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Homework Help: Motor current-time graph

  1. Aug 31, 2010 #1
    1. The problem statement, all variables and given/known data
    Ordinary motor contain a core with coiled electric wire, this coil have inductance and when the
    circuit is close the current will rise gradually. (I still understand the concept to this point)
    After the current rise to a maximun point, the coil have maximum angular acceleration and starts
    to rotate, then the back e.m.f induced by the turning action opose the voltage source and the current gradually decrease to a steady point.

    Howerver, my text book saids during this period of current rise, the coil remain at rest due to inertia.
    I am confused since I think once there are current, there are magnetic force induced, so how
    can the coil remain at rest at this point?
  2. jcsd
  3. Aug 31, 2010 #2

    Chi Meson

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    Without knowing exactly what the textbook says, I am going to guess that it is referring to "real life" examples, as opposed to "ideal" conditions.

    The amount of time it takes for the current to rise from zero to the maximum point is measured in hundredths or thousandths of seconds. In this amount of time, as the current rises, so does the force (torque actually), but there is a moment before motion actually begins due to inertia and friction.

    Once the coil begins to rotate, the back-emf will begin to oppose the initial voltage, and net current will begin to fall. This suggests that the maximum current will occur before the coil actually begins to move.

    Whether this is precisely true in all cases would be hard to measure, since the time interval is so short.

    The point is, the current peaks and subsides very quickly.
  4. Sep 1, 2010 #3
    So is it true that once the motor starts to rotate, the current must decrease?
  5. Sep 1, 2010 #4

    Chi Meson

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    If we are talking DC motors that are supplied by a constant voltage source, then in principle, yes. The coil moving in the magnetic field induces a back-emf, reducing the total emf, which is proportional to current. Also, the coil wires heat up, increasing resistance, reducing current.

    I think that is true for AC motors as well, but I can't say for sure. There are so many little surprising details about induction motors that I've forgotten. Either way, the spike is near instantaneous when the connection is made, and subsequent steady-state is near instantaneous after that (in typical cases).
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