# Energy harvesting using a brushless generator

Joined Jul 18, 2013
24,986
Wait a minute ... this is what I'm beginning to think:
• My favored setup with permanent magnets (let's call it the guitar pickup setup) will produce an offset DC sinewave because the magnetic field acting on the coils never changes direction (although it changes intensity)
Am I right?
The induced current in the string however, changes direction.
Max.

#### cmartinez

Joined Jan 17, 2007
7,627
The induced current in the string however, changes direction.
Max.
At around min 3, the narrator says that an "alternating current" is induced in the coil due to the magnetic field's concentration and relaxation effects, which is exactly what I believe will happen with my arrangement.

And the man makes some pretty informative observations near the end, in which he says that as far as coils go, short and fat makes for a higher (and hence better) inductance than tall and thin (Danny De Vito will be quite pleased when he learns that ... )

Also, the stronger the magnet, the larger the current induced in the coil. Although the string's vibration will be affected because of that, which is not an issue in my application.

So the video has actually reaffirmed my suspicions so far ... many thanks for sharing!

#### Danko

Joined Nov 22, 2017
1,269
Look at this generator. Electromechanical efficiency close to 100%, easy fabrication.

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#### Alec_t

Joined Sep 17, 2013
12,733
With any setup I bet you'll get real AC, albeit not a pure sinewave, because the induced voltage depends on the change of magnetic flux through the coil and that will be both positive and negative.

#### shortbus

Joined Sep 30, 2009
9,322
There is a big difference in the guitar pickup and the reluctance pickup we're talking about here. That difference is that a guitar string is changing direction as it vibrates, so AC is the output. In a reluctance pickup the gear teeth are usually always turning the same direction, so a DC sine wave is the output. Most of those reluctance sensors then use another circuit to "square up" that output, to make a square wave, to 'count' the teeth on the gear.

A sine wave, and I may be wrong, doesn't have to be AC. To my pea brain, AC means changing polarity not necessarily just an sine wave, AC has to go above and below a zero point. If this wasn't true, making a DC to AC inverter would not be such a big deal. And using those inverters to explain how the don't need to be a sine wave look at the modified sine wave inverter output, a series of square(rectangular) waves going above and below a zero point. I'm pretty sure one of you will point out where I'm wrong in my thinking.

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#### cmartinez

Joined Jan 17, 2007
7,627
AC means changing polarity
That is my understanding as well.

In my setup (gear-like rotor, with coiled magnets on the outside) the field expands and contracts as the rotor's teeth approach and move away from each magnet. Now think of it this way: an electric current is induced in the coil every time a magnetic field line cuts through its wire. And as the field contracts, it'll cut it in one direction, and when it expands it'll cut it in the opposite direction.

Now you owe me three beers... one because it'll generate ac, another one for the fun, and the third one for the explanation. And I sincerely hope I'm not wrong, or otherwise I'll owe you an entire case! ... Let's make it a Sam Adams Utopia, shall we?

#### cmartinez

Joined Jan 17, 2007
7,627
Look at this generator. Electromechanical efficiency close to 100%, easy fabrication.
View attachment 149795
Danke, Danko ...

That setup is one that I considered from the beginning, but yours has a couple of differences I hadn't seen in other designs. The first one is that in your scenario all of the coils have been connected in series, which should make it output a much greater voltage. And the second one is that you're using a steel disk on the other side of the magnet arrangement. That should focus their fields on the coils even further and increase the device's efficiency. Maybe its effectiveness at low rpm's could be increased even further by placing a second set of counter-oriented magnets on the other side of the coil arrangement instead of just a steel plate!

Your setup is definitely worth considering, and it might be just as easy to build as the guitar-pickup style. Question, have you actually built and tested this thing?

Many thanks for sharing.

#### Danko

Joined Nov 22, 2017
1,269
... Maybe its effectiveness at low rpm's could be increased even further by placing a second set of counter-oriented magnets on the other side of the coil arrangement instead of just a steel plate!
Here we should separate efficiency from effectiveness (maximum power on predetermined load and RPM).
Efficiency always is close to 100%, independently from magnetic flux value. Lower magnetic flux - higher should be RPM for the same power (Power = Torque * RPM ).
If you need bigger voltage - try at first to decrease wire diameter and increase number of turns (may be you simple have not input mechanical power enough). If your load will dramatically decreases RPM of wind wheel, then mechanical power is not enough.
...Your setup is definitely worth considering, and it might be just as easy to build as the guitar-pickup style. Question, have you actually built and tested this thing?
About 20 years ago I did precision instrumental mechanical load based on similar generator. At the same time I advised children in electronics classes. They built water mill model with 4W fluorescent tube, powered by this generator.

