352 ELECTRICAL EQUIPMENT 



designed to safely withstand double speed, the construction often- 

 times involves considerable difficulties and expense. 



Characteristics. When exciters are to be operated in connec- 

 tion with automatic voltage regulators, as is almost always the 

 case, it is most important that they are designed with this point 

 in view. The densities, especially in the fields, should be fairly 

 low, as with high density the time element required to vary the 

 voltage from one point to another would be so long as to materially 

 affect the regulation. The operating range should, therefore, be 

 below the bend of the saturation curve. 



The exciter should preferably have a time element so that it 

 will be responsive to changes in the field excitation to the extent 

 that, by inserting an external resistance equal to about three times 

 the resistance of the field, the voltage will fall from 125 to 25 volts 

 in from six to eight seconds. An ideal exciter designed along 

 these lines should also give at full field 165 volts and the increase 

 in the field current from 125 volts to 150 volts should not be over 

 50 per cent. 



For alternators operating at maximum inductive load 125 volts 

 is generally required for the excitation, and in order to get a satis- 

 factory regulation when an automatic regulator is used, the 

 exciter must be designed so as to be able to give 165 volts momen- 

 tarily. It is also necessary that the increase in the exciter field 

 current should be small, so that the exciter will respond quickly 

 to the short-circuiting of the rheostat, and thus insure the desired 

 alternator excitation. Should the excitation voltage be any other 

 value than 125, viz., 250 volts, the above values would be pro- 

 portionally changed. 



Shunt vs. Compound Wound. While an exciter can be either 

 compound wound or shunt wound, the former is considered prefer- 

 able for parallel operation with automatic voltage regulation. 



Non-regulating exciters should be more or less highly saturated 

 in order to insure a stable parallel operation. If such exciters 

 were to be used with automatic regulation, they would be rather 

 slow to correspond to the changes in field excitation. If a shunt- 

 wound exciter is designed for a low saturation so as to make it a 

 good regulating exciter, the tendency might be an unstable oper- 

 ation when running in parallel without a regulator. Shunt-wound 

 exciters are, however, generally provided with commutating poles 

 to overcome the above difficulties. 



