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The mystery of the generator?

  1. Feb 5, 2014 #1
    What seems like a simple fix turns out not to be so simple.

    Question, I have an old mill with a waterwheel.
    The shaft turns at 100 RPM. We figure based on what was there a minimum 300HP so 300 x 746 watts per HP, this has the potential to generate 223.8 Kw’s of electricity.

    A 72 pole generator at 100 RPM will give me 60 hz AC.
    So 72 x 100/60 gives me 120 changes per second through the coil. At this number of changes per second, to get 240 volts RMS (339v peak) how many winds of copper wire do I need to do this?

    Wire wrapped around a iron core. Pass a dc current through the wire you get an electro-magnet. This electro-magnet will generate the field for the above coil, how many winds does this magnet have to have?

    Given the above parameters this shouldn’t be that hard to figure out, but I can’t find the formulas to do it, can you help?

    Not interested in putting in a transmission to increase the RPM.

  2. jcsd
  3. Feb 5, 2014 #2


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    You don't say what level of engineering experience and facilities you have. This is a seriously meaty project you are proposing and you will need to be pretty competent to do this safely. It could, of course, be a very impressive and money-earning project.

    Not even a simple belt? You will, at least need a clutch so that you can disconnect the alternator from the wheel. Turning off the water would take too long if you needed to stop the alternator.

    Some basic transmission, at least, would allow you to have the 'electrics' out of the way (in the dry; away from the spray etc.) and, if you really want to get some significant power out, you would be well advised to go for something off the shelf. A 200kW alternator is certainly more than a 'fun project'. If you ever got something like that going, how would you ensure it was mechanically and electrically safe? You have done some basic sums about revs and numbers of poles but that's the easy bit. Have you actually seen a commercial alternator, to get an idea of the scale?
  4. Feb 5, 2014 #3


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    To summarize:

    1. Given a magnetic field, how do you determine an armature design to get 240V into some load.

    2. Given the above, how do you design field coils to produce the magnetic field.

    Do you want a stationary or rotating magnet?

    You should start here:


    Absorb it all, then come back with questions.
  5. Feb 5, 2014 #4


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    That link is pretty comprehensive. Unfortunately, it is actually wrong in Figure 2. It shows a mirror image of the relationship between the direction of current flow and the direction of the magnetic vector. 'Everyone' knows the so-called Corkscrew Rule, which tells you the directions of the current and field vectors follow a right handed screw rule. I tried as hard as I could to make that figure (both of them) fit the correct rule and I couldn't. There are so many google hits about it that I really don't need to put in a link.
    That puts the rest of the web page in doubt - not in principle and qualitatively, perhaps, but things that you read in it should, perhaps be verified from another source, at least when you want to be right about the sign in any prediction.

    Edit: Figure 4 is the wrong way round too. I think the author must be using some alternative way of describing current. Treat everything with care,.
    Last edited: Feb 5, 2014
  6. Feb 5, 2014 #5


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    I find it hard to believe that power estimate.
    What is the diameter? and width? of the water wheel.
    What is the height of fall? and flow rate? of the water.
  7. Feb 5, 2014 #6


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    Nobody seems to want to describe what the OP is asking for. A way to design coils to implement such a beast. I'm not saying that the other issues are not important, but they avoid the initial question.

    I don't have the motor/generator background to even find good material for a starting point (other than what I posted already). I'm pretty stupid about motors.

    as for 223KW, if the mill produces 300Hp .... 5m^3/sec falling 5 meters is 245KW.
  8. Feb 6, 2014 #7


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    Specification AC, 60 Hz, 100 RPM. 100 RPM = 60 sec / 100 = 0.6 second per turn.
    So in 0.6 of a second we see one full turn with 36 full cycles, making 60 Hz.
    How many poles? 36 cycles will require 36 north and 36 south poles per turn. That is 72 magnets or field coils.
    A single phase generator would cog really badly so we need to wind the stator as a balanced 415V three phase machine. That will produce 240V per phase relative to neutral. There will be a total of 3 * 72 = 216 stator coils. Since the stator coils for each phase are in series, each stator coil will need to produce 240V / 72 = 3.33V AC.
    Current per phase will be about 312 amps per phase. That is not a job for wire, it will need copper strap.

