I'm building a generator for a school project. I'm planning to use N42 neodymium magnets with performance of 5100 Gauss, How do I calculate what diameter of copper wire I need to make the coils and how do I determine the best number of turns for that particular set up?

  • \$\begingroup\$ Are we talking about a permanent magnet AC generator? What's the \$μ_r\$ of the core? What sort of budget are we talking about? \$\endgroup\$
    – Hearth
    Commented Nov 8, 2018 at 23:00
  • \$\begingroup\$ Yes it will be a permanent magnet AC generator. I believe the relative permeability would be 1.05. My budget is $100. \$\endgroup\$
    – super95
    Commented Nov 8, 2018 at 23:05
  • \$\begingroup\$ Okay, with a budget like that I have to ask, do you want a "generates usable power" generator, or a "demonstrates the principles behind generator operation" generator? The former may be hard at that budget; the latter less so. \$\endgroup\$
    – Hearth
    Commented Nov 8, 2018 at 23:06
  • 1
    \$\begingroup\$ "I believe the relative permeability would be 1.05." What, you're using plastic with a pinch of rust stirred in? Ferrites for high RF have \$\mu_r\$ of 10 or so, steel is effectively around 200. \$\endgroup\$
    – TimWescott
    Commented Nov 8, 2018 at 23:34
  • 1
    \$\begingroup\$ As it stands this question is too broad, to engineer a motor takes a knowledge of books of information. Best thing to do at this point is experiment \$\endgroup\$
    – Voltage Spike
    Commented Nov 9, 2018 at 16:36

1 Answer 1


This assumes you have an oscilloscope or a sensitive-enough AC voltmeter. Grab any old wire and wind one turn around the core. Don't worry if it's a bit messy. Spin the generator at the design speed and measure the voltage at coil (not after the rectifier, if you're using one). That's how many volts per turn you'll generate at that speed.

Now divide the voltage you need by that number, taking into account whatever losses you'll get from rectification, etc. That's how many turns you need.

Now estimate the area you have available to put those turns into. Divide your area by the number of turns you need. That's the area of the wire you have room for. Multiply it by 0.75 or 0.85 depending on how super-cool you think you are at winding.

Get a wire table (the ARRL handbook has one, there may be at least one on some forgotten corner of the Internet, somewhere). Find a wire size that has the area you need. Get some, and start winding.

And do not be upset if your calculations are way off the first time around -- that just means you're an engineer.

  • \$\begingroup\$ yeah, measuring is also the only way I could approach this task. \$\endgroup\$ Commented Nov 9, 2018 at 0:07

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