Axial alternators with dual stators

A lot of people ask me if it’s a good idea to stack the axial flux alternator with 3 magnet disks and two stators. I have done a page comparing the costs and performance of two alternators built using my recipe.

About hugh

I live off-grid in NW Scotland and have spent my life playing with wind turbines. I also love small hydros. Hands on renewable energy is my thing and I like to learn and to share my experiences.
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27 Responses to Axial alternators with dual stators

  1. shon carter says:

    what about using two stator plates and one rotor sandwiched between them ?

    • admin says:

      A lot of people do propose this as a way to economise on magnets. Unfortunately you don’t win that way. The hard part for the magnets is to push magnetism across an ‘air gap’ (which is the space where the stator is between magnet faces).

      One rotor of magnets cannot throw flux across a double gap as well as two rotors of magnets can across a single gap. It’s a case of no free lunch. If you want to fill a gap with magnetic flux then you have to buy the magnets to do that job, whereas if you double the gap you will find that the magnets you have cannot do that as effectively.

      There is scope for optimisation, and all configurations need to be looked at, but this dual stator with one rotor is not as attractive as you would expect. And there are issues with completing the magnetic circuit.

      Overall you are better putting all of your coils in one stator and keeping the size of the air gap down to one coil thickness rather than introducing a second gap into the path of the magnetic flux.

  2. Adam says:

    If the magnetic rotor plate was placed between two coil stator plates (both sides of the magnet are unobstructed) and the coils were (in theory) “infinitely close” to the magnets, wouldn’t the output be twice as much as a single plate with the same gap?

    • admin says:

      You have to look at the details of the magnetic circuit. It works a bit like an electrical circuit where the magnet is like the battery (voltage or electromotive force) and the air gap is like the load (resistance, called reluctance in this case). Follow the lines of flux around the circuit and you will need to compare the amount of magnet length to the airgap length.

      If you have a stator on each side of a central magnet rotor then there will be two air gaps (coils) in series with one magnet.

      The steel disk is not an obstruction. It is a conductive path that carries the flux to the back of the adjacent magnets without any loss to speak of.

      Put a steel disk with magnets on each side of the stator and you have two magnets lengths for each air gap transition as you trace the magnetic flux path around its circuit. If you used 2 stator with magnets outside and only one magnet int the middle then you would have only 3 magnets for each 2 air gaps so you have less force and therefore less actual flux will be created.

      I am not sure which exact scheme you are proposing but you do not gain anything in terms of magnet efficiency by putting the coils into two stators. Better to have one big stator and then you can have twice the rate of relative motion between magnets and stator.

      Hugh

      • Adam says:

        I have 12 n52 magnets with dimensions of 2in x 1in x 1/2in. I am using 14 gauge wire and will run two strands parallel for my coils. Each coil is expected to have around 40 turns. Each plate will have 12 coils. My magnet plate will be in between both stators and each side of the magnets will be unobstructed (though you say this doesn’t matter?). I am expecting as small as an air gap as possible on each side, perhaps 1mm. Each stator will be fed through its own rectifier circuit. So in theory, I would be doubling the coil area that the magnetic flux penetrates, thus doubling my output. Where is the fault in my logic above? Am I sacrificing magnetic flux strength? If so, how?

        Suppose I was dead set on the method above (which I’m not) how could I do it BEST?

        • Adam says:

          Would it be advantageous to steel plate back both sides of the magnets? If so, how? I am currently researching this topic, so I apologize for my lack of understanding.

        • admin says:

          hi Adam,
          It’s like I said you need to look at the magnetic circuit. So far as I can see your magnets are facing coils on each side, and after passing through a coil the flux is out into space. This can work, but you can greatly increase the flux density by putting steel disks on the outside of the stator (spinning with the magnets) so as to provide a low reluctance path to the next magnet/coil. This helps reduce the reluctance of the circuit and also helps to straighten out the flux lines in an axial direction straight across the gap/coil. Without these disks, much of the flux will go in a short loop from one magnet face to the next with very little cutting of coper wires on the way. The disks have to spin or you will simply create current in the disks and waste power.
          I hope this helps.
          Hugh

          • Adam says:

            If you have time for one more question, could you please explain to me why the metal disks must spin with the magnets? If not for that, I could place my coils on metal plates, requiring much less torque at my rotor, resulting in a higher rpm.

          • Adam says:

            So sorry, got ahead of myself. I see that’s your last sentence. Thank you.

