Quick Electromagnet Question

But if it is enameled, it wouldn't make contact with the battery, and no current would flow. If he had scraped off the enamel, he would know it was insulated!

Well, it is obvious he made contact somehow. Why don't we ask the OP for clarification?

He's not replying. Let's assume he is right. A shorted out winding will produce a magnetic field approximately the same as a straight piece of wire will.

Sure it will, but given the limitations of his power sources he would not have observed that "...the magnetic field was SOOO much stronger...," in his words. His 'D' cells will source only 5-15 amps, max, because of their internal resistance, and a single turn at this current would not produce a strong field; much less a "SOOO much stronger" one.

This is an easy experiment to duplicate. Why not give it a try?

Are you hearing voices? The author said nothing about a HV supply, and did not say 5-15 amps either. I have made electromagnets in the past using batteries. I do not believe D cells are limited to 15 amps. Try it yourself.

I hear voices all the time. You were saying...?

Thanks guys for all of the posts. I thought it would take much longer for anyone to answer. After I read your comments about magnet wire being insulated, I went back and made sure that the wire I was using was uninsulated, which it was. So I'm going to radio shack tomorrow to pick up some magnet wire and a few resistor ranging from 5 - 40 ohms.

Thanks again.

Stephen

"The fact that you managed to get a reasonable electromagnet working using a couple of D cells suggests you were lucky enough to have either some nice, evenly spaced turns in your coil together with a very dirty nail, or some very dirty wire."

Hi John,

My thoughts exactly. Were the turns shorted, the current through the bulk of the coil would travel parallel to the nail and the magnet would not exhibit the kind of pull the OP described.

"Check out the resistance per unit length of the (insulated magnet) wire, and put enough on the nail to get you up to about 20Ω."

The OP may wish to use a smaller wire gauge for his next magnet. 22 AWG wire has a resistance of 16.2 ohms per thousand feet. For $6.59 Radio Shaft Shack sells a magnet-wire set containing three spools: 40 feet of 22-gauge wire, 75 feet of 26-gauge wire and 200 feet of 30-gauge wire. 200 feet of 30-gauge wire would put him in the ballpark of 20 ohms or so:

40 feet of 22-gauge wire @ 16.2 ohms/kft: 0.65 ohms

75 feet of 26-gauge wire @ 40.8 ohms/kft: 3.06 ohms

200 feet of 30-gauge wire @ 103.2 ohms/kft: 20.64 ohms

I would also urge the OP to contact an electric-motor repair/rewinding shop in his area for wire. Oftentimes these folks will sell "spool ends" of a desired gauge at or below cost. I've gotten quite a bit of magnet wire using this route and it's always been good, industrial-quality stuff.

If more windings increase resistance, then how can increasing the number of windings increase the magnetic field strength? I know for solenoids B= μ0 * n*I where n = N/L. But I= V/R so as the resistance increases as the number of windings increase then using I=V/R the current decreases. So why does the magnetic field strength increase when the number of windings is increased along with the resistance?

Thanks for explaining.

Stephen

The objective is not one of increasing the magnetic field, but of increasing the magnet's operating voltage.

We design coil for our relays and see that you have been given some good answers along with some confusion (hope you can separate). To answer your question: each added turn adds both field strength and resistance (which decreases current). At the beginning, each turn adds more field strength than the small decrease in current subtracts. As turns are added, eventually you will hit a peak diameter and field strength, after which each longer turn will subtract more current and field strength than the turns will add.

If you want to keep the 24V supply and not burn your windings up, I suggest you keep the total power around 3W or less (resistance > 192 ohms @ room temp.). There are many possible solutions based on the size of the nail. For example, the maximized field strength for 2" long of windings in a 1/8" diameter nail would be about 7300 turns of (.005 dia. & .423 ohms/ft) #36 magnet wire (to about a .360" dia.). You could replace any fraction of the winding resistance (and volume) with a properly sized power resistor in series with the coil windings, but you will also decrease the field strength by the same fraction. For example, adding a 96 ohm resistor would cut the field strength in half and you would wind only 3650 turns in 1" long X .360" dia. & reduce the field strength by 1/2.

You could use a higher AWG than #36 with fewer turns to achieve the 192 ohms total, but the thin magnet wire would be difficult to wind by hand without breaking.

Have fun!

"At the beginning, each turn adds more field strength than the small decrease in current subtracts."

Because at the beginning, the coil has the smallest circumference and so has the greatest dB/dR (change in field strength vs change in resistance), yes?

