The Transient Reactions of Alternators

Abstract

From these investigations we conclude that the short-circuit currents of alternators are limited by reactances much more complex and much higher than the self-inductive reactances of the armatures, but which are constant for similar units and can be obtained for any size and type of generator by simple low-voltage short-circuit tests. By means of this test value the maximum short-circuit current of a single unit may be readily computed from Im0 = Em0/z (1+¿ -r/x¿)and the maximum torque from equations which show, when developed, that the current per unit in any given short circuit is less as the number of units in parallel increases; that the maximum current always results when the short circuit occurs at the zero point of the corresponding pressure wave, independent of the particular short-circuit conditions; that the maximum instantaneous torque merely varies inversely as the reactance in circuit and hence that instability of the system and generator stresses (except those in end turns) at times of short circuit, are only lessened by reactance coils in proportion to the resulting increase in total reactance; that the torque stress per unit with a given number of units in parallel may be greater or less than that with a smaller number of units, depending on the resistance of the short circuit; and that the maximum torque is entirely independent of the points of the pressure waves at which a three-phase short circuit occurs.