    The number of turns needed to generate 3.33VAC per stator coil_pole will depend on the strength of the magnets and the efficiency of the magnetic path. The rotating field can be made from solid iron with coils or magnets, but the stator poles will need to be laminated from transformer steel and have sufficient cross section to avoid saturation.

    Now, is the rotating field being generated by 72 permanent magnet poles, or is it 72 electromagnets? To quickly regulate the output voltage will need at least some control of the magnetic field. The power output can be regulated by cutting the water flow while maintaining the 100 RPM. That will need to be an automatic control.

    How long will an old water wheel last when driven asymmetrically at 100 RPM by 5 m3/sec. The number of buckets on the wheel determines the cyclic impulse rate delivered to the shaft. Will it set up an oscillation at some critical speed when under power? Will that result in a time variable clearance of air gap in the alternator magnetic path? Does the number of buckets need to be prime with respect to the number of poles to prevent amplification of any oscillation.? Without any transmission the alternator will have to be mounted on the wheel axis. That will dictate the diameter of the alternator and therefore it's length and magnetic geometry.

    This one quarter megawatt alternator is not what I consider to be a wise first project for an amateur.
  9. Feb 6, 2014 #8


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    A flow of 5 m^3/s is 5 tons of water per second, quite a strong flow. This is not some gentle, bubbling brook you are trying to harness.
  10. Feb 6, 2014 #9


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    Only the “223.8 kW of electricity” and “an old mill with a waterwheel” is in the OP,
    the “300Hp .... 5m^3/sec falling 5 meters is 245KW” is a meBigGuy hypothetical.
    The OP power just does not seem right to me, I feel that there is a factor of ten overestimate in the power.

    Until I have a confirmation from newhope_health, re: my post #5, there is little point in designing a ten times oversized hypothetical alternator for a delicate historical waterwheel. The prohibition of a transmission is an interesting way of increasing the cost of both the design and construction. I can't help thinking of all that copper strap at today's market price, and the time it will take to punch and assemble the laminated stator sections.

    It is a pity that all the energy from the peripheral buckets on the waterwheel must flow through such a relatively thin shaft only to be spread out again to the poles. How thick is the existing shaft that caries 250 kW at 100 RPM. That shaft must run through accurate bearings with an enormous side load, the weight of the wheel plus the weight of the water. Now what if the alternator was inside the wheel?

    An alternator for this application can be anything between long and thin or short and fat. One problem is how to adjust the magnetic circuit materials to allow for the topological weaving of the copper strap conductors between the laminated poles. I would prefer to migrate from, or to interpolate between, existing designs than to do a design from scratch. The constraints on design are all external at this stage. Once the power and available space are qualified it will be possible to get down to copper turns at the end of the process. I could guess, say at six turns per coil_pole, but that does not really help because that pre-decides the as yet unknown field strength.
  11. Feb 6, 2014 #10


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    Good reply. That really put some reality on it. 312 amps per phase --- WOW.

    When I think 300Hp I think big block V8.

    There don't seem to be many 100KW+ home generator projects on the web.
  12. Feb 6, 2014 #11


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    That's true but I think you have to look at it from the point of view of any well informed Engineer reading it.
    The OP presents a 'practical' situation and Engineers are practical people. The question was viewed in the same way as "I have a tiger on a hamster wheel in my living room . . . . . . . . ." or "I want to build a Nuclear reactor in my back garden . . . . . ."
    The reactions were only to be expected.
  13. Feb 6, 2014 #12
    Yes there will be a clutch to disconnect the drive, the clutch is no big deal we are only dealing with 300Hp here. There are tons of diesel generators out there much bigger than that out there!

