Figs. 45, 46 and 47.Figs.45, 46 and 47.
Fig. 43 shows a somewhat modified arrangement of circuits. There is in this case but one armature coilE, the winding of which maintains effects corresponding to the resultant poles produced by the two field-circuits.
Fig. 44 represents a disposition in which both armature and field are wound with two sets of coils, all in multiple arc to the line or main circuit. The armature coils are wound to correspond with the field-coils with respect to their self-induction. A modification of this plan is shown in Fig. 45—that is to say, thetwo field coils and two armature coils are in derivation to themselves and in series with one another. The armature coils in this case, as in the previous figure, are wound for different self-induction to correspond with the field coils.
Another modification is shown in Fig. 46. In this case only one armature-coil, asD, is included in the line-circuit, while the other, asC, is short-circuited.
In such a disposition as that shown in Fig. 43, or where only one armature-coil is employed, the torque on the start is somewhat reduced, while the tendency to synchronism is somewhat increased. In such a disposition as shown in Fig. 46, the opposite conditions would exist. In both instances, however, there is the advantage of dispensing with one contact-ring.
Fig. 48.Fig. 48.
In Fig. 46 the two field-coils and the armature-coilDare in multiple arc. In Fig. 47 this disposition is modified, coilDbeing shown in series with the two field-coils.
Fig. 48 is an outline of the general form of motor in which this invention is embodied. The circuit connections between the armature and field coils are made, as indicated in the previous figures, through brushes and rings, which are not shown.
In a preceding chapter we have described a method by which Mr. Tesla accomplishes the change in his type of rotating field motor from a torque to a synchronizing motor. As will be observed, the desired end is there reached by a change in the circuit connections at the proper moment. We will now proceed to describe another way of bringing about the same result. The principle involved in this method is as follows:—
If an alternating current be passed through the field coils only of a motor having two energizing circuits of different self-induction and the armature coils be short-circuited, the motor will have a strong torque, but little or no tendency to synchronism with the generator; but if the same current which energizes the field be passed also through the armature coils the tendency to remain in synchronism is very considerably increased. This is due to the fact that the maximum magnetic effects produced in the field and armature more nearly coincide. On this principle Mr. Tesla constructs a motor having independent field circuits of different self-induction, which are joined in derivation to a source of alternating currents. The armature is wound with one or more coils, which are connected with the field coils through contact rings and brushes, and around the armature coils a shunt is arranged with means for opening or closing the same. In starting this motor the shunt is closed around the armature coils, which will therefore be in closed circuit. When the current is directed through the motor, it divides between the two circuits, (it is not necessary to consider any case where there are more than two circuits used), which, by reason of their different self-induction, secure a difference of phase between the two currents in the two branches, that produces a shifting or rotation of the poles. By the alternations of current, other currents are induced in the closed—or short-circuited—armature coils and themotor has a strong torque. When the desired speed is reached, the shunt around the armature-coils is opened and the current directed through both armature and field coils. Under these conditions the motor has a strong tendency to synchronism.
Figs. 49, 50 and 51.Figs.49, 50 and 51.
In Fig. 49,AandBdesignate the field coils of the motor. As the circuits including these coils are of different self-induction, this is represented by a resistance coilRin circuit withA, and a self-induction coilSin circuit withB. The same result may of course be secured by the winding of the coils.Cis the armature circuit, the terminals of which are ringsa b. Brushesc dbear on these rings and connect with the line and field circuits.Dis the shunt or short circuit around the armature.Eis the switch in the shunt.
It will be observed that in such a disposition as is illustrated inFig. 49, the field circuitsAandBbeing of different self-induction, there will always be a greater lag of the current in one than the other, and that, generally, the armature phases will not correspond with either, but with the resultant of both. It is therefore important to observe the proper rule in winding the armature. For instance, if the motor have eight poles—four in each circuit—there will be four resultant poles, and hence the armature winding should be such as to produce four poles, in order to constitute a true synchronizing motor.
Fig. 52.Fig. 52.
