Fig. 68, 69.Fig. 68.Fig. 69.
Adesignates the field-magnet or magnetic frame of the motor;B B, oppositely located pole-pieces adapted to receive the coils of one energizing circuit; andC C, similar pole-pieces for the coils of the other energizing circuit. These circuits are designated, respectively, byD E, the conductorD''forming a common return to the generatorG. Between these poles is mounted an armature—for example, a ring or annular armature, wound with a series of coilsF, forming a closed circuit or circuits. The action or operation of a motor thus constructed is now well understood. It will be observed, however, that the magnetism of polesB, forexample, established by a current impulse in the coils thereon, precedes the magnetic effect set up in the armature by the induced current in coilsF. Consequently the mutual attraction between the armature and field-poles is considerably reduced. The same conditions will be found to exist if, instead of assuming the polesBorCas acting independently, we regard the ideal resultant of both acting together, which is the real condition. To remedy this, the motor field is constructed with secondary polesB' C', which are situated between the others. These pole-pieces are wound with coilsD' E', the former in derivation to the coilsD, the latter to coilsE. The main or primary coilsDandEare wound for a different self-induction from that of the coilsD'andE', the relations being so fixed that if the currents inDandEdiffer, for example, by a quarter-phase, the currents in each secondary coil, asD' E', will differ from those in its appropriate primaryDorEby, say, forty-five degrees, or one-eighth of a period.
Now, assuming that an impulse or alternation in circuit or branchEis just beginning, while in the branchDit is just falling from maximum, the conditions are those of a quarter-phase difference. The ideal resultant of the attractive forces of the two sets of polesB Ctherefore may be considered as progressing from polesBto polesC, while the impulse inEis rising to maximum, and that inDis falling to zero or minimum. The polarity set up in the armature, however, lags behind the manifestations of field magnetism, and hence the maximum points of attraction in armature and field, instead of coinciding, are angularly displaced. This effect is counteracted by the supplemental polesB' C'. The magnetic phases of these poles succeed those of polesB Cby the same, or nearly the same, period of time as elapses between the effect of the polesB Cand the corresponding induced effect in the armature; hence the magnetic conditions of polesB' C'and of the armature more nearly coincide and a better result is obtained. As polesB' C'act in conjunction with the poles in the armature established by polesB C, so in turn polesC Bact similarly with the poles set up byB' C', respectively. Under such conditions the retardation of the magnetic effect of the armature and that of the secondary poles will bring the maximum of the two more nearly into coincidence and a correspondingly stronger torque or magnetic attraction secured.
In such a disposition as is shown in Fig. 68 it will be observedthat as the adjacent pole-pieces of either circuit are of like polarity they will have a certain weakening effect upon one another. Mr. Tesla therefore prefers to remove the secondary poles from the direct influence of the others. This may be done by constructing a motor with two independent sets of fields, and with either one or two armatures electrically connected, or by using two armatures and one field. These modifications are illustrated further on.
Fig. 70, 71.Fig. 70.Fig. 71.
Fig. 69 is a diagrammatic illustration of a motor and system in which the difference of phase is artificially produced. There are two coilsD Din one branch and two coilsE Ein another branch of the main circuit from the generatorG. These two circuits or branches are of different self-induction, one, asD, being higher than the other. This is graphically indicated by making coilsDmuch larger than coilsE. By reason of the difference in the electrical character of the two circuits, the phases of current in one are retarded to a greater extent than the other. Let this difference be thirty degrees. A motor thus constructed will rotate under the action of an alternating current; but as happens in the case previously described the corresponding magnetic effects of the armature and field do not coincide owing to the time that elapses between a given magnetic effect in the armature andthe condition of the field that produces it. The secondary or supplemental polesB' C'are therefore availed of. There being thirty degrees difference of phase between the currents in coilsD E, the magnetic effect of polesB' C'should correspond to that produced by a current differing from the current in coilsDorEby fifteen degrees. This we can attain by winding each supplemental poleB' C'with two coilsH H'. The coilsHare included in a derived circuit having the same self-induction as circuitD, and coilsH'in a circuit having the same self-induction as circuitE, so that if these circuits differ by thirty degrees the magnetism of polesB' C'will correspond to that produced by a current differing from that in eitherDorEby fifteen degrees. This is true in all other cases. For example, if in Fig. 68 the coilsD' E'be replaced by the coilsH H'included in the derived circuits, the magnetism of the polesB' C'will correspond in effect or phase, if it may be so termed, to that produced by a current differing from that in either circuitDorEby forty-five degrees, or one-eighth of a period.
