EXCITERS 351 



Capacity and Rating. The exciters should have a capacity 

 sufficient to excite all of the synchronous apparatus in the station 

 when these machines are operating at their maximum load and 

 at the true operating power-factor. It is not enough to provide 

 for the excitation when operating at unity power-factor, because 

 the excitation which is required at lower power-factors is con- 

 siderably higher than at unity power-factor. It is considered 

 good practice to make the combined capacity of all the exciters 

 equal to the excitation required for all the generators, when these 

 are operating at their maximum load and stated power-factor 

 (usually 80 per cent), plus a 20 per cent addition for possible 

 variations in the required excitation. 



Auxiliary station apparatus should not be operated from the 

 exciter system, since troubles are always likely to occur in these 

 circuits that may damage the exciters at times when such damage 

 would cause considerable inconvenience in the operation of the 

 station. In many stations, station auxiliaries are now entirely 

 operated by alternating current, and the direct current for the 

 control circuits can be easily taken care of by the use of a small 

 motor-generator set combined with a storage battery. No com- 

 plications are then introduced by voltage fluctuations caused 

 by automatic voltage regulators. Reserve capacity in case of 

 breakdowns should, of course, be provided, the amount depend- 

 ing on the number of units. 



Exciters are now given a maximum continuous Kw. rating 

 based on a temperature rise not exceeding 50 C., as measured 

 by thermometer, above an ambient room temperature of 

 40 C. 



Voltage. The pressure most commonly used for excitation 

 is 125 volts. For A.C. machines of very large capacity requiring 

 a large excitation, it will, however, usually be found more econom- 

 ical to use a 250-volt excitation. This higher voltage will permit 

 the use of smaller exciter and field switches, while leads of reduced 

 size from the exciters to the bus-bars and from the bus-bars to 

 the generator field may be used, and the cross-section of the bus- 

 bars cut in two; all this being of importance in reducing the cost, 

 especially in large installations. A considerable saving can also 

 generally be accomplished in the exciter itself. Machines for 

 125 volts require a commutator twice as large as those for 250 

 volts; and with water-wheel-driven units, where they must be 



