Critical Field Resistance
It is that value of the field resistance at which the D.C. shunt generator will fail to excite.
It is that speed for which the given shunt field resistance becomes the critical field resistance. Critical
field resistance is obtained by plotting the OCC as in fig.2 and determining the slope of the tangent to the
linear position of the curve from the origin. While drawing the tangent, the initial position of the O.C.C is
Due to residual magnetism in the poles some EMF is generated even when If = 0. Hence the curve starts a
little way up. The slight curvature at the lower end is due to magnetic inertia. It is seen that in the first
part of the curve is practically straight. Hence the flux and the consequently the generated EMF is directly
proportional to the exciting current. However at the higher flux densities where it is small iron path
reluctance becomes appreciable and straight.Field windings are connected parallel to the armature and it
is called DC shunt generator. Due to residual magnetism some initial emf and hence some current will be
generated. This current while passing into the field coils will strengthen the magnetism of poles. This will
increase pole flux which will further increase the generated emf. Increased emf and flux proceeds till
equilibrium reached. This reinforcement of emf and flux proceeds till equilibrium reached at some point.
GRAPH between If and Eg* :