Sustained short-circuit phenomena and flux distribution of salient-pole alternators

It is shown in Section IV, that with the ordinary field forms met with in practice the resultant flux wave under s. s. c. (sustained short-circuits) will be extremely distorted, see Figs. 16, (4th wave) 23, 27, 27a, etc., for the simple reason that with the very low voltages obtained under such conditions, the fundamental of the B-curve of the field is reduced so much by the armature reaction that the higher harmonics assume a very predominant rôle and become several hundred per cent of the s.s.c. fundamental. See Fig 26 and Tables XI, XII, and XIII. As a corollary to the foregoing it is found that the B-curve under load will not differ radically from the no-load field form since the fundamental will remain large enough to hold its own. See Fig. 22 and also footnote (4). The cross magnetizing effect of the armature reaction is, of course, to make the B-curve unsymmetrical with respect to the mid-pole axis. Compare Figs. 7 and 22. The magnetic oscillations are studied not only by means of full-pitch stator coils but also by means of rotor coils No. 7 and 8, Fig. 3, and stator coils No. 9, 10, 11, 12, 15, and 16, Figs. 3 and 3a. Attention is called to the following facts for which explanations and theoretical proofs are offered. (1) The ripples at the crest of the e. m. f. waves of the stator coils are due not so much to the flux pulsations as to (a), the to-and-fro flux swing across the pole, and (b) the cutting of a B-curve similar to wave (b), Fig. 6. See equations 22, and 23; also Table I and Note A, at the end. Compare Figs. 7 and 2nd wave Fig. 4. (2) The most important oscillations set up by the armature reaction are 4 and 6 times machine frequency for two- and three-phase machines respectively. See supplementary Notes B and C; also Figs 8a, 8b wave lengths marked B and D, and Fig. 5, wave length marked B, and Table I. (3) Eccentric rotor sets up pulsations once in every revolution, under load or at no-load, but none under s.s.c. See Fig. 4, 2nd wave, Fig. 5 wave length marked A, and Figs. 8a, 8b, and 8c. Also see Table I. (4) The e. m. fs. and fluxes for coils No. 10 at the top of a tooth are very much larger than the e. m. fs and fluxes for coil No. 9, at the top of a wedge. See Tables II and III; also Figs. 10, 11, 12. (5) The spacing of the ripples of the no-load e. m. fs. is unequal, contrary to theory. See Fig. 13 and Table IV. (6) The e. m. fs. and fluxes for search coil No. 5, at the bottom of the slot, are larger than the e. m. fs. and fluxes for the coil No. 1 at the top of the slot. See Fig. 7 and Tables V, VI, and XIV. (7) The third harmonics e. m. fs. of the stator search coils increase with the increasing excitation. See Table IX. (8) Under s.s.c. there is present some cross-magnetizing armature reaction which (a), makes crests 1 and 3 Fig. 27, unequal. See Table XV. (b), it shifts the axis of the flux towards the larger crest, Figs. 23, 27, etc., and thus makes angle B, Fig. 27, less than (1/2) A. See Table XIV. (9) The agreement between the armature reaction obtained by subtracting er from efo, (see Figs. 24, 25, 31 and Figs. 16, 27, etc.) and that obtained by subtracting the equation of the s. s. c. waves as given in Tables XI and XII, from corresponding open-circuit e. m. f. equations as given in Tables VII and VIII, is fairly satisfactory. See last column of Table XIII. (10) The direct component of the armature reaction of an imperfect two-phase machine consists of (a) a regular demagnetizing component, plus (b), a transverse or cross magnetizing component in quadrature to (a). See equations 56, 59, etc. (11) The analysis of the no-load field form gives the 3rd, 5th, and 7th harmonics as 5 to 15 per cent but the (2q ± 1) ths seem to be small. (12) The armature current under s.s.c. is very nearly sinusoidal. See Figs. 15 and 16. Also Table X. The H-curve of armature reaction set up by such a current will be sinusoidal, see equations 32, 33, etc. and succeeding two paragraphs. The B-curve, however, will not be sinusoidal for lack of constancy and uniformity of the permeability at different points of the magnetic circuit, and on account of saturation. See sections III and IV.