PROBLEMS. 395 



stant of an armature winding in which the conductors lie on the 

 surface of a smooth armature core as bands, the width of each 

 band being f of the distance from center to center of adjacent 

 field poles. 



Note. The data of this problem are purposely chosen so that the answer to the 

 problem may not be given in the table on page 124. Assume each band to have an 

 indefinitely large number of conductors. 



79. Using the data in Figs. 117 any 118, referring to a given 

 2,000 kilowatt delta-connected 3 -phase alternator, calculate the 

 half-load regulation of the machine for the case in which the re- 

 ceiving circuits have a power factor equal to 0.6 ; (a) by the 

 electromotive force method, and (b) by the magnetomotive force 

 method. 



Note. The specifications in this problem are purposely chosen so that the answer 

 to the problem may not be included in the examples given on pages 134-137. 



80. A ten-pole alternator having 720 armature conductors supplies 30 amperes to a 

 receiving circuit of which the power factor is 0.866. Calculate the average demag- 

 netizing ampere-turns due to the armature current. Ans. 763 ampere-turns. 



CHAPTER VIII. THE SYNCHRONOUS MOTOR. 



81. Plot curves showing the values of P f , P" and RP for two 

 synchronous alternators in series, the alternator A having an 

 electromotive force of 1,200 volts, the other machine B an elec- 

 tromotive force of 800 volts, the resistance of the circuit being I 

 ohm and the reactance 2 ohms. Result similar to Fig. 127. 



82. An alternator A has an electromotive force of 1,100 volts, 

 a resistance of I ohm, and a reactance of 0.58 ohm. The machine 

 is driven as a synchronous motor with zero load, from i,ooo-volt 

 mains, (a) What is the value of the current ? (b) What is the 

 component of this current which is 90 ahead of the supply 

 electromotive force in phase ? (c] What capacity of condensers 

 would take the same amount of leading current from the 1,000- 

 volt mains at a frequency of 60 cycles per second? Ans. (a) 

 21 1.4 amperes, (b) 206.6 amperes, (c) 548 microfarads. 



83. Two governed engines drive two alternators which are connected in multiple 

 and feed one pair of mains. Engine A drops from 150 revolutions per minute at zero 

 load to 145 revolutions per minute at 100 horse power. Engine B drops from 150 



