Electrical Engineer's Reference Book, 16th Edition

28.12 On-load excitation

Use of the constant unsaturated value of X_d leads to values of E_f higher than those that occur for a practical machine. Thus a cylindrical-rotor generator with X_du = 2 p.u. and carrying rated load at p.f. 0.85 lagging would have E_f = 2.67 p.u. while a practical machine would saturate at around 1.5 p.u. If I_t were read from the airgap line for E_f = 2.67 p.u. it would be about 15% low. At leading power factor, for which saturation levels are low, the error would be small. However, a more accurate estimate of the field current I_f is needed in the design of the excitation system and its cooling, and to determine the open-circuit e.m.f. that would be reached if the automatic voltage regulator failed to limit the excitation ...