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Type of induced/back emf in dc motor's armature coil

  1. Sep 30, 2012 #1
    Actually this not a question, I want to justify whether my concept is right or wrong. So lets start.
    Faraday's law of electromagnetic induction [itex]e=Nd\varphi/dt[/itex] explains the way of
    electromagnetic energy conversions. The induced voltage is always ac because the rate of change of flux is needed.As in transformer we give ac voltage and the induced emf in the coil is also ac . In dc motor though we apply dc supply voltage but again the induced emf/back emf/motional emf is ac? Am I right?

    Thanks for any replies.
  2. jcsd
  3. Sep 30, 2012 #2


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    Yeah you are right however it is only alternating in the coil. The commutator acts as a rectifier in a dc machine when it generates. One of the thousands of available online simulations may help, but basically as a coil is rising and producing the positive half of the ac waveform, it starts to fall and when it goes to zero, rather than go negative, the commutator switches the current direction for the in the same way it switches the current direction during motoring.

    So yeah - the motor when applied with DC generates AC, but at the terminals, it would be seen as DC due to the current switching effect of the commutator.
  4. Sep 30, 2012 #3

    Yeah, in case of dc generator the ac emf is induced and ac current flows through the coil which is rectified by commutator and the terminal end we get the dc voltage as electrical energy. But in dc motor we already give dc supply in the input terminal, where does the rectification happen?
  5. Sep 30, 2012 #4


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    think counter emf
  6. Sep 30, 2012 #5
    Remember you need commutator for DC machines; either motor or generator.
    A rotating coil will generate AC; it will switch the polarity of voltage every 180 degree rotation; so, you have to be clever and switch terminals as well each 180 degrees.

    In the motor; you apply DC voltage. remember if you applied DC voltage to a coil then it would not rotate continiously; the force would switch directions after 180 Degree rotation.
    So, you have to be clever and switch terminals each 180 to make the current direction reverse each 180 so that force is in same direction. So, in motor we do the opposite of rectification; we convert DC voltage to AC (sort of).
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