What is back emf in dc motor ?

When current is flowing through the armature conductor of dc motor, it starts rotating inside the magnetic field of stator pole. As the armature conductors move inside the magnetic field there will be an induced emf in the armature conductor. This emf would be due to Faraday law of electromagnetic induction. The direction of the induced emf will be as per Fleming right hand rule and the induced emf will oppose the applied voltage across the armature terminals. In other words in dc motor the rotating armature will generate an emf as it is rotating under magnetic field, in opposite of the applied voltage and this generated emf in dc motor is called back emf.