The diagram, Fig. 50, differs from the previous one only in respect to the order of connections. In the present case the armature-coil, instead of being in series with the field-coils, is in multiple arc therewith. The armature-winding may be similar to that of the field—that is to say, the armature may have two or more coils wound or adapted for different self-induction and adapted, preferably, to produce the same difference of phase as the field-coils. On starting the motor the shunt is closed around both coils. This is shown in Fig. 51, in which the armature coils areF G. To indicate their different electrical character, there are shown in circuit with them, respectively, the resistanceR'and the self-induction coilS'. The two armature coils are in series with the field-coils and the same disposition of the shunt or short-circuitDis used. It is of advantage in the operation of motors of this kind to construct or wind the armature in such manner that when short-circuited on the start it will have a tendency to reach a higher speed than that which synchronizes with the generator. For example, a given motor having, say, eight poles should run, with the armature coil short-circuited, at two thousand revolutions per minute to bring it up to synchronism. It will generally happen, however, thatthis speed is not reached, owing to the fact that the armature and field currents do not properly correspond, so that when the current is passed through the armature (the motor not being quite up to synchronism) there is a liability that it will not "hold on," as it is termed. It is preferable, therefore, to so wind or construct the motor that on the start, when the armature coils are short-circuited, the motor will tend to reach a speed higher than the synchronous—as for instance, double the latter. In such case the difficulty above alluded to is not felt, for the motor will always hold up to synchronism if the synchronous speed—in the case supposed of two thousand revolutions—is reached or passed. This may be accomplished in various ways; but for all practical purposes the following will suffice: On the armature are wound two sets of coils. At the start only one of these is short-circuited, thereby producing a number of poles on the armature, which will tend to run the speed up above the synchronous limit. When such limit is reached or passed, the current is directed through the other coil, which, by increasing the number of armature poles, tends to maintain synchronism.
Fig. 53.Fig. 53.
In Fig. 52, such a disposition is shown. The motor having, say, eight poles contains two field-circuitsAandB, of different self-induction. The armature has two coilsFandG. The former is closed upon itself, the latter connected with the field and line through contact-ringsa b, brushesc d, and a switchE. On the start the coilFalone is active and the motor tends to run at a speed above the synchronous; but when the coilGis connected to the circuit the number of armature poles is increased, while the motor is made a true synchronous motor. This dispositionhas the advantage that the closed armature-circuit imparts to the motor torque when the speed falls off, but at the same time the conditions are such that the motor comes out of synchronism more readily. To increase the tendency to synchronism, two circuits may be used on the armature, one of which is short-circuited on the start and both connected with the external circuit after the synchronous speed is reached or passed. This disposition is shown in Fig. 53. There are three contact-ringsa b eand three brushesc d f, which connect the armature circuits with the external circuit. On starting, the switchHis turned to complete the connection between one binding-postPand the field-coils. This short-circuits one of the armature-coils, asG. The other coilFis out of circuit and open. When the motor is up to speed, the switchHis turned back, so that the connection from binding-postPto the field coils is through the coilG, and switchKis closed, thereby including coilFin multiple arc with the field coils. Both armature coils are thus active.
From the above-described instances it is evident that many other dispositions for carrying out the invention are possible.
The following description deals with another form of motor, namely, depending on "magnetic lag" or hysteresis, its peculiarity being that in it the attractive effects or phases while lagging behind the phases of current which produce them, are manifested simultaneously and not successively. The phenomenon utilized thus at an early stage by Mr. Tesla, was not generally believed in by scientific men, and Prof. Ayrton was probably first to advocate it or to elucidate the reason of its supposed existence.
Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a part-sectional view at right angles to Fig. 54. Fig. 56 is an end view in elevation and part section of a modification, and Fig. 57 is a similar view of another modification.
