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

         E_b = FZNP / 60A Voltage

• The equivalent circuit of a motor is shown in the Figure A. 
• The armature is represented by source of back emf E_b is in series with armature resistance R_a. 
• The supply voltage V must be large to drive armature current I_a against back emf E_b. 
• The electric work done in over coming this opposition is converted into mechanical power ( E_bI_a ) developed in the armature.

        V = E_b + I_aR_a

             OR

        I_a = ( V – E_b ) / R_a

• If the armature speed is low, the back emf E_b 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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