34 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
315. 
323. 
324. 
325. 
526. 
What sizes of wire are usually employed for the magnet 
winding of series are-lighting machines? 
Give the values of the specific cooling surface to be al- 
lowed for temperature rises of 20°, 30°, and 40° C., re- 
spectively. 
Upon what facts is the calculation of the size of wire for 
a shunt winding based? 
Give the formula for the approximate warm resistance of 
a copper wire of given length and diameter. 
Why does the number of ampere-turns of a shunt magnet 
remain the same, irrespective of the number of turns, 
as long as the same size of wire is employed? 
What formula is used for calculating the cross-section of 
a shunt wire? Explain the symbols employed. 
How is the resistance of the field regulator of a shunt ma- 
chine taken into account in determining the size of the 
magnet wire? 
How does the shunt-voltage, entering the formula referred 
to in Question 320, compare with the output voltage 
of a multipolar machine in which all field coils are con- 
nected in series? 
How is the voltage between terminals of shunt winding 
determined from the output voltage, in case the field 
coils are connected in groups? 
Give the value of the shunt voltage for a 500-volt, 8-pole 
ring type generator if its magnet winding is arranged 
in 2 groups of 4 coils each, allowing 12 per cent. of the 
voltage for overcoming regulator resistance. 
What would be the shunt voltage, if all the coils of the 
machine referred to in the preceding question were 
connected in parallel? 
The size of the shunt wire for a machine having been de- 
termined by means of formula (49), Instruction Book, 
