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Voltage sag

  1. Aug 24, 2008 #1

    In Power Quality, how does starting of large motor will cause voltage sag? How does voltage sag cause the motor to stall and overheat?

    A friend mentioned that it has something to do with sustaining the source power, but I simply don't get it.

    Thank you in advance.
  2. jcsd
  3. Aug 25, 2008 #2


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    Motors draw more current during start up (or if you stall it) than when rotating.
    In normal operation, most of the input power gets converted to mechanical energy.
    In the stopped/stalled condition all the input power gets converted to heat.

    All real world power sources and connecting wiring have impedance (or resistance).
    If you had a power connection with a zero impedance then there would be no voltage sag.
  4. Sep 10, 2008 #3

    If starting a motor consumed high current, then why doesn't the voltage increase too; since V=IR?

  5. Sep 10, 2008 #4
    Motor is an inductive load, so it draws high initial current.
    voltage or the potential is not absorbed, rather electrons are ( current).
    you just need to supply more electrons to take your motor coil to operating level with the same potential.
    the R in ohms law corresponds to resistive real value where as an inductive element possesses reactive component as well.
    Hope this helps.
  6. Sep 10, 2008 #5


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    Due to the Conservation of Energy.

  7. Sep 10, 2008 #6


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    It depends on you're viewpoint or which components of V=IR you are looking at.
    The effective "R" of the motor is variable and very low at startup.
    V is a constant defined by the construction of the source.

    However, the wires connecting the voltage source (and the voltage source) have a value of R associated with them, separate from the motor.
    The voltage drop due to R_source and R_wire does increase with current and must be subtracted from V_source.
    This is seen as a "voltage sag" at the terminals of the motor.
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