a positive charge for an electro magnet? (which is more energy efficient?) Lastly, once charged; how long will an electro magnet remain magnetized even after the energy / electrical supply has been switched off? (for a negative charge and a positive charge separately explained for this question please)

Place a capacitor across the terminals of the electromagnet. This will form a resonant circuit powered by the collapse of the magnetic field in the electromagnet when the current is turned off, and a few cycles of alternating current will be produced, enough to reduce the residual magnetism very greatly. You will need to know the inductance of the electromagnet, and choose a capacitor of such a size that several cycles will be produced before the power stored in the coil is dissipated. The voltage rating of the capacitor should be several times the ninety volts you say you are using, as this inductor will produce a healthy spike when its power is interrupted.

However, the prospective use of an electromagnet in this application should be discontinued or abandoned because it is a dangerous way to stack materials. At least one hazard will arise from materials already stacked being dislodged by magnetic attraction to a load being put down by the energized electromagnet.

Degaussing arrangements based on the principle of resonant circuits have been used to maintain color purity in CRT's for years. They might even be useful in some applications that require the immediate neutralization of an electromagnet's magnetic field. That doesn't mean they should be used to promote or encourage a hazardous industrial operation.

Dear bubbapebi, your answer was impeccably correct. Congratulations!

My approach would be to use a two-stage circuit using diodes and capacitor(s) to achieve an automatic reverse discharge. I think [you] could do this on the cheap using COTS parts and components, but I cannot tell you the parts to use or suggest the circuit without knowing the currents and inductances of your electromagnet.

I'm thinking along the lines of a current-limited 90-Volt capacitor charging circuit in parallel with your electromagnet which ramps up the capacitor voltage when you activate the main electromagnet, so that the magnet strength also ramps up at the same rate (as the current into the capacitor decreases, the current to the magnet increases). When you cut the magnet off, it cuts out instantly and then Diodes, SCRs or maybe power MOSFETS switch the polarity of the Capacitor and discharge it in reverse across the Electromagnet at the same time that the electromagnet inductive kick-back discharge occurs. If synchronized, I think that this should cancel and remove the residual magnetism automatically without switching in AC or doing anything too fancy.

Independant on whatever the application is a complete demagnetization can never reliably done by only reversing the current or by applying constant AC for some time. When the current is switched off noone can guarantee that no small amount of magnetism is still there because you have no correlation between switchoff and AC phase.

I`ve worked a lot with ring cores which had to be demagnetized for precision measurement purposes. The only way for absolutely clean demagnetisation is to use an AC voltage as posted some times, but the magnitude has to be decreased slowly to zero as even a low AC field when broken at a phase angle <>0 will cause a bit of rest magnetism.
Regards Uwe

Not knowing what device you are employing this on, I can only suggest that you dispense with your core and just use a coil, as on an auto accessory drive clutch ( aircon etc). It'll let go as soon as the current is cut. What sort of space restriction do you have?

I will almost guarantee that the steel you want to move won't retain enough magnetism to hold onto your electromagnet once you shut off current to your electromagnet. The steel will stick to the electromagnet when current is applied in either polarity. You can try a demonstration of this with regular magnets and/or DC current relays.

However your idea is very dangerous as I must reiterate. With an electromagnet this strong you're gonna pick up steel debris from a long way off and it could come flying in at a high rate of speed. Electromagnetism is not a new idea and I'm sure similar ideas have been proposed before but have not come to fruition. I would investigate this idea much further before progressing any further.

If you're in the steel business, using electromagnet for stacking structural steel has been the solution for a long time and still nowaday (not all product) but they have different ways to use it depend on the architecture of the mill and depend on the operation design. By the way, I'm just trying to experiment new equipment (DC drive from ABB , GE or Reliance ...) on the old idea (using electromagnet) which is work for our business. If anyone have that experience, please share with me and others. Thanks.

I agree with the posters who said that using electromagnets in the manner described can be dangerous and detrimental to product integrity. However, I disagree that magnets and electromagnets are only suitable for scrap operations and not other material handling needs.

Magnets (moveable permanent and electromagnetic) may often be used for bulk loading and unloading of resilient manufactured parts and materials, such as simple steel stampings and machined parts. Often it is difficult, tedious, or even dangerous (sharp edges, tangling, etc.) to manually handle these types of items. Often they need to be transferred from bulk storage or shipping containers to assembly operations, conveyors, or other feeding equipment. If the transfer distance is relatively short, with little or no vibration, the parts may be transferred easily with little concern for dropped parts or product damage.