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#### cmartinez

Joined Jan 17, 2007
7,627
They built water mill model with 4W fluorescent tube, powered by this generator.
What general dimensions did they use? Magnet size?

#### Danko

Joined Nov 22, 2017
1,269
What general dimensions did they use? Magnet size?
Water wheel was about 300mm diameter. On side of wheel they attached 2 steel rings about 200mm diameter, one with 24 magnets about 15mm diameter.

#### cmartinez

Joined Jan 17, 2007
7,627
I've been browsing through the kjmagnetics website, and they have this couple of beautiful calculators for estimating the magnetic field strength between two magnets (depending on how they're mounted), for visualizing the magnetic field, and more ... the buttons at the top of the page lets one choose the type of calculation to perform ... it's fascinating stuff...

#### cmartinez

Joined Jan 17, 2007
7,627
Look at this generator. Electromechanical efficiency close to 100%, easy fabrication.
View attachment 149795
Danko, in your design, how much would effectivity be affected if the soft iron plate mounted in front of the magnets is removed? I'm sure you placed it there to further concentrate the magnet's fields across the coils, right? I'm only asking for simplicity's sake.

#### Alec_t

Joined Sep 17, 2013
12,733

Here's my guess.
If, for argument's sake, we assume each ferromagnetic component of the magnetic circuit comprising components 1-6 has a relative reluctance of 1 and the coils/air each has a relative reluctance of 10, then the total circuit reluctance is presently 1+1+10+1+10+1 = 24. Removing plate 4 would effectively introduce two air gaps instead, so the total reluctance would then be 1+1+10+10+10+10+1 = 43. In other words I think there would be a significant drop (~40%??) in efficiency.

#### cmartinez

Joined Jan 17, 2007
7,627
View attachment 152115
Here's my guess.
If, for argument's sake, we assume each ferromagnetic component of the magnetic circuit comprising components 1-6 has a relative reluctance of 1 and the coils/air each has a relative reluctance of 10, then the total circuit reluctance is presently 1+1+10+1+10+1 = 24. Removing plate 4 would effectively introduce two air gaps instead, so the total reluctance would then be 1+1+10+10+10+10+1 = 43. In other words I think there would be a significant drop (~40%??) in efficiency.
Wow! ... I didn't think it would be that dramatic!

#### Danko

Joined Nov 22, 2017
1,269
Danko, in your design, how much would effectivity be affected if the soft iron plate mounted in front of the magnets is removed? I'm sure you placed it there to further concentrate the magnet's fields across the coils, right? I'm only asking for simplicity's sake.
Here's my guess.
If, for argument's sake, we assume each ferromagnetic component of the magnetic circuit comprising components 1-6 has a relative reluctance of 1 and the coils/air each has a relative reluctance of 10, then the total circuit reluctance is presently 1+1+10+1+10+1 = 24. Removing plate 4 would effectively introduce two air gaps instead, so the total reluctance would then be 1+1+10+10+10+10+1 = 43. In other words I think there would be a significant drop (~40%??) in efficiency.
Not gaps only.
I will try to draw, what happens without iron plate. Wait some time...

#### Danko

Joined Nov 22, 2017
1,269

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#### cmartinez

Joined Jan 17, 2007
7,627
Thanks! ... that's exactly as I had visualized it. Another question about the proportion of this device. It seems that the width of each coil is about 1/4 of the magnet's diameter. So together, two coils side by side have a width (w) of 1/2 a diameter. Is this proportion a constant? Or can/should it be changed depending on the size of the apparatus or magnets?

Also, what should the coil's thickness (t) be? From your drawing, I'm guessing it's about 1/2 the magnet's thickness. Is this correct?

#### Danko

Joined Nov 22, 2017
1,269
...Another question about the proportion of this device. ...
If magnet diameter = 1, then
-distance between centers of neighboring magnets = 1.625;
-width of two coils together w = 0.625;
-thickness of coil t = 2...3mm (absolutely not depends on magnet dimension).

#### shortbus

Joined Sep 30, 2009
9,322
I'm not so sure that the illustration is correctly showing how the magnetic force would be in the way it's expected. With the magnets arranged like this . If they were arranged with all of the poles the same yes, but with them alternating I don't think so. But could be wrong, and would like proof. I'm basing my thinking on how a magnetic work holding chuck is made, how the lines of force are in them.