ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 93 
voltage drop of an alternator; in fact, it is the decisive 
factor which determines the regulation. 
Roughly speaking, the generator should not have a larger 
drop than 4 of the open eircuit voltage on an inductive 
load, cos P = 0, with normal excitation. Under these 
circumstances the excitation ampere turns for normal 
_ voltage must be three times greater than the ampere turns 
of the loaded armature, assuming that the leakage is zero. 
160. Calculation of Armature Ampere Turns.—The 
following rules are employed to determine the armature 
ampere turns for armatures of different phase numbers. 
Let 7 = the number of armature turns per phase, and 
C = the current in each armature phase winding 
expressed in amperes. 
Then the back ampere turns of a single phase generator 
are: 
AT aOR C1 Tae Baus teed: « (30) 
The back ampere turns of a two phase alternator are ex- 
pressed by: 
yi Ad Agen ah Bh ar Se dae tae aa abe, Sa ree (31) 
And the back ampere turns produced by a three phase 
alternator are: 
PY eee) We oe Bo ae a ee. Pa pear (32) 
The coefficients of the above equations are based on a sinusoi- 
dal current wave form, but they may be applied in most 
eases without any alteration, as only a small correction is 
necessary. 
161. Example 19.—In the case of a single phase generator 
having 672 turns, each carrying a normal eurrent of 120 
amperes, the theoretical number of armature ampere turns 
will be 
A, = 0.9 X 120 x 672 = 72,576. Ans. 
