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Why is counter EMF produced ?

  1. Feb 17, 2013 #1
    Okay so, when a motor spins in an external magnetic field there will be a change in the magnetic flux threading... When this magnetic flux threading is changed there will also be an induced current that is produced but why is this induced current in the opposite direction of the current that is flowing. Could someone please explain to me why the induced current, counter EMF, is in the opposite direction and why this counter EMF is greater than the input voltage?
  2. jcsd
  3. Feb 17, 2013 #2


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    Staff: Mentor

    Back EMF is just a consequence of Lentz' Law. There is no "why", it just is.


    And the back EMF is not "greater than the input voltage". Where did you get that from?
  4. Feb 18, 2013 #3
    @Tommy1995 just be careful as I hope you haven't been reading the "free energy"agenda stuff.
    As with motor they think that the back emf produced when the rotor is spinning is atleast equal or greater then the voltage applied, so they assume that it charges back the batery which feeds the motor +they have a mechanical force that the motor exerts from it's spinning but that would violate the laws of physics and simple reality too as it would turn into a overunity perpetual motion machine.

    None of that is true , a motor has ofcourse much higher starting current and when the rotor achieves significant rpm the current needed to drive the motor can drop 7 or even more times less than the current required at startup.This is due to the back emf but the back emf is never greater.So you can end up having a large motor with no load drawing only like 500mA or 1A or whatever but still it will require more to keep the rpm than you get back.

    The back EMF is something like hitting a rubber tire with a hammer you apply force to the hammer then the hammer hits the tire the tire pushes back but the "kickback" of the hammer from that tire will never be greater than the first strike you made with your force otherwise you could just keep hitting tires and wont get tired yourself. :D

    I hope this clears up the clouds a little bit:)
  5. Feb 18, 2013 #4


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    I see it as yet another manifestation of the Energy Conservation Law - which, I suppose, also "just is". If the back emf were not 'back' then a motor would speed up without any power being supplied. We'd all want one of those!
  6. Feb 18, 2013 #5
    Don't worry @sophiecentaur a hairy homeless looking guy in a low quality youtube video standing in his rent garage already promised me one of those "no BACK emf" motors a while ago.He also mentioned something about buddhism and uniting the world, I invested all my money in this project,just like physics laws the guy just was , yet is no more, now i'm writing this post from a public internet cafe with no shirt on.:D:D:D:D

    BEWARE people free energy is a lie, thank god my story is also.
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