Though the dc generator operates above the knee of magnetization curve, yet the terminal voltage reduces with the increased load”-Why?

In the shunt generator power furnishes to the load. As more devices are connected in parallel, the load on the generator increases; i.e. the generator current increases. Because the generator current increases, the terminal voltage   of the generator decreases. There are three factors that cause this decrease in voltage.

Armature circuit resistance: When no current flows through the armature, there is no IR drop in the armature and the voltage at the terminals is the same as the generated voltage. But when there is current in the armature circuit a voltage drop exist due to the armature resistance, and the terminal voltage is less than the generated voltage. The terminal voltage may be calculated from the following equation:

Vt =Eg-IaRa

Armature reaction: When current flows in the armature conductors a flux surrounds these conductors. The direction of this armature flux is such that it reduces the flux from the field poles, resulting in both a reduced generated voltage and terminal voltage.

Reduction in the field current: The field circuit is connected across the terminal of the generator. When the terminal voltage of the generator becomes smaller the voltage across the field circuit also becomes smaller and therefore the field current will be less.

 For that causes reduces the generated voltage and also the terminal voltage. The generator operates above the knee of the magnetization curve where the magnetic circuit is saturated. Under these conditions it is apparent that a large change in field current is necessary to produce a small change in the induced voltage.