THE DESIGN OF DIRECT-CURRENT 
DYNAMOS AND MOTORS. 
CALCULATION OF MAGNET WINDING. 
70. Series, Shunt, Compound, and Differential Field 
Winding.—Having found the total number of ampere- 
turns required to cause the necessary flux through the 
magnet’ frame, the final calculation of the field winding 
consists in determining the proper number of turns and 
amperes which, when multiplied together, will produce 
the desired magnetizing force. The determination of the 
strength of the current passing through the field winding, 
and, consequently, the selection of the size of the field 
- wire, depends upon the method of field excitation em- 
ployed. 
There are four methods of winding the magnets of continu- 
ous-current machines, the series, shunt, compound, and 
differential winding, each of which has distinct advan- 
tages which make its use preferable for certain purposes. 
Series winding is used chiefly for constant-current arc- 
light generators and for street railway motors, but in the 
latter case, though the field winding as a whole is in 
series with the armature, the separate coils can be. 
variously connected in series or in parallel with each 
other, so as to vary their resistance and magnetizing 
effect. Series winding is sometimes adopted for motors 
applied to hoisting and other purposes where very 
variable speed is required. Series motors may also be 
used for driving fans, pumps, or other machines in which 
the load is fairly constant and is positively connected by 
direct coupling or gearing, so that the motor speed cannot 
become excessive, asit would tend todo if a belt slipped off. 
