DIRECT-CURRENT DYNAMOS AND MOTORS. 
the terminal voltage of the machine must be multiplied. 
For example, if the shunt winding of a 12-pole ring type 
125-volt generator is connected in 4 groups of 3 mag- 
2x 4 
12 . 
E'=% (125—124)=75 volts. Ifin the same machineall the 
magnets were connected in parallel, that is to say, if the 
22032 & 9 
Cae 
and the shunt voltage per magnetic circuit would be 
HE’ = 2 X 1124 = 225 volts. 
The size of wire obtained by formula (48) will furnish the 
required number of ampere-turns irrespective of the 
number of turns employed. If a small number of turns 
is chosen, the current will be high; and if a large number 
of turns is used, the current will besmall. If the current 
is too high, the wire will become overheated and may 
eventually burn out. If the shunt current is taken too 
small, the magnet winding will become too bulky and the 
cost of the machine uunecessarily great in consequence. 
It is therefore seen that it is very important to determine 
the proper strength of the shunt current in each case. 
Designating the watts consumed in the magnet winding by - 
Wm», the shunt current can be expressed as 
nets each, the above factor will be = 4%, hence 
number of groups were 12, we would have 
Since in this equation H’ is known, being the ter.ninal . 
voltage of the machine diminished by 10 to 20 per cens. to 
allow for the drop due to the regulating resistance,, the 
problem remains to find the proper value of w,. The 
power which can be dissipated in the field winding 
depends upon the cooling surface of the magnets and 
upon the temperature rise permitted. If the cooling sur- 
face of the magnet coils is denoted by Sy, and the allow- 
able temperature increase by ¢t, in Centigrade degrees, 
the power dissipated in the magnet winding is given by 
We = 2 X Bais eess4' sin: oe o> (ADE 
