56 THE DIRECT-CURRENT MOTOR CH. II! 



tractive force of x pounds ' available for useful effort,' this 

 does not express the whole force the motor exerts, since it 

 neglects the effort exerted in overcoming the frictional re- 

 sistances of the motor equipment. Thus, if a motor is 

 rated at 1,000 pounds 'available for useful effort,' this 

 would not be the T of our equations ; we must increase the 

 given amount by the equivalent force required to overcome 

 the resistances of the motor equipment. The ratio of the 

 whole force exerted to that ' available for useful effort ' 

 is the measure of the mechanical efficiency of the equip- 

 ment. 



Example 19. A locomotive is to be designed to run 

 at sixteen miles an hour while exerting a force of 3, GOO 

 pounds at the draw bar. Two motors are to be used, 

 connected in parallel and geared to the shafts of the 

 driving wheels with velocity ratio of 4 - 78. The driving 

 wheels are to be 33 inches in diameter. The tension 

 of the line is 500 volts, and the resistance of the motors 

 - 2 ohm each. A force of GOO pounds has to be pro- 

 vided for, over and above the draw bar pull, to overcome 

 the frictional and other resistances to motion in the 

 locomotive itself. To find the induction factor of the 

 motors. As there are two motors the load may be divided 

 equally between them, giving 2,100 pounds as the value 

 of T per motor. Inserting the given values of E, d, v, R, 

 and 8, in Equation 21 we find the induction factor to be 

 36'3. This result may be checked by finding the value of 

 the induced tension. The motor will rotate at thirteen 

 revolutions per second when the train is running at sixteen 

 miles an hour, hence the induced tension will be 472 volts. 

 The current per motor is 111 amperes, and the heat drop 



