Hi

I want to charge a 12V Battery harnessing energy of a rotating shaft having 100-300 rpm which varies frequently. What is the best method to utilize the energy-

1) Using a DC Generator and using a DC-DC converter with feedback to maintain constant voltage to charge the battery or
2) Using an alternator and rectifier setup.

Is there a better way? Kindly comment on the cost of both the systems as well. Thanks

 

Generator does not require a field supply voltage and can self excite , an Alternator needs a field supply from the original battery to start charging when the revs build up.
Alternators give out a better regulated supply at low revs and also higher currents.

 

Generator does not require a field supply voltage and can self excite , an Alternator needs a field supply from the original battery to start charging when the revs build up.
Alternators give out a better regulated supply at low revs and also higher currents.

What you said is not always true.

The generator part only pertains to a permanent magnet generator. The DC generators that were used in vehicles (before alternators came along) used field windings. The field windings were how the output voltage was regulated.

And there are permanent magnet alternators. Most of the household "emergency" alternators are permanent magnet based.

Generators are better at low speed than alternators. But alternators are usually higher amperage output for a given physical size.

But the big difference between the two is the means of making the power. Generators have a stationary field and the power is made in the rotating part. Alternators have the field in the rotating part and the power is made in the stationary part.

 

Thanks. In my application the rotation is not constant so the alternator would keep taking the initial charge every time it starts i guess.
So charging with the dc generator and keeping constant output voltage of 15V using a dc-dc step up converter is feasible right?

 

If it were me, I would use an alternator since they are available in abundance, at the junk yard you can get one, plus a couple pulleys and belt to step up your rpm for the cheapest solution.

 

If it were me, I would use an alternator since they are available in abundance, at the junk yard you can get one, plus a couple pulleys and belt to step up your rpm for the cheapest solution.

+1
An automotive alternator from the junk yard is nearly free. They work best at much higher rpm, like 5000rpm, so a 20:1 gear-up would be about right.
If you don't want to excite the field windings, you can look for a permanent magnet alternator. But controlling the field gives a good control point for controlling the output.

Rectify the AC from the alternator. Then your concern is whether the alternator can ever cause over-charging of your battery. If the battery is big compared to the alternator, this is not a concern. If you need to limit current, there are many options. Come back with more details and someone will know.

 

Great! Thanks guys i'll try out few things and come back

 

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Rectify the AC from the alternator.
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Don't most automobile alternators have built-in rectifier diodes?

 

Don't most automobile alternators have built-in rectifier diodes?

Yes, I think so. The wind power guys often want to use their own rectification schemes though, and remove or bypass them.

 

Some people use the Fisher and Paykel motors for low-RPM generators.

They are a large 3 phase brushless motor used in washing machines, and can be reconfigured to generate a lot of power from low-RPM sources like wind power etc.

 

Thanks I'll look into fisher and paykel

 

here is some very late info. 1) your question is vague. consequently, all replies Must be considered vague, as well.
2) there is a difference between a car alternator and a pma/pma-conversion. a permanent magnet alternator, as the name indicates, has MAGNETS in it.
3) RPMs do not equal POWER. that is Part of the information that you did not provide.
4) it sounds like wind power. if so, you should look at small wind turbines; based on wind speed, etc.
5) any brushed dc pm motor, when used as a generator/alternator, will make dc; and will be less efficient than a machine designed as an alternator/generator.
6) if you want a good pma, you should just BUY ONE. look on ebay / amazon. you should, however, KNOW what you are looking for; in terms of cogging, rpm range, ac or dc output, etc.
7) in the case of a brushless pma, before rectification, the output will be 'wild' ac; which is Not directly usable. however, if cable runs are long, the 'wild' ac can be run to the place where DC/rectification is desired. if a pma has a rectifier preinstalled, it could be removed, if desired.
8) power is WATTS. not volts. example - if pma output is 7 volts @ 15 amps, you would have 105 watts, to 'play with'.
https://www.ebay.com/itm/WIND-PMA-P...611039?hash=item5b2699651f:g:T1oAAMXQUShQ7hJ1