The process of voltage generation in a separately excited and self excited generator vary slightly from each other. The voltage build up process of the mentioned generator types are described below:

How voltage is induced in a separately excited generator:

The operating principle of a generator is based on the interaction between the coils of a rotating armature and the magnetic field of a fixed stator. In a separately excited generator, the magnetic field of the stator are boosted by supplying an external DC. The magnetic poles generate strong magnetic field which produces magnetic fluxes, these fluxes are cut by the rotating armature coils. According to Faraday’s electromagnetic induction, current is produced in the coils.

Faraday’s law of induction states that the magnitude of the rate of change emf induced in a circuit is proportional to the change of magnetic flux, Φ with respect to time, t.

emf= −NdΦ/dt

here,

N= number of turns on the core

Φ= magnetic flux cut by the coils

t= time

From the equation we see, by increasing the no of turns, we can increase the effect of magnetic induction.

How voltage is induced in a Self Excited Generator:

In self excited generators the magnets are energized by own current produced. The current is produced due to the residual magnetism. The residual magnetism in the poles creates magnetic flux which are cut by the armature windings. Initially there is no current in the armature, when the armature is rotated with the help of the prime mover, then the windings cut the flux lines and thus current is induced in the coils. Thus a small amount of electromotive force is generated in the armature. The current then flows through the load, as well as some field current is generated. This current magnetizes the poles more strongly and thus more flux is created due to strong magnetic field. This in turn induces more emf in the armature and thus more current is produced, this process keeps continuing until saturation is reached in the magnets. Thus the current produced in the armature does the function of an external DC supplied to a separately excited generator, that is boost the magnetic fields.