468 ELEMENTS OF ELECTRICAL ENGINEERING. 



94, Find the armature torque in pound-inches corresponding 

 to a tractive effort of 2,000 pounds developed by a street railway 

 motor, the gear ratio between the motor armature and the axle 

 being 14 : 68, and the diameter of the car wheels being 33 inches. 

 Ignore motor, gear, and axle friction in this calculation. Ans. 

 6,794 pound-inches. 



CHAPTER V. POWER LOSSES AND EFFICIENCY. 



95. (a) Find field loss and the stray power loss of the motor 

 specified in problem 78. (&) Find the efficiency of this machine 

 when operated as a generator with an armature current of 50 

 amperes and an electromotive force of 110 volts between its 

 brushes, assuming the stray power loss to be that found under (a), 

 (c) Find the stray power loss and efficiency of the generator under 

 the conditions specified in (fr) making a correction for the difference 

 of induced voltage in the armature. Ans. (a) 286 watts, and 

 329 watts; (b) 85.4 per cent; (c) 347 watts, 85.42 per cent. 



NOTE. The difference between the uncorrected and corrected values of the stray 

 power loss, amounting to only 1 8 watts, is negligible in its effect upon the calculated 

 efficiency of the machine. In the following problems the uncorrected value of the 

 stray power loss is used. 



96. Find the efficiency of conversion and the electrical effi- 

 ciency of the generator specified in problem 95, when its arma- 

 ture current is 50 amperes and its terminal voltage is 1 10 volts. 

 Ans. 94.6 and 90.3 per cent. 



97. Find the true efficiency, the efficiency of conversion, and 

 the electrical efficiency of the generator specified in problem 95, 

 when the armature current is 10 amperes and the terminal volt- 

 age is no volts. Ans. 56.5, 77.2, 73.1 per cent. 



NOTE. In this problem neglect the change of field loss due to the adjustment of 

 the field rheostat which must be made to keep the terminal voltage constant. 



The stray power loss of a generator at light loads is in fact less than at full-load ; 

 this difference depends not only upon the change of induced electromotive force, but 

 also upon the change of distribution of flux in the armature core due to armature re- 

 action. The change of stray-power loss due to armature reaction cannot be calculated 

 and is generally ignored in efficiency calculations. 