Magnets may also be used for stacking or destacking of nested or layered flat parts, once the appropriate magnetic force is determined such that one and only one part will be lifted. Electromagnets may be appropriate as well as permanent magnets that include a stripping, wiping, or other type of mechanical release mechanism. A very common magnetic tool is a hand lifter that uses a permanent magnet encased in a non-magnetic shell, such as aluminum or plastic. The operator is able to retract the magnet from the face of the shell, thus separating it from the load and allowing the load to release. After release, the magnet may be reset against the inside face by release of the spring-loaded retraction mechanism so that another load may be lifted. Good practice would include a simple shake on loading to release any parts back into the container that may be hanging on with only a slight attractive force, so that they might not drop in transit.

I will say again, though, I echo the sentiment that this is a poor application for the use of electromagnets, or any kind of magnetic lifter. The only good use for magnetism in lifting the parts shown would be in stacking/destacking individual parts, and then only for a short distance and not over anything or anyone who could be harmed if the load were to fall. If the parts are clean and smooth vacuum lifters would actually be preferred for that application anyway. Banding or palletizing the loads and lifting from underneath by fork or platform, or from on top by chain. cable, or sling would be the preferred methods to transfer these stacks of materials.

I disagree with the posters who said that using electromagnets in the manner described can be dangerous and detrimental to product integrity. I have worked in a US steel mill for 4 years both in the melt shop and the roll mill. The mill I work for is owned by Nucor who places safety as its priority. In these mills we use hundreds of magnets to handle scrap and finished material. We have no issues with residual magnetism in finished products. Also I would like to point out that all cranes with magnets have a bank of backup batteries to supply power to lower load safely in the event of a power loss. I have visited many other mills in the US and all use magnets because the temperature of the finished product is still warm enough to melt straps and the size and speed it is produced makes any other means of lifting to time consuming(especially in the sheet and bar mills). With enough magnets on a boom any lengths of steel rather it is bars, billets, blooms or sheets can be lifted at a very warm temperature very fast and safely, crane operators use the rule "Once it is not glowing you can use the mag on it"

An Electromagnet, when current is applied, developes a North Pole & a South Pole with respect to the Turns Ratio of the Winding. As long as the Posative vdc is applied, the Posative Pole, remains. When the VDC polarity is reversed, the N & S Polers change to the Oposite Ends, & should almost throw the metal it has picked up away. Trial & Error is an old teacher. Carl

Hi, BigEasy!

I think the answers thus far have lots of interesting possibilities when it comes to building kits, etc. for handling your question.

Why not just choose material for the electromagnet with a low magnetic hysteresis and a high coercivity? This will result in a good electromagnet that becomes nothing to speak of when the power is turned 'off'. You probably don't need to kick the magnetic flux back to zero instantly; but if you did due to a slightly higher material hysteresis, add a small heating coil around the outside of it. The heat will warp the low-hysteresis metal's atoms enough out of magnetic alignment to destroy is magnetism.

Low hysteresis means the metal can't retain magnetic properties well (in some cases no residual magnetism at all!) when the power is shut off; and high coercivity means that the metal is subject to a strong magnetic set of properties when power runs through the windings. These are not conflicting characteristics. Many magnetic materials possess both properties and could fill your bill.

Check out Google for a list of ferromagnetic or other magnetic materials with low hysteresis, high coercivity ratings. They don't have to be soft iron. Make your magnet, and away you go. The guys in the scrap yards are already using this kind of machine, probably made from an M6 iron-silicone alloy or something similar.

Keep it simple.

Mark

the thread goes so far,

Lets look at what is em lift or crane.

where is dangerous?

Hello cnpower

Electromagnet usage in a scrap yard, as your picture shows, gives no problem, providing all persons are not under, and well clear of the suspended load of scrap, so if electromagnet failure occurs, there is no possibility of death or injury.

Power failure with an electromagnet, means almost instant dropping of the scrap steel.

As I said in my earlier post, trouble occurs when the electromagnetic lift of steel is used in a factory or storage situation, and is regarded as unsafe working practice, apart from leaving new steel, or steel objects for sale, with the residual magnetism problem as stated earlier.

Hope that explains it for you.

Kind Regards....

Hi,Sparkstation,

Haha,I trust you. now that you also say it has dangerous, there it is.

