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Calculating energy produced by induction generator

  1. Jan 28, 2011 #1
    If a cylindrical induction generator has:
    a diameter of 4 inches
    500 turns (coils) of #30 wire
    4 2in magnets of unknown magnetic field strength
    and motor generating 3000RPM

    Note that the magnets will be rotating parallel to the turns in the coil. See attachment for concept drawing.

    How much energy can it generate per minute?
    Also how can I determine the field strength of the magnets?

    I need to determine how large of a generator or how many generators I need to build to produce approximately 1200Kilowatts or more per month of energy.

    Attached Files:

  2. jcsd
  3. Jan 29, 2011 #2


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    welcome to pf!

    hi bwshrewsbury! welcome to pf! :wink:

    tell us what equations you think are relevant :smile:
  4. Jan 29, 2011 #3
    Successfully designing and building a PMA (Permanent Magnet Alternator) can be a considerable engineering feat, and from your question I assume you are ill equipped to attempt it from an engineering point-of-view; this does not mean that you are incapable of building a functional PMA.

    In engineering the focus is guaranteeing a product will meet design specifications, typically with particular focus on total cost per unit for a specified production volume. This process involves a complete understanding of every component and their dynamic interaction.

    In the specific example of your proposed PMA, you define the goal as an output of 1,200kW per month. I must assume you mean 1,200kWh. Watts (and kiloWatts) are defined as Joules of Energy per second, while kWh (kiloWatt hours) are defined as the total number of watts that have been produced over a period of one hour. As an example, you could produce 2400kW for half an hour and meet your design criteria of 1200kWh for the month; however, I seriously doubt this would achieve your goal.

    I assume you have some source of mechanical power that will provide the 3000RPM? Perhaps an ICE (Internal Combustion Engine)? It doesn't really make any difference, but knowing a bit about the mechanical source would help define the PMA.

    The next thing you have to consider is what type of electrical output criteria do you require? 12Vdc, single phase 120Vac @ 50hz/60hz, 3 phase...etc etc.

    Next you need to define what type of load you will be driving. A purely resistive load is much easier to drive than a highly inductive load like electric motors.

    Finally, you need to give some serious consideration to the overall efficiency you expect, and this will largely be determined by the mechanical source. For example, if your mechanical source is a river, then efficiency is not as critical as it is if its an ICE since the river is going to flow regardless, but the ICE is going to consume fuel when it is on.

    Anyway, you really need to define some things before anyone can help you much. In the end, you are likely going to be better off using trial and error rather than attempting to fully define the PMA using engineering. A full blown engineering solution started from scratch would likely take considerably longer than starting from an existing design and modifying it through trial and error to suit your needs. I can assure you that your trial and error PMA will not perform as well as a properly engineered approach, but it will be far more cost effective for a "one off" design.

    You may consider starting with the well-documented "Axial Flux PMA" pioneered by Hugh Piggot. A good read: http://www.windenergy.nl/website/files/artikelen/AXIAL_FLUX_HowItWorks.pdf [Broken] You should be able to adapt the basic principals discussed here to DIY a decent PMA without too much effort. A Radial Flux design as you indicated in your OP is certainly the more traditional engineering solution, but the process is difficult to successfully DIY.

    Last edited by a moderator: May 5, 2017
  5. Jan 29, 2011 #4


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    Kilowatts is power, not energy. Do you mean 1200 kilowatt-hours?
  6. Jan 29, 2011 #5
    I want to first say thank you for your replies and also apologize for my incorrect terminology.

    Yes, I am attempting to produce power not energy. As far as the required electrical output I require, it must be able to plug into a standard US wall outlet to supply power back to the grid.

    As far as the mechanical power source providing the 3000rpm it is a small electric motor. I wanted the system to be as green as possible. So no ICE will be used.

    My average power consumption is 1.67kWh per hour. So I want to produce at least 2kWh.
  7. Jan 29, 2011 #6


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    You know I guessed that this was going to be the outcome of this thread from the very first post. It seems that just about every time that someone clueless starts asking questions about generators that it eventually turns out to be free_energy or perpetual_motion related. :grumpy:
  8. Jan 29, 2011 #7
    me too, but I am ever hopeful!


    Please review the implications of the laws of thermodynamics. In a nutshell, what you are attempting to do:

    1) WONT WORK
    2) Violates the laws of Physics
    3) Has been argued a million times, likely thousands in this forum alone.
    4) Will likely get this thread closed.

    At the end of the day you are assuming that you can get more power in the form of electricity out of a generator than you put in mechanical energy into the generator. This by definition is a "perpetual motion machine" and is based on false presumptions. I am certain, like everyone else who has come up with this idea, that you feel you have not properly explained your idea, but let me assure you, I, and everyone else, does fully understand, and your idea won't work.

