48 ELECTRICAL ENGINEERING PROBLEMS 



force; the current is broken in T <j second; required the average 

 induced electromotive force set up, in volts. (| min.) 



8. A coil of 200 turns of wire is threaded by 100,000 lines of 

 force. The exciting current is halved in yj^ second; required the 

 induced e.m.f. If the original exciting current is 5 amperes, what 

 is the inductance of the circuit? (2 min.} 



9. The inductance of a certain dynamo is 40 henries; what will 

 be the induced e.m.f. if the exciting current of 2 amperes is brought 

 to zero in 0.025 second? (1 min.) 



10. What is the average rate of change of current per second 

 during a quarter of a cycle of a 60-period alternating current of 

 100 amperes (effective)? (1 min.} 



11. If in problem 5 each one of the changes of current men- 

 tioned takes place in T ^ second, what will be the e.m.f. set up in 

 each case? (3 min.) 



12. If in problem 6 the current is changing at the rate of 50 

 amperes per second when it passes through one ampere, what 

 e.m.f. will be induced at this instant? (1 min.) 



13. With an exciting current of 7.5 amperes through 1000 turns 

 the reluctance of a magnetic circuit is 0.1257. If the current 

 increases by 2% of this value, the reluctance increases 1%. If 

 this increase of current is at the rate of 15,000 amperes per second, 

 required the e.m.f. induced. What is the average inductance 

 during this increase? (3 min.) 



14. The core of a 25-kv-a., 60-period transformer is 15 X 20 

 cm. and the primary winding has 500 turns. The density, corre- 

 sponding to a magnetizing current of 0.2 ampere, is 5000. Re- 

 quired the average inductance for this current, and the induced 

 e.m.f. if the current were reduced to zero at a rate of 100 amperes 

 per second. (3 min.) 



15. An electromagnet has 1200 turns; with 4.5 amperes the 

 flux is 5 X 10 7 , and with 2 amperes 3 X 10 7 . What is the average 

 inductance? If the density is such that between these limits the 

 magnetization curve can be taken as a straight line, what will be 

 the e.m.f. of inductance if the current change from 3 to. 3.2 amperes 

 in 0.1 second? (3 min.) 



16. A circuit has 1000 turns, the area of the enclosed magnetic 

 circuit is 20 sq. cm. With 4 amperes the density is 10,000 and 

 with 9 amperes it is 14,000. Required the average value of L 

 between 4 and 9 amperes. (1 min.) 



