Critical field resistance

Critical field resistance is a term that is associated with a shunt DC generator. In a DC shunt generator, the voltage induced across the armature, V_a, is directly proportional to the flux acting across it, The flux in a DC generator is directly proportional to the field current, I_f. The critical field resistance is defined as the maximum field circuit resistance (for a given speed) with which the shunt generator would just excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance.

It is a tangent to the open-circuit characteristics of the generator (at a given speed).