What happens to resistance of a motor winding when current or voltage increases

Discussion in 'General Electronics Chat' started by ajosephantony, Jan 7, 2017.

  1. ajosephantony

    Thread Starter New Member

    Jun 8, 2015
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    Hello Guys... Hope you all doing fine...

    So I was asked in an interview these question....

    1. When happens to the resistance of the winding of the motor, when current or voltage is increased....

    2. When happens to the Inductance of the winding of the motor, when current or voltage is increased....

    Well i know that, when current increases, it causes more heating in the winding... which means R and L should remain constant... but my other friend told me that resistance decreases, and he doesnt know about inductance...

    So what is it guys.... I appreciate your time...
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    2,196
    422
    it all depends on what you are talking about.

    The (DC) resistance from the wire is ***largely*** independent of the voltage / current - and may increase slightly as the temperature of the (copper) winding increases (usually).

    The same holds true for inductance.

    A motor is also a generator in that as it is driven by the current flowing through its windings, the movement itself generates a voltage in reverse of the voltage applied to the motor (back emf). What you observe is that as the motor speeds up when a constant voltage is applied to it, its current declines gradually. A perfect motor with no load attached to it will have 0 current going through its windings at equilibrium.

    This may appear to the outside world as if the "resistance" of the motor has increased.

    Obviously, that's just for the typical DC motors.
     
  3. ajosephantony

    Thread Starter New Member

    Jun 8, 2015
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    What about for ac motors
     
  4. ajosephantony

    Thread Starter New Member

    Jun 8, 2015
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    So for DC motors, the impedance remains constant when current increases....

    But for AC motors, when freq is constant, when current increases, what happens to the impedance....

    The answer is impedance remains constant or reduces slightly....
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    4,667
    745
    Impedance of inductor is jwL. It is independent of current. It is independent of voltage. It depends on frequency.
    When, you say, frequency is constant, then impedance is constant.

    However, like others pointed out, you have friction that generates heat, and warmer conductor has lower conductivity, so the impedance can increase somewhat, I am not sure the increase is significant enough.
     
  6. #12

    Expert

    Nov 30, 2010
    18,076
    9,691
    AC doesn't magically change the resistance of copper wire and neither does DC. Heat changes the resistance.
    Neither AC or DC change the inductance of a coil.

    The basic answer to your first post is, "nothing happens". You have to get microscopically picky to find the changes. There is no practical difference in the resistance or inductance of the windings when you apply more voltage.
     
  7. ajosephantony

    Thread Starter New Member

    Jun 8, 2015
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    So my answers are correct.... I said, it remains constant.... :)
     
  8. crutschow

    Expert

    Mar 14, 2008
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    6,859
    The only change in motor resistance would be an increase from IR heating of the wire.
    The only change in inductance would be if the voltage gets high enough to saturate the magnetics, which would cause a reduction in the inductance.
     
    DickCappels likes this.
  9. Dale West

    New Member

    Oct 13, 2018
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    I rarely comment but I had to jump in.

    The Impedance does NOT stay the same, resistance stays the same.

    You cannot avoid the fact that when you have a magnetic field cross a conductor, you induce an EMF.
    So, when the rotor or armature passes a field coil an EMF is induced, this EMF is the Back EMF.

    With a induced Back EMF opposing the original EMF, there is a resultant EMF. i.e. 200V in and 150V as a Back EMF, the resultant voltage is just 50V

    Ohms law tells us that the current from 50V divided by the resistance in the windings will be less than the current from 200V in the winding.

    The Back EMF along with the Inductive Reactance with the Resistance makes up the total opposition to current flow, hence Impedance.

    That is why the current reduces.
     
  10. crutschow

    Expert

    Mar 14, 2008
    23,125
    6,859
    So why do newbies seem to be compelled to do necro-posts such as this one, on a nearly two year old thread? :confused:
    True.
    But this thread was about winding resistance, not impedance.
     
    Last edited: Oct 14, 2018
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