What is critical resistance?, what is the significance of it in a self excited shunt Generator ?

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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