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DIRECT-CURRENT DYNAMOS AND MOTORS. 
41. 
Shunt winding is almost universally employed in motors 
for constant speed with variable load. Shunt-wound 
generators were formerly used for giving constant volt- 
age for electric light, power, and railway purposes, but at 
present compound-wound generators have largely taken 
their place, since the latter automatically maintain a con- 
stant, or even slightly rising, voltage with increase of 
load. 
Compound winding, besides being used for most genera- 
tors, is further applied to motors when a strong starting 
torque is required. 
Differential winding is applicable to generators that are 
required to produce a fairly constant voltage with wide 
variations in speed, as, for example, those driven by 
windmills, or by the axles of railway cars for electric 
train lighting. Differential winding is also employed in 
motors required to run with very constant speed at vary- 
ing load. 
In any given case there is generally little difficulty in decid- 
ing which of the four methods of field winding should be 
adopted, the custom being well defined regarding gene- 
rators and motors for each purpose. 
Determination of the Magnet Winding.—The 
method of winding being decided upon, the next step is 
to find the proper size of wire to employ for the field 
coils. It is obvious that a given number of ampere-turns 
may consist of a few turns carrying a large current, or of 
a great number of turns carrying a small current. 
Whether the former or latter arrangement is employed de- 
pends upon the method of field excitation of the dynamo, 
In a serves machine the total armature current traverses 
the field coils; hence the field winding consists of few 
turns of a conductor having a comparatively large cur- 
rent capacity. In a shunt dynamo, on the other hand, 
only a very small portion of the total current flows ~ 
through the field winding; the latter must, therefore, 
consist of many turns of fine wire. Compound and 
differential machines, finally, have a series winding as. 
