94 ELECTRICAL ENGINEERING PROBLEMS 



Determine the power factors of the line current in each case. 

 (30 min.) 



Note that in these problems an open scale and great care is 

 necessary to get accurate results. The impedance e.m.f. triangle 

 should be drawn to a larger scale than the motor e.m.f. The 

 resistances in these problems are exaggerated as compared with 

 commercial machines. 



7. Repeat problem 6, with a higher excitation, giving a counter 

 e.m.f. of 1200 volts. Determine also the power supplied to the 

 motor in each case. (85 min.) 



8. Repeat problem 6, with a counter e.m.f. of 1300 volts. 

 Determine also the power transformed by the motor to mechanical 

 energy in each case. (35 min.) 



9. In the motor of problem 6 with a load adjusted for each case 

 to 40 amperes, the excitation of the motor is changed so that the 

 e.m.f. 's are respectively 1050, 1150 and 1250 volts. Construct 

 the 3 diagrams for this case; determine the power given to the 

 motor, the copper loss and the power transformed in each case. 

 (35 min.) 



10. The motor of problem 6 is run upon a transmission line 

 having a resistance of 1.2 ohms, the generator e.m.f. is maintained 

 at 1200 volts; construct the 3 diagrams for a counter e.m.f. of 1200 

 volts and obtain the tangents of the lag or lead of the current. 

 (30 min.) 



11. In the motor of problem 6 the resistance is made 2.4 and 

 the reactance 11.6, that is the impedance is doubled. The line and 

 motor e.m.f. 's both being 1200 volts, construct the diagrams for 

 10, 25 and 40 amperes and find the power factors for each case. 

 (30 min.) 



12. A motor is excited to 95 volts, the resistance is 0.1 and the 

 reactance 0.8; the power taken by the motor is constant and its 

 e.m.f. is lagging 15 degrees behind opposition to the current; if 

 the current be 20 amperes, required the terminal e.m.f. With the 

 same terminal e.m.f., what other motor e.m.f. will give the same 

 current? Also, what motor e.m.f. will make the power-factor 

 unity? (15 min.) 



13. With the motor of problem 12 on a 100-volt circuit, find 

 the motor e.m.f. to give minimum current for a load at 3 kw. With 

 the same power transformed and the same motor excitation, what 

 change in terminal e.m.f. would reduce the current to a new min- 



