I have an automotive alternator, and would like to rewind it for higher voltage for wind power application. Since I am going to use it for Battery Charging, should rewinding it for single phase instead of three phase has any advantage ?

I think, voltage will be on higher side in single phase winding and current will be less, what are other advantage or disadvantage I should look out for ?

  • 2
    \$\begingroup\$ Wouldn't it be easier to charge the battery with a 3-phase rectifier? Also what kind of battery are you going to charge? Usually the main problem would be current and not voltage for say lead-acid batteries. \$\endgroup\$ – AndrejaKo Feb 10 '12 at 12:53
  • \$\begingroup\$ 3 phase - or more, will be better. \$\endgroup\$ – Russell McMahon Feb 10 '12 at 14:27

Rewinding as single phase makes no sense. If you have a 3-phase system it is almost always preferable to single-phase:

  • the ripple current caused by 3-phase rectifiers is much smaller
  • the power flow is smooth (vs. single phase which has very high 120Hz power ripple)
  • use of copper is more efficient

Not to mention that I have no idea how you would manage to rewind as single phase. I think for the time + effort + money you spend on rewinding, you could just buy a 3-phase motor/generator in the voltage you need.


I would leave the alternator alone. Lots of engineering and carefully considered decisions went into its design. Nothing good will come out of trying to "rewind" it. You're just going to mess it up.

Either get the right generator for what you are trying to do, or deal with the output of this one. Modern electronic switching power supplies can do pretty efficient conversions. Think of a switcher as a electronic gearbox, just a lot smaller, cheaper, lighter, less messy, and with no parts to wear out.

If you really want to get fancy, you do power factor control on each of the three outputs separately. This will cause the input torque to be smooth over the whole rotation cycle. However, even just rectifying the three phases and then dealing with the resulting DC should still get you where you want to be.

  • \$\begingroup\$ "If you really want to get fancy, you do power factor control on each of the three outputs separately" -- If I wanted to do some PFC, I'd just do single-phase PFC on the rectified outputs (with very reduced DC link capacitors); it's much easier (even easier than regular single-phase PFC because the voltage ripple is greatly reduced) and should be close enough to unity power factor for all practical purposes. \$\endgroup\$ – Jason S Feb 10 '12 at 17:46
  • \$\begingroup\$ @Jason: That does not result in unity power factor, at least if you have the current track the instantaneous voltage. With three phase and constant load, the instantaneous power is constant accross a whole cycle. Current proportional to the max of the three sine waves is better than nothing, but not the same as full power factor control. I suppose you could modify the current to draw constant power, but I don't know what the effects of banging back and forth between phases is, even if the instantaneous power is always constant. Maybe a poor man's PFC? \$\endgroup\$ – Olin Lathrop Feb 10 '12 at 19:29
  • \$\begingroup\$ That's why I said close enough. I'm going based upon intuition here; there's some analogs to 3-phase motor control which I don't have an easy way to summarize briefly. Basically the "banging back and forth" isn't that bad; what happens is that one phase doesn't carry any current but its voltage is near zero, so it's a relatively inactive part of the waveform. Also you raise a point which I glossed over, namely that you'd want the current to be higher when the voltage is lower. But there's still not much variation: rectified 3-phase AC has six 60-degree segments between max and 0.866*max. \$\endgroup\$ – Jason S Feb 10 '12 at 19:37

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