DC motor RPM and voltage

Discussion in 'General Electronics Chat' started by Nathan Hale, May 31, 2016.

  1. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    hello guys hope everyone is fine. I have a DC motor sitting in front of me.
    It is a 12 volt motor with a maximum rpm of 5600.
    Theoretically speaking if I spun the shaft of this motor at 5600 RPM with my fingers would it read 12 volts at the terminals where we supply the power? thank you for the reply
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Not likely unless it's a permanent magnet type.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Are you planning on generator using wind or water?
     
  4. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
    125
    2
    Wind.
     
  5. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
    125
    2
    What if it IS a permanent magnet type? Will turning the shaft at 5600pm produce 12 volts across the terminals?
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The answer is yes, but getting 5600 rpm is hard. One thing you can do is use some kind of gearing arrangement. But, there is a problem, the gearing will introduce additional mas and friction that wind will need to overcome in order to produce rotation of the motor shaft.

    The best way to do this thing is to get permanent magnet motor that does a hundred or a few hundred rpm.
     
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  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    One of the popular types is from a tread mill, but they will generate ~90vdc at 2krpm.
    Max.
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    They also make permanent magnet rotors for automotive alternators. This is/was popular with DIY wind generator community. Some even make their own PM rotors by removing the alternators rotor coils and putting ring type speaker magnets in the coils place.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Wonder why they just didn't use self excitation?
    Max.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Don't know. But they seem to sell a lot of them on Ebay or did.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Seems to sort of defeat the purpose, self excitation can also provide automatic regulation over varying loads and speeds, as is used in an automobile.
    Max.
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Thinking over night about this, they (pm modified alternators) are usually direct drive, blades directly on the shaft. Don't think they get up to a high enough shaft speed to self excite. I agree it makes regulation much harder, but they are using them with shunt regulators, like in many motorcycle alternators.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It is surprising how little a field is needed to initiate self excitation and boot-strap itself up, and I would think they would reach 12v easily as a auto alternator has very high output at low rpm.
    Perhaps no-one tried self excitation, it cannot usually be done with the internal regulator but a very simple circuit is all that is needed, LM311 and mosfet plus a couple of diodes and R/C's.
    Max.
     
  14. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    It might produce close to 12V but...
    When it is running as a motor with 12V on the terminals and the shaft turning at 5600RPM, the voltage generated is subtracted from the 12V and the remainder is (mostly) the voltage needed to drive the current through the resistance of the windings. It all depends on the resistance of the windings and how much current the motor draws.
     
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