Back emf and Back Torque

Faraday's principle applies to every source of emf, not only coils in changing fields: chemical action produces emf in a battery, mechanical pressure in a piezoelectric crystal, a temperature gradient across the junctions of dissimilar metals in a thermocouple, and light above a threshold frequency in a photoelectric cell. A generator specifically uses the motion of a wire through a magnetic field to produce a motional emf.

Since the physical construction of a DC generator and a DC motor is essentially identical, a DC generator connected to a battery will run AS a motor, and conversely a motor turned by an outside agency will behave as a generator. This means that whenever a motor's armature is turning -- even while the motor is doing its normal job of converting electrical energy into mechanical work -- it is UNAVOIDABLY also acting as a generator. By Lenz's law, this self-generated ('back') emf must oppose the very cause producing it, which here is the armature current itself; so the back emf always acts to REDUCE the net current flowing through the armature. …