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Hand-cranked battery charger,looking the helps

  1. Feb 9, 2010 #1
    Hye guys,

    I'm just a beginner in the area..i have a project of hand-cranked battery charger..
    I supposed to improve the current charger in every way possible..
    How could i the miniature generator, to generate the energy by turning the crank..
    Is there any diagram that i can study or any other way so that I can get the clear view about the principle of the charger..

    Looking for your expertise!
  2. jcsd
  3. Feb 9, 2010 #2
    First, you should look at the basic principle of "generator" operation. They operate on the principle of Faraday induction, which basically is V = -(d/dt)∫B·dA. Because of the d/dt operator, the generator RPM should be much higher than the typical hand crank RPM to get the needed voltage, so gearing is needed to speed up the generator.

    Second, you need to decide on what the "generator" is. The old-style generators, a wire-wound armature rotating in a regulated stator field winding with a commutator to get a dc voltage out from the armature, is inefficient. You should consider only a permanent-magnet rotor with the ac voltage taken off the stator (no commutator), or a wire-wound armature rotating inside a permanent-magnet stator, with the dc voltage taken off the armature via a commutator. The former requires rectification via a bridge rectifier, which costs 2 diode drops in voltage (and power) loss. A third generator option is the alternator (like in automobiles), using voltage feedback to the armature via slip-rings and taking a regulated dc voltage off the stator via a bridge rectifier, would get rid of the required dc-dc converter mentioned below. (Alternators are probably only suitable for power outputs over several hundred watts).

    Third, mismatching the generator output voltage to the required battery input charging voltage will waste power and/or damage the battery, so an efficient switching dc-dc buck-boost regulator is required. Look at the available SMPS (switch-mode power supply) IC regulators, including SEPIC (single ended primary inductor converter) ICs.

    Bob S
  4. Feb 17, 2010 #3
    about the generator...can i buy it from the store or should i buy the dc motor and reverse it function to be a generator?
  5. Feb 17, 2010 #4
    If the DC is motor is brushless, you'll need to rebuild it to make it work as a generator. If the motor has a brush, it will be ready to work as a generator. If you can't buy a generator off the shelf, you can use a brush motor.
  6. Feb 17, 2010 #5
    Okay thanks..
    If i use a brush DC motor as the generator, the output of the generator is DC or AC?
  7. Feb 17, 2010 #6
    A brush/commutator motor with a permanent magnet stator will give a dc output voltage that is proportional to RPM. The signal generated in the armature is an ac signal (Faraday's Law):

    V= - (d/dt) B·A = -B·dA/dt where B = magnetic field and A = coil area

    And the brush/commutator rectifies it. Take one of these motors apart and look at the pieces inside.

    Bob S
  8. Feb 17, 2010 #7
    The Brush motors generate DC, but aren't so efficeint. You can find brushless motors for RC use that are much better. You'll be looking at about 2,000-10,000RPM per volt for most of these.

    For the brushless motors, you'll need a three phase rectifier to make it DC. That is, you'll need six diodes. Two on each phase, one to the positive out and one to the negative.

    The type of diodes you choose can make a difference. Over sized schottky diodes will be more efficeint, but you need to disconnect the generator when not in use, because these diodes will leak current slowly away from the battery when not charging.
  9. Feb 18, 2010 #8
    I've already bought the 3v and 9v DC motors that i want to try to make it as generator..
    But if the stator is permanent magnet, can it produce the voltage???

    Those motors have permanent magnet at the stator and winding at the rotar...

    There two things that confusing me...
    1) How to give a current to the rotar winding, so that i can produce magnetic field as my lecturer does not want any electrical source (the only source allowed is mechanical to rotate the hand-crank)....or should i buy some motor with permanent magnet at the rotar..

    2) If i'm not mistaken, to induce voltage, we need to cut the rotating magnetic field by some conductor right...how came the magnet can cut the rotating magnetic field that has been produce by the rotar??

    Please do correct me if i'm wrong...
  10. Feb 18, 2010 #9
    The motor armature (rotor) rotating in the field of the permanent magnet stator will develop a voltage due to Faraday's Law. The higher the RPM, the higher the voltage, so you want to gear up the armature RPM to say 10 x or more times the crank speed. The motor commutator will rectify the voltage. You do not need any battery or other external current or voltage source.

    Bob S
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