This invention as applied to a derived circuit motor is illustrated in Figs. 70 and 71. The former is an end view of the motor with the armature in section and a diagram of connections, and Fig. 71 a vertical section through the field. These figures are also drawn to show one of the dispositions of two fields that may be adopted in carrying out the principle. The polesB B C Care in one field, the remaining poles in the other. The former are wound with primary coilsI Jand secondary coilsI' J', the latter with coilsK L. The primary coilsI Jare in derived circuits, between which, by reason of their different self-induction, there is a difference of phase, say, of thirty degrees. The coilsI' Kare in circuit with one another, as also are coilsJ' L, and there should be a difference of phase between the currents in coilsKandLand their corresponding primaries of, say, fifteen degrees. If the polesB Care at right angles, the armature-coils should be connected directly across, or a single armature core wound from end to end may be used; but if the polesB Cbe in line there should be an angular displacement of the armature coils, as will be well understood.
The operation will be understood from the foregoing. The maximum magnetic condition of a pair of poles, asB' B', coincides closely with the maximum effect in the armature, which lags behind the corresponding condition in polesB B.
It is well known that if a magnetic core, even if laminated or subdivided, be wound with an insulated coil and a current of electricity be directed through the coil, the magnetization of the entire core does not immediately ensue, the magnetizing effect not being exhibited in all parts simultaneously. This may be attributed to the fact that the action of the current is to energize first those laminæ or parts of the core nearest the surface and adjacent to the exciting-coil, and from thence the action progresses toward the interior. A certain interval of time therefore elapses between the manifestation of magnetism in the external and the internal sections or layers of the core. If the core be thin or of small mass, this effect may be inappreciable; but in the case of a thick core, or even of a comparatively thin one, if the number of alternations or rate of change of the current strength be very great, the time interval occurring between the manifestations of magnetism in the interior of the core and in those parts adjacent to the coil is more marked. In the construction of such apparatus as motors which are designed to be run by alternating or equivalent currents—such as pulsating or undulating currents generally—Mr. Tesla found it desirable and even necessary to give due consideration to this phenomenon and to make special provisions in order to obviate its consequences. With the specific object of taking advantage of this action or effect, and to render it more pronounced, he constructs a field magnet in which the parts of the core or cores that exhibit at different intervals of time the magnetic effect imparted to them by alternating or equivalent currents in an energizing coil or coils, are so placed with relation to a rotating armature as to exert thereon their attractive effect successively in the order of their magnetization. By this means he secures a result similar to that which he had previously attained in other forms or types of motor in which by means of one or more alternating currents he has produced the rotation or progression of the magnetic poles.
This new mode of operation will now be described. Fig. 72 is a side elevation of such motor. Fig. 73 is a side elevation of a more practicable and efficient embodiment of the invention. Fig. 74 is a central vertical section of the same in the plane of the axis of rotation.
Fig. 72 and 73.Figs.72 and 73.
Referring to Fig. 72, letXrepresent a large iron core, which may be composed of a number of sheets or laminæ of soft iron or steel. Surrounding this core is a coilY, which is connected with a sourceEof rapidly varying currents. Let us consider now the magnetic conditions existing in this core at any point, asb, at or near the centre, and any other point, asa, nearer the surface. When a current impulse is started in the magnetizing coilY, the section or part ata, being close to the coil, is immediately energized, while the section or part atb, which, to use a convenient expression, is "protected" by the intervening sections or layers betweenaandb, does not at once exhibit its magnetism. However, as the magnetization ofaincreases,bbecomes also affected, reaching finally its maximum strength some time later thana. Upon the weakening of the current the magnetization ofafirst diminishes, whilebstill exhibits its maximum strength;but the continued weakening ofais attended by a subsequent weakening ofb. Assuming the current to be an alternating one,awill now be reversed, whilebstill continues of the first imparted polarity. This action continues the magnetic condition ofb, following that ofain the manner above described. If an armature—for instance, a simple discF, mounted to rotate freely on an axis—be brought into proximity to the core, a movement of rotation will be imparted to the disc, the direction depending upon its position relatively to the core, the tendency being to turn the portion of the disc nearest to the core fromatob, as indicated in Fig. 72.
