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DC Motor RPM Equation Confusion

  1. Jul 3, 2011 #1
    According to http://www.aveox.com/DC.aspx" [Broken] The way to calculate the output of a DC motor in kRPMs is the formula

    RPM of motor: kRPM = (V - RmI) / Kb

    V= Voltage
    Rm = Terminal Resistance
    I = Current
    Kb = Voltage Constant (V/kRPMs)

    This insinuates that an increase in amperage leads to a decrease in RPMs? How is that Possible?

    I know that Current:Torque and Voltage:Speed, but isnt Voltage = I*R, so an increase in current should consequently lead to an incresed voltage?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 5, 2011 #2
    You should read up on back emf of a dc motor. As rpm increases, an opposing voltage aka back emf (@ the armature terminal) will also increases. The back emf opposes the supply voltage and thus reduces the current to the dc motor. Less rpm = less back emf = more current.
  4. Jul 5, 2011 #3


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    Science Advisor

    Yes, the equation shows that if the current increases, all other variables remaining constant, that the speed decreases. The causality is actually the other way around however. If you increase the load (torque) then the speed decreases and this causes the current to increase.
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