US1393141A

US1393141A - Method and means for improving the commutation of dynamo-electric machines

Info

Publication number: US1393141A
Application number: US120689A
Authority: US
Inventors: Jaroslaw K Kostko
Original assignee: Individual
Current assignee: Individual
Priority date: 1916-09-18
Filing date: 1916-09-18
Publication date: 1921-10-11
Anticipated expiration: 1938-10-11

Description

I. K. KOSTKO.

METHOD AND MEANS FOR IMPROVING THE COMMUTATION 0F DYNAMO ELECTRIC MACHINES.

APPLICATION FILED SEPT. 18. 1.9 I6. 1,393,141 Patented Oct. 11, 1921.

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METHOD AND MEANS FOR IMPROVING THE COMMUTATION 0F DYNAMO ELECTRIC MACHINES. APPLICATION HLED SEPT; I8, 1916.

1,393,141. Patented Oct. 11, 1921.

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METHOD AND MEANS FOR IMPROVING THE COMMUTATION 0F DYNAMO ELECTRIC MACHINES. APPLICATION FILED SEPT. 1a. 1916.

1,393,141. Patented Oct. 11, 1921.

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J. K. KOSTKO.

METHOD AND MEANS FOR IMPROWNG THE COMMUTATION 0F DYNAMO ELECTRIC MACHINES. -APPLICATION FILED SEPT. 18. 1916.

1,393,141. Patented Oct. 11, 1921.

s SHEETSSHEET 41 J. K. KOSTKO.

METHOD AND MEANS FOR IMPROVING THE commumnou 0F DYNAMO ELECTRIC MACHINES.

5 SHEETS-SHEET 5- APPLICATION F|LED SEPT. 18,1916- UNITED STATES rarsiar orrics.

JAROSLAW K. KOSTKO, OF ST. LOUIS; MISSOURI.

METHOD AND MEANS FOR Ill/IPROVING THE COMMUTATION OF DYNA1VO-ELECTRIC IMACHINES.

Application filed September 18, 19 l6. Serial No. 120,689.

T 0 all whom it may concern Be it known that I, Janosnrn K. Kos'rno,

a citizen ot the United States, residing'at St. Louis, in the State of Missouri, have invented a certain new and useful Improvement in Method and hleanstor Improving the Commutation of Dynomo-lllectric 'Machines, of which the following is a specifi a' tion.

This invention relates to method and means for improving the commutation of dynamo-electric machines provided with commutators. i

It is well known that in every commutating machine the reversal of current in a coil undergoing commutation is opposed by electromotive forces due to self-induction ofthe coil, to mutual" induction of coils short-circuited by the brush, and to the magnetic field set up by armature coils other than those which are undergoing commutation. Therefore, in order to extend the limit of sparlrless commutation is necessary to produce in the circuit of the short-circuited coil an electromotive force opposite in direc tion to. the above mentioned injurious electrornotive forces. Moreover, if this electromotive force, which I shall call reversing', is not only'opposite in direction, but at every moment, equal to the sum of injurious electromotive forces, then (considering as negligible the ohmic resistances of an armature coil and 01" leads connecting it to the commutator) perfect commutation will result, the so-called straioht line commutation, because in the usual system of graphic representation with time as abscissa and current as ordinate, the current in the short-circuited coil will be represented by a straight line. It is well known that in'this case the current density all along the brush contact surface is constant, and that the energy loss due to the contact resistancebetween the brush and'the commutator is minimum depending only on physical proper.- ties of the brush. However, a straight line curve of the commutating' current is by no means indispensable for perfect commutation. Commutation can be considered as perfect if following conditions are satis fied: 1st, there is no sparking at the edges Specification of Letters Patent.

Patented Oct. 11, 1921.

and even aclose approximation of straight line curve of commuted current can be obtained by intermittent application of a reversing electromotive force, which can be an alternating electromotive force, acting on the shorten-suited coil while its direction is such as topassist the commutation, and

made useful in a different way, but for the same object, during that part of its period when'a direct application would be injurious tor commutation because of the wrong direction.

Some of the objects of this invention, therefore, are to provide method and means to improve the commutation of 'dynamo-' electric machines, by introducing'at suitable periods into the armature coils undergoing commutation an. alternating electromotive force, which is applied in such a manner as to assist commutation and prevent sparking.