Fig. 74.Fig. 74.
This action or principle of operation has been embodied in a practicable form of motor, which is illustrated in Fig. 73. LetAin that figure represent a circular frame of iron, from diametrically opposite points of the interior of which the cores project. Each core is composed of three main partsB,BandC, and they are similarly formed with a straight portion or bodye, around which the energizing coil is wound, a curved arm or extensionc, and an inwardly projecting pole or endd. Each core is made up of two partsB B, with their polar extensions reaching in one direction, and a partCbetween the other two, and with its polar extension reaching in the opposite direction. In order to lessen in the cores the circulation of currents induced therein, the several sections are insulated from one another in the manner usuallyfollowed in such cases. These cores are wound with coilsD, which are connected in the same circuit, either in parallel or series, and supplied with an alternating or a pulsating current, preferably the former, by a generatorE, represented diagrammatically. Between the cores or their polar extensions is mounted a cylindrical or similar armatureF, wound with magnetizing coilsG, closed upon themselves.
The operation of this motor is as follows: When a current impulse or alternation is directed through the coilsD, the sectionsB Bof the cores, being on the surface and in close proximity to the coils, are immediately energized. The sectionsC, on the other hand, are protected from the magnetizing influence of the coil by the interposed layers of ironB B. As the magnetism ofB Bincreases, however, the sectionsCare also energized; but they do not attain their maximum strength until a certain time subsequent to the exhibition by the sectionsB Bof their maximum. Upon the weakening of the current the magnetic strength ofB Bfirst diminishes, while the sectionsChave still their maximum strength; but asB Bcontinue to weaken the interior sections are similarly weakened.B Bmay then begin to exhibit an opposite polarity, which is followed later by a similar change onC, and this action continues.B BandCmay therefore be considered as separate field-magnets, being extended so as to act on the armature in the most efficient positions, and the effect is similar to that in the other forms of Tesla motor—viz., a rotation or progression of the maximum points of the field of force. Any armature—such, for instance, as a disc—mounted in this field would rotate from the pole first to exhibit its magnetism to that which exhibits it later.
It is evident that the principle here described may be carried out in conjunction with other means for securing a more favorable or efficient action of the motor. For example, the polar extensions of the sectionsCmay be wound or surrounded by closed coils. The effect of these coils will be to still more effectively retard the magnetization of the polar extensions ofC.
It will have been gathered by all who are interested in the advance of the electrical arts, and who follow carefully, step by step, the work of pioneers, that Mr. Tesla has been foremost to utilize inductive effects in permanently closed circuits, in the operation of alternating motors. In this chapter one simple type of such a motor is described and illustrated, which will serve as an exemplification of the principle.
Let it be assumed that an ordinary alternating current generator is connected up in a circuit of practically no self-induction, such, for example, as a circuit containing incandescent lamps only. On the operation of the machine, alternating currents will be developed in the circuit, and the phases of these currents will theoretically coincide with the phases of the impressed electromotive force. Such currents may be regarded and designated as the "unretarded currents."
It will be understood, of course, that in practice there is always more or less self-induction in the circuit, which modifies to a corresponding extent these conditions; but for convenience this may be disregarded in the consideration of the principle of operation, since the same laws apply. Assume next that a path of currents be formed across any two points of the above circuit, consisting, for example, of the primary of an induction device. The phases of the currents passing through the primary, owing to the self-induction of the same, will not coincide with the phases of the impressed electromotive force, but will lag behind, such lag being directly proportional to the self-induction and inversely proportional to the resistance of the said coil. The insertion of this coil will also cause a lagging or retardation of the currents traversing and delivered by the generator behind the impressed electromotive force, such lag being the mean or resultant of the lag of the current through the primary alone and of the "unretarded current" in the entire working circuit. Nextconsider the conditions imposed by the association in inductive relation with the primary coil, of a secondary coil. The current generated in the secondary coil will react upon the primary current, modifying the retardation of the same, according to the amount of self-induction and resistance in the secondary circuit. If the secondary circuit has but little self-induction—as, for instance, when it contains incandescent lamps only—it will increase the actual difference of phase between its own and the primary current, first, by diminishing the lag between the primary current and the impressed electromotive force, and, second, by its own lag or retardation behind the impressed electromotive force. On the other hand, if the secondary circuit have a high self-induction, its lag behind the current in the primary is directly increased, while it will be still further increased if the primary have a very low self-induction. The better results are obtained when the primary has a low self-induction.
