All the generators work on a principle of dynamically induced e.m.f. This principle is nothing but the
Faraday’s law of electromagnetism induction. It states that, ‘whenever the number of magnetic lines of
force i.e. flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor
or coil.’ The change in flux associated with the conductor can exist only when there exists a relative
motion between a conductor and the flux. The relative motion can be achieved by rotating the conductor with
respect to flux or by rotating flux with respect to a conductor. So a voltage gets generated in a conductor,
as long as there exists a relative motion between a conductor and the flux.
Such an induced e.m.f. is due to the physical movement of coil or conductor with respect to flux or
movement of flux with respect to coil or conductor is called dynamically induced e.m.f.
So a generating action requires the following basic components to existing,
i) The conductor or a coil
ii) The relative motion between conductor and flux.
In a particular generator, the conductors are rotated to cut the magnetic flux, keeping flux stationery. To
have a large voltage as the output, the number of conductors are connected together in a specific manner,
to form a winding. This winding is called armature winding of a d.c. machine. The part on which this
winding is kept is called the armature of a d.c. machine. To have the rotation of conductors, the conductors
placed on the armature are rotated with the help of some external device. Such an external device is called
a prime mover. The commonly used prime movers are diesel engines, steam engines, steam turbines,
water turbines etc. The necessary magnetic flux is produced by current-carrying winding which is called
field winding. The direction of the induced e.m.f. can be obtained by using Fleming’s right-hand role.