Further objects will appear from the detail description taken in connection with the accompanying drawings, in which:

Figure 1 is a longitudinal section of a commutator Fig. 2 isanend view; Fig. 3 shows a form of generatorsupplying the electromotive force assisting the commutation, in connection with a developed view of the commutator described with reference to Figs. 1 and 2, while F ig. 3 shows the application of this invention to .a machine Fig. 1 shows the curve of the electrometive force generated in this apparatus, together with the curve of magnetic flux re quired for this purpose;

F igs. 5, 6, 7 and 8 show different positions of brushes with relation to commutator described in Figs. 1 and 2, which occur when commutator and brushes are moving relatively to each other;

Fig. 9 shows a general shape of the curve of the current collected froma commutator segment with particular reference to the relative brush and commutator positions shown in Figs. 5 to 8;

Figs. 10 to 141- illustrate the use of this invention in connection with commutators of usual construction, Fig. 10 showing curves of the reversing electromotive force required in such cases, and of the flux producing this electromotive force, and Figs. 11, 12, 13 and 14C, showing different positions of brushes with relation to a commutator of usual construction;

Fig. 15 shows a general curve of the current collected from a commutator segment with reference to brush and commutator positions shown in Figs. 11 to 1%;

Fig. 16 shows an improvement of the arrangement described in connection with Figs. 1 to 9;

Figs. 17 and 18 show the method of applying a reversing electromotive force at any stage of the commutation process other than the beginning and the end;

Fig. 19 shows the curve of the current in a short circuited coil when arrangement of Figs. 17 and 18 is used; and

Figs. 20 to 2% illustrate various methods of generating the reversing electromotive force without a source external to the machine.

This invention can be successfully applied to any kind of commutator, as will be ex plained below, but the best results will be obtained by the use of a specially constructed commutator, shown in Figs. 1 and 2.

All axial commutators e. where the brush area extends in direction generally parallel to the shaft of the machine), whether of V ring construction, or shrink ring type, or any other construction, are, during process of manufacture, compressed in a strong metal ring, in order to form one solid body from the segments and insulating laminae interposed between segments. Fig. 1 represents the longitudinal section and Fig. 2 the end view of a commutator in this stage of manufacture. In' these figures, a is the metal ring, 1, 2, etc, are segments of copper or any conducting material, 5 are insulating laminae. In order to adapt the commutator to the use in connection with this invention, a circular groove (Z slightly inclined on the axis of commutator, as shown, is out out in the end opposite to the end which will be connected to the armature; next, a series of slots 6 in number equal to the number of commutator seg ments is milled in the same end of the commutator, their depth, width and position with reference to the insulation between segments being determined by the electrical features of the design, as will be explained below. The commutator then passes through the regular process of manufacture, as determined by its particular type, until it is assembled and firmly secured on its spider. Groove (Z and slots 0 are then filled with a suitable plastic insulating compound, to which some hard ingredient can be added, to give it desired abrasive properties; then the compound is submitted to such hardening process as its nature may require; finally, the working face of the commutator is finished smooth. The developed face of the commutator appears then as shown in Fig. It can be seen that by this process of construction the insulating compound is firmly anchored in the commutator.

The insulating compound filling slots 6 of the Figs. 1 and 2 forms what may be called

dead segments

1, 2, etc. in Fig. The commutator is shown in this figure as connected to a Gramme-ring armature, but it will be clear from desciption of invention that it is equally well applicable to drum or any other kind of armature, and to lap, wave or any other kind of armature winding. C is the main current collecting brush, preferably made of material having a comparatively low resistance of contact with the material of which the commutator segments are made. This brush can extend axially over the whole working face of the commutator and cover as many segments as required by the general featiilres of the design, as this invention does not prescribe any definite relation between the peripheral width of the main brush and the width of a segi'nent. D is an auxiliary brush preferably made of a material having comparatively high resistance of contact with the commutator segments. In the axial and peripheral directions, the dimensions of the brush I) should not exceed. the corresponding dimensions of a dead segment; it is clear, therefore, that the auxiliary brusn D never shortcircuits directly any of the armature coils. Arrow shows the direction of motion of the commutator relatively to the brushes; it can be seen, therefore, that the process of commutation of a coil is always completed by the auxiliary brush D.

A disk or rotor 9 made from a magnetic n'iaterial, preferably built up from thin iron laminations to the width required for the purpose, as hereinafter described, is keyed on the shaft of the machine in any convenient place. The rotor has projections 71. in the form of teeth and in number equal to the number of the commutator segments. A stator j of magnetic material, preferably built up from thin iron laminations and consisting of the yoke 7c and projections 71, 7c, is shaped and disposed with respect to the rotor, so as to produce the maximum variation of permeability in the magnetic circuit consisting of the stator yoke, projections 7,: and the rotor when the latter rotating with the shaft of the machine. Wound oh the yoke of the stator is a coil m in which tween the main brush C and the auxiliary brush D. A resistance r may be connected in series with the coil n. The line lead is connected to the main brush. When the rotor g is rotating, an alternating electromotive force is generated in the coil a; this electromotive force will be proportional to the exciting current flowing in m if the mag netic circuit is so proportioned that it will not become saturated with the highest possible yalues of current in m.

