76 ELECTRICAL ENGINEERING PROBLEMS] 



circuits with 100 volts; when normally connected, each of the 

 instruments indicates 500 watts, but when the shunt coils are 

 interchanged, each reads 50 watts. Draw a diagram to represent 

 this case and find the currents. (5 min.) 



30. The two wattmeters on a 2-phase system indicate 1500 

 and 200 watts; the e.m.f.'s are both 100 volts and are 90 degrees 

 apart; the currents are 18 and 3 amperes. Draw a diagram for 

 the case and determine the lags (tangents) of the currents. (7 

 min.) 



31. A 2-phase, 3-wire system with e.m.f.'s of 400 volts, 90 

 degrees apart has currents of 25 amperes in each phase lagging 

 30 degrees; what total power would the wattmeter readings show 

 with the two wattmeters placed normally, and also with their 

 series coils both in the common wire; solve graphically. (8 min.) 



32. In problem 31, if one current were 30 amperes, what would 

 the sums of the wattmeter readings become? (10 min.) 



33. Three wattmeters connected in the delta circuits of] a 

 3-phase system show 1120, 1540 and 1780 watts, respectively, the 

 voltages are each 440 and are 120 degrees apart, the currents are 3.2, 

 4.5 and 6.3 amperes. Construct the diagram for this case includ- 

 ing the line currents and give the total power and the power factor 

 of each circuit. (12 min.) 



34. Three wattmeters are used with the voltage coils across 

 the circuits to measure the power in a 220-volt, 3-D 1 ., Y-con- 



nected system. Each reads 1250 wat 4 ^ v ovver factor meters 



show 75% power factor. Construct the diagram for the case and 

 find the line currents and the total power. (7 min.) 



35. The currents in the three leads of a 500-volt balanced delta- 

 connected, 3-phase system are each 173.2 amperes; the readings 

 of the two wattmeters properly connected to measure the power of 

 the system are each 75 kilowatts. What is the power factor of 

 the system? Prove by means of a vector diagram. (6 min.) 



36. With the same system as in problem 35, except that the 

 wattmeters read 75 and kilowatts, draw the diagram and deter- 

 mine the power factor. (5 min.) 



37. Repeat problem 35, except that the wattmeter readings 

 are plus 60 and minus 24 kilowatts. (7 min.) 



38. What would be the readings of two wattmeters with their 

 series coils in the same branches as circuits (a) and (b) of problem 

 17? (10 min.) 



