# Wire size for Electromagnets

Discussion in 'General Electronics Chat' started by wes, Aug 12, 2012.

1. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Well how do you figure out what Wire size to use for a Electromagnet? The reason I ask is because I know you can just look up some AWG table's and figure out what current you will be operating at and then pick the size from there. But how do you figure out what size you need if your wire is going to be operating at high current like 100+ amps and also be very long, like 1 mile plus in length, I assume you would have to take the heat output into consideration and adjust your wire size for that?

Example:

P=I^2xR
I^2=100Ax100A=10,000
#1 guage = .1239 ohms per 1000 Ft x 5.28 (mile = 5280 Ft) = 0.654192 ohms

10000 x 0.654192 = 6,541.92 Watts

So as you can see, 6,541.92 watts is alot of power and would generate alot of heat.

Apr 5, 2008
15,805
2,389
Hello,

Why do you think that a lot of electromagnets are water cooled?

Bertus

3. ### #12 Expert

Nov 30, 2010
16,704
7,351
Because they get too hot?

The first image I got was an oil filled transformer with fins and fans.

The old rule of 1000 circular mils per amp might not work on this one!
It's off my chart but I calculate it as size "0" copper at 10148 feet per ohm.
5,202 watts plus core losses
3.1385 feet per pound=1682 pounds
Last I heard, \$13 per pound.....\$21,870 if you can get it enamel coated.

I think this just fell out of the "hobby" category.

4. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
6.5kW is a lot of heat. If you have a large magnet with a mass of several 100 kilograms, it is not so much actually.

If you examine problems like this, you must consider all factors. If a factor is greater than neglible, it must always be considered.

Also sometimes wire gauge will be uneconomical. The you don't even calculate it, but you change voltage (for instance).

If you are an engineer you would have in mind (or draw on paper if actually required) some kind of flowchart. The top consideration is "economical". Then maybe "size", and "efficiency".

5. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
lol, if what you said #12 is correct about price, then I will have to build a much smaller one. The big reason for asking this about heat was because the way I was thinking the electromagnet was going to be built was multilayered since you cannot just have that many turns in a single layer. So since each wire would be near other wires carry 100+ amps, I figured heat would be a huge issue.

One question though, I think I already know the answer but I just want to make sure.

In a Electromagnet where you want a strong magnetic field, wouldn't you want the most current as possible vs Higher voltage? You could have 10 amps at 10 volts , but wouldn't 100 amps at 1 volt be better since isn't it the current that generates the magnetic field?

I am like 99% sure that the higher the current the better vs higher voltage. The reason I ask is because my friend asked that at the same Wattage, does it matter which is higher since they are equal power and I said I was pretty sure it does because the amperage determines Magnetic field strength.

6. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
The magnetic flux does not directly correlate with current. It is the total product including:

-number of turns
-DC resistance
-magnetic material
-frequency
-voltage

The current is a result of frequency and reverse EMF from the magnetization.
Electromagnets are normally used with DC voltage.

You can find some materials to read here: http://www.coolmagnetman.com/magdcem.htm

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I know there are a bunch of other things that effect the magnetic strength but What i meant was that between the voltage and the current, the current ultimately generates the magnetic field, the voltage just causes the current to flow, right? Or can a high voltage and low current generate the same field as a low voltage and High current? Like does the voltage just cause the low current to move faster and make up for the High Current of the other system and ultimay creat3e the same field strenght?

8. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
1. If you increase voltage, current will increase automatically.

2. Actually for this reason coils intended for higher voltages have more turns, to reduce current.

Important is the total Ampere/turns.

9. ### #12 Expert

Nov 30, 2010
16,704
7,351
"Important is the total amp-turns."

Yeah, voltage is just the driving force, and it doesn't have to be very much. Amps are important but not as important as amp-turns. That's why commercial solenoids have hundreds of turns of wire. 1 amp through 10 turns causes the same magnetic force as 1/10th of an amp through 100 turns.

10. ### Austin Clark Active Member

Dec 28, 2011
409
44
Amp-turns, as stated before, is the only thing that matters. It's impossible to change how much current you're sending through the coil at a specific voltage without changing the diameter (resistance) of the wire itself. You seem to think that you could design so that you'd get really high current at low voltages, so you'd use less power yet keep a strong field going, the only way you can do that is by using thicker cable, which means you can't get as many coils, and thus less amp-turns. In general, the thinner the wire, the better. HOWEVER, the thinner the wire, the higher voltage you'll need. The thinner the wire and the more coils means more worry about heat as well, but you get a more concentrated field. Theoretically, if you had a superconductor, you could use very very little voltage and get enormous current , thus a stronger field without wasted heat, which is why ultra-powerful electromagnets use superconducting coils. I don't understand the physics, but other factors give us a different kind of bottle-neck to maximum field strength and the number of amp-turns we can sustain.

11. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
What kind of magnetic fields are you talking about? At some point (well before you get to the tesla range) you have some other factors to consider, namely hoop stresses. Your magnet is going to literally be trying to tear itself apart.

For large field magnets, mechanical and thermal design is the far more challenging task.

I don't know what the current situation is, because a lot of improvements have been made since I was a co-op student at NBS/NIST in the Superconductor and Magnetic Measurements Group, but that that time superconducting magnets were limited to about 18 tesla while if you wanted to go higher you used copper. The big magnets at MIT's Francis Bitter National Magnet Lab used superconducting magnets to generate the background field (something like 7T) and water-cooled copper magnets to get up to 27T. The claim (tongue-in-cheek) was that a significant portion of the St. Charles river was routed through the magnet when it was at full field.

My understanding is that the current world-record for continuous DC magnets is a 45T magnet at Florida State University and uses a similar hybrid approach with a 'low field' superconducing magnet and 'high field' resistive magnet in the center.

12. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok, I get what your saying, the amp-turns is the important thing to look at but ultimately it is the current that generate's the magnetic field and depending on the wire resistance, you eithier need a higher or lower voltage to get the same current through the wire. So it is the current that is important when generating the magnetic field and also the number of turns and the voltage is only needed to get that current to whatever value it needs to be at.

I didn't plan on it being to strong, in reality it will probably be like 2 feet in diameter and maybe a couple thousand turns at 1-10 amps depending on price of the wire. I will probably use something like 18 guage enameled wire. It will just be a air core wound on a form I make.

I just wanted to know in general like when heat starts to become a big issue, I won't have this on for too long at any given time, maybe 10 mins max.