# Electromagnet help with power

#### Kylehaglock

Joined Dec 13, 2015
6
Hello all,
I built an electromagnet with 18 guage enamel coated wire (about 600 feet) around a 1010 steel bar (1 inch by about 10). When I hooked it up it was pretty weak. I wanted to know if it was possible to hook it up to a variable dc power supply. I know my ohm load is about 3.8 ohms. I originally hooked it up to a wall wart with a 20 ohm resistor in line, as to not fry the adapter. I would like to eliminate the resistor all together. Would a 30, 10amp power supply work for this?

Thanks!

Kyle

#### DickCappels

Joined Aug 21, 2008
10,104
Yes, it is possible to hook it up to a DC variable supply.

#### #12

Joined Nov 30, 2010
18,224
You are still going to have to be careful about heat build up. If you give it the whole 30 volts of DC, you're looking at 237 watts, so it's going to be a few seconds, "on" and a few minutes, "off".

#### Picbuster

Joined Dec 2, 2013
1,045
This calculator computes the force between a solenoid and another piece of ferromagnetic material separated by a gap of distance g.

F = (Fm)2 μ0 A / (2 g2)

F = (N*I)2 μ0 A / (2 g2),
Where:
• μ0 = 4π×10-7
• F is the force in Newtons
• N is the number of turns
• I is the current in Amps
• A is the area in length units squared
• g is the length of the gap between the solenoid and a piece of metal.
The power in wire = i^2 x R + electrical induction power
Now you must be able to calculate all.

#### DickCappels

Joined Aug 21, 2008
10,104
This is a bar-shaped electromagnet. For the above formula, what assumption is made for the flux return path, is it mostly through space or is it guided by to the gap as with a horseshoe magnet?

If it is mostly through space then the length of the magnet should show up somewhere, shouldn't it?

#### Kylehaglock

Joined Dec 13, 2015
6
Dick,
That is the same issue I was having with calculating the power. I tried using similar calculatorsituation but could never account for the gap.

#12,
Iso the high wattage a concern because the dc power source won't have the ability to shed the heat that fast? Should it not be designed to handle it's own maximum output? 10amp x 30v should be 300 watts. Could I install a larger fan or a heat sink?
Thanks

#### GopherT

Joined Nov 23, 2012
8,009
Dick,
That is the same issue I was having with calculating the power. I tried using similar calculatorsituation but could never account for the gap.

#12,
Iso the high wattage a concern because the dc power source won't have the ability to shed the heat that fast? Should it not be designed to handle it's own maximum output? 10amp x 30v should be 300 watts. Could I install a larger fan or a heat sink?
Thanks
The power supply should be ok. However, the bar of carbon steel wrapped with a quarter inch of copper will become a 300 watt space heater. I imagine about 7 layers of copper wire, the outer wraps will insulate the inner layers and the insulation will start to melt. As it melts, you start getting short circuits between wraps and, because of the shorts, generate more heat and more melting and, eventually, a meltdown.

#### Kylehaglock

Joined Dec 13, 2015
6
That's actually a really good point gopher... will using it sporadically make that a non issue? It's only a display for a science class that I teach.

#### BR-549

Joined Sep 22, 2013
4,928
Replace steel bar with iron bar.

#### Kylehaglock

Joined Dec 13, 2015
6
BR,

Totally would if I had access to one, but my father is a machinist, and the 1010 was free. I had heard it was the next best thing to iron was soft steel. It's already wrapped so that's kinda not an option at this point.

#### BR-549

Joined Sep 22, 2013
4,928
Buy a 1000 foot plastic spool of wire, that you can insert bar into.

#### crutschow

Joined Mar 14, 2008
33,364
A better magnet could be made if you wind the wires on a large u-bolt.
The return path provided by the U greatly increases the magnetic force for a given ampere-turns.

#### Kylehaglock

Joined Dec 13, 2015
6
Again, it's already wrapped. I really don't want that much space between the core and the copper anyway.