CHAPTER VIII 

 POLYPHASE SYSTEMS AND POWER 



1. A 2-phase, 3-wire system carries 30 amperes in each outside 

 wire; what will be the current in the common return? (J rain.) 



2. A 2-phase, 3-wire system is supplying 100 kilowatts at 500 

 volts and with 80% power factor; what will be the current in tin- 

 third wire? What is the e.m.f. between the outside wires? (2 

 min.) 



3. A 3-phase, Y-connected system has 4000 volts between 

 wires. The neutral point is grounded. For what voltage must 

 the insulation of the windings be designed? (J rain.) 



4. A 3-phase, Y-connected system with a fourth or neutral 

 wire is designed to use 110-volt lamps between the neutral and the 

 outsides; what will be the e.m.f. between the three lines? ($ / 



5. A 3-phase, delta-connected system carries currents of 50 

 amperes in the delta connections; what is the current in the lines? 

 (I min.) 



6. With 300 amperes in the line of a delta-connected, 3-phase 

 system, what current will flow in the mesh? (J win.) 



7. A 2-phase, 3-wire system is out of balance; the currents are 

 100 and 90 amperes, with an angle of 85 degrees between them. 

 What is the current in the third wire? (2 rain.) 



8. The currents in an unbalanced delta-connected 3-phase 

 system are 80, 90 and 100 amperes; what are the line currents and 

 what are their phase relations (instead of 120 degrees)? Solve 

 graphically. (5 min.) 



9. The currents in the lines of a mesh-connected, 4-phase 

 system are 400 amperes; what will be the currents in the meshes? 

 With 1000 volts between adjacent lines, what will be the e.m.f. 

 between opposite ones? (1 min.) 



10. In the balanced 2-phase, 3-wire system of problem 2, the 

 series coil of a wattmeter is put in the third wire while the pressure 

 coil is across the outside. What will the instrument read and 

 why? (2 min.) 



11. By the revolution of a vector diagram construct the three- 



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