"As turns are added, eventually you will hit a peak diameter and field strength, after which each longer turn will subtract more current and field strength than the turns will add."

At a constant current through the coil, each turn will add to the overall field strength regardless of the net resistance. However, the voltage across the coil will increase in proportion to this resistance provided the current remains constant.

But, as relay coils are typically "voltage mode" devices, at some point the increasing resistance will decrease the current (and therefore the field strength) faster than the increasing number of turns will add to the field strength. It is where these two rates intersect that determines your optimum coil size, yes? This is assuming the larger coil does not "leak" field lines, which is typically not the case in practice.

Would you please show these design relationships here, on this forum, considering that you actually design these things for a living? We can derive the optimal point from a purely theoretical perspective, but in practice theory is only one piece of the total equation, albeit an important one. I'd like to see what other factors contribute to your final, real-life coil designs. You might also wish to show how you calculate the coil's "pull" (traction). This would be instructive, as well. Would you mind posting these things?

Best Regards,

-e

Thank you for this excellent post!

Now you're thinking StephenD420!

The parasitic series resistance of the coil does increase with more wire length and with the same voltage applied to the coil a smaller current runs through the wire. Less current does mean less magnetism per loop. Some algebra and simple calculus of the equations for resistance per length and the magnetic field should tell you how to optimize wire size and number of turns the magnetic field for current used.

But an additional engineering concern should also be considered. Presumably you will be running this coil in a DC continuous manner. (For the nit pickers out there, the time the coil is turned on will be greater than 5*L/R.) During this time the resistance of the coil will be dissipating heat. I assume you don't want your coil to work only once before it turns into a glowing ember. (Remember your power supply is substantial.) So combining the above calculations with the power equation of P=I²*R one can determine the power dissipated per unit length of the wire making up the coil. This equation will now tell you how to make an efficient magnetic that should last a reasonably long time.

One correction though, the relative permeability of iron (your nail) is 4000 to 5000. So B=μ_r*μ₀*n*I produces a considerably higher field until the nail saturates.

Power resistor? I thought a resistor was a resistor. What is a power resistor?

Thanks

Stephen

you seem still a student.

Any resistor with current flowing through it will dissipate power - in the form of heat.

The power in watts is current x voltage, current² x resistance, or voltage² / resistance.

The little things you're probably thinking about, cylindrical with a body about ½" (12mm) long and ¹/₈" (4mm) diameter can only handle about ½W. If you try to get them to dissipate say 4W, they'll glow red hot for a while, then fry (& rupture, like a fuse).

For handling the higher powers, big chunky ceramic or aluminium bodied wire-wound ones are often used. These latter can be obtained capable of handling 100s of watts, and are usually mounted on a metal heat-sink.

"What's in a name? That which we call a roasted resistor by any other name would smell just as bad, if not worse." Bill Shakespeare, Romeo and Juliet Go To Tech School, Act II, Scene II

In other words, it might behoove to use this pièce de résistance, ...

... when our power dissipation would otherwise roast one of these:

wow, thousands of watt can be picked up.

another side, if he would use such resistor, he would not require to wind his winding.

"wow, thousands of watt can be picked up."

Not only, but if I aim it juuuuuust right, I can hear Elvis.

It would actually be very nice in some circumstances if resistors did "pick up" thousands of watts. Unfortunately, they dissipate power. <sigh>

"... if he would use such resistor, he would not require to wind his winding."

You got that right! He could forget that coil altogether and just use the resistor!

If you post the nail size (dia + length) and the magnet wire size you have, I can provide the recommented turns required to maximize your field strength without burning up the windings.

Stephen

I am the guest who posted #52, 57, & 59 and can easily help you if still needed. If you do not post the nail size by tomorrow (2/18/09), I will presume that you have solved your problem and need no further assistance.

Best Wishes!

But don't you think that Stephen will learn more if he does his own homework?

My intent was to walk him through an example of the formulas I provided to show how it works (if he needs it - I know I needed help the first time). An experienced engineer can easily do it without help, but Steven did not give me that impression (no slight intended). Either way, once he hand winds the wire, I bet he will remember his first run in with E/M forever!

wow, you give out a very simple question, but not very easy to answer satisfactly.

you can either build up your winding by bare wire like you did. or by varnished wire or cotton enamel wire etc. no problem. the mag intensity depends on its size, like length and radus. simple written, = winding x coef x area x length/unit x u.( where u =~ 1 in the air).

as you hear hum, because the current is too hight to afforded by power supply. your doing is right by use a resistor, but it s too large at number, try to reduce it to a suit value.( pay attention to its power). or try to use a regulator power, its about 3v engough.

beside, the large distance the large cureent will be absorbed by winding, try to use a core to incread intensity.

