506 ELEMENTS OF ELECTRICAL ENGINEERING. 



spools, which are to be connected in series as usual. The tem- 

 perature of the winding in service is 60 C. Find the size of wire 

 required. Ans. 2,710 circular mils. 



Note. The ampere-turns in a shunt field winding are equal to the quotient of the 

 voltage between the terminals of the winding divided by the resistance of a mean 

 turn of the wire. Of course, a commercial size of wire would have to be used in wind- 

 ing the above coils and the portion of the voltage absorbed in the field rheostat would 

 be slightly different from 20 per cent 



216. The length of winding space on each of the bobbins of 

 problem 215 is 3 inches. Find: (a) Resistance at6oC. of the 

 four bobbins in series when wound with No. 16 Brown and Sharp 

 gauge double cotton- covered copper wire (outside diameter of 

 wire 0.063 inch ; assume no bedding of the layers) ; (ft) find 

 watts dissipated in each bobbin using resistance at 6oC. ; and (c) 

 calculate the approximate rise of temperature of the bobbins 

 above the air. Ans. (a) 51.6 ohms ; (fr) 40.6 watts ; (c) 3i.4C. 

 rise of temperature. 



Note. Dynamo designers estimate the rise in temperature of a field winding by 

 the formula 



in which / is the estimated rise of temperature of the winding above the air in centi- 

 grade degrees, P is the power in watts dissipated in the winding, and A is the area in 

 square inches of the cylindrical surface of the winding. This formula is only roughly 

 approximate, and it is to be applied only to medium-sized field spools with a depth of 

 winding of from 0.75 inch to 2 inches. 



217. A certain 2OO-volt, 5o-kilowatt, 4-pole shunt generator 

 has its four field spools connected in series and it is designed to 

 run at speed of 500 revolutions per minute. This machine is to 

 be driven at a speed of 250 revolutions per minute and to de- 

 velop 100 volts, (a) What change is necessary in the arrange- 

 ment of the field winding ? (b) What is the current capacity of 

 the machine at the reduced speed ? (c) What is the power rat- 

 ing of the machine at the reduced speed ? Ans. (a) The four 

 field spools are arranged two in series by two in parallel ; (ft) 250 

 amperes ; (^25 kilowatts. 



