ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 17 
466. Normal Excitation.—With the application of Equa- 
tion (32), the total value of the effective back ampere 
turns will be found equal to 2.12«15,000=31,800, or 
31,800 
16 
Assuming a short circuit constant of 1.15 with the specific 
- ratio factor of 1.8 to 2, the normal excitation corres- 
ponding to 3,460 volts open circuit pressure will be about 
1.15x1.9x1970=4,300 ampere turns per pole. 
=1970 ampere turns per pole. 
Fie. 97.—PoLtke WINDING or 550 H. P., 3460 VoLtT, SYNCHRONOUS 
3-PHASE Moror. 
46%. Air Gap.—With the object that the normal pressure 
should be below the knee of the no-load characteristic, 
the magnetic densities in the iron parts are kept com- 
paratively low, so that a utilization of about 80 per cent. 
of the total ainpere turns inthe air sap may be expected. 
This 80 per cent. amounts to about 4300 x .8= 3,440 am- 
pere turns ; consequently with a rough approximation by 
meansof Equation (21), theair gap willbe r, x #,=3,440 
_6X0.313 
12 x8 
calculation is taken as 0.2 inch. 
x 4.2 x 10°, where 6=0.25inch, and forfurther 
468. Field Winding Space.—For the determination of the 
pole core height, we will assume that the current density 
