39 6 



ELEMENTS OF ELECTRICAL ENGINEERING. 



driven at constant speed, supply current to the armature of an 

 auxiliary dynamo, of which the field magnet is separately excited. 

 The characteristic of this auxiliary machine at a given speed, con- 

 sidered as a receiving circuit, would be the straight line PP 'P IV , 

 Fig. 28 ; but the counter-electromotive force e of the auxiliary 

 machine increases and decreases with the speed of that machine, 

 so that the line PPP IV moves up and down as the speed of the 

 machine rises and falls. When the circuit is first closed the speed 

 of the auxiliary machine is zero ; e is zero, and the series gener- 



Amperes 



Fig. 29. 



ator builds up to the point Q', Fig. 29, sending current through 

 the armature of the auxiliary machine. This current causes the 

 auxiliary machine to speed up as a motor, thus increasing e. This 

 action continues until the straight line PP'P IV rises above the 

 point Q, Fig. 29, and does not intersect the positive portion of 

 the characteristic of the series generator. Then the series gen- 

 erator immediately loses its field magnetism, the auxiliary machine 

 continues to run because of the momentum of its armature, and, 

 becoming a generator, its electromotive force e now starts a 

 reversed current through the circuit, thus causing the series gen- 



