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Will a contra-rotating stator and armature, generate more power than std elec motor?

  1. Nov 14, 2005 #1
    Hi there.
    Im afraid i know very little about electrical engineering:confused: , hence why I am here, but any help / direction / suggested reading would be much appreciated!
    Like i say, sorry if this is a completely stupid question:blushing: , but if you dont ask you dont get right? and who better to ask...
    The general idea is: I intend to use a small simple electric motor as a generator for a model wind turbine. I realise that the faster you spin a motor, then (up to a point) you can generate more power. What i was wondering is:-
    For a simple electric motor, can you increase the power you achieve by rotating the magnets/stator the opposite way to the armature/windings.
    For example, if in the following link, the magnets were to be rotating counter-clockwise
    My thinking was that having some sort of contra-rotating system would effectively "increase the relative speed" of the motor/generator.
    Thanks for your time,
  2. jcsd
  3. Nov 14, 2005 #2


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    That would increase the relative speed, and hence the generated power, but I don't think that it's a practical way to do it. The mechanics of it would be complicated, requiring another set of slip rings and a reversing drive mechanism. You could achieve the same effect much easier by gearing up the rotor speed.
  4. Nov 14, 2005 #3
    Thanks for your reply!

    Well part of the idea was to consider a contra rotating set of turbine blades, which would provide this extra speed. Furthermore, i thought that (with correct blade and pitch selection) this second rotation may provide power generation in conditions which may not be strong enough to turn the "conventional" blades.

    I have found that some aeroplanes using contra rotating propulsion blades, have fewer blades on the downwind set, and these are also of a higher pitch angle, to optimise the conditions which the first set of blades has set up.

    I am aware that the max efficiency of a turbine is 59%, but more realistically less than this, and that the available power from the wind will be reduced having had some extracted by the first set of blades, but cant help thinking that this may be a way of getting more from what exists already.

    Any thoughts?

    Cheers, Simon
  5. Nov 14, 2005 #4


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    Interesting idea. So I googled it to see if others had investigated this line of thinking. Unfortunately, the initial hits were for a guy who turns out to be a huckster who was trying to push this kind of counter-rotating motor idea as being able to get higher efficiencies than normal. Here's a pretty harsh thumping that one organization gave him after going to one of his promotional shows:


    You might also try more detailed google searches the last one I used was:

    +"counter-rotation" +"electric motor"

    Good luck, and keep on thinking. It's a pretty novel idea in some ways.
  6. Nov 14, 2005 #5


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    Great link, Berkeman. I don't have time to read it all now, but it's a hoot.
  7. Nov 15, 2005 #6
    Thanks for the replies!! Its much appreciated. I will have a read of the link, and also try to google +"counter-rotation" +"electric motor".

    Well, i can assure you, im not a huckster out to sell you guys free energy, but i have a bag of sparks, some tartan paint and some left handed screwdrivers if you are interested, LOL :rofl:

    But seriously, like i say, i dont have much electrical knowledge, and just thought that there may be a very simple reason (for somebody in the know like yourselves:wink: ) why this idea would or would not work. For example, if for some reason counter rotation would put the system out of phase, or if there was some electrical law or rule that would mean this idea would not work.

    Well thanks again for reading this! Please let me know if you have any other ideas and/or reasons why you think this is a good/bad idea. Or if you know any electrical laws which you think may be applicable to this contra rotating idea.

    I dont think i mentioned this before, this is for a university final year project, which is to design and build a model wind turbine. I realise that a small turbine will produce negligible power, but for me this isnt the purpose, it is to find and develop new ideas to try and promote green energy.

    Initially the idea is to produce a small model with pitch control, but i really hope i get the chance to produce some sort of contrarotating turbine to measure the before and after performance (or the single turbine versus contrarotating turbine) in order to find if this idea is feasable, and possibly quantify the differences between the two.

    Again, any input is muchly appreciated.

