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Low RPM model generator

  1. Oct 4, 2016 #1
    Hi all,

    Does anyone know of, or can anyone point me in the right direction for where to find a small, low rpm generator? RPM can be anywhere in the range of 100-300RPM, and size would be around 50mm long.

    To put this into context, I am building a small turbine (~300mm diameter), which will rotate within the limits mentioned above. Therefore, I need something that will convert my mechanical energy of around 16 Watts, to electrical energy that I can monitor.

    The main reason for the low rpm, is that I'd like to avoid the use of a gearbox. I don't have much power to lose. I've looked at some small motor/generators, but they all seem to have very high gearing.

    Any help would be appreciated! Thanks in advance.
  2. jcsd
  3. Oct 4, 2016 #2
    I can't suggest a specific model that might suit this project.
    One thing to bear in mind though, is that at lower rpm, you'll need more powerful magnets and bigger coils to get efficient conversion.
  4. Oct 4, 2016 #3


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    How large can it be? Can you build it yourself? Use a very large number phases on a large diameter rotor.

  5. Oct 5, 2016 #4


    Staff: Mentor

    Your requirements are not that unusual. Wind generators need low RPM generators too.

    I see lots of them listed on alibaba.com
  6. Oct 7, 2016 #5
    I am very limited for space, so I think I will have to increase the rpm via gears, so that I can keep the size of the motor to a minimum

    It needs to be smaller than 60mm diameter, and 60mm in length. I don't think I have the expertise to build one myself

    I had a look, but what I need is much smaller. They generate hundreds of Watts at the least. I'm looking for something that can generate up to 20 Watts
  7. Oct 7, 2016 #6
    How about the kind of dynamos used for generating enough power from a bike wheel to light a few lamps/leds ?
    Is that getting near what you need?
  8. Oct 7, 2016 #7
    My initial thought is - yes, that's what I need. I will have to look into this though. I suspect that the RPM and torque that my turbine provides will be much less than is provided by a large diameter human-powered tyre, driving a small dynamo shaft. Again, I will need to look into this because I haven't had any exposure to the operation of these devices
  9. Oct 7, 2016 #8
    OK. anyway what is your turbine project about?
  10. Oct 7, 2016 #9


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  11. Oct 7, 2016 #10
    Well yes, you can make an electric motor.
    but no you cannot make a motor to drive a generator producing more power than is input.
    The laws of physics as presently undoorstood forbid this, and with good reason
  12. Oct 7, 2016 #11


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    If perhaps you have a local hobby shop that caters to model car and plane clubs, they might get you in contact with some enthusiast that will be willing to assist you in rebuilding a motor that will meet your exact specifications. with just a little skill and learning and very little money you can have the satisfaction of doing it yourself. :smile:
  13. Oct 7, 2016 #12


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    A quick Google tells me bicycle dynamos are mostly 6W so you have over double the required shaft power available.
    Not that it would work as their working rpm it's too high.
  14. Oct 8, 2016 #13
    I still want to know what is your turbine concept.
    It sounds interesting,, but very likely it could be something already tried.
    Anyway I will follow your thread.
  15. Oct 8, 2016 #14
    I'm looking at the difference in efficiency between a standard turbine, and one modified with a diffuser. It has been proven theoretically, but not experimentally.
    It may be the case if I can't find anything off the shelf. It sounds like an interesting challenge.
    I'm very well aware of this. I don't think I ever mentioned building a perpetual motion turbine though..
  16. Oct 8, 2016 #15


    Staff: Mentor

    In that case, I suggest that you completely forget generating electricity. Electricity is not your goat, measuring the power of your turbine is.

    Make instead a small water pump. You can accurately monitor its energy production by measuring how much water it can lift a known height in a known time period. There are dozens of compact and low speed designs useful for water pumps, dating all the way back to the 3rd century BC https://en.wikipedia.org/wiki/Archimedes'_screw. On YouTube I see 318,000 videos on home made water pumps

    As a matter of fact, the goat is to measure the difference in turbine efficiency with and without the diffuser. That means that the absolute efficiency of the water pump doesn't matter as long as the efficiency doesn't change when you add the diffuser to the turbine.
  17. Oct 8, 2016 #16

    jim hardy

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    that's small.
    Is this a home experiment project ?
    I took apart a cordless drill and found it built around a permanent magnet motor about that size. Junkshops are full of them , when the battery goes bad you can get them for just a couple bucks... they use planetary reduction gear so i dont know what speed the actual motor is.
    Car window electric motors are surprisingly small nowadays. There's another possibility. You'll find reduction gears in them too. But poking around junkyards often gives one an idea.

    You didn't say what voltage you want, if just a few volts DC will do give that a try. Get one and spin it with your electric drill, see what it puts out,
  18. Oct 8, 2016 #17


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    He mentioned 20 watts, that's 12 volts and less than 2 amps @ around 300 rpm, seems like a piece of cake to me. :smile:
  19. Oct 9, 2016 #18
    It's for my thesis research. I see what you mean about the pump, but my aim and method are set out in a different way to the concept you mentioned.

    I feel like there is light at the end of the tunnel. I think I will use a 12V DC motor from a hobby/robotics shop. I can put a resistor in my circuit to vary the load.
  20. Oct 12, 2016 #19
    Is this for some kind of remote control plane or boat? What is the goal of your project?

    You do know a permenant magnet dc motor can be a generator also? If you get two motors you can hook them up shaft to shaft and put DC on one and get DC on the other one, different voltage probably but it would transfer power from one form to another. Of course you lose energy to friction but it would work.

    And for your small turbine project, any of those DC hobby motors can be a generator too and they are a lot smaller than 60X60 mm. More like 15X 20mm (just an estimate)
  21. Oct 13, 2016 #20


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    I have just done a similar project, measuring efficiencies of various mixed flow fan and housing designs.
    I started by using fluid power out/electrical power in to measure efficiency, it sounds like you'll be doing electrical power out/fluid power in, so same idea.
    My experience was that this was a waste of time, the change in motor efficiency with RPM swamped changes in fan & housing efficiency. The motor manufacturers characteristic curves were not accurate enough to correct the problem, so I couldn't separate the change in motor efficiency from the change in fan efficiency. Presumably you'll need to operate your turbine & diffuser at a range of RPM, and that the addition of the diffuser will change the efficiency curve and you're looking for that new curve, so I think you'll have the same problem.

    What I ended up measuring was:
    motor torque (Mounting the motor in bearings and using a https://www.amazon.com/Electronic-Balance-Weighing-Sensor-0-300g/dp/B007Q9WMV8)
    motor RPM
    air temp.
    Absolute pressure (atmospheric)
    differential pressure (across Venturi flow meter)
    differential pressure (Fan outlet to atmospheric)
    All via an arduino.

    From those I could plot Electrical power in, shaft power out, volumetric flow rate, mass flow rate, pressures etc and efficiencies.
    Last edited by a moderator: May 8, 2017
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