Heating Problem with DIY A/C Electromagnet

[PJP]

Hello every one I’m a newbie to electrical stuff (I do have basic knowledge) so please be patient with me, please help me!
So, coming to the details of my diy project I’m trying to vibrate a metal string with an electromagnet. so, after an ample amount of research I’ve made the electromagnet with the following specs.

• I’ve took an iron bolt 1-inch in diameter and 5 inches length,
• I had half kg of (about 30 meters) 17 gauge (SWG) enameled copper wire and wound it for about 432 turns.

The specs of this copper wire are as follows:
o Diameter 1.46,
o The ohms per meter is 0.0109,
o It can carry 6.27 amps.

Brought a step-down transformer (A/C to A/C) with the following specs
o 230v input - 24v 5amps output,
o it’s a center tapped transformer (12-0-12)

I’ve fused the transformer with 600 mA fuse (on primary side ) and everything worked fine (well I’ve expected much more oscillation’s in the string) but after around 5 to 10 minutes I could smell some strange enamel and plastic burning, I noticed that the plastic bobbin of the transformer melting,
I switched every thing off, tried to touch the transformer but I could not place my finger for more than 2 seconds as it was so hot!

This is how I connected

• I connected the outer two taps of the secondary to the electromagnet and both the transformer and electromagnet are getting hot pretty quickly,

So,I tried the following.

• I’ve connected the electromagnet this time to the “outer tap and to the center-tap (half winding)” for 12v 5 amps output, and the transformer did not get as hot as before, but the oscillations in the string reduced dramatically.
• I had another 24v 1amp SD CT transformer with 12-0-12, this one also is getting too hot, but when connected with “outer tap and to the center-tap (half winding)” the transformer is not getting as much hot as earlier!

My questions are:

• Why is the transformer getting very hot? (why when connected to outer two taps only!)
• Is something wrong with the electromagnet that i made?(wire gauge/no of turns)
• How do I minimize the temperature in both the transformer and electromagnet?
• Am I missing something in the circuit?
• I want to achieve more oscillations and amplitude in the string (12v 5amps is not achieving the desired oscillations and amplitude).

I want the transformer to work continuously for longer periods (like 10 to 15 hours or more).
please help
please do find a schematic of what I've done, it may not be correct in terms of the symbols, I'm sorry for any errors.

 

[.Scott]

You've got too much amperage on the output side of the transformer - perhaps more than 6 amps.
You need much more impedance in your electromagnet. You may not be able to get it with 17 gauge wire. I would try 1000 turns with 22 gauge. Also, you should probably smooth down the threads on the screw to allow the wire to wind closely and smoothly over it.

 

[Tom.G]

Ping @jim hardy. (The explainer of things magnetic.)

 

[NascentOxygen]

So, coming to the details of my diy project I’m trying to vibrate a metal string with an electromagnet.

Tell us about this metal string...

What is it made of? Is it carrying electricity?
Are you flexible in choice of its length and thickness and especially the force keeping it taut?
You want the vibrations to be fixed at the frequency of the mains, viz., 50 or 60 Hz? So you hope the string will emit a soft hum like a vibrating guitar string, or is its vibration to serve some other purpose?

Please answer each of these questions, not missing any.

 

[Merlin3189]

* I had ... (about 30 meters) 17 gauge (SWG) enameled copper wire and wound ...about 432 turns.
The specs of this copper wire are as follows: o Diameter 1.46, o The ohms per meter is 0.0109, o It can carry 6.27 amps.

So 30m at 0.0109 Ω/m is 0.327Ω

... transformer (A/C to A/C) with the following specs o 230v input - 24v 5amps output, o it’s a center tapped transformer (12-0-12)
I connected the outer two taps of the secondary to the electromagnet and both the transformer and electromagnet are getting hot pretty quickly,

So 30m x 0.0109 Ω/m = 0.327 Ω
Then 24V / 0.327 Ω = 73 Amp. (though the transformer resistance would reduce it .)

