DC Shunt Motor

Discussion in 'Homework Help' started by vincent19-mas, Jun 5, 2013.

  1. vincent19-mas

    Thread Starter Member

    Dec 27, 2012
    83
    1
    Dear all,

    I have some problems in understanding dc shunt motor. Can anyone help me ?

    Why is it when no load, the armature current is zero ? I cant get to understand this. And from the diagram , where is the rotating part actually ? The armature part , Ea ?and V(T) is actually the power supply ?

    Thanks !
     
  2. mlog

    Member

    Feb 11, 2012
    276
    36
    The armature by definition is the rotating component. Even a dictionary will confirm this. First, what do you think the subscript 'a' means under Ea? Second, how do you calculate the armature power, i.e. the load? Hint: Electric Power = what x what?
     
  3. vincent19-mas

    Thread Starter Member

    Dec 27, 2012
    83
    1
    Ea means armature voltage which means internal generated voltage ? Armature power = Ea x Ia.

    Correct me if I am wrong

    Thanks
     
  4. mlog

    Member

    Feb 11, 2012
    276
    36
    Yes! That's right.
     
  5. vincent19-mas

    Thread Starter Member

    Dec 27, 2012
    83
    1
    So, Why is it when no load, the armature current is zero ?
     
  6. gbell12

    New Member

    May 29, 2010
    4
    0
    Old post but just in case someone looks...

    In a motor,

    F = B I l

    Where F = force, B = magnetic field, I = current and l = length of armature

    If we solve this for I:

    I = F/Bl

    If there's no load, then there's no torque required for it to spin, so F = 0, making current I.

    Slightly more intuitively, I think an unloaded motor increases its speed until the back-emf equals the supply voltage - resulting in no potential difference in the armature circuit, and therefore no current.
     
  7. gbell12

    New Member

    May 29, 2010
    4
    0
    Yikes, "so F = 0, making current 0" is what I meant.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    In practice the generated voltage can never usually equal the opposed voltage, (zero current) it requires some to overcome the inertia of the armature and overcome any friction and windage etc, the exact difference will depend on each motor characteristics..
    Max.
     
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