Micro motor/generator as wind power source.

Discussion in 'Power Electronics' started by Raymond Genovese, Mar 4, 2019.

  1. Raymond Genovese

    Thread Starter Well-Known Member

    Mar 5, 2016
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    One of the 13,784 projects that I think about is to use "wind" to generate power to a BLE beacon. Here are two example applications to give the issue some legs.

    1. Have a BLE beacon powered when air is flowing in a vent providing detection/feedback in an HVAC setting. I actually successfully did this using a battery-powered beacon and a sail-switch to connect the battery when there is air flow. That works well and I wrote about it somewhere around here. But, that was with a coin cell. Here I am talking about generating the power with the air flow.

    2. Anemometer beacon that is powered by the wind.

    OK, so that is the general idea.

    Here are two toys that I recently bought.
    FandG IMG_8948r.jpg
    As a brief aside, these ultra-cheap and free shipping purchases always bring me a smile as they take so long to arrive, it is a fun surprise when they do (hey, what's this), followed by an "oh yeah, I remember ordering that".

    The little green one is a fan that runs off of 3v. So, I attached the wires to a bridge rectifier and hit the blades with a hair dryer. The most I got out was...maybe...a volt and that was with the hair dryer going full blast and a few inches from the blades. Definitely a no-go disappointment.

    The yellow one is a toy generator (suitable for adult children). I have not yet hooked it up as with the first one, but all I have to do is give the blades a hefty flick and the LED lights up. An encouraging result. I will hook it up with a rectifier and see if I can get more than 3v out and, if so, the plan is to add a zener (or maybe a 1117) and see if I can power a beacon.

    To get to my question. How do I choose a microgenerator or micromotor that will work well as a generator? I mean, I am not interested in buying a wind turbine, just a small motor/generator. But, I have no idea how to discriminate between the two. What characteristics are there to make the decision?

    Any explanations or links are appreciated, but the simpler the better, if sufficient. If not, tell me that also.
     
  2. mvas

    Active Member

    Jun 19, 2017
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    Why did you put a Full Wave Bridge on the output of the DC Motor with green blades?
    Diodes caused a Voltage Drop.
    Connect your voltmeter directly to the motor output.

    If you remove the green blades and apply 3 volts to that DC Motor, how fast does the shaft spin?
    Now guess how fast you have to spin the shaft of that motor to generate 3 volts.

    Small Wind Turbines are around 30% efficient, in the real world.
     
    Last edited: Mar 4, 2019
  3. Raymond Genovese

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    Because I had one.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This should be DC already?
    Max.
     
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  5. Raymond Genovese

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    OK, I will take my punishment as deserved for what I have done...but...if the rotation is in one direction, is +/- the same as if the rotation is in the other direction?
     
  6. Raymond Genovese

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    Well, I can't guess, but I can tell you that hitting it with the hair dryer spun it MUCH faster than a couple of the AAs.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Even if the generator were making AC, the voltage level is so low that you don't need a bridge to protect the LED, which is a diode after all. LEDs can't tolerate much reverse voltage but I don't think that's much of an issue with this kind of device.
     
  8. Raymond Genovese

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    Of course it is, that is why the LED only lights when the yellow blades are spun in one direction. Thus, my thinking is that you can get voltage and current out if you used two diodes or even four, no matter which way the blades are spinning. So, the bridge that I used to test out the green one was with that in mind. For example, as an anemometer, you would want to power the beacon regardless of the direction of the wind.

    While this is good discussion and potentially helpful, my observation is that the green one sucks as a generator an the yellow one might be much better. That is what I am asking about. Are these two qualitatively different motors? How do I discriminate between the ones that suck and the ones that sick less?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it was a bidirection generator which would be rare, a bridge will ensure the same polarity, regardless of direction.
    Any decent PM motor should generate the same voltage as is required for that particular operating RPM.
    Max.
     
  10. Raymond Genovese

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    OK, so how do you tell a "decent" PM motor from an indecent one other than the observation that I am reporting?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    First the amount of poles, this can be observed by the denser population of commutator bars.
    Price is usually also a give away.
    If you want a super quality one, get one intended for servo usage, they often have skewed rotor lamination's.
    Max.
     
  12. mvas

    Active Member

    Jun 19, 2017
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    No ... REMOVE the Green Blades and attach the 3 volt battery to the motor.
    This UNLOADED shaft speed is how fast you must spin that motor to generate 3 volts.
    3,000 RPM or faster ?
    To spin the motor shaft that fast with air blowing on the green fan blades, you will need to test it from your car window.
     
  13. Raymond Genovese

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    I read your post carefully and, as I said, I can not guess. How do you suggest that I measure the shaft speed? Assuming that you can't derive the measure with the blades on, is there a measure for shaft speed at a voltage available on motor data sheets?
     
  14. mvas

    Active Member

    Jun 19, 2017
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    The Kv rating of the PM Motor gives you the relationship between the RPM ( K) and the Voltage ( v )
     
  15. Raymond Genovese

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    I'm really NOT trying to be snarky and I do appreciate your time an effort explaning...and here is where I am puzzling over your explanation.

    If I connect the green fan to a 3v supply, either batteries or a bench supply, the blades spin, as one expects. If I use a hair dryer, I can also spin the blades (and measure the voltage out with or without diodes).

    I can visually see that the blades are spinning much faster with a hair dryer, as I have stated. It is difficult for me to believe that the shaft is not also spinning much faster. This, because, visually the difference is so notable.

    I reported the observation after your post:
    Based on the observation, I am spinning it (the shaft) much faster. BUT, you are saying that I am not spinning the shaft much faster, I am only spinning the blades much faster? I am not getting 3v out - I am, maybe getting 1v.

    So, I am asking simplistically...am I not spinning the shaft much faster when the blades are moving much faster?

    And, again, how would I measure how fast the shaft is spinning?

    Finally, looking at a data sheet http://www.portescap.com/sites/default/files/17n78_specifications_0.pdf where do I see the relationship that I can use - and, of course, the shaft speed versus blade speed is integral to my understanding.

    Please believe me, I am trying to understand.
     
  16. Raymond Genovese

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    Edited to add: is the torque required much greater on the blade side - is that close? Because I am not getting it if shaft RPM is the measure
     
  17. Yaakov

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    Jan 27, 2019
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    The relationship of the rotational speed of the motor to its output is going to be less than the unloaded speed of the motor when run at the same voltage. Motors are less efficient as generators than as motors. You can calculate the expected output of the motor if you know its resistance and back EMF specifications, and I am sure you can find the proper formula on the web.

    I have heard a rule that generally higher voltage ratings with lower RPM ratings as motors will be more efficient as generators. I have not worked out whether this is correct.
     
  18. Yaakov

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  19. Raymond Genovese

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    That's a helpful link - thanks. I need to digest it for a while...things like "A motor run as a generator will never put out its rated voltage at rated speed. Even with no load, you get the pure back EMF, which is always less than the applied voltage when the shaft isn't being driven externally."
     
  20. Alec_t

    Expert

    Sep 17, 2013
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    Do you have an oscilloscope? Since the motor has a finite number of poles it is likely that there will be periodic peaks in its output when driven as a generator. The 'scope would make these visible and the peak interval would give you the shaft speed, if the number of poles is known. This number is usually discoverable by rotating the motor by hand and sensing the cogging.
     
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