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I have a buggy/go kart project, and am currently trying to decide on a battery charging solution.

It is fully custom, so it is up to me to choose the device and build mounts, pulley ratio, etc. Many doing this same thing have gone with an alternator meant for a John Deere tractor. It is a 20A permanent magnet alternator, which requires an external regulator. I am also considering a mini GM alternator, which is a 35A standard type automotive variety - no magnets, excited with windings and internally regulated. The GM option is actually cheaper too.

The generator will be spun by a ~15HP engine, so, drag is important. The goal is to use the item that produces less drag. 20A will be plenty. I need to keep a battery charged, and run lights now and then (LED lights).

My understanding (possibly incorrect) is that, once full charge is reached, the regulator will "turn off" the alternator, so to speak. I'm not sure if this means there will simply be less draw, or if the magnetic field windings will actually no longer be energized. If they are always energized, I would think this would create more drag than a permanent magnet alternator because it requires power to energize them.

The permanent magnet alternator will always have drag, but not require battery power to energize the windings, so it may in the end ultimately have less drag.

If the regulator detects that peak voltage is obtained and there is minimal draw, will the alternator fully "turn off", resulting in a minimal drag? Or would a permanent magnet alternator be better?

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  • \$\begingroup\$ Regardless of type, external input ones which shuts off charging when you floor the gas pedal exists if you are worried about keeping your 15 HP when you need it. The drag from a permanent magnet one with no load is not zero but low enough. Load is what you should care about. Someone else perhaps know a good keyword for what I refer to? \$\endgroup\$
    – winny
    Dec 28, 2022 at 15:47
  • \$\begingroup\$ @winny Thanks for the comment.... harvesting all of the 15HP isn't actually the primary concern, it is more about drag at idle.... I'm just trying to understand the best option. It could be difficult to tune a low enough idle that doesn't engage the centrifugal clutch with an alternator with a lot of drag. Most of the time, there will be very minimal load, unless we are using lights at night time. \$\endgroup\$
    – slambeth
    Dec 28, 2022 at 15:59
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    \$\begingroup\$ The drag using a permanent magnet alternator will ALWAYS be higher than a coil excited alternator which will only have some very minimal residual magnetic field. I'd suggest you use the alternator with the internal regulator, but this of course requires that you have a battery present. \$\endgroup\$ Dec 28, 2022 at 16:09
  • \$\begingroup\$ this question belongs in mechanics mechanics.stackexchange.com \$\endgroup\$ Dec 29, 2022 at 0:32
  • \$\begingroup\$ @Jason Are you sure? I was really more interested in the inner workings of permanent vs wound created fields in the alternator along with the regulator. I'm not sure the mechanics section would be the best place for that question?? \$\endgroup\$
    – slambeth
    Dec 29, 2022 at 13:17

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The alternator with the excitation winding turned off is as good as it gets: it's just friction and air pumping losses. Nothing beats that. So that's the one I would choose, to minimize prime mover loading in idle conditions.

The regulator can be internal or external to the alternator. An external generic regulator wired into the alternator is more flexible. It can be replaced easily if it fails, without taking the alternator out. You can also replace it with a regulator of your own design. There's at least one EE out there who got tired of replacing the junk stock regulator in his car's alternator, and made a custom one instead that performed better and didn't fail.

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  • \$\begingroup\$ Okay thank you for your answer. The big question mark for me was does it "really" turn off the excitation of the windings, if it does, that certainly seems like it would be better. I will have to run this counterclockwise, so I'm not sure how that'll work out. I think it'll still produce power, but I may need to address cooling. \$\endgroup\$
    – slambeth
    Dec 28, 2022 at 19:39
  • \$\begingroup\$ A follow up on this, if you're still watching... under load, which would have more drag? The permanent magnet alternator, or the windings alternator? Lets say both have a load of 15A, which would require more effort to spin? The wound alternator is rated at 30A, the permanent magnet alt at 20A. \$\endgroup\$
    – slambeth
    Jan 10, 2023 at 16:09

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