Voltage And Power Equations of a DC Motor

 Voltage and Power Equations of a DC Motor

Voltage Equation of the DC Motor

Input Voltage provided to the motor armature can perform the following two tasks:

  • Obtain control on Back E.M.F “Eb” of the Motor.
  • Provide supply to the Ohmic IaRa drop.


V = Eb + IaRa …………………. (1)


  • Eb = Back E.M.F
  • IaRa   = Armature Current X Armature Resistance

This is above related is known as “Voltage Equation of the DC Motor”.

Power equation of a DC Motor

Multiply both sides of Equation (1) by Ia , we get,

VIa= EbIa +Ia2Ra ………. (2)


  • VIa = Input Power supply (Armature Input)
  • EbIa = Mechanical Power developed in Armature (Armature Output)
  • Ia2Ra= Power loss in armature (Armature Copper (Cu) Loss)

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  1. Hello sir I want the full equation…..

  2. R.J.Jochems says:

    In the article “Voltage And Power Equations of a DC Motor” the iron losses are not mentioned.
    Those losses can not be neglected.
    Would be interesting to see those incorporated in the calculations.

  3. R.Prabhakar Rao says:

    I need power knowledge for the selection of motors and VFD.

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