Winding a strong electromagnet

[Xaverius]

X - u da man!

When it comes to math - often I'll skim through it to get to the conclusions/summary at the end.

Having it translated into English is most excellent.
(I sucked at Spanish - and that's really what math is - another language.  A useable translation means an increase in the ability to 'speak' and understand it)



And yes - (line 1) .5 watt: 1.12V x .48A = .54 watts.  That's the thing - seems crazy strong for so little juice. Still seems something is out of wack?
Holding test resulted in: 3/4 lb. calculated Gauss: 4875
3/4" x 1/4" N42: 18 lb (stated spec - not tested) Gauss: 13,200
So the offered calculation would seem to imply a stronger holding force than the observed 3/4 lb?
(edit: then again, there is a difference is surface area 1/2" EM core vs. the neos 3/4"... and?)

As to application - I'm pulsing the EM in repulsion against a PM over a small airgap.
However - texts state attraction forces are stronger than repulsion.  The flux gets 'squeezed out the sides' in repulsion.  Just some side-thinking on the maybes.....

Hi, yes, attraction is more efficient and it keeps the permanent magnet from degaussing, which is a hazard of repulsion.

Yes, it is a strong attraction for a small amount of electricity but the key is the number of turns and the permeability.

For the N42 with a surface area of 3/4"  I calculated 58 pounds, and for 1/4" it was 6 pounds.  From my understanding the permanent magnets don't always have the apparent pulling power because of geometric constraints, such as shapes and sizes of attractants.

For the EM I calculated 3.5 pounds.  Are you using ferrite?  Something is wrong with the parameters, did you obtain the 4800 gauss figure from the chart?  Needs more investigation.

[capthook]

Got my ferrite rods - will do some testing next few days....

- - -
Magnetic Design Formula (math stuff someone might find useful - it's all Latin to me  )

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Voltage-Flux Density-Frequency (Faraday's Law).
Flux density must be determined to assure proper operating level. The level of flux density changes the value of Rac, the core loss resistance, and has an effect on the permeability as shown below. Flux density can be calculated from:
Bmax=[(Erms * 108)/(4.44*A*N*f) ]
   (4.44 for sine wave, 4.0 for square wave) where:
Bmax=Maximum flux (gauss) density
Erms=equivalent rms voltage across coil
N=number of turns
A=core cross section (cm2)
f=frequency (hertz)


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Ampere's Law
Ampere's Law relates magnetizing force to peak current, number of turns and mean magnetic path length. H=(0.4*pi* N*I)/l  where:
H=magnetizing force (oersteds)
N=number of turns
I=peak current (amperes)
l=mean magnetic path (cm) 
The magnetizing force determines the estimate of flux density using the normal magnetization (B/H) curves. The relative permeability at that magnetizing force can then be determined by:
µ=B/H  where:
µ=relative permeability
B=flux density (gauss)
H=magnetizing force (oersteds) 

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Inductance Considerations
The inductance of a wound core can be calculated from the core geometry using the following formula: L=(0.4*pi*µ*N2*A*10-8)/l where:
L=inductance (henries)
µ=core permeability
N=number of turns
A=core across section (cm2)
l=magnetic path length(cm) 

AL is the Inductance Factor of a core, and is expressed as nH/turn2 (or mH/1000 turns).
L(nH) = N2 * AL
where:
L = inductance in nH for N turns
AL = core inductance factor
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Magnetic Formula Conversions
The following table lists conversion factors for magnetic units.
oersteds * 0.795 = amp-turns/cm
gauss * 0.0001 = tesla
mH/1000 turns * 10 = uH/100 turns
in2 * 6.425 = cm2
circ mils * 5.07 x 10-6 = cm2
watts/lb * 17.62 = mW/cm3

[capthook]

Well - ferrite sucks as an EM core........

Wound a quick coil - has like 1/4 the holding power of the hex bolt.

Guess that's why that make filters out of the stuff - it's high permeability sucks up flux great - and doesn't transmit it well?

Oh well - only cost me $20.........

Maybe I'll see if I can get some electrical steel from a junkyard - an old car alternator or microwave or something this weekend.  (actually - I have a junk riding mower - maybe it's got something)

- - -

As to the discrepancy of the holding power of line 1 in the chart - turns out I was testing the hold on a much smaller coil - 40' of #22 - not 125'...... DOH 

[capthook]

May have finally found a US supplier for premium EM core material:
"CMI specializes in ultra low carbon electromagnetic iron, not merely low carbon steel."

http://www.cmispecialty.com/products.cfm

"CMI-C Magnetic Core Iron Cold Drawn Rod and Bar per ASTM A-848-01
Diameters:  .2500, .3125, .3750, .4375, .5000, .5620, .6875, .7500, .8750, 1.000, 1.250, 1.500, 1.750, 2.000, 2.250, 2.500, 2.750, 3.000, 3.250, 4.125, 4.500,  6.000"

"PRODUCT DESCRIPTION
CMI-C Electromagnetic Iron Rod is specially processed with a critical strain for optimum uniformity. Maximum magnetic properties are achieved following suggested final anneal applied to fabricated parts.

• BENEFITS
Hi-permeability, low coercivity. Low loss provides highest force/watt input."

Will call tommorrow for prices etc......sounds expensive and like it comes in 6' sections min.

[Freezer]

Perhaps someone could try a pyramid type electromagnet coil design.  See the video for an explanation.

http://www.youtube.com/watch?v=tR_8f0DYK5s&feature=related