  9. Jan 29, 2011 #8
    I am not concerned with perpetual motion. Every where I have looked for the information I needed someone brings up perpetual motion. Then the threat goes on a tangent and dies.

    So right now lets end all of the talk of perpetual motion. This is not the thread for it.

    All I am concerned with is the formulas for calculating power generated from a generator based on the known factors I previously stated. For any factors that I am missing, if there are formulas for calculating them please let me know.

    Trial and error is fine but time consuming. I do not want to build a generator to plug it in and realize a month later that it produced insufficient power.

    "At the end of the day you are assuming that you can get more power in the form of electricity out of a generator than you put in mechanical energy into the generator."

    If that statement were true then the power companies would not use generators to produce power; alternators could not produce power for vehicles; and wind turbines would not work. However, with a simple cardboard box, some wire, a small magnet, and a LED, you can produce enough power manually to power the light. Therefore your quote that an electric motor driving generator cannot produce sufficient power is false.

    Everyone please keep to the facts and formulas and if you can help, then please do.
    Last edited: Jan 29, 2011
  10. Jan 29, 2011 #9
    Power = Torque * Angular Velocity (RPM)

    See This: http://en.wikipedia.org/wiki/Torque

    In a generator, Power in the form of Mechanical Power is converted to Electrical Power using:

    Electrical Power = Mechanical Power * C

    C = 0 to 1 typical values ranging from 0.2 to 0.8

    For an Electric Motor Power, in the form of Mechanical Energy is a function of Electrical Input Power:

    Mechanical Energy = Electric Power * C

    C = 0 to 1 with typical values ranging between 0.2 and 0.8

    SO, to power your generator from an electric motor we might have:

    Electric Power Out Of Generator = (Electric Power To Motor * 0.75) * 0.75

    A specific Example:

    Electric Motor Input = 2.2kW

    2.2kW * .75^2 = 1.2375kW

    SO, your idea, that I obviously do not understand, will produce your 1.2kW output with a mere 2.2kW input.

    I know you are going to argue that you will use a much smaller motor to turn the alternator, and I am trying to explain to you that it simply won't turn the generator. It just won't. You and 10,000 other people who do not have a clue what you are talking about want other people to explain to you how to create energy, and you simply refuse to accept the answer.

    I have showed you clearly the salient points and formulas. In my previous post I suggested you review the laws of thermodynamics because they clearly define the error you are making. So get angry, get glad, get whatever makes you happy, but what you are suggesting is a perpetual motion machine in direct violation of the laws of physics. The harder you argue the worse it gets, please take the time to learn basic physics.

  11. Jan 29, 2011 #10


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    Sorry, you don't get a choice: you must produce both at the same time. However, you can't reference one and then use the units for the other!
    1.67 kWh per hour is 1.67 kW. kwh/h=kW
    Supplying power back to the grid is much more complicated than that and requires coordination with the power company.
    Er, no. As others said, conservation of energy applies. As I said in another thread just last week, there are two common errors that typically lead people to believe this is possible:

    1. Failure to realize that the torque a generator imparts on its drive shaft is determined by the amount of power it is generating. It's not just a free-spinning shaft. Power in=Power out (at best).
    2. Gears/pulleys use normal mechanical advantage (like pulleys and levers) and so if force is multipled, distance is divided (and vice versa) and therefore work or power doesn't change. I'm not sure if this one applies since you didn't say if you were using gears/pulleys.
    An alternator produces a tiny amount of power compared with the output of the engine that powers it. And power companies use generators to produce power and those generators get an input of mechanical energy that is larger than the output. Heck, just look up any self-contained generator online and you'll be able to verify this. For example, here's one that has a 13 HP motor and produces 7kW of power. I'll let you do the math to verify the applicability of conservation of energy there...

    Frankly, it's pretty irritating (and a little laughable) when someone who doesn't even understand the difference between power and energy tells a bunch of engineers and physicists they don't know what they're talking about wrt conservation of energy - one of the most fundamental principles in science.

    Taking a step back: if what you suggest were possible, don't you think a bunch of people would already be doing it? I know I'd love an easy way to get rich selling electricity! A little critical thinking would tell you that since no one is doing this, it's probably not possible.

    I don't know how the other guys saw this coming, but I didn't and I'm usually pretty good at smelling this sort of thing. Yes, for some reason we've had a rash of these lately, probably half a dozen in the past month whereas I don't think we normally get more than 1 or 2 a month.

    Anyway, this isn't going to go in a productive direction so I'm locking it.
    Last edited: Jan 29, 2011
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