ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 125 
20%. 
208. 
V =the peripheral speed in feet per minute; 
K =a constant which varies somewhat with 
different types of construction. 
T, = temperature rise in degrees Fahrenheit. 
The factor K may be taken equal to 100 when the winding 
depth does not exceed 1.5 inches and for larger coils K 
should be about 120, assuming that the coils consist of 
insulated wire. For flat edge wound strip coils, this 
factor is as low as 85. 
To provide good ventilation and cooling, it is further re- 
quired that the coils should not be too close together on 
the periphery, and that the air gap should not be too 
small. 
Determination of Air Gap.—In the determination of 
the clearance or air gap the amount of necessary field 
ampere turns in the first instance is decisive. 
For a machine of a certain capacity at a certain speed, the 
most favorable amount of armature ampere turns is fixed 
in accordance with the data given in Par. 182. 
From the conditions of regulation, the ratio between the 
armature and field ampere turns is given, so that the 
number of the necessary field ampere turns can be eal- 
culated for the normal open cireuit voltage. As a general 
rule about 75% of the same should be utilized in the 
air gap. 
The exact predetermination of the air gap is not an all 
important condition, as it is easily altered in the workshop 
if required, but one would do well to take a somewhat 
smaller gap for a new type of alternator, so that if any 
alteration is required it simply means the enlarging of 
the same by turning material off the pole shoes. 
Advantages of Large and Small Air Gaps.—The 
application of a small air gap results in the employment 
of a small number of field ampere turns and reduces the 
leakage of the magnet system. 
