Generator coil output - Sorry if it's a stupid question 6

[nattyb52]

Hi, I'm new here and I apologize up front if my terminology is incorrect. I've been a Civil Engineer for over 30 years, but am very far from an expert in electrical. Sorry if this is an elementary school level question for you guys!!!!

I am hoping someone here can either give me a hand or point me in the right direction.

I have a proposed electric coil design and was wondering if I am doing my power potential calculations correctly.

My "coil" at this point will be wound on an 2.5" long by 1/2" wide by 2" high iron or ferromagnetic core or spindle. I am looking at about 70 meters of AWG#20 magnet wire for the windings.

If I pass a neodymium magnet, Grade 42, gauss 13,200, 3" long, 1/2" wide and 1/4" thick directly under it, at a velocity of 2 m/s and 90 degrees to the long side of the coil I would like to be able to verify what amount of voltage I can get out of it. And then, based on the proposed circuit resistance, what power and current I can expect.

I've found and used the following HyperPhysics page to get an approximation. I will be supplying approximately 300 pounds of force to push the magnet past the coil.

My best guess, using the formulas from the above web page, is that I can get 730 Volts. And applying 200 ohms of resistance to the circuit will lower the push force required down to 300 lbs. This gives a current of 3.65 amps and a power output of 2674 watts.

I realize the calculations are only theoretical and there are probably many other variables that come into play, but if any of you could help me verify the numbers, or point me in the right direction, I would really appreciate it.

Thanks much in advance for any help you can offer.

 

[OperaHouse]

Try fieldlines.com There are a lot of people there with practical experience with winding generators.

 

[electricpete]

fwiw, I did a very quick and dirty calc with what I thought were generous assumptions (to predict high voltage) and came up with 67 volts. This is based on 500 turns. You might want to double check it. I suspect actual voltage would be lower. Also of course it only lasts briefly. And you have a challenge to control to motion of your magnet so it doesn't get sucked onto the coil iron.

Desc	Symbol	Value	Units	Formula
Length in axial direction	Length	2.5000	INCHES	2.5
Width	Width	0.5000	INCHES	0.5
Height	Height	2.0	INCHES	2
Perimeter around W and H	Perimeter	5.0	INCHES	=2*(Height+Width)
Fractional Increase in mean perimeter due to coiling	X	1.2	unitless	1.2
Expanded permiter	ExpandedPerim	6.0000	INCHES	=X*Perimeter
Number of turns	Turns	5.00E+02	unitless	500
Total length of conductor	Length	3.00E+03	INCHES	=Turns*ExpandedPerim
Diameter of conductor	Dconductor	3.40E-02	inches	=0.034
Areas based on conductors	Aconductor	9.08E-04	inch^2	=PI()*Dconductor^2/4
Total area of all condutors	Atotal	4.54E-01	inch^2	=Aconductor*Turns
area calculated from X	Acalc	4.00E-01	inch^2	=Width*Height*2*(X-1)
velocity	Velocity	2.00E+00	m/sec	2
Rate of change of area wrt time	dAdt	1.00E-01	m^2/sec	=Velocity*Height/39.97
Flux density	B_	1.34E+00	T	=13400*0.0001
Voltage	V	6.71E+01	volts	=B_*dAdt*Turns

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[electricpete]

Hmmm. Maybe I did not properly account number of turns. Let me try that again later and post back. How many turns are you assuming?

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[nattyb52]

Thank you both, I do appreciate it

 

[nattyb52]

Pete,

Right now I have it as 60 turns per layer and a total of 12 layers.

 

[nattyb52]

It will also cycle past the coil 4 or 5 times a second. I've already got the mechanism for controlling the magnet path and mechanics worked out.

 

[electricpete]

Attached is the same calculation, except I have changed the definition of X and of the calculated area to be more sensible. I added a figure to explain what I was doing. It is mostly a simple geometry problem to figure out how many turns you can wrap in that geometry... then simple Faraday's law. (simple doesn't mean I haven't made a math error). I didn't sharpen my pencil on flux density, just used your value for magnet flux density (I assume it represnts flux density at surface of magnet in free air)... which I assume is somewhat an upper bound for flux density in the core since there is still a lot of free air and core is some distance from the magnet (did you use a different value for coil core flux density?). Still comes out with only 63 volts which is much lower than your result. I would say one of us must have made an error.

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[electricpete]

Also my number is still dc, so does not translate directly to ac. First cut would be divide by sqrt2, but that is iffy..need to look closer at what kind of motion the magnet will have in relation to the coil boundaries and whether it will "act" like a sinusoid with peak velocity 2 m/sec

Also, I'll mention that MacGyverS2000 is a valued regular and just excercizing his perogative to check up on your motives (we have seen plenty of students).

