68 
ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
116. 
117. 
where F is the resultant of the various items of reluctance 
contained in the magnetic circuit of the machine. The 
above equation represents the open circuit characteristic. 
The ratios between the corresponding ampere turns and flux 
are the variable values of &, and are known as the apparent 
magnetic resistances of the alternator. These values are 
indicated in Fig. 34, plotted in terms of the field ampere 
turns or excitation current. The curve may be plotted on 
a larger or more suitable seale if desired. 
Later on, in Paragraph 131, it will be seen that the absolute 
values of R are required for the predetermination of the 
no-load characteristic, and the form of the apparent re- 
luctanee curve is sufficient for the determination of the 
voltage drop from the test data. 
Example 15.—Method of plotting the apparent reluc- 
tance curve from the test data. 
Fig. 34 gives the open circuit characteristic of a 235 KVA. 
three-phase generator which has a normal terminal pres- 
sure of 5,200 volts with a star connected armature. By 
dividing the values of the excitation currents 30, 20 and 10 
amps. by the corresponding values of the E. M. F., 6,500, 
5,400, and 3,300 volts, we find the numbers whose relative — 
values indicate the course of the apparent magnetic re: 
sistance in terms of the field excitation, and Fig. 34 shows 
that the same approaches a constant value for zero ex- 
citation. 
The magnetic resistaney R, of the non-excited alternator 
plays an important role in the determination of the voltage 
drop. 
Short Cireuit Characteristic.—The short circuit 
characteristic Fig. 34, gives the current of the short-cir- 
cuited armature in terms of the excitation ampere turns 
when the speed of the generator is. kept constant. The 
