ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 11 
a smaller number, because: (1) the value of the E. M. F. 
factor is in inverse relation to the number of slots; (2) an 
increased number of slots means the application of more 
insulation material, thus a loss of winding space; and 
(3) it implies the use of several sizes and forms of coils 
which necessitate the manufacture and stocking of dif- 
ferent sizes of formers. 
24. Decreased Temperature Rise.—The specifications of 
the permissible temperature rise, insures the long life 
of the machine and prevention of breax downs originated 
by excessive heating. The necessary data is based on the 
quality of insulation materials used in modern practice. 
The station engineer prefers, of course, to take a machine 
with a temperature rise as low as possible, consistent with 
the price, which of course means a larger alternator. 
This, from the manufacturer’s point of view, is unreason- 
able, especially when all modern means of cooling and 
ventilation have been applied in the construction of the 
properly designed machine. 
25. Increased Short Cireuit Current.—A greater short 
circuit current than three times the normal is usually econ- 
sidered an unnecessary evil, because, as already mentioned, 
it may injure the generator. 
First of all, in case of an accidental short circuit, the exees- 
sive current may burn out the armature winding, which 
is naturally dimensioned for the normal current only. 
Secondly, it will cause the occurrence of heavy surging 
currents through the armatures of alternators running 
in parallel. 
A generator with a large short cireuit current is expensive 
and demands the use of an excessive amount of. field: 
copper. 
26. Increased Insulation.—A higher safety factor of insula- 
tion to withstand a test greater than that given in Par. 
