The back emf is the electromotive force generated by the rotation of
a motor armature in a magnetic field. As soon as a magnetisable part moves in a magnetic field, a force is generated which counteracts the EMF, the back EMF. In a motor, the back emf is almost as large as the voltage applied to the motor and is only affected by minor losses. The polarity of the back emf is opposite to that of the voltage applied to the motor armature.