But you can see that a transformer which is designed to pass 5 A, is going to be very unhappy with even half of 73 A. The heat generated in the winding is proportional to the square of the current, so even 10A makes 4x as much heat as it should handle, 20A would be 16x too much, etc.

So,I tried the following.

• I’ve connected the electromagnet this time to the “outer tap and to the center-tap (half winding)” for 12v 5 amps output, and the transformer did not get as hot as before, but the oscillations in the string reduced dramatically.
• I had another 24v 1amp SD CT transformer with 12-0-12, this one also is getting too hot, but when connected with “outer tap and to the center-tap (half winding)” the transformer is not getting as much hot as earlier!

So halving the voltage will halve the current and quarter the heating. You have improved the situation.
Now you could keep reducing the voltage and the current until you reached a value which matched the transformer design, but as you note, the lower current is less effective.
Now, similar transformers to the 24V 5A would be 12V 7A, or 6V 10A, 3V 14A etc, but I don't think these low voltage transformers would be readily available and you're still short of the 70A which worked for you with your electromagnet.

So Tom's idea of increasing the turns on the EM would be better.

My questions are:

• Why is the transformer getting very hot? (why when connected to outer two taps only!) Too much voltage => too much current
• Is something wrong with the electromagnet that i made?(wire gauge/no of turns) Not enough turns
  
• How do I minimize the temperature in both the transformer and electromagnet? Lower current
  
• Am I missing something in the circuit? No
  
• I want to achieve more oscillations and amplitude in the string (12v 5amps is not achieving the desired oscillations and amplitude). Nascent is going into this

I want the transformer to work continuously for longer periods (like 10 to 15 hours or more).

please do find a schematic of what I've done, it may not be correct in terms of the symbols, I'm sorry for any errors. No, you've explained much better than many questioners do. Thank you.

I'll see what you say to Nascent.
One thing that might help is to put the EM near the middle of the wire (assuming it's not there already.)

 

[Asymptotic]

an iron bolt 1-inch in diameter and 5 inches length

The specs of this copper wire are as follows:
o Diameter 1.46,
o The ohms per meter is 0.0109,
o It can carry 6.27 amps

Where did the 6.27 amp number come from? Is this the wire manufacturer's rating?
Coil DC resistance (based on 0.0109 ohms/meter x 30 meters) is 0.327 ohms.

230 volt AC primary may suggest 230V household mains at 50 Hz line frequency.
Inductive reactance (AC resistance) = X_L= 2πfL.
The big question is, "What is your coil inductance"?

How many winding layers? 5 uniform (not scramble wound) layers ought to fit on a 5 inch length.

You know most of the factors involved - wire diameter, number of turns, coil diameter - but a major unknown in the inductance formula is the relative permeability (μ_R) of the "iron bolt" used for your core. μ_R is very nearly 1.0 for air, but ranges from about 5000 for pure iron to 100 for mild carbon steel. This is to say, the largest variables under your control is what exactly the core is made of, and details of how the coil is wound (number of layers, scramble wound versus winding it in even layers, etc.).

The primary fuse didn't cut loose, and 600 mA is about 115% of rated transformer capacity (24V x 5A = 120 VA; 120VA is 0.522 amps at 230V). The secondary had enough current flowing through it to get excessively hot, so the 6 amps @.Scott mentioned seems about right. Disregarding for the moment transformer regulation (the rated 24V fell to a lower, and perhaps quite a bit lower value with your solenoid load hooked up to it), the effective AC resistance would have been on the order of 24V/6A, or 4 ohms. If it were much higher the transformer wouldn't have overheated, and would have cleared the primary fuse if load impedance was much lower.

I'm having a hard time with this (I'm rusty, and quite possibly figuring it incorrectly) because this suggests an inductance of 0.018 henry and an effective core permeability on the order of 2, given 432 turns of 1.46 mm diameter wire on a 26mm diameter coil form, and far from the 1.3 henry inductance and 420 ohm coil impedance a 100 μ_R core would provide.