In Figs. 54 and 55,Adesignates a base or stand, and B B the supporting-frame of the motor. Bolted to the supporting-frame are two magnetic cores or pole-piecesC C', of iron or soft steel. These may be subdivided or laminated, in which case hard iron or steel plates or bars should be used, or they should be wound with closed coils.Dis a circular disc armature, built up of sections or plates of iron and mounted in the frame between the pole-piecesC C', curved to conform to the circular shape thereof. This disc may be wound with a number of closed coilsE.F Fare the main energizing coils, supported by the supporting-frame, so as to include within their magnetizing influence both the pole-piecesC C'and the armatureD. The pole-piecesC C'project out beyond the coilsF Fon opposite sides, as indicated in the drawings. If an alternating current be passed through the coilsF F, rotation of the armature will be produced, and this rotation is explained by the following apparent action, or mode of operation: An impulse of current in the coilsF Festablishes two polarities in the motor. The protruding end of pole-pieceC, for instance, will beof one sign, and the corresponding end of pole-pieceC'will be of the opposite sign. The armature also exhibits two poles at right angles to the coilsF F, like poles to those in the pole-pieces being on the same side of the coils. While the current is flowing there is no appreciable tendency to rotation developed; but after each current impulse ceases or begins to fall, the magnetism in the armature and in the ends of the pole-piecesC C'lags or continues to manifest itself, which produces a rotation of the armature by the repellent force between the more closely approximating points of maximum magnetic effect. This effect is continued by the reversal of current, the polarities of field and armature being simply reversed. One or both of the elements—the armature or field—may be wound with closed induced coils to intensify this effect. Although in the illustrations but one of the fields is shown, each element of the motor really constitutes a field, wound with the closed coils, the currents being induced mainly in those convolutions or coils which are parallel to the coilsF F.
Fig. 54, 55.Fig. 54.Fig. 55.
A modified form of this motor is shown in Fig. 56. In this formGis one of two standards that support the bearings for the armature-shaft.H Hare uprights or sides of a frame, preferably magnetic, the endsC C'of which are bent in the manner indicated, to conform to the shape of the armatureDand form field-magnet poles. The construction of the armature may be the same as in the previous figure, or it may be simply a magnetic disc or cylinder, as shown, and a coil or coilsF Fare secured in position to surround both the armature and the polesC C'. The armature is detachable from its shaft, the latter being passed through the armature after it has been inserted in position. The operation of this form of motor is the same in principle as that previously described and needs no further explanation.
Fig. 56.Fig. 56.Fig. 57.Fig. 57.
Fig. 56.Fig. 56.
Fig. 57.Fig. 57.
One of the most important features in alternating current motors is, however, that they should be adapted to and capable of running efficiently on the alternating circuits in present use, in which almost without exception the generators yield a very high number of alternations. Such a motor, of the type under consideration, Mr. Tesla has designed by a development of the principle of the motor shown in Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In the construction of this motor he employs an annular magnetic frameJ, with inwardly-extending ribs or projectionsK, the ends of which all bend or turn in one direction and are generally shaped to conform to the curved surface of the armature. CoilsF Fare wound from one partKto the one next adjacent, the ends or loops of each coil or group of wires being carried over toward the shaft, so as to formU-shaped groups of convolutions at each end of the armature. The pole-piecesC C', being substantially concentric with the armature, form ledges, along which the coils are laid and should project to some extent beyond the the coils, as shown. The cylindrical or drum armatureDis of the same construction as in the other motors described, and is mounted to rotate within the annular frame J and between theU-shaped ends or bends ofthe coilsF. The coilsFare connected in multiple or in series with a source of alternating currents, and are so wound that with a current or current impulse of given direction they will make the alternate pole-piecesCof one polarity and the other pole-piecesC'of the opposite polarity. The principle of the operation of this motor is the same as the other above described, for, considering any two pole-piecesC C', a current impulse passing in the coil which bridges them or is wound over both tends to establish polarities in their ends of opposite sign and to set up in the armature core between them a polarity of the same sign as that of the nearest pole-pieceC. Upon the fall or cessation of the current impulse that established these polarities the magnetism which lags behind the current phase, and which continues to manifest itself in the polar projectionsC C'and the armature, produces by repulsion a rotation of the armature. The effect is continued by each reversal of the current. What occurs in the case of one pair of pole-pieces occurs simultaneously in all, so that the tendency to rotation of the armature is measured by the sum of all the forces exerted by the pole-pieces, as above described. In this motor also the magnetic lag or effect is intensified by winding one or both cores with closed induced coils. The armature core is shown as thus wound. When closed coils are used, the cores should be laminated.