It is well known that sparking can occur both along the edge of the brush where commutator segments enter under it and along the edge where-segments leave it. Along the'leading edge of the segment an excessive current and energy density can be produced either by application of a too strong reversing electromotive force, when the current decreases at a too rapid rate, or by shortcircuiting the coilin a strong field of aid verse polarity, when the current at the beginning of commutation increases above its normal value; this usually occurs when the brushes are moved too close to the pole tips. ith brushes set on neutral. ant. in absence of any artificial means of commutation sparking never occurs along the leading edge of the segment, especially if the armature coils possess highselt induction, as in case of high voltage machines, to which this inventionis especially applicable. Therefore, in most cases it is suf'licient to introducethe reversing electromotive force to ward the end of the commutation period only, leaving the commutation process to follow its natural course at the beginning.

F lto 9 illustrate the application of this invention to such a case.

In Fig. i let F represent the total magnetic tl-ux'linked with the coil a (Fig. 3) as tunctionof time; E is the curve of derivatives of the curve F and can, therefore. be taken as representing the electromotive force induced in the coil n and acting betweenthe main brush C and the auxiliary brush D. As the number of teeth ot the rotor gis equal to the number ot comn'iutator segments, the period of the alternating electromotive force generated in the coil or is equal to the time during which the commutator rotates through anangle correspond ing to one segment. Considering now more particularly the moment when the auxiliary brush has just left the segment 1 (Fig. 5), the above mentioned period of the electromotive force E, immediately following this moment, can be decomposed into three parts as follows: First, auxiliary brush ison the dead segment 2-, main brush covers part oi": segment, 2; beginning and end of this part of the period are represented respectively D; the bulk of the in Figs. 5 and 6 ,(where, as well as in Figs. 7 and 8, the coil n and the rest of generating apparatus shown in Fig. 3 is omitted for simplicitys sake). Second, auxiliary brush and main brush both cover part of the ment 2; beginning and end of this part of the period are represented in Figs. 6 and 7.

Third, auxiliary brush covers part of seg-. ment 2; main brush has left this segment and covers part of segment 3; beginning and end of this part of the period are shown in Figs. 7 and 8. I

ssume now that the rotor g has been adjusted on the shaft so that the three parts of the period described above are made to correspond respectively to parts u, 2) and w of the elcctromotive force curve E Fig. 4. The action of the whole combination on the commutation of an armature coil 0 connecting

segments

2 and 3 will then be as follows: From the moment when the coil 0 is shortcircuited by the main brush to the moment when the auxiliary brush has just left segment 1 (Fig. 5), the process of commutation is not interfered with. During the part of the period represented by u, auxiliary brush is on the dead segment 91"; circuit of the coil u is open, and the current in the coil 0 still follows its natural course. of period corresponding to o, auxiliary brush and part of the main brush bear upon the same segment 2. Should the connection between the two brushes contain nothing but a resistance, then, with the main brush of low contact resistance, only a small part of would flow by the way of the auxiliary brush current would still flow through the main brush, and high final density and sparking under this brush mi ght result. .To prevent this the electromotivc force generated in the coil 41. and corresponding to the period c is introduced between the main and the auxiliary brushes and so directed as to tend to produce-in the connection between the brushes a. current of the same direction as that which is collected by the auxiliary brush from the segment 2. Considering the current produced by electromotive force F as independent from, and superposed upon, the currents existing in the short-circuited aii'mature coils. it can be seen that the total current due to E flows through the auxiliary brush, resistance r, and coil 71, but in the main brush it divides itself in severa. branches, one of which closes through the contact resistance of the main brush and the metal of tho segment'Q,

while all others close through thecontact During the partand resistances and reactances ot the, short-cirof the path through the segment 2, practically the whole current will flow through this segment. It is true that toward the end of this part of the period the contact resistance of the main brush with the segment 2 increases rapidly, but the electromotive force E is very small then, and lasts only a very short time, so that no apgreciable amount of current will pass in the path of the short-circuited cells. The action of the electromotiye force E is, therefore, such that the resultant density of current between main brush and segment 2 will be equal to the dil'li'erence oi? densities due to the current collected from the armature and the current circulating; in the local path through segment 2, while in the auxiliary brush the re sultant density will be equal to the sum of the above densities; in other words, the eftect oi the electromotivc force E is to transfer the current collected from segment 2 from the main brush to the auxiliary brush. The action can he further improved y insertion. of a sma i selt' ind action 1 (h in the circuit of the coil a, as will be explained below. During the part of: the pe riod 0.0 the segment 2 is in contact with the auxiliary bru'a only. i lloctrmnotiye force l) is now reve d in direction and assists the reversal of the current in the sl1ort-eircuited coil 0," it can be so adjusted as to produce zero density at the moment when the segment leaves the auxiliary brush. perfectly sparhless commutation is thereby obtained.