Fig. 75, 76.Fig. 75.Fig. 76.
Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76 is a similar diagram of a modification of the same. In Fig. 75 letAdesignate the field-magnet of a motor which, as in all these motors, is built up of sections or plates.B Care polar projections upon which the coils are wound. Upon one pair of these poles, asC, are wound primary coilsD, which are directly connected to the circuit of an alternating current generatorG. On the same poles are also wound secondary coilsF, either side by side or over or under the primary coils, and these are connected with other coilsE, which surround the polesB B.The currents in both primary and secondary coils in such a motor will be retarded or will lag behind the impressed electromotive force; but to secure a proper difference in phase between the primary and secondary currents themselves, Mr. Tesla increases the resistance of the circuit of the secondary and reduces as much as practicable its self-induction. This is done by using for the secondary circuit, particularly in the coilsE, wire of comparatively small diameter and having but few turns around the cores; or by using some conductor of higher specific resistance, such as German silver; or by introducing at some point in the secondary circuit an artificial resistanceR. Thus the self-induction of the secondary is kept down and its resistance increased, with the result of decreasing the lag between the impressed electro-motive force and the current in the primary coils and increasing the difference of phase between the primary and secondary currents.
In the disposition shown in Fig. 76, the lag in the secondary is increased by increasing the self-induction of that circuit, while the increasing tendency of the primary to lag is counteracted by inserting therein a dead resistance. The primary coilsDin this case have a low self-induction and high resistance, while the coilsE F, included in the secondary circuit, have a high self-induction and low resistance. This may be done by the proper winding of the coils; or in the circuit including the secondary coilsE F, we may introduce a self-induction coilS, while in the primary circuit from the generatorGand including coilsD, there may be inserted a dead resistanceR. By this means the difference of phase between the primary and secondary is increased. It is evident that both means of increasing the difference of phase—namely, by the special winding as well as by the supplemental or external inductive and dead resistance—may be employed conjointly.
In the operation of this motor the current impulses in the primary coils induce currents in the secondary coils, and by the conjoint action of the two the points of greatest magnetic attraction are shifted or rotated.
In practice it is found desirable to wind the armature with closed coils in which currents are induced by the action thereon of the primaries.
In the preceding descriptions relative to synchronizing motors and methods of operating them, reference has been made to the plan adopted by Mr. Tesla, which consists broadly in winding or arranging the motor in such manner that by means of suitable switches it could be started as a multiple-circuit motor, or one operating by a progression of its magnetic poles, and then, when up to speed, or nearly so, converted into an ordinary synchronizing motor, or one in which the magnetic poles were simply alternated. In some cases, as when a large motor is used and when the number of alternations is very high, there is more or less difficulty in bringing the motor to speed as a double or multiple-circuit motor, for the plan of construction which renders the motor best adapted to run as a synchronizing motor impairs its efficiency as a torque or double-circuit motor under the assumed conditions on the start. This will be readily understood, for in a large synchronizing motor the length of the magnetic circuit of the polar projections, and their mass, are so great that apparently considerable time is required for magnetization and demagnetization. Hence with a current of a very high number of alternations the motor may not respond properly. To avoid this objection and to start up a synchronizing motor in which these conditions obtain, Mr. Tesla has combined two motors, one a synchronizing motor, the other a multiple-circuit or torque motor, and by the latter he brings the first-named up to speed, and then either throws the whole current into the synchronizing motor or operates jointly both of the motors.
This invention involves several novel and useful features. It will be observed, in the first place, that both motors are run, without commutators of any kind, and, secondly, that the speed of the torque motor may be higher than that of the synchronizing motor, as will be the case when it contains a fewer number of poles or sets of poles, so that the motor will be more readily andeasily brought up to speed. Thirdly, the synchronizing motor may be constructed so as to have a much more pronounced tendency to synchronism without lessening the facility with which it is started.
Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view of the synchronizing motor; Fig. 79 an end view and part section of the torque or double-circuit motor; Fig. 80 a diagram of the circuit connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of modified dispositions of the two motors.
Fig. 77.Fig. 77.
