-ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 47 
the bearing does not exceed the usual practical limit of 
about 600 lbs. per square inch. 
When using formula (9), one must not forget that through 
inaccurate work in the shops or wear in the bearings, an 
eccentric displacement will likely occur between the arma- 
ture and the magnet wheel, which involves a onesided 
magnetic pull. This additional pressure may amount to 
a value as high as 60% of the weight of the magnet wheel. 
It is, therefore, advisable for the designer to allow an 
ample margin when calculating the friction losses. 
87%. Iron Losses. The iron losses in an alternator are in- 
‘fluenced by a large number of factors. The hysteresis loss 
is proportional to the frequency, is increased by increas- 
ing the magnetic density, and is influenced by the wave 
form of the armature current. When calculating eddy 
eurrent losses a fourth factor has to be considered, namely, 
the thickness of the sheet iron used for the armature core. 
Experience has proved, that in order to obtain the minimum 
eddy current losses a lamination of 0.02 to 0.01 inch is 
the most satisfactory for all requirements, because the. 
employment of a finer gauge would only increase the cost 
of the stamped material and the labor in assembling, with- 
out the return of any appreciable advantages. Almost 
the whole iron losses are located in the armature iron as 
only a small loss takes place in the field system when 
excited by continuous current. The iron losses in the pole 
pieces are smaller, the smaller the opening of the arma- 
ture slots. ; 
It is advisable to use a lamination having a thickness of 
0.04 inch for the pole shoes, regardless whether open slots 
or closed holes are stamped in the armature. 
88. Magnetic Densities.—The values of magnetic densities 
in the armature core vary according to the frequency. 
Table 9 gives the limits of magnetic densities in magnetie 
