DC generator vs Alternator

Discussion in 'The Projects Forum' started by Sudeep_Banerji, May 15, 2013.

  1. Sudeep_Banerji

    Thread Starter New Member

    May 15, 2013
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    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:)
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,993
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    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.
     
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  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    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.
     
  4. Sudeep_Banerji

    Thread Starter New Member

    May 15, 2013
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    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?
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    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.
     
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  6. wayneh

    Expert

    Sep 9, 2010
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    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.
     
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  7. Sudeep_Banerji

    Thread Starter New Member

    May 15, 2013
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    Great! Thanks guys i'll try out few things and come back
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Don't most automobile alternators have built-in rectifier diodes? :confused:
     
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  9. wayneh

    Expert

    Sep 9, 2010
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    Yes, I think so. The wind power guys often want to use their own rectification schemes though, and remove or bypass them.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    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.
     
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  11. Sudeep_Banerji

    Thread Starter New Member

    May 15, 2013
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    Thanks I'll look into fisher and paykel :)
     
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