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Changing motor speed with external magnets

  1. Jan 12, 2016 #1
    In the below video it is shown that rpm is increasing by using external magnetic field . can i know on which principle it is based on?







     
  2. jcsd
  3. Jan 13, 2016 #2

    NascentOxygen

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    If you weaken the field of a DC motor, it has to spin faster to generate the back emf to oppose the applied voltage. Here, it seems he is using external permanent magnets to oppose or cancel some of the manufacturer's designed field, thereby forcing the motor to spin faster. The motor is running under no load, so it's not doing a whole lot of useful work besides overcoming bearing friction and windage, etc., though suffering shortened life because the bearings are not appropriate for this out-of-spec operation.

    An interesting demonstration, but nothing exciting.
     
  4. Jan 13, 2016 #3

    meBigGuy

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    He is trying to imply that he increased the energy output of the motor with less energy input, when in reality he decreased it.
    If he had loaded the motor, with a significant load that slowed it below its no load rpm, then it would have performed poorly.
    It is a very deceptive, almost fraudulent demonstration (since he didn't explain what he actually did and its real effects). Here is a PF thread that address the effects of external magnets on PMDC motors.

    https://www.physicsforums.com/threads/external-magnets-to-pmdc-motor.851333/
     
  5. Jan 13, 2016 #4

    NascentOxygen

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    I see nothing deceptive here. The video shows how an improvisation can increase the speed of an unloaded motor. It would be a useful demo for electric motors classes, IMO.

    I'd recommend the technique only in a pinch, as over time it's likely that the external magnets are going to weaken the motor's permanent magnets. If you need a higher speed motor, buy one designed for what you need.
     
  6. Jan 13, 2016 #5

    meBigGuy

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    He calls it a "stargate" effect and implies he is getting something for nothing. I think he actually says that. And, he does not explain he is increasing the no load speed at the expense of the motor's power capability. He says nothing about weakening the effective stator magnets.

    If it actually explained what was going on and why, then I would agree. But he is deceptive about what he is doing and intentionally misleads. Of course it would be a good experiment for a motors class since it is demonstrating a real effect. But the OP had to ask what was going on. It was not done to be educational.
     
  7. Jan 13, 2016 #6

    NascentOxygen

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    Stargate means nothing to me; but if it has implications then I can see some people jumping to conclusions.

    He explains what the external magnets are doing on the video I viewed.

    Many youtube videos demonstrate something without explaining principles behind it, probably to garner interest. It's often the case on youtube of here's a video I made, you go figure it out. The "Mythbusters" series took it upon themselves to confirm or debunk some of the more perplexing.

    When you produce your meBigGuy series of educational videos we'll know to have very high expectations!
     
  8. Jan 14, 2016 #7

    NascentOxygen

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    In summary, the video demonstrates standard DC machine characteristic. It is predictable and well-understood physics. There is nothing original here, and no magic.

    Thread closed.
     
  9. Jan 15, 2016 #8

    f95toli

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    "Stargate" was a sci-fi movie that later spawned a number of TV shows. As far as I remember the "gates" actually consisted of large rotating discs that opened up portals to other solar systems (etc) .
    So yes, calling something a "Stargate motor" will definitely raise a few eyebrows.
     
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