EE EE 
ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 67 
sponding excitation, as the E. M. F. is directly proportional 
to the number of magnetic lines of flux which cut the 
armature coils. The no-load curve indicates the saturation 
of the magnetic circuit of the generator. 
114. Saturation Point.—In alternator design, it is impor- 
tant to have a well saturated magnetic circuit within the 
working limits, and the normal pressure should be located 
over the knee or bend of the curve. If the normal work- 
ing pressure is situated cn the lower part of the curve 
- which is under the knee or bend, the voltage drop will be 
large, producing an unsteadiness of the voltage, due to a 
comparatively small alteration of the excitation producing 
a considerable variation in the degree of saturation. 
On the other hand, to work far above the knee or bend has 
also its disadvantages, namely, it involves a wide range of 
excitation regulation to effect a certain rise in the voltage 
of the armature, and such a wide range to work over is 
found ineonvenient by the dynamo attendant when looking 
after the machinery. 
From past practice it has been found best to aim at a normal 
saturation, where one per cent. voltage variation requires 
about 2.2 per cent. regulation in the exciting cireuit. 
4115. Relation Between Field Ampere Turns and Ar- 
mature Flux.—lIt must be clearly understood that the 
voltage on the armature terminals will always depend on 
the number of magnetic lines of flux which enter the plane 
of the armature winding. In an unloaded alternator 
there is no magnetomotive force in the armature which 
will oppose the effect of the field ampere turns and all the 
magnetic lines pass through the armature winding, except 
those which find their way through the leakage paths. 
It has ‘been found that the relation between the field ampere 
turns A, and the useful armature flux F’,, or its equivalent 
aay ‘EK. M. F,, is ‘as follows: 