The photo is from a book of 40years ago. I hv never seen such equipment in fact. However I think its more expensive for a home factory as well.

there are in fact many ways to move the object, one is as same as said above by most people, bind them with rope or cable as same as peasants binding cutted hay. its an simple way to lift and move the steel.

CNPOWER,

Interesting to hear you say you have never seen such magnet lifting apparatus. Nonetheless, given China's vast area and in relation to that area how little it is yet industrialized, I am not surprised.

Nor am I surprised to hear you speak of your enterprise as a "home factory."

That of course explained why you were talking about designing and building such gear. If that picture was from a book forty years old I can tell you that the equipment looks exactly the same today as then.

I live in Atlanta, Georgia in the U.S., and we have here any number of junk yards where old cars are crushed, bits and pieces of various metals trucked in, etc. I would bet within a ten mile radius of where I live we could find a couple dozen of those crane magnets. They are of course too big for the use your pictures suggest.

One of the pictures I posted was of a rolling gantry. Those things come in various sizes, are not awfully expensive, and indeed not that hard to build. Given an electric or manual chain hoist you would have it made.

I found your comment about a home shop of special interest because I remember Chairman Mao's exhortations for everybody to build a small steel furnace in their back yard.

Unfortunately, while masses of people can quite effectively build, say, a dam, masses of individuals working each alone at a small furnace in their yard does not work out although furnaces almost that small were common here in the States when it was still a British colony. Indeed, there are preserved such only about a hundred miles from here in the Birmingham, Alabama area. But then again, the American colonialists did not have to satisfy the huge numbers you there in China must.

In an art studio at Georgia State University, we did indeed have such a home-built furnace which could melt about a hundred pounds of metal at a time. Reducing iron ore in such a furnace would have worked too.

Regards,

Jack Jersawitz

Are you curious that I hanvt seen that thing? Im also suprising at your words at present. There are junk processing sites at your place dont mean there are such sites everywhere. Not everyone has seen that equipment, I believe.

I dont know if you have ever seen how to plant crop,say wheat etc, if not, then I say your country near starve to death? willl you laugh at my this words?

Im odd where I hve speaked that my enterprise is a home factory? Can you show me clearly?

Back to your words even if it looks like a home factory, is there something to do with you? Is it very funny?

Have you ever invested one cent?

To be truth Im very very poor indeed. I hvnt no job no more income, only earn 1 or 2usd/per day when do a labour job.

Could you so kind to find me a more income job in your district to solve my living problem at present?

Of cause I hv to introduce myself at first, I cannt even lift a 15kg thing to go up stair, but I can calculate 1+2 =3 and 2x2=5. very wise.

I can also operate remote controller of tv set to let people around to watch wonderful program.

Are you working in that art studio? you are painter? or craft artist?

haha, In my lab, I hve only a electric searing-iron which is using for welding pcb.

oh, jack,

I forget to tell you I cannt design the chain block, neither manual nor electric drived.

because I dont know strains of cable or chain material and stress and strengh of them. But the peasants in the district can manufacture such things, they qualified certification of iso9000 and exort many such sets to europe and north america. there may be many products in your district coming from them.

if yu hope to use the set to make em lift, I can offer you very cheaper one than you buy on market.

Regards

Hi all,

Just two points:

1. About safety: Even in the case you use slings or any other mechanical means to attach the load, usual safety measures recommends not to stand below the load under no circumstances, nor passing the load over people who may be working nearby. I think BigEasy is aware of this issue.
2. Regarding demagnetization, it depends on the magnetic permeability of the material. Of course, partial demagnetization can be made by using AC with decreasing current, but this method will get certain degree of magnetization (just look at any B-H magnetic hysteresis loop).

The only way to completely demagnetize the parts are to heat them above the corresponding Curie temperature.

Regards !!

I fully agree with bubbapebi #13:

<< Place a capacitor across the terminals of the electromagnet. This will form a resonant circuit powered by the collapse of the magnetic field in the electromagnet when the current is turned off, and a few cycles of alternating current will be produced, enough to reduce the residual magnetism very greatly. >>

A dying-out-circulating Energy is required for de-magnetizing.

This principle is used in "De-Magnetizers"

<< The Gregg #14

My approach would be to use a two-stage circuit using diodes and capacitor(s) to achieve an automatic reverse discharge. >>

This principle is used in "Manetizers" to ensure that no REVERSE current flows through Manetizing Coil to

de-manetize.

A Clamping Diode is used in parallel with "Manetizing Coil" to absorb all the Back-EMF-energy across the coil when power is diconnected.