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Electrical load power

  1. Dec 2, 2011 #1
    Hi all.

    This question has been bugging me for a while now so hopefully you can help.

    I want to know how an electrical load (such as a motor) maintains its power, like when the voltage drops the current increases.

    I know all the formulas ( in this case P = V x I ) but want to know how it works.

    At nominal operation the motor is spinning fine and sees a current passing through its terminals. Then there is a voltage drop or dip and so it draws more current.

    How does it (know how to) draw more current? A motor requires a current to induce a magnetic field and it (if it had a brain) does not know anything about the source. All it wants is a particular current to produce the magnetic field. Where does the voltage relationship come into this.

    I have searched and searched but all I can find is it maintains P = V x I, but not how it does this.


  2. jcsd
  3. Dec 2, 2011 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    When the input voltage drops to a motor, the output power of the motor drops.

    The only common constant-power loads are DC-DC converters. They use switched currents through magnetics to transform an input voltage into an output voltage at the load current. The input current depends inversely on the input voltage, since DC-DC converters are basically "constant power converters".
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