12 ELECTRICAL ENGINEERING PROBLEMS 



magnetic path including the armature is 30 cm. To do this, the 

 density through the air must be 8000 gausses, which will require 

 11,000 in the magnet and armature. How many turns will be 

 necessary with an exciting current of 3 amperes? What would 

 the density become when the armature was pulled up? (6 min.) 



9. The magnetic circuit of a dynamo is made up as follows: 

 cast-iron ring, length of path 25 cm., density 6000, two wrought- 

 iron cores each 10 cm. long, with density of 16,000, two gaps 

 5 mm. long, with mean density of 8000, the path through the 

 laminated armature 15 cm. long with a density of 10,000. Re- 

 quired the ampere-turns necessary for the excitation. (4 min.) 



10. Given a cast-steel ring, 8 inches inside diameter, 2 inches, 

 diameter of iron; B is 15,000 gausses. Required the maxwells, the 

 oersteds and the exciting current if there are 150 turns. (8 min.) 



11. Given a cast-iron ring 100 cm. in mean diameter. Required 

 the number of amperes necessary to make B equal to 4000, 6000, 

 8000 and 10,000 if the winding be of 854 turns. (6 min.) 



12. In the case of the last example, if the winding has a resist- 

 ance of 3.76 ohms, what will be the power necessary to produce 

 each density? Note great increase in power with increase in 

 density. (5 min.) 



13. In order to raise a certain weight the density in a horse- 

 shoe-shaped forged hoisting magnet must be 16,000 gausses; the 

 length of the iron circuit is 50 cm. If there are 1428 turns carry- 

 ing two amperes, how near to the weight must the magnet be 

 brought to lift it. If the current were increased to 7.14 amperes, 

 how many turns would be necessary? (7 min.) 



14. A dynamo magnetic circuit is made up of the following 

 parts: 27 cm. in the armature with a density 7000; two clearances 

 of 3.4 mm. each, with B equal to 12,000, and a cast-steel field of 

 53 cm. with a density of 15,000. If the magnetizing current be 

 10 amperes, how many turns will be necessary? (4 min.) 



15. A 2-kw. transformer has a magnetic circuit whose section 

 is 7.57 X 5.22 inches and length 22.3 inches. If there are 800 turns 

 and a maximum density of 5000 is necessary, what will be the 

 maximum magnetizing current? (4 min.) 



16. The hysteresis loop for a sample of sheet steel is carried 

 to 13,750 gausses. The coordinates are chosen so that one inch 

 represents ten units of magnetizing force and 10,000 gausses. 

 The area of the curve is found to be 0.513 sq. in. What would be 



