588
DYNAMO
the armature, but further increase the useful flux, and compensate for the loss of volts over their own resistance and that of the armature. The machine will then give for a constant speed a nearly constant voltage at its terminals, and the curve ot the external characteristic becomes a straight line for all loads within its capacity. Since with most prime movers an increase of the load is accompanied by a drop in speed, this effect may also be counteracted • while, lastly, if the series-turns are still further increased, the voltage may be made to rise with an increasing load, and the machine is “over-compounded. The question of the commutation1 of the current in a section of the armature winding during the time when it is short-circuited brush resting simultaneously on both the trailCommuta- by the anq iea(]ing sectors, turns primarily on the varying tion and contact-resistance between the brush tip and the comsparkmg. inutator sectors.2 If the brushes are of the same width as the sectors, and T = the total time of the short-circuit depending on the speed of rotation, the area of contact between the brush and the leading sector at any time t (reckoned from the commence^ ^ ment of short-circuit) is a , while that between the brush and the trailing sector is oA; hence as the brush slides over from the leading to the trailing sector (Fig. 36), the contact-resistance of the leading sector gradually increases from R, to <», and at any time t within the period of short-circuit is R/ while the contact-resistance of the trailing sector gradually diminishes T from oo to R/} and at any time is R,-. If cx and c2 be the instantaneous values of the current in the leading and trailing sectors c T respectively, the current densities and s2 are cc ^ and -A If R — the resistance of the coil and its commutator cont nexions, and f {t) be the E.M.F. induced in the coil by its rotation through the external field, either positive or negative according as it is moving on the trailing or leading side of the neutral line of zero field, the direction of the old current in the coil being reckoned as positive, the complete equation to the short circuit is by Kirchhoff’s laws— -L' How when there is no induced E.M.F., or f(t) = 0—in other words, if the short-circuited section is moving in zero field—the current persists in the section by reason of c. c. its inductance, and does not fall in propor- 2P; ) tion to the amount by which the brush passes over on to the trailing sector ; hence the current density of cx in the leading sector is greater than the density of c2 in the trailing sector. There is thus a difference in the two values in the bracket, and s1 being greater than s2, there is a greater fall of potential between sector 1 and the brush than Fig. 36. between sector 2 and the rest of the brush. Further, c2 is negative, and the brush tip being necessarily throughout at the same potential, there results a potential difference acting round the circuit in the direction 1 - coil — 2, i. e., against the positive decaying current of the coil, and assisting the current which is growing up in the trailing sector. Some part, therefore, of the self-induced E.M.F. is spent in overcoming this negative E.M.F. until equality is reached, and then the current becomes negative simply in virtue of the E.M.F. set up by the unequal current densities. If the equation as above expressed holds good unconditionally, and no secondary effects come into play, it will be found that the current in the short-circuited coil will be reversed to the exact value that is required in the new direction, or be automatically commuted at the end of the period of short-circuit by the mere action of the brush-contact resistance; and further, that this will be the case wherever the brush be placed, and whatever the direction 1 Cp. Swinburne, “The Theory of Armature Reactions in Dynamos and Motors,” and Esson, “Some Points in Dynamo and Motor Design,” Journ. Inst. Elec. Eng. vol. xix., Feb. 13,1890; Esson, “ Notes on the Design of Multipolar Dynamos,” Journ. Inst. Elec. Eng. vol. xx. p. 265 ; J. Fischer-Hinnen, E.T.Z., No. 5, 1893, p. 53; Sayers, Journ. Inst. Elec. Eng. vol. xxiv. p. 122 ff. ; Mordey, Journ. Inst. Elec. Eng. vol. xxvi. p. 532ff. ; Allen, “Sparkless Reversal in Dynamos,” Journ. Inst. Elec. Eng. vol. xxvii. p. 209 ; J. Fischer-Hinnen, E.T.Z., 1898, Nos. 51, 52; Prof. Arnold and Dr Mie, E.T.Z., 1899, Nos. 5, 7, 8; Everett and Peake, Electrician, vol. xl. p. 861, and vol. xlii. p. 328. 2 Housman, Joum. Inst. Elec. Eng. vol. xxii. p. 399 • Thorburn Reid, American Inst. E.E., Dec. 15,1897, vol. xv. p. 33.
