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For a school project, I am building a small hydroelectric generator. It uses a 10v, 600RPM/v brushless Tower pro 3520-7t-8 motor, which, when geared up (I'll spare you the calculations here), should be spinning at around from 150 to 800 RPMs. So, in other words, it will be putting out, depending on its speed, approximately 0.2 to 1V.

I will run the output from the motor through a three-phase rectifier (it uses six diodes) and a capacitor for some smoothing, and then on to the 5V regulator, which I am asking about here. The final idea is to have a USB output to, for example, charge a phone.

To summarize, what is the easiest and most efficient way to get from unregulated 0.2-1V to regulated 5V?

Motor

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    \$\begingroup\$ Why do you have to rectify the low-voltage signal? It might be easier to use a transformer to multiply the low voltage up to say 6 or 7 V unregulated, then rectify and then regulate down to 5 V. Admittedly the 5:1 dynamic range of your input makes this possibly a bit messy, but it will also make boost solutions tricky. \$\endgroup\$ – The Photon May 17 '16 at 1:59
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    \$\begingroup\$ The bridge rectifier will eat up all of your voltage. A diode drops a few hundred millivolts at best, a volt or more at worst. \$\endgroup\$ – pipe May 17 '16 at 2:07
  • \$\begingroup\$ @The Photon Since I am using a brushless motor, (the type from a radio control plane), it outputs three-phase AC. \$\endgroup\$ – willem.hill May 17 '16 at 2:08
  • \$\begingroup\$ This is the motor I'm using, just so you know what I'm talking about: imgur.com/MB8oHBm \$\endgroup\$ – willem.hill May 17 '16 at 2:19
  • \$\begingroup\$ @ThePhoton See above ^ I didn't include your username properly. And thanks for the comments everyone. \$\endgroup\$ – willem.hill May 17 '16 at 2:25
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Basically, you're in a bad position.

Using your specified approach, a simple 3-phase rectifier just won't work: at the lower end your AC voltage is not nearly enough to overcome diode drops in your rectifier. Even using Schottky diodes a minimum peak voltage less than about 0.5 volts will not produce a useful current out of the rectifier.

ThePhoton has what seems like the obvious alternative, which is to boost the generator output using a transformer and then rectifying and regulating the output. There are two problems here. The first is the relatively large voltage range (5:1) of the generator output, although this can be dealt with. Much more of a problem is your generator speed. 150 to 800 rpm will give generator output frequencies in the range of 2.5 to 13.3 Hz, and finding transformers which will operate at these frequencies will be a good trick. At the least, they'll be big.

I'd recommend increasing your gearing ratio by a factor of 20 to give a range of 3000 to 16000 rpm. This will give an output frequency in the range of 50 to 267 Hz, which fits nicely in reasonably standard transformers, which you can get with 50 Hz to 400 Hz capability. In doing so, you'll up the output voltage to 4 to 20 volts, and you should be able to make a fairly standard DC supply from that. Depending on your exact current requirements, it may well be necessary to use a slightly higher gear ratio to get adequate voltage at the low end, but you're at the low end of the available frequency range anyways, so that should not be a big problem electrically. Mechanically may be another story, but hey - it's just gears, right?

You won't be able to get standard 3-phase transformers in the low power range you want, but you won't need them. Just hook your bridge to the generator directly.

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The motor you're using is designed to run at around 6000 RPM (10 V x 600 RPM/V). It will generate very little power at 800 RPM, and essentially nothing at 150 RPM.

Ideally, you shouldn't be using this part. It's designed primarily as a motor, not a generator, and even so it's designed with a very different operating range in mind than you need. Consider using an automotive alternator instead; those run at speeds closer to what you're describing.

If you must use this motor, though, you will need to gear up the input significantly to make it operate efficiently.

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  • \$\begingroup\$ Random, slightly off-topic question - will a 12V motorcycle starter I have sitting in my barn work well for this? I don't have it on me right now, so I can't say anything for specs... \$\endgroup\$ – willem.hill May 17 '16 at 3:13
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    \$\begingroup\$ Impossible to say without looking at it, but probably not. Most starter motors have a solenoid integrated that disengages the motor from the shaft when the motor is unpowered. \$\endgroup\$ – duskwuff May 17 '16 at 3:33

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