166 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
311. Voltage Drop.—tThe voltage drop being specified as 4% 
for non-inductive full load and 12% for the cos P = 0.85 
load, with reference to Fig. 5, we may assume a voltage 
drop of 18% for an armature current corresponding to the 
full load, but having a power factor, cos P =0. Pro- 
vided that the normal open cireuit pressure is on the knee 
of the curve the above cos P =O regulation will be 
obtained by making the normal excitation ampere turns 
about 2.75 times greater than the normal effective back 
ampere turns of the armature. . 
Fies. 75 AND 76.—PoLE AND SLoT WINDINGS OF 175 KVA., 
220 VoLT, 3-PHASE ALTERNATOR. 
312. Field Ampere Turns.—The ampere conductors of the 
armature have been ealeulated as 16,560 per phase, so that 
the number of ampere turns per phase is 8,280 and the 
effective back ampere turns will be 2.12 X 8,280 = 17,500, 
see Equation (32). Therefore, the minimum field ampere 
turns corresponding to the normal open ecireuit voltage, 
are equal to 2.75 X 17,500 = 48,000 total, or 4,000 per pole. 
313. Winding Space Available.—For this excitation, we 
will assume a current density of 1,300 amperes per square 
inch of the field winding and a space factor of 0.4; so with 
