The counter-electromotive force (abbreviated counter emf, or CEMF ) 1
is the voltage, or electromotive force, that pushes against the current which induces it, is caused by a changing electromagnetic field. It's represented by Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. Counter emf is a voltage developed in an inductor network by a pulsating current or an alternating current. 2
The voltage's polarity is at every moment the reverse of the input voltage 3 4
In a generator using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator.. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced. 5
Pretty self explanatory dont you think, i added this as the previous entry was rubbish and he's obviously visited an engineer who doesn't know what back EMF is.