292 ELECTRICAL EQUIPMENT 



of the machine and dangerously increase the severe mechanical 

 strains produced by the same on the apparatus, as these increase 

 with the square of the current. A highly saturated machine, 

 on the other hand, is detrimental to the use of automatic voltage 

 regulators. With these regulators a close inherent regulation 

 machine is not necessary as a good regulation of the system can, 

 nevertheless, be maintained. The regulator automatically in- 

 creases the field excitation as the load increases and thus main- 

 tains a constant terminal voltage. If desired, it can also be 

 adjusted so as to increase the voltage with the load and com- 

 pensate for the line drop. 



Modern water-wheel-driven alternators have a regulation at 

 unity power factor of around 20 to 25 per cent. This is con- 

 sidered entirely satisfactory as the voltage regulation is best taken 

 care of by automatic voltage regulators. 



Short-circuit Current. In speaking about the short-circuit 

 current of an alternator, distinction must be made between the 

 instantaneous short-circuit current and the sustained or permanent 

 short-circuit current. 



The sustained short-circuit current of an alternator is limited 

 by the armature resistance and reactance, as well as its reaction 

 on the field. It is equal to 



where E is the generated e.m.f. corresponding to the field excita- 

 tion, and Z s the " synchronous impedance, " representing the 

 combined effect of the above three factors. This formula, there- 

 fore, gives the value of the sustained short-circuit current, while its 

 instantaneous value will be very much higher. This is due to the 

 fact that in the first instant, when the generator is short-circuited, 

 the current is limited only by the resistance and self-induction of 

 the armature circuit, while a time lag of sometimes a few seconds 

 takes place before the armature reaction becomes effective. The 

 armature resistance and reactance are thus the only two quan- 

 tities that limit the instantaneous short-circuiting current. This 

 limiting effect is, however, not constant, but decreased slightly 

 with high short-circuiting currents due to their saturation of the 

 magnetic field. 



Fig. 163 represents an oscillogram of a typical three-phase 



