Q. Define and explain the critical field circuit resistance and critical speed of a DC shunt generator.
Ans. The OCC and R - line for a DC shunt generator. As, Rf is increased the line slope increases, till at critical Rf value the coincides with the linear part of OCC. At and higher than this value of Rf VOC reduces to merely the residual value (negligible), which implies that the generator fails to excite.
Also, if for fixed Rf the generator speed is reduced, at critical speed the OCC comes to a position where its linear part lies along the Rf line and the generator fails to excite.