For clarification, is your 50 loop electromagnet made from 22 gauge non-insulated copper wire and a small nail.....or are you trying to use the wire and nail to hook the power supply to the electromagnet?

Just a couple ideas that you can test with a multimeter....

1. The power supply may have been damaged by attaching it initially without resistance.

2. If the electromagnet is built using the nail and non insulated wire, the power supply may maintain sufficient voltage that it arcs to the nail and back to the wire, or from winding to winding. If this is occurring and was not occurring with the power supplied via the battery, it could explain the difference in magnetic flux.

3. Perhaps the power supply is AC and not DC? Never hurts to ask/check.

Why is nobody blinking at the mention of UN-insulated wire?? If the wire is really UN-insulated, you have a maximum of ONE turn around the core, and your power supply is trying to drive a dead short! Even 50 turns of 22 gage wire is practically a dead short.

The best answer came from europium: get very small (30 gage) magnet wire and put a few HUNDRED turns on it -- you'll get a fine magnet. But know that magnet wire is NOT un-insulated! It's coated with some kind of varnish which provides insulation. The reason for this is that it allows the turns of wire to be very closely packed together (because the layer of insulation is extremely thin), which improves the magnetic flux density by minimizing losses.

i blinked.. i definitely blinked. I'm still blinking.

#2 is in direct response to my blinking......

No insulation...leading it to arc to the nail and back or to adjacent loops.

blink blink blink

wink wink

say no more, say no more

I have a 24V 24A DC power supply that I am trying to hook a 50 loop electromagnet using 22 gauge non insulated copper wire and a small steel nail. Every time I hook the electromagnet to the power supply it sparks and Humms.

Non insulated sounds like a short circuit to me

"Non insulated sounds like a short circuit to me"

Yes, it does, and it would be a short-circuit were the wire non-insulated.

More likely, though, is that the wire is enameled. Why? The OP says, "When I hooked my electromagnet to two D batteries that I tapped together using electrical tape the magnetic field was SOOO much stronger than when I hooked it up to the power supply."

Were the windings shorted, the OP would not get such a strong magnetic field. The problem, then, must be that his power supply simply cannot sustain the kind of load his magnet presents. His magnet windings are not shorted, but simply present a very low impedance to the power supply (I'm guessing that the resistance is around 50 milliohms).

At 24 volts, the OP's supply would have to be able to source a whopping 480 amps to remain in regulation! The OP said his supply is rated at 24 amps, well below this figure. The supply is evidently dropping out of regulation. It simply cannot support the kind of load the OP is presenting.

In summary, the windings are not shorted because his electromagnet actually works, but insofar as his power supply is concerned, the load might as well be a short-circuit!

For a: "50 loop electromagnet using 22 gauge non insulated copper wire and a small steel nail" , make sure the turns are all tight together ... coat the entire assembly with a good, thorough coat of "Never-Seez" ... and use 4 automotive-type batteries in series for your power supply.

Make sure you have a video camera rolling, either for AFHV or UTube...

awright... Duh! ever hear of MAGNET WIRE...??? <click<<<

Yeah? So what's your point?

Uh... my *point*...?
even tho the original post stated, quite clearly:
"I am trying to hook a 50 loop electromagnet using 22 gauge non insulated copper wire and a small steel nail"...
YOUR POST , mr guru, (#2) said:
"I'm guessing that you've got around 40" of 22 AWG enameled copper wire in your electromagnet." Which comment led the next few down the wrong trail...!

go reread Post 1, Post 2, Post 3, Post 4 (who at LEAST made mention of the potential problem...), Post 5 (who also took notice)... then you will realize that #6 and #7 were simply trying to turn-up-the-lights so to speak, and (hopefully) elicit some appropriate feedback from Stephen (the originator).

That was all---

add a "PS" to the above:

"Drnk coffee! Do stupd thngs faster with mor energi!"

That really pushed your button, yes?