 

[Merlin3189]

Sorry PJP and thanks Asymptotic, I must be asleep today ! I'd completely ignored the inductance!

It is interesting that Asymptotic suggests an inductance of 18mH, when the standard formula gives something of the order of a Henry. I had the same shock myself when I realized my error (after seeing his post) and did similar calculations.
I wonder if PJP has perhaps used a stainless steel bolt ? Some of these go down to very low permeability (single digit) (link)
That may not be likely, since it would be a pricey lump of scrap metal, but 1" bolts are not that common anyway, so it was probably scrounged from some esoteric source. (I assume scrounged as, if I wanted to buy a 1" iron bar, it would be v.much easier to buy exactly that than any sort of 1" bolt.)

 

[Asymptotic]

Sorry PJP and thanks Asymptotic, I must be asleep today ! I'd completely ignored the inductance!

It is interesting that Asymptotic suggests an inductance of 18mH, when the standard formula gives something of the order of a Henry. I had the same shock myself when I realized my error (after seeing his post) and did similar calculations.
I wonder if PJP has perhaps used a stainless steel bolt ? Some of these go down to very low permeability (single digit) (link)
That may not be likely, since it would be a pricey lump of scrap metal, but 1" bolts are not that common anyway, so it was probably scrounged from some esoteric source. (I assume scrounged as, if I wanted to buy a 1" iron bar, it would be v.much easier to buy exactly that than any sort of 1" bolt.)

Not to worry, Merlin ... I'm finding it difficult to keep it all straight in my own head :). I used an online calculator (at https://www.eeweb.com/tools/coil-inductance) based on the following formula where N is # of turns, D is coil diameter, and d is wire diameter.

The 18 mH value came from solving for L based on the ~6 amp (~4 ohm) solenoid load (L=X_L/(2*pi*50 Hz), then punching different permeability values into the calculator using 432 turns, 1.46 mm diameter wire, and 26mm diameter coil variables until arriving at a permeability of 2 to yield 18 mH.

I like your idea that the bolt may be stainless. A table in the Wikipedia entry for permeability places austenitic stainless relative permeability nominally at 1.003, but it may go up to 7 depending on whether is has been cold-worked and/or welded (which makes it partially martensitic, per http://www.bssa.org.uk/cms/File/SSAS2.81-Magnetic%20Properties.pdf). This would certainly explain why the inductive reactance and inductance appear unexpectedly low.

 

[Tom.G]

So Tom's idea of increasing the turns on the EM would be better.

Where credit is due: That was @.Scott who suggested more turns.

 

[Merlin3189]

Thanks Tom. I was trying to give credit, just got it wrong.

Despite my wildly wrong calculation, the overheating really has to be too much current, even if it's only 6A rather than 4A .
If it is a non-magnetic bolt, then absolutely must change that. Otherwise More turns is likely to help.
I hope PJP comes back with more details, as there may be other steps as well.

 

[JRMichler]

How do eddy currents and hysteresis in the bolt affect inductance? I designed an electromagnet once, and needed to use thin (0.015") laminations to control eddy currents. And electrical (I forget the correct term) steel to control hysteresis losses. But I never did find what would have happened if I had tried to use a solid block of ordinary steel.

 

[PJP]

You've got too much amperage on the output side of the transformer - perhaps more than 6 amps.
You need much more impedance in your electromagnet. You may not be able to get it with 17 gauge wire. I would try 1000 turns with 22 gauge.

hey scott thanks! will a 22 gauge be able to handle 5 amps on the secondary of the SD transformer, as the amp carrying capacity of 22 gauge is only 1.57 amps??
i choose 17 gauge thinking that i can pump 24 v 5 amps without over heating the coil (as 17 gauge it is rated above 6 amps)
"from what i understand the 17 gauge wire is sucking power out of the transformer, and thereby overheating it". is this correct?
i did try the electromagnet with some other gauge wire, I'm not sure of the gauge (it was salvaged from an old fan) but it did not heat the transformer.
what if i want to increase the strength of the electromagnet?, let's say instead of 24v may be 48v then should i again go for a different gauge of wire?