    Cheers!! Si
  8. Nov 16, 2005 #7


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    It appears that I misunderstood your original post. I thought that you meant to have the stator/case assembly rotating opposite to the rotor. If you just intend to have contrarotating turbines driving the rotor, that's a different (and much simpler) story.
    It will be beneficial, but perhaps not for the reason that you suspect. Keep in mind that the primary reason (usually) for contrarotation is for balance/restriction of reaction torque. To make your assessment properly, you should also try running two turbines turning in the same direction. I think you'll find that the torque increase will be about the same as with contrarotation, because the main benefit will be from having more vane surface in the airstream. The speed of rotation will depend upon your vane pitch, and shouldn't change regardless of how many turbines you have. The higher torque of more wheels would allow you to run steeper pitch and gearing, though, so the maximum speed of the rotor could depend upon it. (Make sure that you don't overdrive the generator, though. You could shell a bearing or suffer too much internal wear.)
    Double-check this with others before taking my word for it, but I think that I've got it right.
  9. Nov 16, 2005 #8
    Sorry, i think my last post may have confused matters, thats my fault. I will try to clarify, but im afraid i am not very articulate, so sorry if it sounds like im "dumbing it down" thats not my intention.

    How i believe a wind turbine generator works is say the armature/coils of a motor are fixed to the same shaft as the turbine blades which rotates say clockwise in adequate weather conditions. This runs through a fixed (not moving) set of magnets/stator, and provides a rotational speed.

    However, if the magnets were rotating in the same direction and speed as the blades, then no electricity would be produced, because relatively the rotor and stator would not be moving with respect to each other (this obviously would not happen.)

    However, if the magnets/stator were to rotate in opposite directions, then the relative rotational speed would be the addition of the (clockwise) + (anti-clockwise) speeds.
    for example 15rpm (clockwise rotor) + 15rpm (anti clockwise magnets) = 30rpm total

    i.e. if a car going at 30mph hit a stationary car, the impact would be equivalent to a 30mph smash, but if both cars were travelling towards each other at 30mph then this becomes 60mph in total

    So for a standard "windmill" the blades turn the armature, and the magnets remain stationary. The idea i was thinking of was to have a second set of blades behind the first (at the opposite pitch, to allow the two sets of blades to spin in opposite directions), only now the magnets would be attached to the second blades. This means that the magnets would now be rotating in the opposite direction to the front blades, and hence in the opposite direction to the armature. And so i thought that the addition of the anti-clockwise rotational speed to the usual clockwise rotational speed would become the total rotational speed which would be larger than simply the rotor spinning inside a stationary set of magnets.

    A further application of this idea relates to the "cut in" windspeed of the "windmill" For a given hub height, blade diameter, No. of blades etc the blades will begin to rotate above a certain windspeed, say 5mph.
    For the rear (contra-rotating) blades we could select the the required dimensions and properties to commence spinning at a lower windspeed, which would mean electricity would now be produced at a lower cut in speed (even if the front blades have not yet started turning). I believe this would work because the effect of rotating the armature inside the magnets, in theory is the same as rotating the magnets around a stationary armature. Hope i made sense? I just mean that the back blades will produce power from say 3mph upwards, but in addition to this the front blades would begin to rotate AS WELL from 5mph upward.

    Sorry if i have just confused matters more? And your input is definately much appreciated! So thankyou again!

    Last edited: Nov 16, 2005
  10. Nov 16, 2005 #9


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    No need to apologize; that post cleared it up completely. Both of my previous interpretations were 1/2 right; put those halves together and voila!
    Your idea seems sound in principle. I don't think that there'd be an advantage over just gearing the turbine shaft up 2:1 to double the speed, but that's no reason not to try it. The difficulty, as I mentioned in my first response, will be the additional bearings and slip rings needed for free rotation of the stator (ie. rotor2). That's not all that hard to to, but you'll probably start cussing midway through the project. :biggrin:
  11. Nov 16, 2005 #10
    Heh heh yeah i can imagine!

