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

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

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

(OP)
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.

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

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

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

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.

CODE

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
 

=====================================
(2B)+(2B)'  ?

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

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?

=====================================
(2B)+(2B)'  ?

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

(OP)
Thank you both, I do appreciate it

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

(OP)
Pete,

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

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

(OP)
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.

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

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.

=====================================
(2B)+(2B)'  ?

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

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).   

=====================================
(2B)+(2B)'  ?

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

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

=====================================
(2B)+(2B)'  ?

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

(OP)
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.

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

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

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

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.

 

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

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.

=====================================
(2B)+(2B)'  ?

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

2
Aw, man, you mean I missed someone ripping me a new one? winky smile

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

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

(OP)
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.

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

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

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

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

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.

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

Quote:

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.
 

=====================================
(2B)+(2B)'  ?

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

(OP)
Waross, thank you for your input.

What you are describing sounds like a solenoid. Is that right?

I know a toroid is one of the most efficient shapes for controlling flux density, but I thought one of the important points for electricity generation was to have the magnet poles pass at 90 degrees to the coil windings. If the magnet were to be passed through the core/windings as you describe, how would you orient the magnet poles in relation to the wire? Maybe I'm not visualizing it correctly.

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

I guess there are potential benefits and potential disadvantages of that construction.   It definitely could work well depending on the geoemtry.  If diameter of the cutout hole is same as width of the original ring, then the bypass path is narrow/limited and may even tend to saturate, which would help.  And the benefit I'm sure Bill was envisioning is reduced airgap (the equivalent airgap when magnet aligned along the ring would be the width of the magnet)

=====================================
(2B)+(2B)'  ?

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

As clarification, my 10 May 11 11:21 post was followup to my 10 May 11 11:16 post, not to  nattyb52's post.
 

=====================================
(2B)+(2B)'  ?

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

Hi guys. The purpose of my suggestion was to illustrate the relationship between MMF, total flux, flux density and the length of the magnetic path.
Pete, imagine the cutout circle to have a 1/4" slot cut midway across it and the magnet (i/4" thick) placed in this slot. No iron bridging across the ends of the magnet but of course some leakage flux.
We have a ring that is 1" across. Not the diameter but the external radius minus the internal radius.  We have cut out of this ring a 1" circular piece and inserted the magnet into this piece. We have replaced this piece and are now rotating it. When the magnet is at right angles to the ring there will be maximum flux in the ring. 180 degrees later the flux will be maximum but in the opposite polarity. A wire coil wound around the ring (the wire passes through the hole in the center of the ring on each turn) will have a voltage induced in it by the varying magnetic flux in the ring. The ring may be replaced by a rectangular shaped core.  

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

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

Thanks Bill. I agree, once it was clarified that the hole will be approximately the same width as the ring.

=====================================
(2B)+(2B)'  ?

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

(OP)
Pete and Waross, thank you for all your help. I really appreciate it.

I know that increasing the velocity will increase voltage, but is it proportional or exponential?

When you talk about cross sectional area, are we discussing only total winding cross section or is it core cross section, or both?

Thanks again

Jim

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

Proportional.
Core area.

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

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