318 ELECTRICAL EQUIPMENT 



changing the number of circuits. The above is particularly true 

 of generators with large diameters and a great number of poles. 

 Small machines with few poles can, as a rule, be reconnected or 

 rewound for any voltage up to and including 2300. It is a com- 

 mon but erroneous idea that machines wound for 2300 volts, 

 delta connected, can be simply reconnected to 4000 volts Y. 

 While this is all right so far as mere voltage is concerned, the slot 

 in the armature may not be large enough to accommodate the 

 extra insulation required for the higher voltage. In large machines 

 the above change may sometimes be made without much diffi- 

 culty, but small machines require as a rule, new coils and fre- 

 quently new punchings. 



Parallel Operation. In order that an alternating-current gen- 

 erator shall be able to carry a load, a current must flow corre- 

 sponding to this load. The e.m.f. required to generate this cur- 

 rent is the resultant of the terminal and the induced e.m.f. 's of 

 the generator, the displacement between these e.m.f. 's being due 

 to the impulse of the prime mover. In the same manner when 

 two or more generators are operating in parallel the division in 

 load between the different units is entirely dependent on the 

 turning efforts of the prime movers, and a change in the field 

 excitation, as with direct-current generators, will have no effect 

 whatsoever. 



For a satisfactory parallel operation it is important that the 

 e.m.f. 's of the generators are the same and that they are operated 

 in perfect synchronism, as if this is not the case cross currents 

 will flow between the units. These cross currents may be either 

 wattless or they may represent a transfer of energy, depending on 

 whether they are caused by a difference in the e.m.f. or a speed 

 variation of the machines. 



When two alternators are operating in parallel at the same 

 speed, their e.m.f. 's are naturally in opposition as shown in Fig. 186. 



Let OA be the e.m.f. of generator No. 1 and OB the e.m.f. of 

 generator No. 2, the difference in their values being caused by a 

 stronger excitation of the latter machine. The resultant e.m.f. 

 OC will be in phase with OB, and, being impressed on the syn- 

 chronous impedance of the two generator armatures in series, it 

 will produce a cross current, lagging nearly 90 behind the e.m.f. 

 of generator No. 2 and leading nearly 90 in advance of the e.m.f. 

 of generator No. 1. This is practically true, as the impedance 



