476 ALTERNATING CURRENTS 



4. What is meant by Y-connection? How many wires are necessary? 

 What is the numerical relation and the phase relation of the line voltage to 

 the coil voltage in this system ? The line current to the coil current? What 

 relation exists among the three-coil currents if thriv is no neutral wire? 



6. At unity power-factor what is the total power generated in a Y-con- 

 nected generator in terms of coil volts and coil current? If the power-factor 

 is other than unity? What is the line power in terms of line current, line 

 voltage and power-factor? 



6. Why is the line power-factor the cosine of the coil power-factor angle? 

 What significance has power-factor in an unbalanced system? 



7. Why is the delta-connection not a short-circuit for the three coil 

 voltages? What is the numerical relation and the phase relation which 

 exists between the coil current and the line current? 



8. What is the total power produced by a delta-connection equal to in 

 terms of coil voltage, coil current and coil power-factor? In terms of line 

 voltage, line current and coil power- factor? 



9. Sketch the connection of the three-wattmeter method of measuring 

 power, (a) When the neutral of the system is accessible. (6) W r hen the 

 neutral is not accessible. To what is the total power equal in terms of the 

 wattmeter readings? 



10. Illustrate the principle of the Y-box and state the conditions under 

 which it can be used. 



11. Sketch the connections of the two-wattmeter method of measuring 

 power. Under what conditions do the wattmeters read the same? 

 Different? 



12. Under what conditions does one wattmeter reverse? Give two 

 methods of obtaining power-factor from the two-wattmeter readings alone. 

 Under what conditions can the two- wattmeter method not be used to 

 measure power in a three-phase system? 



13. What phase relations exist between the voltages of a two-phase 

 system? Show the connections of four different types of two-phase system. 

 What relations exist among all voltages in each of these systems? 



14. Sketch two methods of connecting the coils of a two-phase generator. 

 What relation exists between the coil voltage and the line voltage in each 

 of the two systems? Between coil current and line current? 



PROBLEMS ON CHAPTER IV 



49. A certain three-phase Y-connected alternator is rated at 500 kv-a. 

 2,300 volts, at unity power-factor. What is its current rating per terminal? 

 What is the rated coil current and coil voltage? 



60. What is the current and voltage rating of the machine in problem 



49 if it is changed over to a delta-connection? 



61. A Y-connected alternator delivers a balanced three-phase load of 



50 amp. at 230 volts and 0.8 power-factor to a delta-connected motor. Find 

 the current and voltage per coil in both the generator and the motor. What 

 power is involved? 