It is evident that a pulsatory as well as an alternating current might be used to drive or operate the motors above described.
It will be understood that the degree of subdivision, the mass of the iron in the cores, their size and the number of alternations in the current employed to run the motor, must be taken into consideration in order to properly construct this motor. In other words, in all such motors the proper relations between the number of alternations and the mass, size, or quality of the iron must be preserved in order to secure the best results.
In that class of motors in which two or more sets of energizing magnets are employed, and in which by artificial means a certain interval of time is made to elapse between the respective maximum or minimum periods or phases of their magnetic attraction or effect, the interval or difference in phase between the two sets of magnets is limited in extent. It is desirable, however, for the economical working of such motors that the strength or attraction of one set of magnets should be maximum, at the time when that of the other set is minimum, and conversely; but these conditions have not heretofore been realized except in cases where the two currents have been obtained from independent sources in the same or different machines. Mr. Tesla has therefore devised a motor embodying conditions that approach more nearly the theoretical requirements of perfect working, or in other words, he produces artificially a difference of magnetic phase by means of a current from a single primary source sufficient in extent to meet the requirements of practical and economical working. He employs a motor with two sets of energizing or field magnets, each wound with coils connected with a source of alternating or rapidly-varying currents, but forming two separate paths or circuits. The magnets of one set are protected to a certain extent from the energizing action of the current by means of a magnetic shield or screen interposed between the magnet and its energizing coil. This shield is properly adapted to the conditions of particular cases, so as to shield or protect the main core from magnetization until it has become itself saturated and no longer capable of containing all the lines of force produced by the current. It will be seen that by this means the energizing action begins in the protected set of magnets a certain arbitrarily-determined period of time later than in the other, and that by this means alone or in conjunction with other means or devicesheretofore employed a practical difference of magnetic phase may readily be secured.
Fig. 58 is a view of a motor, partly in section, with a diagram illustrating the invention. Fig. 59 is a similar view of a modification of the same.
Fig. 58, 59.Fig. 58.Fig. 59.
In Fig. 58, which exhibits the simplest form of the invention,A Ais the field-magnet of a motor, having, say, eight poles or inwardly-projecting coresBandC. The coresBform one set of magnets and are energized by coilsD. The coresC, forming the other set are energized by coilsE, and the coils are connected, preferably, in series with one another, in two derived or branched circuits,F G, respectively, from a suitable source of current. Each coilEis surrounded by a magnetic shieldH, which is preferably composed of an annealed, insulated, or oxidized iron wire wrapped or wound on the coils in the manner indicated so as to form a closed magnetic circuit around the coils and between the same and the magnetic coresC. Between the pole pieces or coresB Cis mounted the armatureK, which, as is usual in this type of machines, is wound with coilsLclosed upon themselves. The operation resulting from this disposition is as follows: If a current impulse be directed through the two circuits of the motor, it will quickly energize the coresB, but not so the coresC, for the reason that in passing through the coilsEthere is encountered the influence of the closed magnetic circuits formed by the shieldsH. The first effect is to retard effectively the current impulse in circuitG, while at the same time the proportion of current which does pass does not magnetize the coresC, which are shielded orscreened by the shieldsH. As the increasing electromotive force then urges more current through the coilsE, the iron wireHbecomes magnetically saturated and incapable of carrying all the lines of force, and hence ceases to protect the coresC, which becomes magnetized, developing their maximum effect after an interval of time subsequent to the similar manifestation of strength in the other set of magnets, the extent of which is arbitrarily determined by the thickness of the shieldH, and other well-understood conditions.
From the above it will be seen that the apparatus or device acts in two ways. First, by retarding the current, and, second, by retarding the magnetization of one set of the cores, from which its effectiveness will readily appear.
Many modifications of the principle of this invention are possible. One useful and efficient application of the invention is shown in Fig. 59. In this figure a motor is shown similar in all respects to that above described, except that the iron wireH, which is wrapped around the coilsE, is in this case connected in series with the coilsD. The iron-wire coilsH, are connected and wound, so as to have little or no self-induction, and being added to the resistance of the circuitF, the action of the current in that circuit will be accelerated, while in the other circuitGit will be retarded. The shieldHmay be made in many forms, as will be understood, and used in different ways, as appears from the foregoing description.
