IX. Magnetic Characteristic or Saturation Curve 20. The dependence of the generated e.m.f., or terminal voltage at open circuit, upon the field excitation is called the magnetic characteristic, or saturation curve, of the synchronous 1000 2000 3000 4000 5000 FIG. 69. — Synchronous generator magnetic 7000 characteristics. machine. It has the same general shape as the curve of mag- netic flux density, consisting of a straight part below saturation, a bend or knee, and a saturated part beyond the knee. Gener- ally the change from the unsaturated to the over-saturated por- tion of the curve is more gradual; thus the knee is less pronounced in the magnetic characteristic of the synchronous machines, since the different parts of the magnetic circuit approach saturation successively. The dependence of the terminal voltage upon the field excita- tion, at constant full-load current through the amature into a 148 ELEMENTS OF ELECTRICAL ENGINEERING non-inductive circuit, is called the load saturation curve of the synchronous machine. It is a curve approximately parallel to the no-load saturation curve, but starting at a definite value of field excitation for zero terminal voltage, the field excitation required to maintain full-load current through the armature against its synchronous impedance. dF dE The ratio -«• -=- ~FT r Hi is called the saturation factor s of the machine. It gives the ratio of the proportional change of field excitation required for a change of voltage. The quantity 5 = 1 is called the percentage saturation of the machine, as it shows the approach to saturation. In Fig. 69 is shown the magnetic characteristic or no-load saturation curve of a synchronous generator, the load satura- tion curve and the no-load saturation factor, assuming E = 1000, I = 100 as full-load values. In the preceding the characteristic curves of synchronous ma- chines were discussed under the assumption that the saturation curve is a straight line ; that is, the synchronous machines working below saturation. 21. The effect of saturation on the characteristic curves of synchronous machines is as follows: The compounding curve is impaired by saturation; that is, a greater change of field excita- tion is required with changes of load. Under load the magnetic density in the armature corresponds to the true generated e.m.f. EI, the magnetic density in the field to the virtual generated e.m.f. EI. Both, especially the latter, are higher than the no- load e.m.f. or terminal voltage E of the generator, and thus a greater increase of field excitation is required in the presence of saturation than in the absence thereof. In addition thereto, due to the counter m.m.f. of the armature current, the magnetic stray field, that is, that magnetic flux which leaks from field pole to field pole through the air, increases under load, especially with inductive load where the armature m.m.f. directly opposes the field, and thus a still further increase of density is required in the field magnetic circuit under load. In consequence thereof, at high saturation the load saturation curve differs more from the no-load saturation curve than corresponds to the synchronous impedance of the machine. SYNCHRONOUS MACHINES 149 The regulation becomes better by saturation; that is, the increase of voltage from full load to no load at constant field excitation is reduced, the voltage being limited by saturation. Owing to the greater difference of field excitation between no load and full load in the case of magnetic saturation, the improvement in regulation is somewhat reduced.