84 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
part of the circuit under consideration. In this manner 
the values of at,, and at, in (3'7) can be obtained, the 
former by one single operation if the field frame is all of 
the same material, or by adding several products if vari- 
ous portions of the magnet frame are made of different 
materials. 
The exact calculation of the compensating ampere-turns, 
at,, is shown in Par. 65; for preliminary calculation of 
the magnetizing force, however, it is sufficient to increase 
the sum of the gap, armature, and frame ampere-turns 
by 75 or 20 per cent., to make up the demagnetizing 
tendency due to armature reaction. 
64. Magnetization Curves.—lIn Fig. 27 the magnetization 
65. 
curves are given for cast tron, wrought tron, and cast 
steel, the values being averaged from tests of various 
brands of each kind of iron. These curves are well 
adapted for the student's use, but the practical dynamo 
designer will obtain closer results by using curves plotted 
particularly for the iron whichhe employs. For the con- 
venience of the student the values contained in Fig. 27 
are given in Table 26, 
Magnetizing Force for Compensating Armature 
Reactions.—The armature current, in magnetizing 
the armature core, exerts a double influence upon the 
magnetic circuit: (1) a direct weakening influence upon 
the magnetic field, due to the lines of force set up by the 
armature winding, and (2) an indirect, secondary influ- 
ence by shifting the magnetic field in the direction of the 
rotation in case of a generator, and in the opposite direc- 
tion in case of a motor, thereby causing greater magnetic 
density to take place in those portions of the polepieces at 
which the armature leaves the pole than in those at which 
it enters. 
The direct weakening effect of the armature current, or the 
number of field ampere-turns which are neutralized by 
the influence of the armature current, is equal to the back 