Inasmuch as neither motor is doing any work while the current is acting upon the other, the two armatures are rigidly connected, both being mounted upon the same shaftA, the field-magnetsBof the synchronizing andCof the torque motor being secured to the same baseD. The preferably larger synchronizing motor has polar projections on its armature, which rotate in very close proximity to the poles of the field, and in other respects it conforms to the conditions that are necessary to secure synchronous action. The pole-pieces of the armature are, however, wound with closed coilsE, as this obviates the employment of sliding contacts. The smaller or torque motor, on the other hand, has, preferably, a cylindrical armatureF, without polar projections and wound with closed coilsG. The field-coils of the torque motor are connected up in two seriesHandI, and the alternating current from the generator is directed through or divided between these two circuits in any manner to produce a progression of the poles or points of maximum magnetic effect. This result is secured by connecting the two motor-circuits in derivation with the circuitfrom the generator, inserting in one motor circuit a dead resistance and in the other a self-induction coil, by which means a difference in phase between the two divisions of the current is secured. If both motors have the same number of field poles, the torque motor for a given number of alternations will tend to run at double the speed of the other, for, assuming the connections to be such as to give the best results, its poles are divided into two series and the number of poles is virtually reduced one-half, which being acted upon by the same number of alternations tend to rotate the armature at twice the speed. By this means the main armature is more easily brought to or above the required speed. When the speed necessary for synchronism is imparted to the main motor, the current is shifted from the torque motor into the other.
Fig. 78, 79.Fig. 78.Fig. 79.
A convenient arrangement for carrying out this invention is shown in Fig. 80, in whichJ Jare the field coils of the synchronizing, andH Ithe field coils of the torque motor.L L'are the conductors of the main line. One end of, say, coilsHis connected to wireLthrough a self-induction coilM. One end of the other set of coilsIis connected to the same wire through a dead resistanceN. The opposite ends of these two circuits are connected to the contactmof a switch, the handle or lever of which is in connection with the line-wireL'. One end of the field circuit of the synchronizing motor is connected to the wireL. The other terminates in the switch-contactn. From the diagram it will be readily seen that if the leverPbe turned upon contactm, the torque motor will start by reason of the difference of phase between the currents in its two energizing circuits. Then when the desired speed is attained, if the leverPbe shifted upon contactnthe entire current will pass through the field coils of the synchronizing motor and the other will be doing no work.
The torque motor may be constructed and operated in various ways, many of which have already been touched upon. It is not necessary that one motor be cut out of circuit while the other is in, for both may be acted upon by current at the same time, and Mr. Tesla has devised various dispositions or arrangements of the two motors for accomplishing this. Some of these arrangements are illustrated in Figs. 81 to 85.
Fig. 80.Fig. 80.
Referring to Fig. 81, letTdesignate the torque or multiple circuit motor andSthe synchronizing motor,L L'being the line-wires from a source of alternating current. The two circuits of the torque motor of different degrees of self-induction, and designated byN M, are connected in derivation to the wire L. They are then joined and connected to the energizing circuit of the synchronizing motor, the opposite terminal of which is connected to wireL'. The two motors are thus in series. To start them Mr. Tesla short-circuits the synchronizing motor by a switchP', throwing the whole current through the torque motor. Then when the desired speed is reached the switchP'is opened, so that the current passes through both motors. In such an arrangement as this it is obviously desirable for economical and other reasons that a proper relation between the speeds of the two motors should be observed.
In Fig. 82 another disposition is illustrated.Sis the synchronizing motor andTthe torque motor, the circuits of both being in parallel.Wis a circuit also in derivation to the motor circuits and containing a switchP''.S'is a switch in the synchronizing motor circuit. On the start the switchS'is opened, cutting out the motorS. ThenP''is opened, throwing the entire currentthrough the motorT, giving it a very strong torque. When the desired speed is reached, switchS'is closed and the current divides between both motors. By means of switchP''both motors may be cut out.
Fig. 81, 82, 83, 84 and 85.Figs.81, 82, 83, 84 and 85.
In Fig. 83 the arrangement is substantially the same, except that a switchT'is placed in the circuit which includes the two circuits of the torque motor. Fig. 84 shows the two motors in series, with a shunt around both containing a switchS T. There is also a shunt around the synchronizing motorS, with a switchP'. In Fig. 85 the same disposition is shown; but each motor is provided with a shunt, in which are switchesP'andT'', as shown.
We now come to a new class of motors in which resort is had to condensers for the purpose of developing the required difference of phase and neutralizing the effects of self-induction. Mr. Tesla early began to apply the condenser to alternating apparatus, in just how many ways can only be learned from a perusal of other portions of this volume, especially those dealing with his high frequency work.
