502 ELEMENTS OF ELECTRICAL ENGINEERING. 



armature, A A, are also of wrought iion, 2x2 inches square, and 

 the air gap between the armature and the ends of the cores is 

 ^ inch. Find the total number of ampere -turns required to 

 give a pull of 934 5 pounds on the armature. Ans. 6,175 ampere- 

 turns. 



Note. Neglect magnetic leakage and reluctance of joints between cores and 

 yoke. Assume length of magnetic path in yoke and in armature each equal to 9 

 inches. 



APPENDIX B. CHARACTERISTIC CURVES. 



201. Given the following data, specifying the external char- 

 acteristic of a series generator : 



E x 65.1 121 o 138.0 150.0 158.3 163.5 ^4.3 volts 

 I 12 24 30 36 42 48 54 amp 



The combined resistance of armature and field winding is 055 

 ohm. Plot the external characteristic from the above data, and 

 derive and plot the total characteristic, ignoring the demagnetiz- 

 ing action of the current in the armature. Ans. Coordinates 

 of one point E a = 1 54.5 volts, I a = 30.0 amperes. 



202. The speed of the above generator is doubled ; derive and 

 plot the altered external characteristic, and the altered total 

 characteristic. Ans. Coordinates of one point, E a = 292.5 volts, 

 I x 30.0 amperes. 



203. The field of the series generator of problem 201 is 

 wound with 0.8 as many turns of wire as in problem 201, the 

 wire being smaller in size so as to have the same resistance. 

 Derive the data for the external characteristic of the new machine 

 from the data of problem 201, ignoring the demagnetizing action 

 of the current in the armature, and plot the new external 

 characteristic. Ans. Coordinates of one point, E x = 133.87 

 volts, 7. = 37.5 amperes. 



204. Derive the data for the external ampere-ohm character- 

 istic of the series generator of problem 20 1 , and plot the external 

 ampere-ohm characteristic of the machine. Ans. Coordinates 

 of one point, 7. = 30 amperes, R x = 4.6 ohms. 



