ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 9 
439, 
440. 
441, 
Flat V Curve.—From theabove it follows that the flatter 
the V characteristic curve, the steadier will be the running 
of the motor. This condition tends to cheapen the design, 
as the number of armature ampere turns may be higher or 
the number of field ampere turns less. In the case of a 
large armature reactance, it is not desirable to drive the 
saturation in the iron parts too high, otherwise the range 
of regulation will become too excessive in order to obtain 
a certain amount of phase displacement. 
In the case of a large armature reactance, it is well to take 
the pole arc ratio somewhat smaller than usual, to keep the 
saturation in the pole and rim at moderate values and to 
make the air gap larger so that the latter will utilize the 
principal part of the necessary ampere turns. 
The saturation of the teeth and armature core is determined 
by the specified efficiency. 
Determination of V Curve.—TIn order to determine 
the V curve of the synchronous motor the no-load and short 
circuit characteristics of the machine, when it is driven at 
normal synchronous speed as a generator, should be known. 
As already mentioned, the V characteristic gives the armature 
current in terms of the excitation, for a constant speed, 
terminal voltage and mechanical load. These conditions 
of constancy imply, that the watt component of the arma- 
ture current, C,cos mg must also be a constant value, 
because the mechanical output is proportional to the input in 
K.W., which is expressed by K.W.=mx E, Xx (C,cos 9). 
Ampere and Excitation Values.—The constancy of 
the voltage, load and watt component of the armature 
current, is represented in Fig. 94, by the line PP* which is 
parallel to the line OO*. In Fig. 94, the ampere values as 
well as the terminal voltage are substituted by the corre- 
sponding ampere turns. 
The length OA (ampere turns) indicates the constant ter- 
minal pressure of 3500 volts, the length A P (ampere turns) 
corresponds tothe armature current, of which the minimum 