The action of my device is illustrated in 9 shoaing the current 2'75 collected from segment 2 during; period a o w in function oil time. Curve IPlii shows the current for the commutation without auxiliary brush. Under ordinary circumstances spar-lo inp; would occur in M \"vhere the density represented as infinitely great, tangent to the curve 270 being ierpendicular to the axis of time. Curve l-l I-L-hil shows the current 2 70 in the case when an auxiliary brush is used, but without any eectromotive force between this brush and the main brush.

Curve IKL-l.i represents the case when an electromotire force E is used and adjusted to give Zero density at the moment when the auxiliary brush lcayes -he segment 2; the curve 270 is then tangent to the axis of time in M. 4

If the specific contact resistance of the main brush is much lower than that of the auxiliary brush, then the current density between the main brush and segment 2, due to the armature current, rapidly increases toward the end 01": the period 1). In order to compensate it by means of a current set up in. the opposite direction by the coil n it would be necessary to generate an electromotive force E rising to a considerable value in an exceedingly short time. Now it is diticult to do this by means of a generator such as described in Fig. 3; it is, therefore, preferable to insert a certain amount of selfinduction (shown as coil 1 in Fig. 3) in the circuit of the coil a; then at the end. the period c the high density due to the current from the armature will be counterbalanced by the discharge current due to the energy stored in self induction 1. This latter, oi course, can be incorporated in the coil n or the resistance 0'. As explained above, the impedance of the armature will prevent this discharge current from flowing through the short-circuited coils rather than through the segment 2.

It is well known that by Virtue of the law of induction the area. of the curve of E (Fig. l) above the axis of time must be equal to the area below this axis; on the other hand, the reversing eleetromotive force during time '10 must be considerably greater than E during the time 0). One way to obtain suitable values for both is to extend the inte .vals u and o and shorten the interval to, but this can be done only at the expense oi some desirable features of electrical de- Another way of solyinp; the difficulty is to design the generator of the reversing electromotire force with very poor regulation;- it will be noted that it acts generator, delivering a current, only during interval o; during; 10 it acts as a motor, receiving the current. During o, s soon as the brush contact surfaces become suilicient, a fairly large current can be allowed to flow between the main brush and auxiliary brush inside of the segment 2gif the regulation of the a 'iparatus is poor, then, to equal upper and lower areas oi? the curve R will correspend very unequal areas of the curve E re resenting the terminal voltage or" the apparatus, 0., the voltage directly applied to If a resistance '1" is connected in series h the generator, the ohmic drop will still further diminish E" during Q0, thus increasing the effect ol the poor regulation oi": the generator. As the amount of energy involved in the commutation process is very small, the loss in Q will be en tirely n eligible, the more so because it occurs outside of the commutator and does not affect the heating of the machine.

there must he as many stators 7' spaced around the disk 9 (Fi a 3), as there are brush sets in the machine; but, it the winding so designed that armature coils are always in the same phase of the commutation process under several sets of brushes, then the number or stators y can be reduced, each stator arryinp; several independent coils l-lowever, in many cases it will be found of adrantage to have a special stator j for each brush set, and excite the corresponding coil or by the current, or the frac is clear that in the most general case tion of the current, of this brush set only, instead of the full armaturecurrent. The electromotive force E will then be always proportional to the current which it is sup: posed to reverse, regardless of unequal distribution of current between different brush sets; the numberof equalizing connections can thenbe reduced, or they can even be dispensed with entirely.

It is well known that, in order to obtain maximum output from a given size of a1 former action in the short-circuited coils of alternating current commutator motors. Sparking which results from circulating curren s set up by these electromotive forces cannot be prevented by commonly used com: mutating devices, such as commutating poles; second: with several coil sides concent 'ated in the same slot the elements determining the value of the'reversing electromotive force, such as self induction, mu-.

tual induction, etc, are different for different positions of the coil in the slot; therefore, in most cases, the best that can be done is to adjust the reversing electromotive force'to an average or a maximum value;

it frequently happens thenthat commutator segments are blackened at regular i11- tervals. In accordance with this invention, however, to each armature coil corresponds a special projection 71. (Fig. 3) it is, therefore, possible to adjust the reversing electromotive force to suit the requirements of each individual coil by proper selectionof the length and shape of the air gaps in the generating apparatus of Fig. 3, or by any:

other method of controlling the. rate of change of flux linked with coils a. The limit of sparkless commutation will thenbe considerably extended.

