what is back emf

Back EMF, also known as counter-electromotive force (CEMF), is the electromotive force (EMF) that opposes the change in current which induced it. It is the voltage generated by a running motor that acts to counter the supplied voltage. Any rotating coil will have an induced EMF, and in motors, this is called back EMF since it opposes the EMF input to the motor. Back EMF is the generator output of a motor, and so it is proportional to the motors angular velocity. It is zero when the motor is first turned on, meaning that the coil receives the full driving voltage and the motor draws maximum current when it is on but not turning. As the motor turns faster and faster, the back EMF grows, always opposing the driving EMF, and reduces the voltage across the coil and the amount of current it draws. The term back EMF is also commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature and the magnetic field produced by the motors field coils or permanent magnet field, thus also acting as a generator while running as a motor.