DIY Electromagnet Design Questions

Hi there,

I am building an electromagnet to work in my workshop to secure steel parts during fabrication.

I did some research on electromagnets and bought some supplies - I understand that current is more important to magnetizing ferromagnetic materials than voltage.

I have 60 meters of 1.5 mm diameter enameled copper wire for the coil - bought to handle high current if needed.

I have several 60W power supplies that supply 5 VDC at 12 Amps - they were cheap.

I have several pieces of soft iron for the cores - for its ferromagnetic properties and high permeability.

I plan to encase everything is epoxy resin.

Here is my question:

If I want to maximize the current through my setup should I:

a) hook up the power supplies in parallel with resistors to share the load using a ONE 60 meter coil around ONE core to increase current.

or

b) make THREE 20 meter coils and hook up a separate power supply to each coil and use all THREE coils around ONE core to increase total current.

or

c) something else.

I am thinking the 5 VDC power supplies may struggle with the long coil.

Any suggestions would be helpful - I have yet to power anything up yet.

Thanks.

Welcome to ETO!
Assuming you intend 1.5mm diameter and not 1.5mm² cross-sectional area, the resistance of 60m of 1.5mm diam copper wire is only 0.41Ω, so a 5V supply would push 5V/0.41Ω = 12.2 Amps through it. One 5V 12A supply theoretically won't cope. The use of two supplies would require balancing resistors so that they share the load safely.

Cubicle_22 said:

I am building an electromagnet to work in my workshop to secure steel parts during fabrication.

Click to expand...

Here is a good explanation for making a metal holding/bending machine, from the inventor of the MagnaBend. While it's meant for a magnetic bending brake, it applies to just about any metal clamping "machine". He gives an explanation of how to chose the wire size and coil, and it's not quite the same as most people think. https://aaybee.com.au/Magnabend/Building Your Own Magnabend.html

If your doing this to hold stuff while milling you may want to reconsider. As a life long machinist/die maker,(55years as a living and 2 years in high school) I've never seen magnetic chucks, work holding devices, for anything but grinders. The reason being that the steel chips don't get out of the cutting path and cause much trouble . But suit yourself.

Hi there,

Thanks for the advice.

Yes, 1.5 mm diameter enameled copper wire - quite thick stuff - from aliexpress.

I tried measuring the resistance of the wire with my multimeter but it was too low to be sure if it was reading properly.

Here's a thought:

What if I cut the 60 meter wire into three pieces - 20 meters each - hooked up a separate 5V power to each 20 meter coil - combined all three coils around the same core - will this let me run around 36 amps total around a single core instead of 12 amps from two power supplies wired up in parallel? - say I added a resistor in series with the copper wire to increase the current slightly through each coil up 12 Amps (theoretically)?

The higher the current the stronger the electromagnet.

Hmmmm?

Thanks.

Cubicle_22 said:

I did some research on electromagnets and bought some supplies - I understand that current is more important to magnetizing ferromagnetic materials than voltage.

Click to expand...

The key factor is Ampere-Turns; current multiplied by the number of coil turns it flows in.

eg. 50 turns with 12A current gives the same field strength as 100 turns with 6A current or 250 turns with 2.4A current.
All 600 Ampere-turns.

Great,

Thanks for the valuable information - I have some reading to do now.

If you cut the 60m wire into 20m pieces, this will reduce the resistance by two-thirds and INCREASE the current needed by 3×.

This is true for a fixed supply voltage. BUT assuming you are still using a total of 60 meters of wire you will have a third of the number of turns. (One turn of three conductors in parallel still counts as one turn.) As you have reduced the length of each conductor from 60 m to 20 m the resistance of each conductor will be one third of it's original value. Now as you have three conductors in parallel the total resistance is a third of the resistance of one conductor. So now the the resistance is one ninth of the original value. So to get three times the original current the voltage has to be reduced to one third of the original voltage.
So Nothing has changed other than the supply voltage. The power is the same and the ampere- turns is the same.

Les.

I have never seen a magnetic work holding device that works on 12V.

I think I have a better plan now for making my electromagnet - I will ditch the DC power supplies and go with 110 VAC rectified to 100 VDC.

I verified the length of my 1 kg of 1.5 mm diameter enameled copper wire - comes to around 60 m - in hindsight I should have made better calculations but too late now.

I filed the ends of the 60 m wire and hooked it up to my bench power supply - 5 VDC gave me 7.3 Amps - giving me 11.4 ohms / km - which seems to check out with the tables.

The 1.5 mm wire is rated up to 28 amps - I read that somewhere.

The length of one turn of the coil will be 2 m - long e-type design for electromagnet.

Resistance of 60 m of wire comes to 0.684 ohms.

Resistance of 6 x 60 m (360 m) is 4.1 ohms

At 100 VDC rectified the 360 m coil should draw 24 amps.

I have 220 VAC but that would give me too much current.

3800 ampere-turns (minimum requirements for this electromagnet design) divided by 24 amps gives 158 turns.

360 m of wire should give me 180 turns.

I will solder the lengths together - shouldn't be a problem? Basically have 6 coils around the same core wired in series - solder joints insulated and outside the core itself..

I think these figures will work?

I can massage the values by tinkering with the length of the coil if needed.

Anything I am missing this time? My brain did start to hurt a bit crunching these numbers - been decades since I was in a physics class.

Cubicle_22 said:

Anything I am missing this time?

Click to expand...

Yes, sorry...

Wire current ratings are for a single wire or core in open air.
With more than one together, they must be derated, as heat cannot be dissipated as easily.

To cut it short, you would be putting a lot of power (heat) in to the coil and it would cook quite quickly.

The maximum continuous current in a multilayer coil is going to be somewhat less than half the open air rating of the wire.

Understood completely,

Going to (at the least):

Increase length of wire to 540 m (6.1 ohms)
number of turns to 270
corresponding current draw of 16 amps (using 100 VDC from rectifier)- which is manageable on a low duty cycle with a timer circuit.

I had a research magnet at my disposal at one time. 30 killogauss. it's more important to regulate current. The magner was water cooled. It would be wise to monitor temperature.

We had a water pressure switch wired to a bell. Not ideal, but it made sure the water was on.

16 amps at 100vdc gives you 1.6KW. That's quite an elaborate electric heater your building there!
Plain old enamel wire can handle more heat than the poly-something-ethene type but I don't imagine it's up to that power level without some help!

Cubicle_22 Did you even look at the link I gave? It's from a guy who made these work holder, magnetic bending brakes for a living, he's the actual inventor of them. And many of them have been sold by him and many of them have been made by others from his website instructions. What he shows and the formulas he gives can be made in any size you want or need, so why keep going at this blind?

Thanks for info - reading material was helpful - I had to modify the wiring schematics to work at 110 VAC - component list given is for 220 VAC - component values have to be adjusted - I have a lot of 1. 5 mm enameled wire at my disposal - thermocouples are available in my parts bin - I am actually cobbling this together from stuff at hand - I will probably have to hook up a power resistor to lower the current - magnet might be too strong for what I need - I have my reading glasses on now - thanks - I know I am coming at this from the wrong direction - but thanks to you guys I will tame this monster - all good.