4/6 ELEMENTS OF ELECTRICAL ENGINEERING. 



ing the same percentage of the armature periphery as before, the 

 length of the air gap being the same, and the field intensity in 

 the gap space due to the field winding alone being 8,000 gausses. 

 What is the greatest permissible current in the armature of the 

 machine if it is assumed that sparkless running requires the 

 resultant field intensity under the leading pole tips to be at least 

 25 per cent, of the field intensity due to the field winding alone? 

 This four-pole drum armature winding has two paths between 

 positive and negative brushes. Ans. 163.6 amperes. 



122. The pole faces of the generator specified in problem 121 

 are widened so as to cover 7 5 per cent, of the armature periphery 

 and the gap space is increased to 0.7 centimeter. Find the 

 greatest permissible current output of the machine if it is assumed 

 that sparkless running requires the field intensity under the lead- 

 ing pole tips to be at least 25 per cent, of the field intensity of 

 8,000 gausses which is due to the field winding alone. Ans. 178 

 amperes. 



123. Find the armature demagnetizing ampere -turns per mag- 

 netic circuit and the number of turns required in each series field 

 coil to balance the same, in a four-pole I oo-kilowatt, 5oo-volt 

 generator, having a four-path armature winding with 500 con- 

 ductors ; forward lead of the brushes is 10, and 0.868 of the 

 armature current flows through the series field coils. Ans. 1,389 

 ampere -turns and 4 turns. 



124. The field coil of a dynamo rises 46 centigrade degrees 

 above a room temperature of 35 C., what would be its rise of 

 temperature above a room temperature of 25 C. ? Ans. 44.39 

 centigrade degrees. 



NOTE. Let ^ be the temperature rise above 25 C. and t" the temperature rise 

 above 35 C. In the first case the actual temperature of the winding is 25 -f- & and 

 its resistance, R t ', is ^ [i -}- ^(25 -j- /')]. In the second case the actual temperature 

 of the winding is 35 -f- 1" and its resistance, R", is R^\i + /3(35 -J- /")] The 

 rates at which heat is generated by the given current in the two cases are propor- 

 tional to R f and R' f respectively, and, since the rise of temperature is proportional 

 (very nearly) to the rates of generation of heat, therefore 



t' : t" : : R' : R". 



