260 



ELEMENTS OF ELECTRICAL ENGINEERING. 



series motor (a negative booster), M t as shown. The power de- 

 veloped by M is belted back to the main generator G. When 

 the elevator motor takes but little current the voltage, E 2 , is high 

 and the battery is charged. When the elevator motor takes a 

 large current, the current flowing in the series field coil, S, of the 

 auxiliary motor, M, causes it to develop an increased counter 

 electromotive force so that E 2 is decreased and the battery dis- 

 charges. The motor, M, produces an effect which is equivalent 

 to a voltage drop in a long pair of feeders, and the battery operates 

 exactly like a floating battery. 



The motor, M, keeps the output of the main generator, G, much 

 more nearly constant if it is provided with two field windings, S 

 and S' ', which oppose each other. In this case the windings, vS 

 and 5', may be made to balance each other when the elevator 

 motor takes its average current. Then if the elevator motor takes 

 less than its average current the coil, S f , predominates, M becomes 

 a generator, increases E v and causes the battery to be charged ; 

 if the elevator motor takes more than its average current the coil, 



Fig. 151. 



S, predominates, M becomes a motor, decreases E y and causes 

 the battery to discharge. The use of the additional field winding, 

 S', which opposes S, makes it possible to use many more turns 



