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BLDC pump suddenly runs dry...what happens next?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Flyback, Apr 13, 2014.

  1. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Ok and where does that come from?
     
  2. Flyback

    Flyback Well-Known Member

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    Battery/Solar or mains originally
     
  3. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    :facepalm: ..................................................................................................:facepalm:
     
  4. dave

    Dave New Member

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  5. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Sorry dont get it. Is there any info on the web about these power systems?
    What else runs off this power system?
     
  6. Flyback

    Flyback Well-Known Member

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    Agricultural world tends to be very secretive, this is the world I live in and must accept
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    1) Since you have this motor set up with your regulated current supply in the lab, can you use a dynamometer/ adjustable brake to vary the load and measure the effect on current/voltage/speed?
    2) Why, when the IC provides a SpeedFB voltage output, do you go to the trouble of applying the tacho output to a frequency-to-voltage converter?
     
  8. Flyback

    Flyback Well-Known Member

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    I don't think the speed-representing -voltage actually comes out to a pin of ML4425?
    So we use the LM2907 frequency to voltage converter

    http://www.ti.com/lit/ds/symlink/lm2907-n.pdf

    we have a dyno so can use it as you say when we have it built
     
  9. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    But you said its a "typical agricultural dc bus"
    So what is a typical agricultural dc bus?
    Is there other equipment that runs off it? or is it just for the pump?
    I am really trying to understand exactly what you are doing, so I can understand why you are doing it the way you are trying to.
     
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The datasheet (re the internal speed control via PWM) says "The speed command is compared with the sensed speed from SPEED FB through a transconductance error amplifier."
    SpeedFB is available at pin 20 (DIL version of IC).
     
  11. shortbus=

    shortbus= Well-Known Member

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    No, I don't agree. If you have a clothes washing machine, stand by it when it is in the rinse cycle. The pump keeps running and the water as it gets drained, starts to get less and less. The pump is just pumping air until the cycle is over.

    When your BLDC pump is running out of water it will start to try to speed up. But since the load has diminished the rotor and stator will go out of sync, and the driver if configured AS IT SHOULD BE, will throw a fault and shut down. This is controlled by you or who ever sets up the driver. No one else is involved. If you don't do your part, the driver can't do its part.

    If you haven't built one yet, how do you even know how it will react?
     
  12. shortbus=

    shortbus= Well-Known Member

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    Boy, if only BillyMayo or DexterMcCoy was still around, you and him could work this out (snap) like that!.:p:D:rolleyes:
     
  13. Flyback

    Flyback Well-Known Member

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    its a fixed speed water pump.
    As in the following attachment

    Coil "on time" is the full commutation period, ie 2.5ms, not 1.05ms as the diagram below says
     

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  14. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    What agricultural industry are you referring too? I live in rural agricultural America and I can say that unless you are raising illegal crops the industry is not the least bit secretive. :rolleyes:

    Personally I think you just make this stuff up as you go along being no one can confirm any of it.
     
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  15. Flyback

    Flyback Well-Known Member

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    OK, but this is a Fixed speed pump.
    Why wouldn't the following work for controlling this fixed speed pump?
     

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  16. kubeek

    kubeek Well-Known Member

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    Maybe because it is unnecesarily overcomplicated. The most important part is how the 8000RPM is achieved. First you need to ramp the motor up - you cannot just out of sudden go from 0 to 8000 RPM, especially when the pump is already loaded. Second thing, if you unload the pump, the RPM will try to shoot because the torque will act on the unloaded momentum of the impeller until someting reacts. This means the motor will most likely get out of sync when there is sudden load change.
     
  17. BobW

    BobW Active Member

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    You seem to be obsessed with the fact that this is a fixed speed pump. FYI, in industry, most pumps are fixed speed. It's not the big deal that you're making it out to be. What is a big deal is that you're attempting to run a motor off of a constant current source, which several of us have tried to explain to you (with much banging of heads against brick walls), is totally asinine.

    Likewise.

    I guess the only reason I bothered to post anything, is that, maybe, it would save others from wasting their time.
    I'd never heard the monkey/football thing before. It's certainly appropriate.
     
  18. alec_t

    alec_t Well-Known Member Most Helpful Member

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    So, it transpires from post #114 that the supply is not a constant-current source. Rather, it is a duty-cycle controlled current source regulated according to sensed motor speed. That makes more sense.

    Edit:
    Why more complexity? Why not feed the speed sensor signal directly to the opamp?
     
    Last edited: Apr 22, 2014
  19. kubeek

    kubeek Well-Known Member

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    Or use an RC filter or integrator if you want that long response time?
     
  20. Flyback

    Flyback Well-Known Member

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    ...you mean from the pin of the ML4425?
     
  21. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Yes. Any reason why not? And shouldn't the opamp have a speed-set signal (no, not the one from the ML4425 :)) as a second input?
     

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