    Why no transmission or stock off the shelf generator head? To get 1800 RPM that's an 18 to 1 ratio loosing all that HP. Stock, off the shelf 72 pole, haven't come across any. Mechanical is my strong point. This thing has an 8" solid steel shaft! Runs smooth at a consistent 100 RPM measured on a tachometer under load.

    It should be very obvious that it will be a rotating electro-magnet hence the second part of the question. Trying to put that much amperage through a set of brushes would be silly. No this will be brushless making control much easier, regulate the DC to the stator controls the AC.

    This thing can be 4' or bigger in diameter, this was an old factory so space isn't much of an issue.

    So, coil of wire, how many winds to get 339V, (assume the obvious laminated steel and a 1/8" air gap between magnet pole and coil)?

    Soft iron core wrapped with copper wire creates magnetic field that passes by the above coil 120 times per second, how many wraps of wire to create the required magnetic field for the above coil.
  14. Feb 6, 2014 #13


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    HP = torque * RPM. A gearbox transforms the ratio of Torque to RPM. The reduction in torque will be balanced by an increase in RPM. There would be relatively little loss of power in a transmission with two stages of reduction.

    This appears contradictory. Do you really mean DC = stator = field winding?

    339V peak is 240 VACrms?
    Why do you insist on wiring the 72 poles per phase in parallel? With coils in parallel, an error of one in the turns count will result in a circulating current that will heat all the coils involved. It is effectively a shorted turn.

    How many phases are you considering? How many coils per phase?

    What will you do with the power generated? If you inject it into the grid then you should consider a self excited induction machine. That will eliminate brushes and the continuous problem of synchronisation, along with the damage possible when it all gets out of sync, which it will do at some time, but maybe only once in it's lifetime.

    With good magnetic design and correctly chosen lamination material, an engineer will get about 2.0 volts RMS per turn.
    My guess is that a first design will make between 0.5 and 1.0 VAC / turn.
  15. Feb 6, 2014 #14
    This seems like a pretty high RPM for an big old waterwheel. I imagine these things creeping along at a couple rotations a minute.

    Would be really cool if you could post a photo or video of this beast in action.
  16. Feb 9, 2014 #15


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  17. Feb 10, 2014 #16


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    300 hp? You should post a picture of your water wheel.

    The weight, speed and output of the most powerful water wheel ever built. (that I could find)

    From the image, I would estimate that it was 60 feet in diameter, and 17 feet wide.


    Wiki claims it put out 500 hp, and was 22 feet wide.
    And a graph in the reference goes all the way to 1100 hp.

    My guess is that Larry is not talking about a water wheel.
  18. Feb 10, 2014 #17


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    International communication fail, probably. To the Brits here, "an old mill with a waterwheel" suggests something built a few centuries before even the Pilgrim Fathers had been invented, not something that was high tech in 1915.

    If this really does run at 100 RPM with an 8 in solid steel shaft, I'm more inclined to think it is a water turbine not a traditional mill wheel. If the plumbing ran down the side of a small mountain, 300 HP might not be unreasonable.
  19. Feb 10, 2014 #18


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    The 8” shaft is the clue that it is high torque. If it really is 100 RPM then it must be a turbine, because a big open wheel could not hold together at 100 RPM. I accept that a turbine in a closed pipeline would generate 300 HP without problems.

    Is it a horizontal shaft? The earliest turbines had horizontal shafts, later they tended to vertical shafts.

    The motor used as an induction generator would be excited by the grid connection, and so would need to be driven at between about 3% and 8% faster than zero slip. That would be about 950 RPM. The 300 HP could be extracted by using a 60Hz, 4 pole, (900RPM), three phase, 300 HP induction motor and a gearbox with a ratio of about 1 to 9.50 . All that energy would need to go somewhere. That suggests grid connection would be needed, unless there was a good alternative. That type of motor is available between $3k and $10k, second hand, or $500 from a friendly scrap dealer if you are lucky.

    Finding the right gearbox will be more difficult but an industrial scrapyard will probably have some with about that ratio or slightly higher. It pays to make friends with the electricians and mechanics at your nearby paper mill.
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