It has been assumed above that the disk 9,

was rotating at the same speed as the comniutator; the number of projections h (Fig.

8) was then made equal to the number of commutator segments. In case of slow speed machines the shaft of the disk can be geared to the shaft of the commutator: the

number of projections it will then bearto the number of commutator segments the same ratio as the speed of the commutator shaft to that of the disk shaft. It is clear that the advantage resulting from assigninga specially shaped tooth to each individual armature coil can be retained by suitable selection of ratio of speeds of the commutator and the disk.

Such as described above, the application of this invention involves the use of a spe cially' constructed commutator. It is possible to retain most of its advantages in connection with commutators of usual construc tion, 'such as the one shown indevelopment in Figs. ll to 14, z. e., in which segments are separated from each other by thin insulating laminae s of uniform thickness. This can be accomplished in the following way: current collecting apparatus consists, as before, of a main brush C (Figs. 11 to 14) and an auxiliary brush D connected by a COll'fl in which an alternating electromosuch as shown in Fig. 3. A resistance r inductive or not, can be connected in series with n if desired; external current is taken 1 from the brush by means of a lead Selection of brush properties and relative poance. Main and auxiliary brushes may extend over the whole working face of the commutator and must be set at such distance from each other that at the moment when the segment 2" is leaving the main brush, the auxiliary brush has already completed the commutation of the coil 0 and'bears V entirely on segment 2 Then the time during which the short-circuited coil 0? is subject to the action of the commutating device can be divided in three parts: first; designated by v in Fig; 10, during which the auxiliary brush bears wholly on the seg, ment 2, a portion. of the main brush also covering this segment; Figsll and 12 show relative positions of the commutator and brushes at the'beginning and end of this period; second; intervalw during whlch only auxiliary brush is in contact with the segment 2 and this segment only; Figs. 12 and 13 represent the beginning and end of this period; third; interval a during which the auxiliary brush is in contact with. seg

ments 2 and beginning and end represented in Figs. 13 and 14:.

Beginning now with the moment when the segment 1 has just left the auxiliary brush (commutator moving with relation to the brushes in direction of the arrow f), the action will be'as follows: during-period o the action will be exactly as described'in connection with a commutator with dead segments, Fig. 3; electromotive force IE will tive force E is generated by variation .of flux F (Fig. 10) 1n a suitable generator,

be made of material of high contact resistsimply transfer the armature current from rapidly to such value asto completely reverse the current in the short-circuited coil 0 and, preferably, make it to reach a value somewhat in excess of normal armature current, as shown at N in Fig. 15, which represents the variation of current '17: collected from the segment 2 during the time o +w +u After reaching its maximum value, the electromotive force E rapidly decreases and remains practically zero during the short period u when the coil 0 is shortcircuited directly by the auxiliary brush. As the contact resistance of the latter is a.- sumed to be high, and the current in the coil 0 has already very nearly its final value, no sparking can result from this direct short-circuit oi? the coil.

Thus a sparkless commutation can be ob taincd even while it is impossible to introduce a reversing electromotive force in the circuit of a short-circuited coil at the precise moment of termination or" the commutation process.