  3. Yura says:

    Hello? dear frienfs! PLEASE HELP ME.. I want to make Axial Flux Genetator with 2 Stators or 3 starots and 3 rotors… or 4 rotors (using 3 phases in each stator – STAR scheme) but i dont know HOW TO CONNECT STATORS BETWEEN EACH OTHER? How to connect same phases for different stators in whole chain?
    For example… we have 3 stators with 3 phases (A, B, C) in each!

    For 1st stator we have phases A1,B1,C1:
    A1-start A1-end
    B1-start B1-end
    C1-start C1-end

    For 2nd stator we have phases A2,B2,C2:
    A2-start A2-end
    B2-start B2-end
    C2-start C2-end

    For 3rd stator we have phases A3,B3,C3:
    A3-start A3-end
    B3-start B3-end
    C3-start C3-end

    ————————————————–
    HOW TO CONNECT Same PHASES From different stators BETWEEN EACH OTHER?
    maybe like this…
    —————————————-
    —————————————-
    Phase A
    A1-start – TO OUTPUT; A1-End + A2-Start; A2-End + A3-Start; A3-End – TO OUTPUT
    Phase B
    B1-start – TO OUTPUT; B1-End + B2-Start; B2-End + B3-Start; B3-End – TO OUTPUT
    Phase C
    C1-start – TO OUTPUT; C1-End + C2-Start; C2-End + C3-Start; C3-End – TO OUTPUT
    —————————————

    Am i right? do you can help?

    • hugh says:

      hi Yura,

      YOur first challenge is to find out whether the phases are the same in each but if they are contructed the same then they should be ok. Just check that the magnets are lined up and the coils are lined up axially as if there is a rotational shift between them they will not be in the same phase at all.

      Yes you can connect them in series as you have described. This will give you a higher voltage (x3).

      Or you can take each stator to a rectifier and then parallel all of the DC outputs. This will give the possibility of more current at the same voltage.

      I will also repeat again that this is not the best way to use these magnets and coils. Build a larger rotor and put all the magnets on that so they work with all the coils and you will get much better performance.

      cheers
      Hugh

  4. goodness says:

    Hi, I am a novice in magnetism, but as a engineer, I made my first wind turbine which works very well in the wind but it lacks electricity. So I made a 90 turns winding, 3 coil stator with 3 round ceramic magnets at the other side, but no electricity. I don’t get the idea of magnetism. My turbine is just useless without electricity. Pls advice me. thanks

  5. goodness says:

    Must the coils and magnets sit on steel plates?

    • hugh says:

      Yes they must sit on steel plates to get strong magnetic field. It’s much easier to follow one of my designs than to start with your own design. I can maybe help if you send pictures but the alternator is probably too small to be useful.

  6. goodness says:

    My wind turbine is the first ever but its hand made. ie. with little or no machine. I only have local welders in my locality who may not give a good circle steel plates for my project.
    So,
    what if I put 6 or more neodymium magnets to the back of non metallic disc directly behind the stationary 3 windings, then the rotor should have 6 or more PM on the same type of disc that rotates. I think it should create more flux right?
    This method is to avoid use of steel plates. does it make sense.

    • hugh says:

      hi Goodness,

      To be able to help you I will really need a lot of detailed information with sizes and voltages and diameters and everything you can think of. I understand that you have built an axial flux alternator using steel(?) disks, and you sent me a picture of the completed turbine (thanks) but I cannot see the layout of the magnets nor the gap between plates nor the shape and number of coils use or anything much from that. I am sorry but I do need a lot of detail and it has to be in one message because I do not have time to search for messages you may have sent me over the last few months.

      Thanks in advance for the full details.

      cheers
      Hugh

      • goodness says:

        OK thanks. That picture i sent to you does not have any alternator attached to it. so I am trying to build a alternator so I can attach it and what you saw was a non metallic disc holding the blades in place. My wind turbine is made of PVC.
        Thanks.

        • hugh says:

          Think of a magnet as like an everlasting battery that creates a current (flux actually) in a circuit. Flux goes from one pole of the magnet to the other. Steel is like a copper wire with low resistance, and air is like a high resistance. So if the path from one pole face of the magnet to the other is mostly steel then you will get a high flux level but if it is mostly air then you will get a low flux. It’s a little bit like ohm’s law. Less resistance means more current. Use steel disks, and keep the space between magnet faces short and you will have the best flux. Flux also depends on the grade and “length” (thickness) of the magnets as well as the area of the magnet faces. A “long magnet” (thick one) is like a higher voltage battery that can push flux across a bigger air gap.

