196 



ELECTRICAL ENGINEERING 



are shown in Fig. 164. Assume that the prime mover of (1) runs 

 for an instant at a slightly increased speed; the voltage of (1) 

 rises and it takes more than its share of the load; the voltage of 

 (2) falls because its load is decreased and its series excitation is 

 decreased. Machine (1) therefore takes more of the total load 

 and its voltage rises higher until it supplies all the load and in 

 addition drives (2) as a motor. Since the current in (2) is re- 

 versed the m.m.f. of its series winding is also reversed and it runs 



Load 



S 2 



FIG. 163. 



Amperes 



FIG. 164. Voltage characteristics. 



as a differential motor driving its prime mover at a high speed 

 until the load on (1) becomes so great that the protective appa- 

 ratus opens the circuit and shuts down the system. 



To get over this difficulty the equalizer connection ee, Fig. 165, 

 is used. It is a conductor of low resistance connecting in multiple 

 the series windings of the two machines. Now if the prime mover 

 of (1) runs above normal speed the voltage of (1) rises and it takes 

 an increased load. The increase of current does not all go through 

 the series winding of (1) but divides between the windings of (1) 

 and (2) in inverse proportion to their resistances and so prevents 

 any decrease of the voltage of (2). Thus with an equalizer con- 

 nection (2) will still hold its load. The resistances of the series 

 windings must be adjusted so that the load current will divide 

 between them in such proportion that each machine will supply 

 its proper share of the load. 



Before connecting machine (2) in parallel with (1) which is 

 delivering power, first close switches S 2 and 83, Fig. 165, and 

 adjust the shunt field of (2) until its terminal voltage is the same as 

 that of (1). The excitation of (2) is now provided partly by its 

 shunt field and partly by its series field carrying part of the load 

 current. After checking the polarity to see that it is correct close 



