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Some help with numbers please?

  1. May 19, 2004 #1

    megashawn

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    some help with numbers please???

    I'm trying to make a very small generator. I had in mind using this engine.

    It is a wankel engine, designed for rc airplanes and cars.

    Here are some specs on it:

    Displacement: 0.303 cu in (5.0 cc)
    Practical rpm: 2,500-18,000
    Output: 1.27 bhp(ps)/17,000 rpm
    Weight: 11.8 oz (335 g)

    Now, perhaps you can see my reasoning for wanting to use this engine. My questions start when I try to figure out how much power I can make with this thing. I understand that 1 horsepower is roughly equivalent to 746watts. So does this mean this engine would be capable of generating somewhere around 1kw of electricity?

    Basically I need some help determing what kind of motor I would use for the generator. For instance, could I use a motor with stats such as this?

    Operating Voltage: 16-24v
    No Load RPM 24,000
    Amp Draw @ Max Efficiency 21.3a
    Max Efficiency 82%

    (info borrowed from here )

    Basically, would using such a motor provide me with the ~1kw the mini-wankel can produce (If I'm correct about that?) or would anyone have any suggestions to get more power in a better manner?
     
  2. jcsd
  3. May 20, 2004 #2
    So, in short, you want to use the Wankel internal combustion engine to run the electric motor as a generator.

    Of course the only power you will get out of the Wankel engine is mechanical.

    The maximum amount of electricity you could get out of the motor (as a generator) would depend on the wire guage, number of windings, the RPM's, and the strength of the magnetic field. Where the motor has a max rating of 24 volts and 21.3 amps, it seems to me that the max you would get is about 500 watts. That's only if it can produce 24 volts and 21.3 amps. The motor is rated to draw that much power, but without knowing the construction of the motor, the power out of it can only be determined by running it.

    That's my opinion anyway.

    LB
     
  4. May 20, 2004 #3

    megashawn

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    Basically thats what I'm trying to do.

    I just tossed the motor stats out, as I had them readily availible. Would it be beneficial for me to try to make my own generator?

    Basically, with 1.27 hp at the crank or the wankel, how can I establish the limit of the motor I wish to use as a generator?
     
  5. May 20, 2004 #4

    Cliff_J

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    Typically the current of an electric motor is the lowest at full no-load rpm and highest current demand is at zero rpm (stall). The maximum efficiency is usually at 1/2 maximum RPM, something like 50% is typical (1/2 output 1/2 wasted as heat).

    Its a neat idea, maybe not practical but a neat use of the RC stuff. Too bad an OS engine isn't cheap. A more economical way to test this would be to use one or two RC electric motors to spin another as a generator. Join them with a pulley?? Or two pinion gears for an RC car? Anyways with a less expensive test setup you could load the 'generator' down and find out the potential power output of the motor acting as a generator. I'd guess no more than 100W electrical but that is just a wild guess. Where do I guess from? These motors do run for a few minutes with a 1200mAh battery pack and get warm at that power level, attempt to extract too much more and I think you might have issues.

    Cliff
     
  6. May 21, 2004 #5

    megashawn

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    Well, for what I'm trying to do, I'll probably end up using a chainsaw engine. I can get those a dime a dozen from a friend.

    I was just curious how to determine what size motor I could run with an engine producing a certain power. 1.27hp in the case of the wankel.

    I was looking at some portable generators in production, and I noticed a 1400w generator has a 3.5 hp engine. 1400 watts is close to 2 hp (my earlier 1hp=746w is correct right?), so I assume the extra power is to overcome the losses and what not.

    So do you think probably my best bet will be trial and error? I've tried making generators like you describe before, only I used a dremel tool to drive the generator. Using a small radio shack motor rated at 6-14v (I believe) I was reading 18v as an output. However, the dremel tool is a beast when it comes to rpm (I believe 18,000 or so) which is why the wankel peaked my interest.

    Anyhow, I thank you all for the help so far, and any which you may provide.
     
  7. May 21, 2004 #6
    Hello megashawn,
    If you're looking for a good working rule of thumb to go by then two horsepower per killowatt will do the job. As for the generator; you're wanting the power to be 1000 watts, but what type of voltage/current do you desire?
    For example, is it to power AC household appliances, charge batteries, or run low voltage DC equipment?
     
  8. May 21, 2004 #7

    megashawn

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    Well, essentially I'm wanting to charge batterys. I've got another idea to assist in charging them, but I want this to be the main battery charger. From the batterys, I can just run an invertor to do most anything else I need. Of course, it would be nice to be able to plug a stereo, small tv or even a laptop into it, but not essential for my current goals.

    As to my power requirements, I've not really determined them yet, as I'll be limited by how much energy I can produce in as small an area as possible, which is why I was looking for the rule of thumb you provided me with.

    But I'm sure I'll be back with more questions. I'll probably just bump this thread when the time comes.

    Again, thanks for the help
     
  9. May 21, 2004 #8
    For something inexpensive and readily available might I suggest using a small engine such as a Briggs & Stratton coupled to an automotive alternator/regulator? True that you will lose somewhat in efficiency due to the field current that must be provided, but there are bonuses too. Chainsaw engines can be rather noisy if you have to run them at high rpms. I prefer a 5-hp small engine with a large pulley running at a slower rpm. I have in days past built a number of these devices and could quite likely unearth photos of a completed working unit. Do you have any welding skills for constructing a frame ?
     
  10. May 22, 2004 #9
    If portable power is what you are looking for, check this out:

    http://www.donrowe.com/inverters/xpower_400_p.html

    I picked one up at Sears for $80 on sale. It's rechargable from a solar panel, or from your vehicle's cigarette lighter, or from household current, or a gas-powered generator.
     
  11. May 22, 2004 #10
    I saw a tiny 2-cycle portable generator capable of DC or AC output at a tool store a few years ago. It was so cute I wanted to buy it just have it on hand to look at from time to time. It was smaller than any of the ones below but I can't find a link. It was made by Chicago Electric IIRC. Anyway, just to have something to look at for size/information here they are;

    http://www1.bottomdollar.com/search_getprod.php/masterid=1359885

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=11771&item=3814787668&rd=1

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=11771&item=3814801127&rd=1

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=64005&item=4128602946&rd=1
     
  12. May 22, 2004 #11

    megashawn

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    my friend is the welder, he's still learning, but pretty good.

    I'm actually hoping for this thing to be quite a bit smaller then any of the ones you linked to. The wankel I linked to is pretty small, I've been told it sits in the palm of your hand. I've also considered using a recipricating engine for a model airplane, they are a bit cheaper, and abit more powerfull.

    Basically, I'd like to be able to strap this thing to a dirt bike fender.
     
  13. May 22, 2004 #12
    I'm thinking you may not be able to achieve more than a few amperes of battery charging current with something so tiny. I'd look for low voltage permanent magnet motors (electronic surplus) and see what is available. I picked up two motors taken from a photostat machine that were quite impressive (could output over 10-amps at 14V). They were about 4" in diameter and 2" thick. I mounted one in a homemade exercycle and it worked quite well in that capacity, the other I used for a fan motor.
     
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