DIRECT-CURRENT DYNAMOS AND MOTORS. 49 
the armature determines the approximate rise of temper- 
ature of the armature. : 
If the latter is too high, the ratio of power loss to the cool- 
ing surface must be correspondingly decreased, either by 
increasing the cooling surface or by decreasing the power 
loss, or by a combination of both expedients. The cool- 
ing surface is increased by increasing the size of the 
armature, and at the same time the latter is enabled to 
accommodate a larger size of conductor, thus reducing its 
resistance and lowering the power loss in the winding. 
An excessive rise of the temperature is usually the result of 
having assumed too great a winding depth; in re-design- 
ing the armature, particular attention should therefore 
be bestowed upon this point. The winding depth may be 
decreased either by reducing the number of layers of the 
armature wire, or by so subdividing the armature con- 
ductor that a suitable winding depth can be obtained by 
a proper rearrangement of the wires. 
43. Power Consumed in Armature Winding.—The 
power lost in the armature winding of a dynamoor motor 
is the square of the current in the armature multiplied by 
the armature resistance, or in symbols: 
ree Eh 05d. cas eth ls wala ok (28) 
where w, = power dissipated in armature winding, in 
watts; site 
C = total current flowing in armature, in am- 
peres; 
Tr, = armature resistance, warm, in ohms. 
In the case of a series machine, C in formula (28) is iden- 
tical with the given external current; in shunt and com- 
pound-wound machines, however, allowance for ‘the 
shunt excitation must be made. The amount of current 
passing through the shunt winding is the terminal 
K. M. F. of the machine divided by the resistance of the 
shunt circuit. The latter not having been determined at 
