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
equivalent-circuit-of-dc-motor.png

  • The electric work done in over comming 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 aramture 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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