Certain laws govern the action or effects produced by a condenser when connected to an electric circuit through which an alternating or in general an undulating current is made to pass. Some of the most important of such effects are as follows: First, if the terminals or plates of a condenser be connected with two points of a circuit, the potentials of which are made to rise and fall in rapid succession, the condenser allows the passage, or more strictly speaking, the transference of a current, although its plates or armatures may be so carefully insulated as to prevent almost completely the passage of a current of unvarying strength or direction and of moderate electromotive force. Second, if a circuit, the terminals of which are connected with the plates of the condenser, possess a certain self-induction, the condenser will overcome or counteract to a greater or less degree, dependent upon well-understood conditions, the effects of such self-induction. Third, if two points of a closed or complete circuit through which a rapidly rising and falling current flows be shunted or bridged by a condenser, a variation in the strength of the currents in the branches and also a difference of phase of the currents therein is produced. These effects Mr. Tesla has utilized and applied in a variety of ways in the construction and operation of his motors, such as by producing a difference in phase in the two energizing circuits of an alternating current motor by connecting the two circuits in derivation and connecting up a condenser in series in one of the circuits. A further development,however, possesses certain novel features of practical value and involves a knowledge of facts less generally understood. It comprises the use of a condenser or condensers in connection with the induced or armature circuit of a motor and certain details of the construction of such motors. In an alternating current motor of the type particularly referred to above, or in any other which has an armature coil or circuit closed upon itself, the latter represents not only an inductive resistance, but one which is periodically varying in value, both of which facts complicate and render difficult the attainment of the conditions best suited to the most efficient working conditions; in other words, they require, first, that for a given inductive effect upon the armature there should be the greatest possible current through the armature or induced coils, and, second, that there should always exist between the currents in the energizing and the induced circuits a given relation of phase. Hence whatever tends to decrease the self-induction and increase the current in the induced circuits will, other things being equal, increase the output and efficiency of the motor, and the same will be true of causes that operate to maintain the mutual attractive effect between the field magnets and armature at its maximum. Mr. Tesla secures these results by connecting with the induced circuit or circuits a condenser, in the manner described below, and he also, with this purpose in view, constructs the motor in a special manner.
Fig. 86.Fig. 86.
Fig. 88, 89.Fig. 88.Fig. 89.Fig. 87.Fig. 87.Fig. 90.Fig. 90.
Fig. 87.Fig. 87.
Fig. 90.Fig. 90.
Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of an alternating current motor, in which the present principle is applied. Fig. 87 is a central section, in line with the shaft, of a special form of armature core. Fig. 88 is a similar section of a modification of the same. Fig. 89 is one of the sections of the core detached. Fig. 90 is a diagram showing a modified disposition of the armature or induced circuits.
The general plan of the invention is illustrated in Fig. 86.A Ain this figure represent the the frame and field magnets of an alternating current motor, the poles or projections of which are wound with coilsBandC, forming independent energizing circuits connected either to the same or to independent sources of alternating currents, so that the currents flowing through the circuits, respectively, will have a difference of phase. Within the influence of this field is an armature coreD, wound with coilsE. In motors of this description heretofore these coils have been closed upon themselves, or connected in a closed series; but in the present case each coil or the connected series of coils terminates in the opposite plates of a condenserF. For this purpose the ends of the series of coils are brought out through the shaft to collecting ringsG, which are connected to the condenser by contact brushesHand suitable conductors, the condenser being independent of the machine. The armature coils are wound or connected in such manner that adjacent coils produce opposite poles.
The action of this motor and the effect of the plan followed in its construction are as follows: The motor being started in operation and the coils of the field magnets being traversed by alternating currents, currents are induced in the armature coils by one set of field coils, asB, and the poles thus established are acted upon by the other set, asC. The armature coils, however, have necessarily a high self-induction, which opposes the flow of the currents thus set up. The condenserFnot only permits the passage or transference of these currents, but also counteracts the effects of self-induction, and by a proper adjustment of the capacity of the condenser, the self-induction of the coils, and the periods of the currents, the condenser may be made to overcome entirely the effect of self-induction.