LOL

Don't know if this has anything to do with this but it appears to me that my cell phone charges much faster when connected to my truck cig.lighter (then when charging on the 120-AC/DC charger). Another interesting item is the report from the inventor Joseph Newman that he can/has demonstrated run an electric 'motor' on (a set of 4 or 6 AA) batteries that have been discharged PRIOR to installation to his 'system'. He tells us that the 'motor' runs on voltage & not on current. Interesting too is that the depleted batteries he stated with will get 'recharged' even though the batteries used are NOT rechargeable (type). Newman also offers an explanation (I'm pretty sure) of what you are experiencing. Good luck. Carlos This (your experience) is telling us that any device meant to harness 'magnetic power' needs to concentrate on the 'conditioned' energy of a DC/battery source. Which brings to mind- if battery chargers were run (taping or using a fully charged battery) as a direct source to/for the depleted battery- it would charge faster- maybe even better (improved molecular & atomic rearrangement)! Somebody try it please.

"The choices you make might be mistakes but it is never too late to turn around." Tony Lang- musician

A GA for you, Sir!

"Ladies and Gentlemen, don your tinfoil hats!"

StephenD40,

First, Bravo on your honesty about not understanding what's going on.

You have two basic problems in your configuration that can be solved in one simple stroke. But more practically will require two steps. The bare copper wire your using is shorting out to the nail and adjacent turns of the magnet. The power supply you are using can provide 576 watts of power. If you use all of it something will get very hot. (Good intuition about putting the series resistance to stop the shorted electromagnet from shorting out the power supply. But any series resistance will rob power from the electromagnet.) If you obtain some very thin gauge insulated magnet wire you will significantly increase the resistance and prevent inter-turn shorting and increase the number of turns available around your small nail. I expect your best bang for the buck would be to use this wire from Allied http://www.alliedelec.com/Search/ProductDetail.aspx?SKU=214-3590&MPN=8058&R=214-3590&SEARCH=214-3590&DESC=8058?utm_source=Allied&utm_medium=merchandising&amp;utm_campaign=Allied_Top_Sellers&utm_content=214-3590 36 AWG wire has about 0.4 ohms per foot. So if no other series resistance exists (more on this later) ten feet of this wire wrapped around your nail will draw six amps from your twenty four amp capable supply. However, this will mean that your small nail and coil will have to dissipate 144 watts of the available 576 watts of power. Just applying some hand waving thermodynamics, your small nail will have to be water cooled to survive this for any long period of time. I suspect though that the six amps of current with all the turns that ten feet of cable around your small nail will saturate the magnetic permeability of your small nail. So reducing the current to one ampere can now be done by placing a 20 ohm power resistor capable of dissipating 20 watts http://www.alliedelec.com/Search/ProductDetail.aspx?SKU=8954452&MPN=RH05020R00FE02&R=8954452&SEARCH=8954452&DESC=RH05020R00FE02 This will mean that your coil and wire package will now have to dissipate a plausible 4 watts of power. But with 4 watts of power your nail will still get quite hot.

I hope you appreciate this extrapolation of what seems like a simple problem. Knowing the size and composition of the nail along with how much space is available around the nail can permit predicting how much wire can be utilized around your nail. Thus the magnetic field expected at the nail's end can be predicted. Also real thermodynamic analysis can be done to prevent insulation breakdown and/or fire from occurring.

Stephen,

maby your power supply is current limited and the coil with out the resistor looks like a short. Also when you have the 2 D cell batterys running the magnet do they heat up? I am thinking its shorting between coils because your using non insulated wire.

have a nice day.

Hey Stephen,

It sound like you in the process of learning some good stuff! Here is an excellent resource to help you learn some of the background physics involved in what you're doing:

Main page

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

E&M

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Solenoid

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

none pointed out that the winding turns is too little to work for such so higher supply?

why not increase turns up to 5000 to 10000 with more thin coated wire?

Your answer reminds me of a joke in an electronics magazine several years ago. A man was building a do-it-yourself kit. The caption at the bottom read "Now wind the secondary - 24,282 turns."

With that many turns he would not get much current, hence not much magnetic field.

haha, I cannt make sense of the humor.

how many turns is the primary?

"haha, I cannt make sense of the humor."

In a word: ludicrous. Try winding 24,000-odd turns on a coil yourself, by hand. You'd still be at it, fingers blistered and raw and not on this forum posting silly things.

"With that many turns he would not get much current, hence not much magnetic field."

Yes, but the author goes on to say, "Now connect the 300 kV supply you built from last month's issue. Wear gloves."

3ookv? haha,

I wonder even you that dont remember ampere loop law?

need I mention a simple practice formula? NI= phi x R where R is not resistor, but magnetic resistance. from required magenetic force, we can get I then N,

You do realize that the top line is a quote from an earlier post by someone else?

It might be a joke. america style amusing?

StandardsGuy shown us a (seemingly) hv vessel for his icon. he might know the way of winding.

