I want to roll my own generator

Discussion in 'Electrical' started by shell shock, Mar 22, 2011.

  1. shell shock

    shell shock Member

    Hey there MBc, for the past while I have been meaning to put a full lighting system on my MB (turn signals, breaks head lights ect, ect) but i do not wish to drain the engine of its power. I looked to ebay to see if i could find some means of a generator. My search yielded some results.This one company in particular caught my attention. Here is a link:

    now to me, 25+s&h is steep to pay. So here is my question; is this not just a iron cored coil? and have any of you had luck with it? thanks in advance


  2. RedBaronX

    RedBaronX Member

    I'm currently building a 12v system that includes a rechargeable battery (recharge with home outlet power, not bike generator). I would like to eventually turn it into a self-contained system with a generator, but I personally wonder if a wheel-turned dynamo might be more reliable than one powered by the magneto.
  3. brynmn13

    brynmn13 New Member

    e-bay generators

    I bought one of those e-bay 12 volt generators and first off it didnt fit around the magneto right it was to close with no room to move it back the glue and tape they used to put the gen together caused the magneto to rub against it causein the motor to stall I took it apart used a dremel tool to shave a 1/16 of an inch of the metal core so there will be more room between the magneto and gen so they dont touch . if you want to go that route and buy one from e-bay be prepaired to completely re-do the whole thing.by the way the 12 volt gen cost right at 45.00usd it was a real pain in the butt!
  4. shell shock

    shell shock Member

    ah, danke, very helpful. if what you described is true, im just going to DIY it.
  5. loquin

    loquin Active Member

    In order to build one that will work (without huge losses due to eddy currents,) you need to use a laminated core (layers of sheet metal, glued together, with insulating shellac between the layers) and the gap between the moving magnet and the core has to be very small. These requirements imply a relatively high level of precision, which makes it difficult for the home builder...

    Note that you can 'cast' a core by using slow set epoxy, and mixing in as heavy a load of iron filings as you can get. Essentially, you want to end up with an iron filing 'paste.' The efficiency of a cast core will be lower than if you use laminated steel, but you can possibly live with this, as you can use a sanding drum the same diameter as the magneto magnet to attain a very close fit between core and magnet The coil is wrapped around the core.

    The main thing to keep in mind is, as the as the magnet moves past the core, the moving magnetic field is routed through the laminated core (and is routed through the center of the coil with the core.) The smaller the gap between magnet and core, the less the magnetic field loses strength when it 'jumps' this gap. As the magnetic field which cuts through the core changes in strength, electrical current is induced in the coil.

    You'll have to do a lot of experimentation, changing the number of turns of wire and wire size in the coil, until you get the voltage you need and the current you can live with. Also, make sure you use magnet wire, as it has a thin layer of shellac or lacquer insulation. (the thinner the insulation, the greater the number of turns of wire you can fit.)
    Last edited: Apr 8, 2011
  6. shell shock

    shell shock Member

    not to sound ungrateful, but could you provide me with a cross section diagram? that would help me immensely, as i seem to get what your saying. thank you so much for your reply

  7. loquin

    loquin Active Member

    Here's a simple sketch.

    The laminated core is normally made of many layers of thin, soft iron. These layers are insulated; either with shellac, or, as the sheet metal is annealed, an oxidation layer forms that is enough insulation, as the voltages in the eddy currents are very low.

    The lamination layers can be as thin as 1/64th of an inch, although thicker layers are more common.

    The core is used to direct the magnetic field through the 'bobbin' of wire that is the coil. The reason that the tolerances are tight, is that the greater the air gap the magnetic field has to take, the greater the loss of strength of the magnetic field. So, the air gap distance between the magnet in the flywheel and the face of the core needs to be as short as possible.

    The reason that you need a laminated core is that the magnetic field, as it sweeps past the core, induces currents within the core itself. If they are allowed to move through a large distance, uninterrupted, they can be quite large (in terms of amperage) and they will heat up the core (and causes a lot of losses) By forcing the eddy current paths to be short, the losses are much lower. There are trade-offs involved. You want as much iron in the core as possible and to do that, you need thick (or no) layers. But, you need layers to reduce eddy currents, and the thinner the layers, the lower the eddy current. Designers walk a tightrope with layer thickness...

    Think about the open-body fan motors that are used in oven vent fans and bathroom exhaust fans. Their construction is very similar to what you want to do.

    One final point. This 'generator' is actually an alternator, so the output is AC (alternating current) and not direct current. So, you will need to add a full-wave rectifier, consisting of 4 diodes, to convert the AC output from the coil to DC.

    Attached Files:

    Last edited: Apr 14, 2011
  8. shell shock

    shell shock Member

    this diagram is perfect. thank you so much loquin! i am familiar with rectifiers, so the conversion should be no problem.
  9. GearNut

    GearNut Active Member

    Just a thought, every hair dryer I tore apart as a kid used a brushed DC motor for the fan motor. Get one from a second hand thrift store and you will have 4 heavy duty diodes all ready wired up and ready to go. Just remove the motor and use it's input terminals as your output leads. The dryer's AC input leads to the diode pack will still be the input leads you will need to use from the "generator".
    Last edited: Apr 14, 2011
  10. shell shock

    shell shock Member

    great idea! totally forgot to scavenge for this [project. an to think, i was gonna spend 4 dollars for em. and now ill have a motor as well
  11. loquin

    loquin Active Member

    There is one reason that you might want to buy some diodes...

    The alternator you build just isn't going to have a lot of voltage/power to waste. If you use standard, silicon diodes for the bridge rectifier, you're looking at losing apx. 1.4 V across two diodes in the bridge current path (the two that are dropping the voltage alternate during the AC cycle,) at any one time.

    However, if you use Schotkey diodes in your bridge, they have a forward voltage drop that's about half that of standard diodes, so you'll end up with about .7 volts more at the load, and half the diode power loss.

    Cars have power to burn (relatively) and the 60-70 watts they can lose in the diodes is very small in the scheme of things. But, if your alternator only puts out 40-50 watts total, 6 watts watts in the diodes is substantial - 12% of the total! Less than three (with schotkey diodes) would be about 6%...
  12. GearNut

    GearNut Active Member

    Thank you loquin! I have never heard of Schotkey diodes before. They sound like quite the ticket for these low power alternators.
  13. davidstipek

    davidstipek Member


    Remember if you use a wheel driven Dynometer (Like our parents had an me when I was a kid. It is an AC power supply. You need to use a Rectifier of some sort to change to DC then you can charge battery. don't forget to install switch to disconnect battery when not being used.

    I have been using this system to prove out wiring on chopper bike prior to engine install.

    Note: You can also use flashing 12 volt LEDS for turn indicators. then no relay or flasher needed. I used red/blue in front with blue light lenses filtered out 95% of red. Same as back used red lenses and it filtered out 95% of the blue.

    If you use 12V system it works. Also works with 6V system as minimum bightness difference.

    Taillight we used white led mounted in center of lens and set 6V or 12V halogen flashlight bulb behind it for brake light.

    you can also get dynamic headlight by using same Halogen flashlight bulbs in low and high beam (note use a diode to enable both to be used in High beam!) You can get bar mounted switches from Electric bike dealer or use bar mounted M/C switches, you can get on/off Hi/Low turn indicators and horn all in one

    see photos:


    Attached Files:

  14. shell shock

    shell shock Member

    ok, so my build has been halted as my colleges machine shop is under reno this summer. bumer XP