  7. goodness says:

    1. How do I use my multimeter to measure the voltage in my stator? If it produces AC, Should I set the multimeter to AC?
    Like I said, I am still trying to understand magnetism but my questions are going to be a bit silly if you don’t mind.
    2. I tried to experiment with a bulb, copper winding and neodymium magnets, but no light at all. are the videos I watch online fake?

    Thanks Hugh for your last response.

    • hugh says:

      to make a voltage you need quite a lot of wire turns and quite a lot of powerful magnets moving quite fast. So maybe you did not have enough of everything? Yes you need to set the multimeter to ACV and use a low range to detect the voltage you may see at low rpm with only a few weak magnets and a few coils in series. Again, I can’t be specific unless you tell me some facts.

  8. goodness says:

    Thanks Hugh for your responses. But am not certain about any fact now, most especially the voltage I need. I am looking up to not less than 5v for charging mobile phones and powering led lights. I am just doing this experiment for fun of it.
    Sir, do you mean that hard disk magnets are not powerful enough?

    • hugh says:

      hi Goodness,

      There a quite a few people asking me questions all the time. Unfortunately my memory is very bad, and although I know that you have sent me many messages, I cannot remember the details of them and I do not have time to search for them each time I hear from you. That is why I ask you to tell me the details, including the number of magnets, size of magnets, number of coils, number of turns per coil, space between disks and as much of this stuff as possible in a list, in the message, so I can try to help you.

      thanks
      Hugh

  9. Karana Olivier says:

    Hi Hugh

    I studied with Dan in Colorado and I have a project that involves using electronic waste to produce 60-watt turbines. I have a small working model I can send in a pic. It used dual stator and a single two-sided rotor that flies between the two stators. We tested each stator at 7.5 volts using a drill press at 300 rpm. (This model had 12 2-inch hard drive magnets (two poles, lying in sequence around the rotor), 120 winds per coil, 6 coils per stator, an air gap of 3 mm (I can tighten it). In this model, the stators are not embedded in any resin.

    I am back in Africa again and attempting to replicate the model using other even cheaper parts for teaching purposes. I am using neodymium magnets from hard drives, 21 gauge wire (recovered from a local junkyard here in Abidjan, Côte d’Ivoire). I am using six coils with 80 turns each. Magnets are 2.5 inches long. The model I am making now was going to be dual stator but may stick with single. It just needs to produce 7-8 volts.

    My main questions

    1) how do you link the two stators? Every time I try, I seem to be canceling them out and it shorts. I can feel the drag immediately.
    2) I just read your proposal to use a synchronized turning metal plate behind the rotor. Was that advice only for the proposed twin stator idea or in general. I am not sure how I would incorporate the moving metal backing on the other side of stators (one side is no problem but not both).

    • hugh says:

      hi

      You can link your two stators by putting them on separate rectifiers to feed the battery in parallel.

      It is a good plan to install a second disk on the other side of the stator. There is no benefit to using two stators. Put all of your magnets on one side of a large disk and all of your coils into the stator. Cast the stator as thin as possible (not extra plastic disk) and mount it between the magnets and the second steel disk that spins with the magnets.

      cheers
      HUgh

  10. Karana Olivier says:

    Hi Hugh,
    I am referring to the small dual stator model that I emailed you recently. In your comments, you recommend “Cast the stator as thin as possible (not extra plastic disk)” – I do not understand your reference to as thin as possible. Do you mean the coils should be wound wide (so thin) rather than high (thick)? I did not understand the reference to the plastic disk.

    • hugh says:

      Hi Karana,
      You need to study magnetism and the idea of the magnetic circuit. The magnetism does not like to travel a long way in air and so it becomes weak. If you can make a short gap between the magnets and the steel disk on the other side then you will get stronger magnetic flux and more voltage. Do this by keeping the stator compact, and keeping the steel disk close to the magnets. Get rid of the plastic disk you attach the coils to (because it wastes space), and cast the coils in resin instead. If you take a look at the way that Dan Bartmann and I make alternators then you will see the difference. This is going to work better than the way you did it up to now.
      cheers
      Hugh

      • Karana Olivier says:

        Hi Hugh. I have a rudimentary understanding of magnetism. I now understand that you are referring to the air gap being smaller. Got it. I am working with what I can. Casting coils resin may not be an option in some conditions. I am designing these to operate on what is available locally around two refugee camps. If I can find the resin they use to repair cars then I will use that, but in a couple of areas, even cars are rarely available, so we will see. It may not be possible to remove the plastic housing for the coils which have a thickness of 1.5 mm. Every millimeter counts, I know. Thanks again for your time and guidance. Cheers, Karana

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