    Thanks again for your comments. I am now thinking that the simple gearing would be easier, but will keep my options open :smile:

    Like i say, if you have any fantabulous ideas, feel free to post them :wink: Thanks again Danger!!!!
  12. Nov 16, 2005 #11


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    No worries, mate. Keep us informed of your progress. :smile:
  13. Jul 30, 2008 #12
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    spongebob, This was some time ago you posted this but as I was also asking myself the exact same question when I found your post I thought I should reply with my thoughts.
    On the face of it this seemed a good idea.
    However for this to work you must consider the torque generated by the two contra rotating props and the torque transfered from one to the other though the generator. Prop torques must be the same or almost the same and that’s not going to be easy.
    If there is a large difference in the force generated by the 2 props I think you would find that because considerable torque is required to spin the generator the prop generating most torque would spin, pulling the motor magnets pulling the stator pulling the second prop against the wind.
    The resultant motor speed would be less than the speed of the fastest prop giving the reverse effect of what you desire.
    You can test this out if you like by constructing a simple contra rotating prototype similar to toy kids put on sand castles only with 2 props folded in opposite directions coupled by a friction pad between props (to simulate the generator) all on the end of a stick.
    You should then see the problem. (I have not tried this so try and prove me wrong)
    If you use a desk fan as a wind source with some considerable care you may be able to adjust the friction pad and the props to get perfect contra rotation at one wind speed. Now speed the fan up or slow it down and I would expect everything to go wrong again. You need to adjust the pad or props again to maintain balance.

    (((A more thorough test If your not put off yet would be construct a larger outdoor prototype again with a friction pad between the props and free running on an axle. Put a bycycle odometer on each contra rotating blade. Build this and build also a conventional single prop with larger blade area with a same friction pad design against a plate fixed to the axle. Note if you put 2 props on a pole the pole cant support props of as big a size as a single prop because theres more weight and more drag. At a guess Id pit 2 2ft props against a 3ft prop basically something of similar visual impact and drag.)))
    When you compare the idea to an aircraft with contra rotating blades this is not the same. The aircraft props are connected by gears so as one prop turns the other must turn in opposite direction. Your idea is actually analogous to a single prop aeroplane. As the prop rotates there’s a tendency for the whole aircraft to rotate in the opposite direction. The pilot must compensate for this by trimming the aircrafts wings to produce an opposite torque. This is actually very easy for a human. Just like its easy to ride a bicycle and keep in balance. However it’s more difficult to create an automated system to balance the torques. I believe that the contra rotation prop for aircraft design is actually used on unmanned aircraft to make them easier to control remotely and avoid need to trim plane in this manner which is difficult when your not actually in the plane and the plane is out of visual range, all you have to go on is the camera on the plane. Some simple cheap toy helicopters ( $20 type) have a geared contra rotating balance or prop for similar reason.
    All this said I believe there is still merit in the design if one of the props can be trimmed to keep the torques equal. It’s evident though the smaller you make the windmill the quicker the props can speed up and slow down so the harder it is to balance. Also the greater the overhead in terms of cost!
    note: You may read that a generators produces more than twice the power when rpm is doubled. This looks like a clear reason to double the rpm in the way you mention. You must also realize it takes considerably more than twice the effort to turn the generator at double rpm. Your 2 prop design will not spin the motor twice as fast.
    Of course its certainly true that generators have an optimal rpm range and that most generators are designed to work at higher rpm. Typically automotive alternators are designed to perform best around 2000-3000 rpm. A specialized generator for wind turbine use should perform at much lower rpm It will have permanent magnets so will not have a start up energy consumption overhead to magnetize its coils. There will be other changes to design.
    Also consider the power loss that would be created if you were to just use a single prop with gearing to increase generator speed. This is a rough estimate but Id account for a 4% loss of efficiency for driving the generator from a simple 2 cog gearbox. large cog on prop, small cog on generator. Now consider the potential loss of efficiency in your 2 prop design In efficiency will be introduced by the turbulence generated by the 2 props rotating in opposite directions.
    If the props are close together then this will be more than likely cause more than the afore mentioned 4% loss.
    Its possible to do all the math and maybe come up with a workable design.
    Maybe something with a large T with a prop on each end of the head however I am pretty certain that a design with a contra rotating magnet motor and twin conventional props is a non starter for a small home wind turbine.
    As a large commercial wind generator though where the props can be bigger + further apart and the cost in terms of energy loss and initial outlay for any control mechanism technology would be less as a proportion of the energy produced and build cost the idea may have some merit.
  14. Aug 6, 2008 #13
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    What few realise is that much work on quite large contra rotation wind turbines was done by the Third Reich Wind Energy Ministry, 1935-45, and later by Trimbles Windmills, see this URL


