说明：  This simulation illustrates the current and voltage buildup in a self-excited dc shunt generator driven at constant speed.Examine the effects of: a) changing the residual voltage magnitude and polarity (vres) b) varying the field resistance Rf =[145 200 250 300 350] c) varying the load conductance GL=[0.16 0.5 ] d) changing the field inductance Lf e) modifying the armature parameters Ra and La