Question

what is dynamo? how it works. And what is its principal

Answer 1

In German "dynamo" is synonymous with electric generator, but in English, "dynamo" is archaic for all but astrophysical electric generators.Here the Mechanical energy is changed into electrical energy.Dynamos were invented majorly for replacement for batteries.The range in size of natural and man-made dynamos or electric generator- motors is awesome, 46 orders of magnitude, from the smallest (common) motor, and electric watch, to the awesome galactic black hole accretion disk dynamo.principle of dynamo is  Self-excitation.The process of generating a magnetic field by means of an electric current is called self-Excitation.

Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a magnetism when the generator is turned off.Self-excited generators must be started without any external load attached.

Answer 2

dynamo is a gadget that converts mechanical energy into electrical energy. A simple example is the dynamo attached to the wheel of a bicycle or motor cycle.
When the wheel rotates, the axle in the dynamo is caused to rotate. This rotary motion is used to induce electricity in a circuit by electromagnetic induction principle.  The circuit has a bulb connected to it.
As long the wheel moves and axle of the dynamo moves, there is current flowing in the circuit.