Critical resistance : The resistance of field-circuit for which
around value build up process is possible is known as critical resistance
.(After the value build up process is possible)
The significance of critical resistance in a self
excited shunt generator:-
Due to residual magnetism in poles, some emf and hence
current would be generated .When this current flow through the field coils will
strengthen the magnetism of the poles.This will increases the pole flux which
will further increase the generated emf that means further increase current
which further increase the flux and so on. This phenomena will be happening
till the generate voltage equal to terminal voltage. This phenomena also
depends on the resistance of the field circuit. If field resistance is
increased, then slope of the resistance line will be increased, and hence the maximum voltage
in which the generator will build up at a given speed decreases. If the field
circuit resistance too much that the resistance line doesn’t cut the no load
magnetization curve then there were no build up voltage. The resistance value
if where build up processing possible is the critical resistance and above this
value no voltage build up process is possible.
In the
following figure OI, OC, OA, OB are the
resistance line. OA resistance value is larger than OB
and OI too much resistance value. ON is the no load magnetization curve. OI
doesn’t intersect the ON curve that is OI is critical resistance. OM is the generated voltage corresponding point P as
referred to OA resistance. OB is less than OA
so in here voltage generated increases rather than OA resistance line. SO in
voltage build up process critical resistance has a significance.
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