It is preferable on account of the undesirability of using sliding contacts of any kind, to associate the condenser with the armature directly, or make it a part of the armature. In some cases Mr. Tesla builds up the armature of annular platesK K, held by boltsLbetween headsM, which are secured to the driving shaft, and in the hollow space thus formed he places a condenserF, generally by winding the two insulated plates spirally around the shaft. In other cases he utilizes the plates of the core itself as the plates of the condenser. For example, in Figs. 88 and 89,Nis the driving shaft,M Mare the heads of the armature-core, andK K'the iron plates of which the core is built up. These plates are insulated from the shaft and from one another, and are held together by rods or boltsL. The bolts pass through a large hole in one plate and a small hole in the one next adjacent, and so on, connecting electrically all of platesK, as one armature of a condenser, and all of platesK'as the other.
To either of the condensers above described the armature coils may be connected, as explained by reference to Fig. 86.
In motors in which the armature coils are closed upon themselves—as, for example, in any form of alternating current motor in which one armature coil or set of coils is in the position of maximum induction with respect to the field coils or poles, while the other is in the position of minimum induction—the coils are best connected in one series, and two points of the circuit thus formed are bridged by a condenser. This is illustrated in Fig. 90, in whichErepresents one set of armature coils andE'the other. Their points of union are joined through a condenserF. It will be observed that in this disposition the self-induction of the two branchesEandE'varies with their position relatively to the field magnet, and that each branch is alternately the predominating source of the induced current. Hence the effect of the condenserFis twofold. First, it increases the current in each of the branches alternately, and, secondly, it alters the phase of the currents in the branches, this being the well-known effect which results from such a disposition of a condenser with a circuit, as above described. This effect is favorable to the proper working of the motor, because it increases the flow of current in the armature circuits due to a given inductive effect, and also because it brings more nearly into coincidence the maximum magnetic effects of the coacting field and armature poles.
It will be understood, of course, that the causes that contribute to the efficiency of condensers when applied to such uses as the above must be given due consideration in determining the practicability and efficiency of the motors. Chief among these is, as is well known, the periodicity of the current, and hence the improvements described are more particularly adapted to systems in which a very high rate of alternation or change is maintained.
Although this invention has been illustrated in connection with a special form of motor, it will be understood that it is equally applicable to any other alternating current motor in which there is a closed armature coil wherein the currents are induced by the action of the field, and the feature of utilizing the plates or sections of a magnetic core for forming the condenser is applicable, generally, to other kinds of alternating current apparatus.
If the field or energizing circuits of a rotary phase motor be both derived from the same source of alternating currents and a condenser of proper capacity be included in one of the same, approximately, the desired difference of phase may be obtained between the currents flowing directly from the source and those flowing through the condenser; but the great size and expense of condensers for this purpose that would meet the requirements of the ordinary systems of comparatively low potential are particularly prohibitory to their employment.
Another, now well-known, method or plan of securing a difference of phase between the energizing currents of motors of this kind is to induce by the currents in one circuit those in the other circuit or circuits; but as no means had been proposed that would secure in this way between the phases of the primary or inducing and the secondary or induced currents that difference—theoretically ninety degrees—that is best adapted for practical and economical working, Mr. Tesla devised a means which renders practicable both the above described plans or methods, and by which he is enabled to obtain an economical and efficient alternating current motor. His invention consists in placing a condenser in the secondary or induced circuit of the motor above described and raising the potential of the secondary currents to such a degree that the capacity of the condenser, which is in part dependent on the potential, need be quite small. The value of this condenser is determined in a well-understood manner with reference to the self-induction and other conditions of the circuit, so as to cause the currents which pass through it to differ from the primary currents by a quarter phase.
Fig. 91 illustrates the invention as embodied in a motor in which the inductive relation of the primary and secondary circuits is secured by winding them inside the motor partly upon the same cores; but the invention applies, generally, toother forms of motor in which one of the energizing currents is induced in any way from the other.
LetA Brepresent the poles of an alternating current motor, of whichCis the armature wound with coilsD, closed upon themselves, as is now the general practice in motors of this kind. The polesA, which alternate with polesB, are wound with coils of ordinary or coarse wireEin such direction as to make them of alternate north and south polarity, as indicated in the diagram by the charactersN S. Over these coils, or in other inductive relation to the same, are wound long fine-wire coilsF F, and in the same direction throughout as the coilsE. These coils are secondaries, in which currents of very high potential are induced. All the coilsEin one series are connected, and all the secondariesFin another.