74 ELECTRICAL ENGINEERING PROBLEMS 



current curves of a 100-ampere, 3-phase, Y-connected generator; 

 also draw circuit diagrams showing the values and directions of 

 the instantaneous currents in each circuit for a = 0, 30, 45, 

 60 and 90 and observe that the sum of the currents is zero in 

 each case. (Hand -in the vector diagram.) (10 min.) 



12. Let the armature of the last problem be delta-connected 

 with 100 amperes in the outside circuits; construct the vector 

 diagram for the armature and line currents and by its revolution 

 the sine curves for all the currents. Use the conventions that 

 currents are positive in the mesh when counterclockwise and in 

 the line when directed away from the junctions. Draw the five- 

 circuit diagrams as in problem 11. (Hand in the vector diagrams.) 

 (20 min.) 



13. A 100-volt, delta-connected, three-phase system has in 

 each branch resistance and reactance as follows: circuit (a), 3 and 

 3 ohms; circuit (b), 6 and ohms; circuit (c), 1 and 5 ohms. 

 Construct the diagram and determine the currents in the meshes 

 and leads. (10 min.) 



14. A 200-volt, delta-connected system has currents of 27, 43 

 and 18 with power factors of 100, 86.6 and 50%. Determine the 

 currents in the leads and their phase relations (tangents) to the 

 e.m.f.'s. (10 min.) 



15. Each circuit of a 120-volt, Y-connected, 3-phase system 

 has a resistance of 3 and a reactance of 6 ohms. Construct the 

 diagram and determine the line currents and their power factors. 

 (8 min.) 



16. The three circuits of a 150-volt, Y-connected system are 

 made up of groups of incandescent lamps having resistances 

 respectively of 5, 8 and 10 ohms. Determine the current in each 

 lead and the voltage to neutral around each, circuit. (15 min.) 



17. The three circuits of problem 13 are connected Y instead 

 of delta; required the current and voltage of each and the phase 

 angles (tangents) between the currents and line e.m.f.'s. (20 min.) 



18. If in problem 16 a fourth, or neutral, wire is used, what 

 will be the current in each of the four lines? (4 min.) 



19. If in problem 17 a fourth wire is used, what will be the 

 current in each of the four lines? (8 min.) 



20. A two-phase, three-wire system takes 200 amperes, with a 

 power factor of 86.6 in each phase, from a 100-volt generator and 

 is supplied through lines, each of which has a resistance of 0.1 ohm. 



