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.
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