April 2016
I have a question about an article I saw where they had a platter of coils and a platter of magnets. They spun the magnets over the coils producing energy. My question is: How do you determine the correct wire size and number of turns, size and the number of coils, and the size and number of magnets to get to a desired voltage and amperage?
Is there a best way to figure out what combination works?
I appreciate any insight or direction.
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You can find the American Wire Gauge (AWG) from this formula that I found in the Magnetics Incorporated Ferrite Core catalog and handbook:
AWG = -4.31*ln(1.889*I/C) where
I = the AC or DC current
C = the current density in amperes/cm2 ; 400 is conservative or you can use 800.
To do any calculations you will need to know the magnetic strength of the magnet, symbolized by H and measured in Oersteds. The magnet manufacturer should be able to give you that info. The H varies with distance so you need that data and how far the coil (I am assuming solenoid coil) is from the magnet. Given the number of turns in the coil and the speed of the magnet, it is possible to calculate the voltage output. That is a complicated procedure and I don’t know how to do it.
For a one off project, it may be better to do trial and error: choose a core, wind a coil, spin the magnet, and measure the voltage. You will need an oscilloscope. Add or remove turns as needed; pretty straightforward approach!
I think the coil should not be much bigger than the magnet and if you have a magnet for each coil and connect the coils in series, the voltages will add. However, you cannot produce a DC voltage that way. You could use a diode to produce pulsating DC to charge a battery.
There is going to be math. SERIOUS MATH! Number of turns will be given by Faraday's law, at least to a first estimate. https://en.wikipedia.org/wiki/Faraday's_law_of_induction Look about half way down the page where it says, "Faraday's law states that the EMF is also given by the rate of change of the magnetic flux..." And http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html There are many more internet sources, but I did say that this will only give you a first estimate. You are going to have to factor in things like the coil resistance and the current that is actually flowing (I^2 R loss) and perhaps others to come up with the final figure.
Wire size: If you are just experimenting with a crude apparatus, like you refer to, then almost any wire size you use will be OK. This is because you will find it very hard to generate any really useful level of current. I would suggest starting with a #24 solid, enamel insulated wire and see if you can even burn it out, I doubt it. But, there are tables for the ampacity of different wire sizes. Since this will be a coil which limits ventilation, you will need a table that takes this into account.
In difficult cases you may have to do the calculations for heat flow from the interior of the coil to the external atmosphere. And, of course, larger wire sizes will have smaller losses (I^2 R) in the coil so that is a factor if you are generating a serious amount of power. https://en.wikipedia.org/wiki/Ampacity
All of these calculations are interrelated so in a serious design you would probably have to go through the calculations several times to arrive at a final solution. And build a prototype to verify them.
This is how I would do it, but I’m devious: Start by figuring out how much voltage and current output you need. Mark these up by 30-50%. Get a brushless DC motor that runs from that much voltage and current. Take out the motor driver circuit and replace it with a polyphase rectifier. (sounds intimidating but really just three to six diodes) You are done.
