Magnetic induction generator: how to design?
Matias N. wrote 06/11/2017 at 15:28 • 1 pointHi,
I would like to understand how the different variables relate on a magnetic induction generator. I would like to use this on a bicycle and see what can I power with it. Having a no-battery wireless hall sensor (for a speedometer) would be nice. Also, I can use the same magnet use for sensing for power.
However, I do not know what characteristics the coil and magnets should have, how much current I can get per unit of time, what voltage do I get, etc.
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Start with thinking about how many Watts you generator should have. Do you want something like a regular bike dynamos which nominally have 6V/3W for running the lights or something that can translate the whole 150-250W that you may bring to the pedals in electricity or something that can just power some tiny piece of electronics with a few mA.
You can trade Volts for Amperes by choosing a thicker wire which results in less turns in the coils.
Main thing you need to understand is: A generator also is a motor and a motor also is a generator. Both are the same thing and both effects are happening at the same time.
So in a motor the rotation induces a voltage that prevents current from flowing through. Because of this an electric motor only uses the electric the power that is actually taken from the output shaft (and by friction and other losses).
The same way the current you pull from a generator creates a breaking force (that's were the power comes from). It also creates a magnetic field in the coils that weakens the main field. So if you use permanent magnets there is a limit for the power you can get at a given RPM determined by the layout and the magnets.
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Well,.. yeah. You will intrinsically pull more amps if you produce less voltage and maintain the same amount of power draw. Also yes, increasing the wire size will provide the ability to pull more amps (though you should size your wire by maximum expected current)
But, I think there might be an incentive to keep current production as low as possible. Smaller amperage means less heat, possibly less weight with the smaller wire, and to highlight Florian's comment: less resistance torque.
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That's not quite right. From a magnetic POV it doesn't matter what kind of wire you use. The only thing that matters is the size and geometry of the coil and the the sum of all currents going around in the windings. So 5 windings with 10A each is the same as 500 windings with 0.1 A each. If you are using wire with a 100 times smaller cross section the size of the coil (ignoring things like isolation) and also the losses will also be the same.
So using different wire keeps the power and magnetic flux produced the same but only trades Voltage for Amperes in a way the product stays the same. Think about the induced overall current as a single loop that is split into the single windings and then glued together to get the one loop voltage multiple times.
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The point I was trying to make is conductor size doesn't effect power output... (outside of the small amount of internal resistance.) Size your wire to prevent over-heating... not to effect power output
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It depends on how you look at it. Yes, just changing the wire thickness only improves efficiency. But in practise you can't do that. The coils are limited in size. So the amount of copper you can use in the coil is fixed. Keeping the amount of copper the same and changing the wire thickness also changes the number of windings. So everything I have said assumes you use all available space no matter what wire is used.
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Haha, okay.. I think I understand where you are coming from now Florian. Are you a physicist by chance? ..I would approach this problem completely different.
Let's say your goal is to power a 6V @ 0.5A device.
- I would pick a speed that you want to power your device at. As part of the power supply, I would actively regulate voltage output from the generator to 6V and cut power generation off when you are going slower than that. (Like a wind turbine)
- I would buy the heaviest neodymium magnets that my budget would allow. And then, crunch the numbers to determine the number of coils needed based on factors like velocity, magnet strength, rotor diameter...
- then,. I guess I would size my wires according to the output amperage.
Honestly, copper mass never enters my mind until the bike gets too heavy. But even then, i'm thinking of ways to reduce the number of coils by increasing the magnetic strength. I would still try to meet that 6V target.
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The layout is even more complicated. As the current is creating a magnetic field countering the one from the magnets things are dependent of the load. The optimum is half current. There are two worst cases with zero power output:
* Short ciruit: zero Volts, max current.
* Open: Max voltage, zero current.
Between these there is a parable in with maximum in the middle.
So generators with magnets behave more like current sources - assuming they turn fast enough. This is even more pronounced for real bike dynamos which iron parts. They basically produce 500mA at any voltage assuming high enough speed.
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ugh.. okay! How good is your calculus? lol.. I have a simple explanation and then I can elaborate with some simplified power engineering formulas.
Our goal is to control the power output characteristics: voltage, current, and frequency.
Frequency: I'm guessing we won't focus on this one since you'll be rectifying to DC? But, it's good to know that the speed of rotor and number of magnets determine frequency..
I think the formula is something like: frequency = [(rpm)(#of poles)]/120
Voltage: The voltage turns out to be the sum of voltages in all of the coil turns. So, the more coils the better.. higher magnetic field strength the better..
This formula is: voltage = (velocity x Magnetic Field Strength) * length of coil
Current: The current will be determined by your load. How much current do your devices want to pull out of your generator to function?! The more current you draw though, the harder it's going to be to rotate the rotor (or peddle). If you draw too much current, it's going to start messing with how fast you can peddle and which will mess with the frequency output (but you are rectifying to DC anyway).
If you want a formula for an advisable current range.. I'd guess to say look up the formula for electromagnets and then do some algebra... should look something like this:
current = [(strength of magnetic field * length of coil)] / (number of turns in coil)
If you'd like to study/verify this stuff for yourself.. i'd recommend researching the following topics: Faraday laws, Lorrentz force, Electromagnetism, Magnetic Induction, Electro-Mechanical Energy Conversion
Hope that helps!! I'll follow you too, just in case you need more help. It's great to put my education to use.. especially to help fellow clean energy hobbyists :)!
Source:
I've built a number of DIY renewable energy generators. Here is a little 3-phase generator that I built.
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tl;dr? I would focus entirely on reaching the intended load voltage. e = (v x B) * L.
Strong magnets, lots of turns of wire, little distance between magnets on rotor and coils of wire on stator.
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