2/4 ELEMENTS OF ELECTRICAL ENGINEERING. 



tained is made exact by the use of a pair of adjustable resistances 

 connected from the middle main to the outside mains. 



(d) The motor-generator balancer. The two generators shown 

 in Fig. 156 maybe replaced by a single 22O-volt generator of 

 the ordinary type and the current that comes into or flows out 

 of the station on the middle main may be taken care of by a 

 small motor-generator, consisting of two small shunt-wound 

 dynamos , P and Q, with their armatures mounted on one shaft 

 and connected electrically, as shown in Fig. 159. Consider the 



ioo amperes 



10 Amperes 



go amperes 



Fig. 159. 



particular case in which the upper main carries an outward current 

 of ioo amperes, the middle main a return current of 10 amperes, 

 and the lower main a return current of 90 amperes as shown in 

 Fig. 159. Part of the current which enters the station on the 

 middle main will then flow down hill, as it were, through P to 

 the negative terminal of the large generator, thus causing P to 

 to act as a motor and deliver mechanical power to Q which will 

 act as a generator and pump the remainder of the current in the 

 middle main up hill, as it were, to the positive terminal of the 

 large generator. When an outward current flows in the middle 

 main, Q is operated as a motor by current that flows down hill 

 from the positive terminal of the large generator, and P operates 

 as a generator and pumps current up hill from the negative ter- 

 minal of the large generator. In order to keep the potential of 

 the middle main at the proper value so as to divide the electro- 



