26 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
#1. Cyclical Variation.—In the case of direct coupled alter- 
nators working in parallel, there are two alternatives to 
secure the necessary minimum eyclical variation. Either 
the ordinary engine flywheel can be retained and the alter- 
nator magnet wheel placed side by side with the flywheel, 
or a part of the rotating weight is put in the alternator 
wheel to obtain the total amount of flywheel effeet. The 
latter arrangement is the most economical and requires less 
space. 
Direct coupled alternators, for low, medium and high speeds, 
are usually of the flywheel type with a revolving magnet 
wheel. It is necessary to make the diameter of the alter- ~ 
nator of ample dimensions, to allow the application and 
arrangement of the necessary revolving mass, which should 
be placed as near as possible to the periphery. 
52. Periodicity.—As will be seen from practical examples, 
large flywheel generators are specially suitable for 50 and 
higher periodicity. It will be found diffieult to design an 
economical high or medium speed alternator for 25 cycles, 
because the most favorable diameter would not permit the 
arrangement of rotating flywheel mass. 
53. Knowledge of Workshop Methods.—In order that 
the designer shall arrive at the best design as regards 
economy in manufacture, he must be able to calculate the 
labor costs on different parts of the machine; therefore 
he must be aware of the different workshop methods, espe- 
cially if they are out of the standard, as is the case with 
large generators. For standard machines, the designer has 
generally data and empirical rules for caleulating the ma- 
terial, as well as the labor costs. 
With reference to the method of machining the magnet wheel 
of a heavy alternator, it should be noted that the hub is 
first bored and faced on a powerful horizontal boring and 
milling machine; afterwards the wheel is clamped on a 
