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Building an Alternator

  1. Jun 3, 2013 #1
    I understand that this question has been asked before, but I've seen so many different formulas that I'm questioning which one is applicable to my situation. What are the correct formulas I need to build an alternator to meet certain requirements? I need to meet a specific voltage and amperage requirements and that's it. I need to know how many coils, what gauge wire in the coils, how many magnets, what size magnets, what gauss magnets, air gap... Essentially everything. Is there anyone that can help me calculate all of this?
  2. jcsd
  3. Jun 3, 2013 #2
    Are you literally building this or is this a paper exercise.
  4. Jun 3, 2013 #3
    Literally building it.
  5. Jun 3, 2013 #4
    Is this a one-off hobby thing or are you building this for mass production.
    Last edited: Jun 3, 2013
  6. Jun 3, 2013 #5
    One-off hobby thing, for now.
  7. Jun 3, 2013 #6
    Sounds like this can be an interesting project for you.

    The more info you can post about your goals the better. For example:

    Permanent magnet or electromagnet field?
    Designing and building your own cores or purchasing something?
    How much power? What is driving it? How fast will it be spinning?
    Have you done any power-in power-out analysis? For example you will not be able to get 1KW out if it you are hand-cranking it.
  8. Jun 4, 2013 #7
    Yes it should be interesting and I'm looking forward to building it. As for your questions, not much is set in stone, the only requirements I need to have are voltage and amperage output but here are the answers:

    - I was initially thinking permanent but as I was saying if this needs to change for some reason it can.

    - I was also planning on building my own cores but if I would be better off purchasing certain items I will.

    - A lot of power (Hahaha!), wind (But this is not a typical application), and it will be spinning at a relatively steady 25 MPH.

    - No I have not done any power-in power-out analysis.
  9. Jun 4, 2013 #8

    jim mcnamara

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    Do you mean 25rpm? Windmill rotation is usually not in mph.
  10. Jun 4, 2013 #9
    No, I mean MPH. As I was saying this is not a typical wind application, and an RPM cannot be calculated yet because I have not designed the diameter of the alternator yet. So, it will have an air velocity of 25 MPH spinning it at a currently unknown RPM.
  11. Jun 4, 2013 #10
    Sorry for not being clearer.
  12. Jun 4, 2013 #11
    You may be approaching this the wrong way. The size of motor is not going to dictate how much power you are capable of extracting from the wind. You first need to determine how much power is available to be captured from the wind based on wind speed, blade size, friction, etc. This will tell you how big of an alternator you need.

    In other words you don't want to optimistically build a big 1KW alternator only to find out that the air cannot move it.
  13. Jun 5, 2013 #12
    I agree and that was my plan as an approach I just wasn't quite sure what calculations to do to get there. The wind speed is set at 25 MPH and cannot change, as for everything else I'm really just trying to determine everything before I build this rather than trial and error. I've found one formula that seems relatively legit but I wasn't sure, plus I had a couple questions about it. The formula is:

    N = V((Tesla*Magnet Area)/Seconds per Revolution) where N = The number of coil windings

    I'm under the impression you can divide N by the number of magnets that are in the alternator and therefore have an equal number of windings on each magnet that total to N. So I would then assume that when calculating magnet area you only include one magnet, because adding the total area would be redundant and incorrect, right? Lastly, I also assumed when you determine tesla (which I understand to be: 10,000 gauss = 1 tesla) you use the gauss of just one magnet as you do in area, because gauss doesn't change with multiple magnets, does it?

    So if I assume that all this is correct, now I have my coil winding number. Now I need the wire gauge for my coiled wire, how do I determine that?

    Then I need to determine air gap, and anything else I need to make this work and I'm not quite sure where to find how...
  14. Jun 5, 2013 #13
    Also, I know there is going to be power loss to friction, heat, etc... And I'm not exactly using a blade, I'm going to direct the air flow so it's more of a paddle wheel type shape. (Imagine something similar to a water wheel.) I know it sounds weird, don't worry... Hahaha!
  15. Jun 5, 2013 #14


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    But have you got yourself a suitable turbine to work this thing? Without a suitable source of mechanical power you will be disappointed. The turbine (and the availability of the wind, of course) is the crux and not the alternator design. Whilst it's not trivial, the problem of designing an alternator is very approachable. You have to ask yourself what rotation speed you want to work with, too. This is strictly 'Engineering' and needs to be approached accordingly.
    "Ha ha" doesn't make things work, I'm afraid. It will just cost you money for no results if you approach it this way.
  16. Jun 5, 2013 #15
    I have the wind power. I have a big window for rotation speed so that's not at the top of the priority list. Believe me, I fully understand this is an "Engineering" matter, especially since I am an engineer. I'm just not a EE or very familiar with building alternators. I'm a MECHE that works in the subsea oilfield business so as you can imagine EE is not something I encounter everyday. So as I said, don't worry. I just need some help with these formulas.
  17. Jun 5, 2013 #16


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    Fair enough. But are there any applications, similar to yours, which use such an unusual form of turbine? 25mph doesn't tell the whole story about how much power is available. Is this flow of air over an unlimited cross sectional area? I was concerned with the way you presented the original problem - which was 'unconventional' and, hence, a bit suspect. You would have got a much more positive response of you had just given an input shaft power and rotation rate. Do you have this information - purely to indulge me?
    BTW, this wouldn't involve air flowing from a tunnel or mineshaft, by any chance, would it?
  18. Jun 5, 2013 #17
    Completely understnadable, I've always been an unconventional type of person, and I appreciate your assistance. Yes, the air flow is over an unlimited cross sectional area; which I'm planning to reduce down to an unknown area. So with the area reduction it will be flowing through somewhat of a tunnel, but it's not wind from a tunnel or mineshaft. I'm not sure what kind of format for shaft input power you would like, so if you could give me an example of what you mean I can work on it. As for the rotation speed of the shaft, the shaft isn't designed yet. So there is no know diameter for the shaft, and therefore no known rotation speed. However, the velocity of the shaft will be 25 MPH minus the loss of speed due to friction. I was attemting to find a formula for all these things so I could "plug and play" with them so I could then narrow in on aspects like shaft diameter.
  19. Jun 5, 2013 #18
    no known diamter**
  20. Jun 5, 2013 #19


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    To be perfectly frank, I find the thought of you being a mechanical engineer to be insulting. You need to stop wasting our time and start acting like an engineer or this thread will be locked:

    Shaft speed is RPM, not MPH: what is it?
    Power is kW: What is it? You said you have voltage and amperage: what are they? You know you can use them to find power, right?

    Shaft speed can be changed with gears, but it is best to start with the rpm of the prime mover for simplicity and efficiency of design.

    But if you don't know input power, you can't design an alternator and this is a mechanical engineering issue, not an electrical one.
    Last edited: Jun 5, 2013
  21. Jun 5, 2013 #20


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    Unacceptable response by kdixon has been deleted. Thread is closed.
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