JNTU B.Tech II ELECTRICAL MACHINES I Previous Question Paper (August/September - 2022 - R16)

Self-excitation is the process by which a DC generator builds up its own magnetic field using the EMF generated by its own armature. In this process, the field winding is connected in series or parallel with the armature, allowing the generated voltage to drive a current through the field coils, which in turn strengthens the magnetic flux.

The necessary conditions for self-excitation are:

1. The machine must possess residual magnetism in its field poles to initiate the process.
2. The field winding must be connected to the armature in such a way that the current flowing through it produces flux in the same direction as the residual magnetism (cumulative connection).
3. The field circuit resistance must be less than the critical field resistance at the operating speed.
4. The machine must be driven at a speed greater than the critical speed.

The buildup of EMF in a DC generator is a cumulative process driven by the interaction between the armature rotation and the field circuit. The steps are as follows:

1. When the armature rotates, the conductors cut the small flux produced by the residual magnetism, inducing a small residual EMF in the armature.
2. This small voltage causes a current to flow through the field windings connected to the armature.
3. The current flowing through the field coils produces an additional MMF, which increases the magnetic flux in the air gap.
4. The increased flux leads to a higher induced EMF in the armature, which further increases the field current.
5. This regenerative cycle continues, with the voltage rising along the linear portion of the magnetization curve.
6. The process eventually stabilizes at a point where the field circuit resistance line intersects the magnetization curve, known as the point of equilibrium. At this point, the terminal voltage remains constant for a given load and speed.