One of the most important advantages derived from the use of any satisfactory device for improving the commutation is economy effected by using fewer commutator segments and stronger armature, 6., with more ampere-conductors along the periphery. These conditions lead to designs with high self induction per coil, and the commutation is liable to be somewhat sluggish during that part of the commutation process when no external means of assisting it are used. The reversal of the current will then take place toward the end of the short-circuit; this will require considerable care in adjusting the electromotive force E to its proper value and wave shape; also the distribution of current under the brush will be far from uniform, and a part of the main brush will be more or less useless for collection of the current. From another point of view; it is well known that as long as the current in the short-circuited coils follows straight line law of variation, it has no effect on the main field, action of the current flowing in one direction being counterbalanced by the action of the current flowing in the opposite direction; but if the commuted current either always ez-zceeds the ideal straight line value, or is always below it, it begins to react on the main field in such a way that in case of under commutation the main'field of a motor is strengthened, while in -a generator it is weakened. This action is 'ather useful in a motor, because it makes the operation stable and prevents hunting, especially in motors built for adjustable speed duty; but it is in every respect detrimental in a generator. rm these reasons, in many cases it may be of advantage to accelerate the reversal of the current not only at the end, but also at the beginning or at any other part of the commutation process. This invention can be easily applied to such cases. It the special commutator of Fig. 3 is used, the reversing eleetromotive force can be introduced at the beginning of the commutation period by an arrangement shown in Fig. 16. It is similar to that of Fig. 3, but a second auxiliary brush D covering less than a dead segment, is added; it is connected to the main brush C by a resistance 1"", inductive or not, and by a coil 71. in which a suitable alternating electromotive force is generated by means of an apparatus such as shown in Fig. Connections and proportions of the other auxiliary brush D are similar to those oi? the brush D of the Fig. 3. Line lead Q2 is attached to the main brush C The action of the brush 1) is, briefly, as follows: during the interval of time when the main brush C and the auxiliary brush D are on the adjacent segments, as shown in Fig. 16, the electromotive force in the coil or gives a sharp impulse in the direction oi? the reversal of the current in the short-circuited coil 0 When, immediately afterward, brushes and D touch the same egment 2 the electromotive force in a, acting now in the opposite direction, is used for transferring the current from the zuixiliary to the main brushso as to gradu ally annul the current in the auxiliary brush, ratory to its entering pon the dead the case of commutators of usual con struction, without dead segments, this invention can be applied in a similar way, except that the reversing electromotive force 'ai'inot be introduced in the circuit of a shortcircuited coil at the precise moment of the beginning or the end of the commutation process, but it can be introduced at any other moment during this process, and as closely as desired to the beginning or the end. Figs. 17 and 18 show arrangement by means of which a reversing electromotive force acts twice upon a short-circuited coil while it is und oing commutation. The brush consists oi: three parts, C C G which, for simplicitys sake, are represented as equidistant and at the same width-less than the width of a segment-although many other arrangements and proportions can be conceived, all based on the principle explained below. the only limitation being that the distance between any two consecutive brushes mustbe less than the width of a segment. External current is taken from the brush C by the lead 5 Every two consecutive brushes are connected by a coil, such as n and a. Resistances and self inductions can be used in series with these coils, or with the brushes, if desired, although, in this case, it is preferable to use a generator with poor regulation to avoid losses due to the main current flowing in resistances. Alternating electromotive forces of suit-able phases, wave shapes and intensities are induced in coils n and 12. Action is as follows: while brushes occupy positions as in Fig. 17, z. 6., when two consecutive brushes are separated by an insulation between the segments, electromotive forces in coils n-iand u give an impulse in the direction ofthe reversal of the commuted current, the values of these electromotive forces being preferably considerably in excess of the sum of all injurious electromotive forcesopposing the commutation in respective coils.- When brushes are in a position such as in Fig. 18, i. 6., when coils n and 12- are shunted by commutator segments, forces, acting in the opposite directions, help to transfer the current from one brush to the other. Electromotive forces are preferably arranged so as to have a large interval of Zero value, during which the commutation is completed without external assistance, the commuted current having reached nearly its final value. This also precludes the possibility of an electromotive force of wrong direction being applied to the coil because of an imperfect contact between the brush and the commutator.

In Fig. 19,HG is the curve of the coinmuted current during a complete commutation period T of a coil: current passes from a value +ia to the value 'ia; parts of the curve marked H correspond to positions such shown in Fig. 18; parts marked G to positions such as shown in Fig. 17. J shows the ideal straight line commutation curve. It can be seen thus, that. a commutation closely similar to the straight line can be obtained, so that currents in the short-circuited coils will have little or no influence on the main field- It is clear that instead. of three brushes, any number could be used in the same. way, {giving correspondingly greater number of reversingimpulses during commutation pc- In difficult cases of commutation best results can be obtained by the use of arrangeriod.

ments similar to Fig. 3 or Fig. v16, but with main brush subdivided, as in Figs. 17 and 18. In all preceding descriptions of various methods of applying this invention, it has been assumed that the alternating; electromotive force was generated in anapparatus similar to the one shown in Fig. 3. It must be understood. however, that any kind of apparatus generating an alternating electromotive force of suitable frequency, wave shape and intensity can be used.

Investigation of the commutation process of a normally proportioned machine shows that the energy output of agenerator producing the elect-romotive force used in the way described above, is exceedingly small the electromotive.

Fig". 20. already comparatively to theoutput of the machine itself. In many cases, therefore, it will be possible to do away entirely with a separate generator and produce the electromotive force E in the machine itself. This can be accomplished by introducing a variable resistance in a. circuit acted upon by an elecobtained by variation of contact area according to a predetermined law, combined. it

necessary, with variation of specific contact "csistance of materials along the area of contact.

