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, R_{f} is increased the line slope increases, till at critical R_{f} value the coincides with the linear part of OCC. At and higher than this value of R_{f} V_{OC} reduces to merely the residual value (negligible), which implies that the generator fails to excite.
Also, if for fixed R_{f} the generator speed is reduced, at critical speed the OCC comes to a position where its linear part lies along the R_{f} line and the generator fails to excite.