2 October 2017

Working Principle of DC Motor

  • An Electric motor is a machine which converts electrical input energy into mechanical output energy.  
  • When a current carrying conductor is placed in magnetic field, it experiences a mechanical force whose direction is given by Fleming's left hand rule. The magnitude of the force experienced by the conductor is given by
         F = B I L sin q
        Where F = Force in Newton
                  B = Maximum flux density in Weber / meter2, it is a vector quantity
                  I = Current in Ampere
                  L = Length of conductor in meter
                      q  = Angle between position of conductor and direction of magnetic field
  •  When q = 90° ( conductor and magnetic field are mutually perpendicular ) force    produced on the conductor is maximum. 
  • If q  = 0° ( conductor and magnetic field is parallel ) force produced on the conductor is zero.

How does the rotor (armature) rotate?
  • When the DC supply is given to the motor, a direct current passes through brushes, commutator and armature winding. 
  • When it passes through commutator, it converts DC into AC. Therefore the group of conductors under the influence of N – pole  and S  pole carry currents in the reverse direction.


  • The two pole DC Motor is shown in the Figure C. 
  • Let the current in the armature conductors be outward under the influence of N – pole ( Shown by dots ) and inwards under the influence of S  pole ( Shown by Crosses ). 
  • The direction of force on each conductor can be determined by applying Fleming's left hand rule.
Fleming's Left hand rule
  • If the first finger shows a direction of magnetic field and middle finger shows a direction of current, thumbs indicates the direction of force. 
  • The conductors experience force in the clockwise direction as shown in the Figure D. 
  • These forces collectively produces a driving torque resulting armature rotates in the clockwise direction. 
  • When the rotor rotates, there is always an equal and opposite torque on the stator that is why it is very important to bolt a motor securely.
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