2 October 2017

What is Back Emf ?


  • As soon as the armature starts rotating, dynamically induced emf is produced in the armature conductors. 
  • The direction of induced emf is found by Fleming's Right hand rule
    which is direct opposition to the supply voltage
  • This emf is referred as back emf or counter emf. 
  • The expression of the back emf is given by
         Eb = FZNP / 60A Voltage


  • The equivalent circuit of a motor is shown in the Figure A. 
  • The armature is represented by source of back emf Eb is in series with armature resistance Ra
  • The supply voltage V must be large to drive armature current Ia against back emf Eb
  • The electric work done in over coming this opposition is converted into mechanical power ( EbIa ) developed in the armature.
        V = Eb + IaRa
             OR
        Ia = ( V – Eb ) / Ra
  • If the armature speed is low, the back emf Eb will be less and more armature current flows. 
  • Similarly at high speed of the motor, less armature current flows through armature winding. 
  • At no load condition, small torque is required to overcome the friction and windage losses therefore the back emf is almost equal to supply voltage.
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