    See also
    which were the basis for the Trimblemill "blade" design, subsequent rotor engineering shown here:

    Model tests for Trimble indicated that energy collected from each rotor was additive, so "Betz Limits" could be exceeded, not so far achived on larger amchine, probably due to incorrect blade spacing.

    If the Kloss paper is correct [see reference in earthtoys] then contra rotations WTs can be grid liniked automatically without electronics. "Contra" rotation frequency & voltage is maintained, but individual rotors adopt different speeds according to wind conditions. Ye to be demonstrated, [probably was about 1943 but report lost]

    This has major cost implications for wind energy, especially offshore, as many small rotors - on the basis of a rotor being an area for income but a volume for cost - can be less costly than one rotor of the same overall swept area. The principle of "maximum economic diameter" discussed by Betz. Solar CSP "dishes" are ganged up on this principle.

    Further as contra units are self centering on the wind, they can be point suspended, so transferring all overturning moments normally found with bottom supported WTs into a downward sructural load. Major implications for offshore wind farms, which could probably be barge mounted.

    See also www.grunweb.org.uk
  15. Aug 6, 2008 #14


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    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    Something else to check out from 1945,

  16. Aug 7, 2008 #15
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    For what its worth I would invisage a working contra rotating design would look something like (not exactly same as) these pictures I quickly found on the net.



    I havent read about these turbines but I would imagine there would need to be some contra rotation to keep the things balanced.
  17. Aug 7, 2008 #16
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    geebee, I know the Selsam set up, it demonstrates the benefits of multirotor "arrays" to keep costs down. As far as I am aware they are single rotor.
    For a picture of an actual operating contra machine - timblemill - there is one on the earthtoys URL I have a lot of other data/pictures available. I was the original concept designer based on Woods Guide to Fan Engineering !!!
    see also the grunweb site which I look after for the Grünhaus project
  18. Aug 7, 2008 #17
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    Quote: I was the original concept designer based on Woods Guide to Fan Engineering !!!

    Do you have any decent facilities to build a small turbine?
  19. Aug 7, 2008 #18
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    How small ? Original Trimble test unit was 1.5m dia, normal machine 6m, rated 5 kW @ 10m/sec wind
    Might find a you a second hand Trimblemill in Ireland for you to play with ?
    Or UK manufacturer of axial flux alternators might help, after all a contra rotating WT is but an "alternator with wings"
    Re sailwing blades, redesign wanted here. In WWII the Hawker Hurricane fighter had cloth wings on a metal from, and flew at 400 mph + !
    Can give you direct contacts to both the above

    I have no manufacturing/testing facilities of my own now. Am 81 !!
  20. Aug 7, 2008 #19


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    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    An idea that has been in my mind for many years, you might know if any others have considered this.

    Instead of passenger pods or seats, have airfoil sections of some size, on a ferris wheel type design. The lift produced at any place around a 360 degree circle will produce a torque at the generating unit(s), the stronger the wind the more generators engaged. A slow but very powerful force can be transmitted.

    I have not looked at your site yet, please forgive if this is already mentioned or discussed somewhere else.


  21. Aug 7, 2008 #20
    Re: Will a contra-rotating stator and armature, generate more power than std elec mot

    Something like what you are proposing was put forward by Honneff about 1933. I will try and find a reference, but I suspect that there is nothing about this on line
    Henneffs 1932 book Windcraftwerke may have it in. Only place I know of a copy is in the Science Museum Library, London His design was for 5 by 7 MW 150 m dia.contra rotors on one tower !!

    Large wheel type wind energy collectors, barge mounted for offshore appeared in the 1970's as well. I think I have a paper on this filed somewhere, could send illustrations if you want them.
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