An arrangement of this kind is shown in A is a commutator with dead segments similar to the one described with reference to F ig. 3. l) are two brushes set approximately symmetrically with reference to the center line Po-Po midway between poles N and S; thesebrushes may consist of ahomog'eneous material all along the area of contact, or they may be composed of materials of different specificv contact resist-- anccs, such as (Z and (Z in Fig. 20; part (Z is made of a high contact resistance material, such as carbon; part (l is made of a. low contact resistance material, such as copper or copper carbon combination; the total variation of resistance is thereby greatly increased. Current collecting brushes and auxiliary brushes. which would be set on the line PoPo and could be of any variety described above,are, for the sake of clearn'ess, omitted in Fig. 20.

It s clear that with the brushes D" located as above, the electromotive force generated between these brushes depends only on the armature current, and will be practically proportional to it if the brushes D are set so as to embrace only non-saturated teeth. This electromotive force acts on the coil. m through arcsistance r"; the latter can be so adjusted as to give any desired value to the time constant of the circuit consisting of the coil m resistance 1 contact resistance of brushes D* and impedance of the armature winding bctweenthe brushesD. It will be found that in normally proportioned dynamo-elcctric machines the time constant of this circuit can be made very small without excessive loss; an excessive damping by the self induction can thus be avoided. Coil m the same slot the. electromotive force in a" vided in two parts C and can be made to suit each individual coil in the slot by varying" the peripheral width of dead segments. As a rule, current taken "from this device will be too small to cause sp t-riding under brushes l).

A similar device can be used in connection with a commutator of the usual type, 2'. 6. without dead segments. It is based on the following principle: it is well known that except in the case of straight line commutation, the distribution of the current along the brush is not uniform. but can be inn aginedas a superposition of a. uniformly distributed current, equal to the current delivered by the brsh to the brush stud, (called main current hereinafter) and of circulating; currents flowing in the local circuits consisting of the short-circuited coils.

' contact resistances of the brush and the body of the brush; at no load and with brushes on magnetic neutral between the poles these circulating currents depend on the strength of field produced by the armature and on the salt-induction of the short-circuited coils; they are, therefore, proportional to the an mature current: rith the brushes shifted from magnetic neutral toward either pole. they depend also on the strength of the field produced by the poles.

Suppose now that the main brush is di- J, Figs. 21 and 22 connected by a resistance r and a coil 022?. Current collected from the commutator is conveyed to the brush stud by conductor q attached to the brush C. Figs. 9 1 and 22 represent two extreme positions of the brushes C and (l with reference to the commutator, supposed to be rotatingin direction ot the error; f; in Fig. :21 brushes cover two adjacent segments 52 and 3. It is clear that in Fig. 91. the part of the main and the circulating currents corresponding to the brush C flow toward. or awa from the conductor and the. liuush C respec tively through the coil m and, resistance but in the position shown in Fig. 22 this path is shunted. by the double contact resistance of brushes C and with the segment 8. Current in the coil m ill, therefore, pulsate periedically and can be utilized for generating by induction an alternating electromotive force in a coil a which connects the brush C with an auxiliary brush thus assisting the commutation in the way described in connection with Figs. 11 to ll. t is clear that a brush for assisting the commutation at the beginning of comn'uitation process could also be used, alone or in C011- nection with D The elliciency of this device, as well of that shown in Fig. 20 can be materially improved by connecting acondenser 0 (Figs. 21 and 22) across the terminals of the coil m; with suitable wave shape of the current in the coil m an exchange of energy between brush the condenser and the self-induction of the coil will be set up, through medium of a current circulating in the circuit of the coil m and the condenser, but not passing through the resistance r"; pulsating iux set up by the coil m will be thus intensified without any loss in resistance.

lly varying; the relative size of brushes C and. C any desired traction oi the main current can be made to flow in the coil m The law of variation of the current in m can be regulated by giving an appropriate shape to th areas of contact of brushes C and C with the segment 3.

This device has the advantage of being not sensitive to the brush position, because a wrong position of brush intensifies the circulating currents and, therefore, the reversing electromotive force. When electromotive "forces of dili'erent phases or different wave shapes are desired, the main brush can be divided in more than two parts, every two consecutive parts being connected by coils such as 077;.

