490 ALTERNATING CURRENTS 



112. Power generated in a station by a three-phase, 2,500-kv-a. alternator 

 at 6,600 volts, is transformed for transmission to 33,000 volts by a delta- 

 Y-connected transformer bank. The voltage is then stepped down by a 

 delta-delta bank to 13,200 volts for distribution. What is the current, 

 voltage and kv-a. rating of each transformer? Give the current and the 

 voltage rating of the primary and secondary of each transformer. Neglect 



113. A certain sub-station receives power at 6,900 volts and this power 

 is stepped down to 2,300 volts by two 200-kv-a., V-connected transformers. 

 If the power-factor of the secondary load is unity, what is the maximum 

 power which the transformers can deliver without exceeding their ratings? 

 What is the current and the voltage of each winding? 



114. Repeat problem 113 for three 200-kv-a. transformers connected 

 delta-delta. Compare the per cent, increase of capacity with the per cent, 

 increase of investment. 



116. Power is received at a certain factory at 2,300 volts, three-phase. 

 It is desired to transform it to 230 volts, two-phase, four-wire. If the total 

 power is 50 kw. at 0.8 power-factor, what should be the ratings of the trans- 

 formers if the Scott connection is used? Make a sketch showing the method 

 of connecting these transformers. 



116. If the transformers of problem 115 were used to obtain a 230-volt, 

 two-phase, three- wire system, indicate the currents and the voltages in each 

 part of the system. Make a sketch. 



117. A single-phase line delivers 60 kw. at 1,100 volts. An ammeter, a 

 voltmeter, an indicating wattmeter, and a watthour meter are all necessary 

 in operating this circuit. Sketch the connections of the instrument trans- 

 formers and the instruments. Give the ratios of the transformers and the 

 factor by which each instrument reading must be multiplied in order to 

 obtain the corresponding value of the current, voltage, power, etc., existing 

 in the high-voltage circuit. 



QUESTIONS ON CHAPTER VIII 



1. Describe a simple experiment illustrating the underlying principle of 

 induction-motor action. Show that the tendency of the rotor to follow 

 the inducing magnetic field is another illustration of Lenz's law. Why 

 cannot the rotor attain the speed of the inducing magnetic field. 



2. Make a sketch of a two-phase gramme-ring winding and sketch 

 the position of the magnetic field for three or four different values of the 

 currents. Repeat for a three-phase drum winding. What is the relation 

 between the space-advance of the magnetic field and the time-change of 

 the currents? 



3. What is meant by revolutions slip? Per cent, slip? Show how the 

 rotor frequency is related to the slip. 



4. Upon what three factors does the torque developed by an alternating- 

 current motor depend? Plot a sine of wave current and a sine wave of 

 flux about 45 out of phase and then plot the resulting torque curve. 



