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Homework Help: Concept of dc motor and a.c. generator

  1. Nov 18, 2011 #1
    1. The problem statement, all variables and given/known data
    I want to ask how dc motor and ac generator work and how they are used.

    dc motor is a device that converts electrical energy into mechanical energy. dc motor consists of magnet, coil, split ring commutator, brush and dc supply. dc supply will give direct current to the coil and magnetic force will act on the coil, make it rotates, let say clockwise. The split ring commutator will reverse the direction of the current round the coil every half-turn so that the coil will keep rotating clockwise.
    My questions:
    1. We call it dc motor because the supply used is dc? If the supply is AC then it is AC motor?
    2. What is the application of dc motor? If the function is to convert electrical energy to mechanical energy, it means that we use the mechanical energy for something, what is it?

    Now about ac generator. ac generator converts mechanical energy into electrical energy. ac generator consists of magnet, coil, brush, and slip rings. The coil will rotate and produce electricity.
    My questions:
    1. I don't understand why the coil can rotate. I suppose the concept is the same as dc motor, using magnetic field and current to produce force and then produce turning effect. But if it is true, then the coil must be connected to power supply to get current so we basically change electrical energy to mechanical energy then to electrical energy. I am sure my concept is wrong but I don't know what the mistake is.

    2. ac generator used concept of electromagnetic induction to produce electricity. How does it apply the concept? My opinion is because the coil rotates, it will cut the magnetic field so emf will be induced in the coil. Slip rings will transfer the emf produced by the coil to external circuit. And because the emf induced is alternating, the current produces also alternating. Am I correct?

    3. If my opinion above is correct, then the coil in dc motor will also cut the magnetic field and emf will be induced on it. So dc motor also can produce electricity?

    Ahhh, I'm so confused :grumpy:
    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 18, 2011 #2
    You got it correct !

    The answer is yes. If you have not understood properly the concept of simple AC generator then you should not jump to http://en.wikipedia.org/wiki/AC_motor" [Broken].

    Yes always remember that motors convert electrical energy to mechanical energy. DC motor has various applications ! Used in computer disk drives , dvd drives and fans , windmills and other various appliances which rotate.

    Generator generates electricity. You rotate the coil or in big generators it is rotated by machinery. This cuts the magnetic flux and hence induces current.

    Well done. I really appreciate that you admit your mistake. We generate AC current in the coil by rotating it between the two opposite polarities of the two different cylindrical magnets or it is rotated by machinery. The other way is the changing of current continuously. But the former one is preferred.

    Watch this tutorial :


    No the current flows in the coil . The coil rotate lets assume clockwise right ? Ok so when coil rotation makes right angle it must stop (magnetic lines of two opposite polarities of the two different cylindrical magnets and coil become parallel). Due to the angular momentum gained by coil it make 180 degrees and hence the split rings reverse the direction of current and hence coil continue to rotate in clockwise direction.
    The concept behind the working of DC mortor and AC generator is the Fleming's left hand rule and Fleming's right hand rule. Look them up.

    Is this a homework question ?
    Last edited by a moderator: May 5, 2017
  4. Nov 18, 2011 #3
    How can it be used in windmill? It is more obvious to me that windmill is used to turn the coil in ac generator to produce electricity. Windmill can rotate because it uses wind energy. How to relate dc motor to windmill?

    Yes, the current from the supply flows in the coil and the coil itself rotates so it cuts magnetic field. Till this point, Am I correct or wrong?

    If I am correct, then emf must be induced in the coil. I also feel this is weird because the coil has already had potential difference on it because it is connected to the supply but my mind keeps thinking that the coil cuts magnetic field and another emf should be induced on it.

    No, it isn't. I'm just trying to learn about this and looking for explanation :smile:
  5. Nov 19, 2011 #4
    I am referring to the mills which are used to remove husks of grains. Some people refer them as windmills.

    And yes windmill which is used to convert wind energy to electricity works on the principle of ac generator.

    Your statement related to DC motor is correct.

    Uhh DC motor. The coil is connected to a battery. When circuit is closed DC current flows which rotates the coil because coil experiences torque or specifically couple when it is a conductor between two cylindrical magnets of opposite poles. At 0o and 180o it experiences the highest torque. Now what you say is when it rotates it cuts the magnetic flux (magnetic flux is magnetic lines of force passing through a cross sectional area). Hence emf other than of battery must be induced in coil , right ?

    You are pretty correct in this. Two types of emf are developed :
    1. Applied emf between two electrodes in battery.
    2. Induced emf by the reason just as you said.

    This induced emf developed induces current other than DC in the coil ! You know that direction of coil is changing continuously. Hence at one flick of second the induce current flow in same direction as the DC current while at another flick the induced current flow in opposite direction of DC current. You must also remember that intensity of that induced current is very less than DC current. So the net result becomes very negligible. That is why we don't consider all this.

    To further prevent all this we laminate the coil in somewhat cylindrical shape to :
    1. Prevent jerky motion of coil.
    2. To prevent http://en.wikipedia.org/wiki/Eddy_current" [Broken].
    3. To prevent energy loss due to induced current if it is necessary.

    Does this make sense ?

    Last edited by a moderator: May 5, 2017
  6. Nov 27, 2011 #5
    It really makes sense. Thank you very much for your help :)
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