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[electricpete]

My calc was based on open circuit conditions, also. Under load, terminal voltage would be even lower.

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[nattyb52]

Thank you very much Pete. I made a mistake, yes. My numbers were based on 4 coils and magnets, because that is what my design is based on. I had used 270 meters of wire and a corresponding number of turns and layers.

And I apologize for sounding off. It's been a long rough year.
Thanks so much for your help.

 

[waross]

Going by the sketch posted, it looks like you are not completing the magnetic circuit. A long flux return path through air is inefficient and may drop the flux density by several orders of magnitude. Simple problems in flux density are sometimes worked on the basis of amp turns per inch of the magnetic circuit.
If the length of the iron portion of a magnetic circuit is doubled the flux density resulting from the same magnetizing force is halved.
Keep your iron and magnet path as short as possible.
If part of the iron path is replaced by an air gap, the ratio may be over 10,000 to 1. Air gaps are bad. Air gaps are kept as short as possible.
That is the main reason that horseshoe magnets are made in a horseshoe shape.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

After you've settled the electromagnetic calculations, I recommend that you double check the answer to make sure that the estimated electrical energy output is not greater than the calculated mechanical energy input.

If the output exceeds the input, then either a Nobel prize awaits, or perhaps there's an error in the assumptions.

 

[electricpete]

The flux density 13200G that you mentioned is probably Br, which would roughly be the flux density if you connected a super-high permeability iron strap from one face of the magnet to the other. Your actual flux density it going to end up orders of magnitude less with all the air in your magnetic circuit as waross mentioned.

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[electricpete]

This calculator may give you an idea of the order of magnitude of flux density that you can expect

http://www.dextermag.com/magnetic-field-calculators

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[MacGyverS2000]

Aw, man, you mean I missed someone ripping me a new one? ;-)

Now that we've established you're not a student, I'm curious as to the project's purpose... always interested in seeing what other fields are doing and what for.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[nattyb52]

The coil design is far from set in stone. In fact, it can be modified in a variety of ways as needed to improve the performance. The actual packaging of the design is wide open right now and can be adjusted or modified as needed. I'm just a little too inexperienced to know the exact route to follow. There are no size or shape restrictions, on the coil or spindle design. I can even pass a second magnet over the top if that would improve the output. If the flux path is important, I can shorten the spindle and increase the windings. I can enclose the windings in the core if that would help. I can even stack several "coils" right next to each other if needed.
Would any of these things improve it in any way. Thanks in advance.

 

[waross]

Here is a simple design to show the effects of changes in dimensions.

Imagine a ring 1 inch thick and 6 inches in diameter with a 4 inch hole in it.
For a given magnetizing force, increasing the mean diameter will decrease the total flux.
For a given magnetizing force, increasing the cross sectional area will increase the total flux.

Now imagine a slot slightly greater than 1/4 inch cut across the ring. You may pass your magnet through this slot. There must be a small air gap to allow the magnet to pass, however one inch of air gap may be equivalent to about 10,000 inches of iron. (Check the magnetic property curve of the iron for an exact figure at the working flux density.)

To refine this a little further, Cut out a circular part of the ring that may be rotated in the ring. Mount the magnet in the center of this part and then rotate it in the hole from which it was cut.

The voltage will be proportional to speed of movement of the magnet or the speed of rotation.

Early magnetos had a mechanism that would stop the movement of the magnet and compress a spring. The spring would release and the magnet would rapidly pass the core/coil for a good spark. This gave a good spark when hand cranking an engine to start it. It had the added advantage of retarding the spark to avoid backfiring. Once the engine started centrifugal force would disengage the spring action mechanism.
oral:
The speed of the magnet past the core/coil is important.
The speed of the magnet does not have to be constant through-out its travel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nattyb52]

I'm doing this on a personal basis, not as a Civil Engineer. I'm wandering far afield and feeling very lost

Playing with some ideas to see if anything comes of it, but I've got so much to learn and am still stumbling my way along.

 

[electricpete]

To refine this a little further, Cut out a circular part of the ring that may be rotated in the ring. Mount the magnet in the center of this part and then rotate it in the hole from which it was cut.

You were better off with the magnet in an airgap. if the magnet sits in a hole surrounded by steel, than the steel forms a short circuit path for the magnet, which causes increased flux through the magnet, which costs MMF drop without any useful purpose.


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