A simple calculation shows that the current density under brush C and segment 3* has always a finite value as long as the specific contact rcsisance otlf this brush is dilifercnt from zero; excessive current density and sparking under the brush C can therefore, be always eliminated.

lTi 2 :3 and 24; show a method of applying" my invention especially suitable for machines in which the strength of combined main pole and armature reaction lield is approximately proportional to the armature current, as in series wound traction motors. C is main. current collectiirc" brush, ot any desired width; D and D are auxiliary brushes, of the width less than a. segment, and made of a material of equal or higher contact resistance than the main brush. These auxiliary brushes are connected to the main brush respectively by coils n and on" which are in inductive relation to each other. For the direction of rotation indicated by the row 1'' an armature coil begins the COH1I11U tation process under the brush l) and com pletcs it under the brush 1). Current collected from the brushes is carried to the stud by conductor g attached to the main brush. The action of this combination is as follows:

During the time when the commutator moves from the position of Fig. 2 to that of l ipg. 245; the armature coil 0 is short-circuitcd by the coil m; current in the latter incre ses from a minimum value to a certain IDGXUOTUTI value at the end of this period; after that coil m" is shunted by the segment 3 and the current decreases to the minimum value, current collected. from the connector .2 passing from the auxiliary brush to the main brush Without external assistance, because the specilic resistance of the auxiliary brush is at least as high as that of the main brush. A unidirectional pulsating current will flow in the coil m and if the coils m and n are so proportioned that the action of m is predominant, an electromotive force will be generated in n which can be used for completing the commutation of another armature coil 0 in the same way as has been explained in connection with Figs. 11 to 14.

Within its proper sphere of application this arrangement is an electrical equivalent of improving the commutation by shifting the brushes, inasmuch as the reversing electromotive force in the coil a depends (indirectly) on the strength of the field ahead of the main brush, same as in motors with brushes shifted against rotation; yet, the

line of symmetry of the main brush can remain on the magnetic neutral, and the advantage of reversibility is not lost; it is only necessary to bring the terminals of the coils m and n -either directly, or by means of transformers-to the reversing drum of the controller, so as to reconnect them when the direction of rotation of the motor is reversed. Another advantage is the possibility, or even the advisability of using a large polar arc, which leads to an economical design. a 1

Of course, for a correct application of this arrangement, it is necessary that the electromotive force induced in the armature coil 0 while it is short-circuited through the coil m" has such direction as to assist the reversal of the current in this coil; that is, in case of a motor, the field of the main poles must be stronger than the armature field in the region where the armature coil 0 is located during this interval.

It is obvious that various changes may be made in the details of construction and arrangement of parts, within the scope of the claims, without departing from the spirit of this invention. It is, therefore, to be understood thatthis invention is not to be'limited to the specific details and arrangements shown and described.

Having thus described the invention, what is claimed is:

1. In a dynamo-electric machine, an armature having armature coils, a commutator connected to said armature coils, brushes bearing on said commutator, and means for applying an alternating electromotive force between said brushes, said parts being con structed and arranged that when the applied electromotive force acts. in a directionsuitable for reversal of the current in the short-circuited coil, the circuit containing the electromotive force closes through said coil, while when the electromotive force acts in a direction opposing the reversal of the current, the circuit containing the electromotive force is shunted by a commutator segment, for the purpose set forth.

2. In a dynamo-electric machine, an armature having armature coils, a commutator connected to said armature coils, sets of brushes bearing on said commutator, the adj acent brushes of a set being spaced peripherally less than the width of a commutator segment, and means for applying an alternating electromotive force between the brushes ofa set, the electromotive force acting in one direction while the brushes of a set between which the electromotive force is applied are separated by the insulation between adjacent segments, and acting in the opposite direction while said brushes bear on the same segment.

3. In a dynamo-electric machine, an armature having armature coils, a commutator connected to said armature coils, sets of brushes bearing on said commutator, the adjacent brushes of a set being spaced peripherally less-than the width of a commutator segment, means for applying an alternating electromotive force between the brushes of a set, the electromotive force acting in one direction while the brushes of a set between which said electromotive force is applied are separated by the insulation between adjacent segments, and acting in the opposite direction while said brushes bear on the same segment, and a resistance and a reactance in circuit with said brushes and said means.

4. In a dynamo-electric machine, an armature having armature coils, a commutator connected to said armature coils, sets of brushes bearing on said commutator, a con ducting connection between certain of said brushes, and a conducting connection between certain of said brushes and in inductive relation with said first conducting connection, and resistances and reactances in one of said conducting connections, said first conducting connections being arranged so as to be intermittently shunted by the commutator segments, for the purpose set forth.

5. In a dynamo-electric machine, an armature having armature coils, a commutator connected to said armature coils, sets of brushes bearing on said commutator, a conducting connection between certain of said brushes, and a conducting connection between certain of said brushes and in inductive relation with said first conducting connection, and resistances and reactances in series with and capacities in shunt with one of said conducting connections, said first conducting connections being arranged so as to be intermittently shunted by the commutator segments, for the purpose set forth.

In testimony whereof I afiix my signature this 17th day of April, 1916.

' JAROSLAW K. KOSTKO.