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How back emf helps in energy conversion in a motor?

  1. Aug 20, 2008 #1
    How back emf helps in energy conversion in a motor???

    hello,
    I am an engineering student and during the past few days i was searching on the net to clarify my doubts on "motors".So i came across an article and on reading one part of it a doubt arosed in my mind.

    Q1)

    "To take a practical example, an electric motor running under no load
    uses very little power. If the motor were frictionless and
    superconducting, it would use no power. This is because the back emf
    opposes the imposed voltage."

    This was the part of the article.My doubt is that If the motor was frictionless and
    superconducting will it rotate and produce mechanical energy??

    Q2)Can you tell me how the electrical work done by the applied voltage in overcoming and causing current flow against back emf is possible??

    Practically i know that when someone is pushing me in one direction inorder for me to oppose it i need to give an equal and opposite force.That is a push is required to oppose a push..
    So using this idea in motors our applied voltage must do an equal amount of work against this back emf.Hence our applied voltage must provide an equal amount of electrical energy to oppose the electrical energy produced by back emf..So if this was the condition can you tell me how this back emf aids in converting electrical energy to mechanical energy???
    Q3) What will happen if there was no back emf?


    I hope to get a reply soon...

    Thank you
     
  2. jcsd
  3. Aug 20, 2008 #2

    mgb_phys

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    Science Advisor
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    Re: How back emf helps in energy conversion in a motor???

    As soon as you try and remove any mechanical energy it will slow down, reduce the back emf and so draw current from the supply.

    The original statement is saying that if you start a (frictionless) motor and leave it runnign with no load the back emf cancels the supplied voltage and so is equivalent to just disconnecting the motor with a clutch.
     
  4. Aug 23, 2008 #3
    Re: How back emf helps in energy conversion in a motor???

    mgb phys:
    Can you please give a detailed explanation???
     
  5. Aug 23, 2008 #4
    Re: How back emf helps in energy conversion in a motor???

    A motor under load looks, for the most part, like a resistor. You can just as well consider the back emf as the voltage dropped across a resistor.

    The more load you place on the motor, the less its effective resistance, resulting in increased current and power.
     
  6. Aug 23, 2008 #5
    Re: How back emf helps in energy conversion in a motor???

    Back or counter emf opposes the terminal voltage and limits armature current. As long as terminal voltage is greater than the induce CEMF current will flow, which it will be.
     
  7. Aug 23, 2008 #6
    Re: How back emf helps in energy conversion in a motor???

    If there was no CEMF, such as when starting a DC motor, armature current would be extremely high. In this case, starting resistors can be used to limit the initial current surge.
     
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