ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
436. 
437. 
438. 
In standard alternator designs, the above ratio is about 2 or 
more; that is, with the normal excitation, the machine can 
stand a momentary overload which is about double the - 
rated capacity. 
Maximum Load Rule.—In the design, it will be suffi- 
cient to work with the approximate rule, that the maximum 
Joad with normal excitation is equal to 
K.W.n=mXH,XCs, - - - - (42) 
where K.W., is the maximum power expressed in K.W.; 
E, the terminal pressure per phase; and C, the short circuit 
current with normal excitation. 
From the above rule it follows, that an alternator designed 
for good regulation, will have a larger overload capacity 
than one with a large voltage drop. 
Small Overloads.—In cases where sudden overloads 
are not possible, as for instance when the synchronous 
motor has to drive a continuous current dynamo for 
accumulator charging, it will be sufficient to design for 
1.25 overload capacity. Consequently, in such cases, the 
armature will receive a great number of ampere turns, - 
while the value of the field ampere turns remains standard, 
but at the same time the iron part will be greatly reduced. 
In the case of the 450 H.P. three phase motor under dis- 
137.5 ae 
59.5 ay 
that is, the maximum mechanical load with 6,300 ampere 
turns excitation will be equal to 2.8x450=1035 H.P. 
The V Curve.—The V curve of a synchronous motor, 
indicates what alteration is necessary in the excitation, 
in order to obtain a current supply with power factor 
cos p= 1 or cos p=< 1, either with lagging or leading 
displacement. | 
With regard to steady running, as well as a safeguard in 
case of false regulation, it is desirable that the alteration 
in the phase angle should result in a wide range of the 
excitation regulation; that is, the armature reactance and 
also the voltage drop should not be chosen too small. 
cussion, the overload capacity is equal tc 
