ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 211 
407%. 
408. 
409. 
Causes of Cross Currents.—With regard to the 
parallel running of alternators, the designer should re- 
member the following points: 
Alternators which have to work in parallel, must have similar 
wave forms in order that no cross current shall appear in 
the armatures, and here again the most favorable condi- 
tions are offered by a sinusoidal wave form. 
Two or more alternators running in parallel have a certain 
control over each other, and if there is a difference in 
their speed, then one transfers load to the other, which 
reveals itself by a cross current between the armatures. 
This control or synchronizing power is larger, the smaller 
the reactance of the armature, that is, the better the regu- 
lation. 
Alternators with low voltage drop are liable to heavy eross 
eurrents if the machines are accidentally thrown out of 
step. 
Speed Variation.—There is little trouble with alternators 
| driven by water wheels or steam turbines, owing to their 
uniform rotation. — 
In the case of reciprocating steam engines as prime movers, 
a good engine governor is a most important thing, and 
every precaution should be exercised in order that the 
angular variation of the speed shall be kept within small 
limits. 
Such a condition must receive the close attention of the 
designer, especially if a low voltage drop is specified. 
Flywheel Effect.—To keep the angular variation within 
small limits it is necessary to apply a certain amount of 
flywheel effect and it is the best practice to put as much of 
the rotating mass in the alternator as possible. 
Very often the designer is compelled to increase the diameter 
of the magnet wheel in order to find room for the revolving. 
mass, which necessitates a sacrifice in the electrical proper- 
ties of the alternator. 