 

[Merlin3189]

You can continue to make progress with these trial & error expts, but I think it would help a lot if you could glean a bit more solid data from them. Can you not get hold of a cheap multimeter and make a few measurements? I routinely use a very cheap multimeter I've had for 20yrs. It cost me a fiver and I see them online for as little as 3 quid these days. It will measure 10A AC and all the voltages you need.

Without knowing what your iron core is, you can't reliably calculate the inductance, so a few measurements could help. If you know any more about your bolt, please say.
You could check it with a magnet to see if it is ferromagnetic (SS might not be.)
If you have any other bits of iron, even if they are not suitable for your application, experimenting with them might show whether the material you are using could be improved on.

I'm working on a project to vibrate an iron string with an electromagnet.

This is helpful, as Nascent and I thought it might be a current-carrying wire. Even more detail would be useful - size, tension, length, where you are placing the EM, etc. - as Nascent asked earlier.
The purpose would also be helpful to show us what your other options might be. Eg. If you want a stronger magnet, you need more amps or turns or a different core. But you may not need a stronger magnet. You may be able to make other changes, say to the metal string or the shape of the EM, which would achieve your object with a weaker EM.

 

[sophiecentaur]

A simple solution to the over current problem could be just to use a nice fat power resistor in series with the coil. If you want no more than 5A through the secondary then a resistor of 12/5 = 24Ω [Edit 2.4Ω: I didn't re-read that before going out to the shops!] in series will achieve that, whatever the coil resistance happens to be. A bit of trial and error would get the best result. A different (much lower voltage) transformer would also help. But that could be hard to find for free.

 

[.Scott]

hey scott thanks! will a 22 gauge be able to handle 5 amps on the secondary of the SD transformer, as the amp carrying capacity of 22 gauge is only 1.57 amps??
i choose 17 gauge thinking that i can pump 24 v 5 amps without over heating the coil (as 17 gauge it is rated above 6 amps)
"from what i understand the 17 gauge wire is sucking power out of the transformer, and thereby overheating it". is this correct?
i did try the electromagnet with some other gauge wire, I'm not sure of the gauge (it was salvaged from an old fan) but it did not heat the transformer.
what if i want to increase the strength of the electromagnet?, let's say instead of 24v may be 48v then should i again go for a different gauge of wire?

As has been addressed earlier in this thread (@Asymptotic), you need to make sure that the bolt you are using is made of something that is strongly magnetic. If it isn't, nothing will work.
If your bolt is good, then hopefully what will be reducing the current is not the resistance in the wire, but the inductance. To get more inductance, you need a good iron bar and lots of tight windings. 22 gauge wire will allow you to keep those windings tight. What is "sucking power" from the transformer is that lack of inductance.
If you want more power from the electromagnet, up the voltage and the number of windings proportionately. In fact, you may want to increase the windings slightly more than proportionately. But first get it to work at the lower voltage.

 

[Rive]

You would be able to save some problems by salvaging a transformer with an E core and an existing winding for the electromagnet.
If you rearrange the core with using only the 'E' parts on one side and abandon the 'I' parts entirely then you would get a far stronger electromagnet.
If the winding is already for 36-48V or above then you have a good chance that no further parts will be needed to set the current.

 

[jim hardy]

Ping @jim hardy. (The explainer of things magnetic.)

I've been away , at kids' house with no internet sorry i missed you @Tom.G (@ not working again)

I think you guys have hit the bolt on the head . Not enough inductance to counter the applied voltage.

L = μμ₀N^² A/_Length but the flux path is only half iron, return path is air , so it'll have maybe twice the inductance of an air core solenoid.
Furthermore for a nonlaminated core like a bolt , relative inductance oops, make that relative permeability μ is far less for AC than for DC . In my experiments it was in the tens rather than the hundreds you get for DC

If your winding surrounds the whole five inches of bolt length
area is pi/4 square inches, length is five inches
Change those dimensions to to metric units and calculate inductance of an air core solenoid that size with 432 turns.