of the field in which the short-circuited coil is moving. If, however, it be moving in the old field, or f(t) is positive, the current-density under the leading brush edge will be enormously great, in order to counterbalance both the E.M.F. of self-induction and that impressed by the field. There is, however, an important secondary effect, which comes into play when the current-density becomes very high, and which causes sparking at the brushes when the position of the brushes is not correctly adjusted. "When a circuit is broken by an ordinary switch, the area of contact is diminished more or less rapidly, and when the current-density over the decreasing contact-surface rises beyond a certain limit, the local development of heat is so great that some portion of the metal forming the contact becomes incandescent and eventually is volatilized, and an arc results. In exactly the same way, under a very high current-density, the trailing edge of the leading sector or the leading edge of the brush becomes fused so that as the brush tip leaves the leading sector the current still finds an outlet for a short time through the heated vapour, and a series of small arcs are started every time that a sector passes from under the brush. Now, in the dynamo, if the brushes are kept fixed on the line of symmetry, and current be taken out of the armature, the distribution of the field is so distorted by the action of brushes the cross ampere-turns of the armature that the neutral line of zero field, where the lines just dip into the armature core and immediately leave it again, is displaced ahead of the line of symmetry. As the coil is now moving in the trailing field, the E.M.F. set up by it is in the old direction and tends to keep up the old current, hence the excessive currentdensity is magnified, and the more*so as the current is increased. The higher the brush contact-resistance, however, the less the abnormal density, and the less the difficulty of commutation. One remedy, then, is to employ a brush of high contact-resistance, and for this purpose the only suitable material that has been discovered is carbon moulded into hard blocks. Even if some little sparking does take place with carbon brushes, the damage done thereby to the commutator surface is very much less than would be the case with metal brushes. The only objection to their use is that in virtue of their high resistance they necessarily increase the normal loss of volts between the armature winding and the external terminals. If brushes of copper wire or gauze are used, it is not possible to maintain them on the line of symmetry, when the current reaches any great amount, without excessive sparking, and the only remedy is to shift the brushes forward in the direction of rotation through an angle of lead, which increases roughly as the armature current is increased. By the shifting of the brushes the armature ampere-turns become divisible into the two groups of back- and cross-turns, as shown in Fig. 31. The latter are progressively reduced, so that though the total flux may not be so large as at no load, the distortion becomes less ; and if the shifting is carried far enough, the diameter of commutation eventually overtakes the neutral line, and finally the short-circuited coil is moving in a reversing field, or f(t) is negative. The reversing E.M.F. then rapidly causes the old current to decay, starts a reversed current, and if the adjustment is correct, the strength to which the new current is raised at the end of short-circuit is precisely that of the armature current in the portion of the winding which it is to join, so that on opening the short-circuit there is no sparking. From the moment that a dynamo begins to run with excited field, heat is continuously generated by the passage of the current through the windings of the field-magnet coils and Heating the armature, as well as by the action of hysteresis effects. and eddy-currents in the armature and pole-pieces. Whether the source of the heat be in the field-magnet or in the armature, the mass in which it originates will continue to rise in temperature until such a difference of temperature is established between itself and the surrounding air that the rate at which the heat is carried off by radiation, convection, and conduction is equal to the rate at which it is being generated. Evidently, then, the temperature which any part of the machine attains after a prolonged run must depend on the extent and effectiveness of the cooling surface from which radiation takes place, upon the presence or absence of any currents of air set up by the rotation of itself or surrounding parts, and upon the presence of neighbouring masses of metal to carry away the heat by conduction. In the field-magnet coils the rate at which heat is being generated is easily determined, since it is equal to the square of the current passing through them multiplied by their resistance. Further, the magnet is usually stationary, and only indirectly affected by draughts of air due to the rotating armature. Hence for machines of a given type and of similar proportions, it is not difficult to decide upon some method of reckoning the cooling surface of the magnet coils Sc, such that the rise of temperature above that of the surrounding air may be predicted from an equation of the form t° — —rr.—, fee where W = the rate in watts at
