POLYPHASE SYSTEMS AM) /'Oil/-;// 



75 



What will be the voltage on each circuit and the phase angle 

 (tangent) between it and the current? Not< th, . ti.-ct of the 

 common return in unbalancing the circuit. (S // 



21. A 4-wire, 3-phase system is supplying 150 amperes at 

 unity power factor to synchronous motors conn* < -t <1 delta, and 

 85 amperes each to three groups of incandescent lamps connected 

 to the fourth wire. Find the current in each wire and the motor 

 voltage if the lamps are for 125 volts. (8 win.) 



22. A 3-phase, delta-connected system has 5 ohms non-induc - 

 tive resistance in each mesh circuit. There are also 3 ohms in each 

 main circuit outside of the delta. With 110 volts applied to the 

 system, what will be the current in each circuit and tin- c.m.f. 

 around the delta? (6 min.) 



23. Same as problem 22, except that 2 ohms reactance is added 

 to each of the mesh circuits. (8 min.) 



24. Same as problem 23, except that 2 ohms reactance is added 

 to each of the main circuits. (8 min.) 



25. Given a ring-wound armature placed in a 2-pole field and 

 tapped at 0, 90, 120, 180, 240 and 270 degrees. With 250 volts 

 across opposite brushes, required the voltage across adja 

 brushes (90 degrees apart) of 2-phase and across the 3-phase ! 



If the generator is to give 100 kilowatts, what will be the current 

 in the armature conductors for the single, 2-phase and 3-phase? 

 Also what in the line? (8 min.) 



26. If the armature of problem 25 is wound with 2 flat wires 

 50 X 400 mils and 600 circular mils per ampere be allowed, what 

 would be the capacity as single, 2 and 3-phase generators? (6 

 min.) 



27. Find the wire area and the B. & S. size for the lines to 

 transmit 430 kilowatts at 22,000 volts, single-phase, 3-phas. 

 2-phase, 4-wire and 3-wire. Using the B. & S. numbers ivqnir.'d. 

 at 20 cents per pound, find the cost per mile of circuit in each case. 

 Take 1200 circular mils per ampere as giving a satisfactory drop. 

 (15 min.) 



28. Given a 3-phase motor with 400 volts between the 1 

 power factor of 85%, efficiency of 80% and with 10 horse-power 

 delivered; required the current in the lines and also in the con- 

 ductors of the delta-wound armature. (3 min.) 



29. Two wattmeters are placed in the outside wires of i\ bal- 

 anced 2-phase, 3-wire system feeding two similar non-inductive 