Since half the magnetic path length is iron, by applying the simple reasoning that iron is a short-circuit for flux,
we can estimate reluctance of the magnetic circuit is roughly halved by wrapping your coil on an iron bolt instead of a wooden form.
So flux only about doubles.
That puts a limit on your inductance of roughly twice what you calculated for air core.

Then calculate reactance and impedance. I think you'll find as others have suggested, you are connecting your transformer to a near short circuit.

That's why it overheats.

Any help ?

 

[sophiecentaur]

Not enough inductance to counter the applied voltage.

Which means the voltage is too high. A dropping resistor or a different transformer is needed. Let's face it, the right transformer is always the way to start.
It's always hard to decide the optimum design for an electromagnet - particularly when it's not a good basic design - like a bar magnet. Many turns are hard to achieve unless you are pretty experienced so it needs few turns and lots of current (= Low Voltage supply)
You want a strong field, the answer is a 'horseshoe' as the original magnet makers found. If it can't be a horseshoe then a yoke round the outside will increase the strength of a magnetic actuator. But a single iron rod is never going to perform impressively.

Actually, having looked at the OP again. I see that a horseshoe core would be quite possible, giving many times the field that's around the end of a bar.

 

[jim hardy]

Actually, having looked at the OP again. I see that a horseshoe core would be quite possible, giving many times the field that's around the end of a bar.

He might experiment with a gigantic "C Clamp" for adjustable air gap.

 

[sophiecentaur]

Having established that there is some Field from this simple solenoid, I would say that it's worth spending a finite amount of money on improving on this. There are many transformer kits on the 'constructors' market and the iron laminations are ideal for this sort of thing. 'C' cores of various sizes are available and there are formers too, to make winding possible. A bit of shopping around and the OP could even find ready wound bobbins with specified numbers of turns and resistance. I would bet they would beat any 'amateur' hand wound coil. A really excellent driver could be made for vibrating the string.
A good addition to an experimenter's kit is a strong 'super magnet'. They aren't cheap but they have many uses in addition to actual experiments.
In my A level classes, I used to feed the output of a lab signal generator along the wire itself and put it between the poles of a pretty strong horseshoe magnet. It was easy to make the wire vibrate, doing things the other way round from the OP's system. The permanent magnet was better than most home wound electromagnets would ever be. The wire, being a virtual short circuit was well matched to the low voltage / low source resistance of the Audio Oscillator.
As it's clear that 12V is obviously too high and one or two volts would probably be more suitable, why not use two transformers in tandem, mixing and matching ratios from mains down to an intermediate voltage and then from that down to 2V. (Something like two 10:1 transformers in tandem) Quite safe as long as the second transformer is connected as step-down too.

 

[Asymptotic]

I was hoping for more detail from @PJP on the vibrating string aspect before commenting further, but was thinking it may be in some ways analogous to a material vibrator feeder. A feeder vibrator uses a laminated E core with a winding around the center limb which pulls a laminated flat piece toward it. This flat piece is coupled to the material trough via an elastomeric bushing through the vibrator housing.

When fed with AC they vibrate weakly, but supplied with half-wave and product fairly shoots from them. Thought I'd once sketched out the SCR controller used on the vibrators in question ... not having any luck finding it, but the circuit wasn't much more than this one courtesy https://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/silicon-controlled-rectifier-scr/.

 

[Averagesupernova]

Why not do something like the soldering guns do and have very few turns for the secondary? It could drive the wire to vibration with a permanent magnet, or wind a coil and drive that.

 

[jim hardy]

Why not do something like the soldering guns do

Great thought.

A secondhand soldering gun perhaps ? Usually under five bucks at thrift shops around here.

 

[george brack]

if the bolt is part of the problem and is of reasonable quality it should have markings on the head showing its tensile strength .They could be numbers or lines radiating from the centre or both .If a number begins with the letter 'A' it's stainless steel .If there no markings it's probably just mild steel.