"none pointed out that the winding turns is too little to work for such so higher supply?"

Why too little? Even a single winding would produce a substantial magnetic field provided you pumped enough current through it and if we ignore the fact that the required current in this case would vaporize a 22 AWG wire.

The OP's 24-volt supply is shutting down. That much is obvious. Funny that his magnet works with two 'D' cells, though. Last time I checked, two 'D' cells in series comes to about three volts. You can be sure, though, that with a load like his the voltage is much, much less when his load is connected. So what is driving the magnet? Current.

Many on this thread seem to think the OP's coil is shorted because he says the wire is "not insulated," whilst completely neglecting the fact that with a functional power source (D-cells in this case), the damn thing actually works. The proof is in the pudding, as they say. And to one not familiar with different types of wire insulation, enameled wire can look as though it were bare.

So what have we here? Which of the OP's observations carries more weight? His speculation that the wire is not insulated, or the hard fact that his electromagnet works?

As part of an experiment some years ago I built an electromagnetic nail-gun that would bury a large finishing nail (sans head) about 1-3/4" into a block of wood.

The nail-gun dumped the charge from surplus 300-volt, 60,000 uF capacitor banks (2700 Joules each) into a series of coils that were switched sequentially. Each coil was wound from 80 turns of #4 AWG enameled magnet wire (square cross-section). The coils operated in series pairs that were energized in the sequence: 1-2, 2-3, 3-4, etc.

At the midpoint of each coil pair was a photo-transistor that switched to the next coil in the sequence (using IGBTs) when the tip of the nail broke the beam. Residual current from each de-energized coil was then dumped by a second IGBT to a power resistor in order dissipate the remaining energy from each bank. This prevented inductive spikes and acted to prevent the newly de-energized half of the pair from "pulling back" on the nail once it passed the midpoint.

I used pairs of IGBTs which together functioned as a single SPDT make-before-break switch. Immediately before the first IGBT was switched off the second one connected the resistor to the coil to maintain an unbroken current path. These coils produced some pretty hellish magnetic fields, to be sure - and with relatively few windings.

Wow! That nail gun sounds pretty cool! Was there a long charge time before driving the next nail? Did you get tired of having to cut the heads off a box of nails?

(Not recommended for people with pacemakers.)

Would you agree that, theoretically, it could also work with aluminum nails?

I haven't given aluminum nails much thought -- although the heads would certainly be easier to snip off (I ground the heads off the steel ones) -- but I did consider using long, cylindrical samarium-cobalt magnets. Talk about moving like a bat outta hell! Sadly, the impact would very likely have shattered them - and they're expensive!

Maybe I should've asked for a DARPA grant, ya?

I wouldn't use aluminum nails because they're not the right shape to make a nice current loop. Better to use aluminum -- or better yet, copper -- rings and launch them from the iron core of an AC coil. You outta try that sometime. Works like gangbusters!

Yes, this thing has "weapon" written all over it!

Anyhow, I remember being astounded in college when they demonstrated the effect of the rotating magnetic field, and showed that even an aluminum can would spin like mad in there!

Some kinds of motors use a copper disk as the rotor. Aluminum would work as well (being only slightly more resistive) and is probably used where a high-torque, low MOI rotor is needed.

ludicrous? fine. learn a fun word. haha.

50 turns winding can has enough air insulation, why you are sure it will be shorted?

it s odd.

he use 3v cells, why dont think the cell soon exhaust after connect with the winding?
if it would be a good power supply, it will be burned as well.

50turns, how much is it resistances? cannt be calculated?

Regards

How you made your Electromagnet-Fixture.

Is there no such thing?

All the turns are around <a small steel nail> & you are connecting the ends of wire to Power Supply output?

Your questionis very ambiguous.

---------

50 loops [turns] of uninsulated 22AWG/SWG wire [even if wound carefully not to short-cicuit

will draw enormous current to bring the supply to CL or Clamp the supply almost to few mAs

[If your supply is a professional one. Such supplies go to almost 0V output & very small

current to protect the supply.]

Most of the professional supplies may also have +Crow-Bar-Protection; which shuts down

& does not re-inste unless short-circuit is removed.

Can you give full specs of suuply.

Note:

Magnetizing needs indepth knowledge of Magnetizer /Demagnetizers, otherwise a dangerous mishap may occure.

Very Strong construction of such Fixtures is needed; otherwise @ 24V to 50 turns

[around a loose Fixture may explode it]

Safety is the 1st priority for human life.

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