As a modification of his type of motor with "shielded" fields, Mr. Tesla has constructed a motor with a field-magnet having two sets of poles or inwardly-projecting cores and placed side by side, so as practically to form two fields of force and alternately disposed—that is to say, with the poles of one set or field opposite the spaces between the other. He then connects the free ends of one set of poles by means of laminated iron bands or bridge-pieces of considerably smaller cross-section than the cores themselves, whereby the cores will all form parts of complete magnetic circuits. When the coils on each set of magnets are connected in multiple circuits or branches from a source of alternating currents, electromotive forces are set up in or impressed upon each circuit simultaneously; but the coils on the magnetically bridged or shunted cores will have, by reason of the closed magnetic circuits, a high self-induction, which retards the current, permitting at the beginning of each impulse but little current to pass. On the other hand, no such opposition being encountered in the other set of coils, the current passes freely through them, magnetizing the poles on which they are wound. As soon, however, as the laminated bridges become saturated and incapable of carrying all the lines of force which the rising electromotive force, and consequently increased current, produce, free poles are developed at the ends of the cores, which, acting in conjunction with the others, produce rotation of the armature.
The construction in detail by which this invention is illustrated is shown in the accompanying drawings.
Fig. 60, 61.Fig. 60.Fig. 61.
Fig. 60 is a view in side elevation of a motor embodying the principle. Fig. 61 is a vertical cross-section of the motor.Ais the frame of the motor, which should be built up of sheets of iron punched out to the desired shape and bolted together with insulation between the sheets. When complete, the frame makes a field-magnet with inwardly projecting pole-piecesBandC. To adapt them to the requirements of this particular case these pole-pieces are out of line with one another, those markedBsurrounding one end of the armature and the others, asC, the opposite end, and they are disposed alternately—that is to say, the pole-pieces of one set occur in line with the spaces between those of the other sets.
The armatureDis of cylindrical form, and is also laminated in the usual way and is wound longitudinally with coils closed upon themselves. The pole-piecesCare connected or shunted by bridge-piecesE. These may be made independently and attached to the pole-pieces, or they may be parts of the forms or blanks stamped or punched out of sheet-iron. Their size or mass is determined by various conditions, such as the strength of the current to be employed, the mass or size of the cores to which they are applied, and other familiar conditions.
CoilsFsurround the pole-piecesB, and other coilsGare wound on the pole-piecesC. These coils are connected in series in two circuits, which are branches of a circuit from a generator of alternating currents, and they may be so wound, or the respective circuits in which they are included may be so arranged, that the circuit of coilsGwill have, independently of the particular construction described, a higher self-induction than the other circuit or branch.
The function of the shunts or bridgesEis that they shall form with the coresCa closed magnetic circuit for a current up to a predetermined strength, so that when saturated by such current and unable to carry more lines of force than such a current produces they will to no further appreciable extent interfere with the development, by a stronger current, of free magnetic poles at the ends of the coresC.
In such a motor the current is so retarded in the coilsG, and the manifestation of the free magnetism in the polesCis so delayed beyond the period of maximum magnetic effect in polesB, that a strong torque is produced and the motor operates with approximately the power developed in a motor of this kind energized by independently generated currents differing by a full quarter phase.
Up to this point, two principal types of Tesla motors have been described: First, those containing two or more energizing circuits through which are caused to pass alternating currents differing from one another in phase to an extent sufficient to produce a continuous progression or shifting of the poles or points of greatest magnetic effect, in obedience to which the movable element of the motor is maintained in rotation; second, those containing poles, or parts of different magnetic susceptibility, which under the energizing influence of the same current or two currents coinciding in phase will exhibit differences in their magnetic periods or phases. In the first class of motors the torque is due to the magnetism established in different portions of the motor by currents from the same or from independent sources, and exhibiting time differences in phase. In the second class the torque results from the energizing effects of a current upon different parts of the motor which differ in magnetic susceptibility—in other words, parts which respond in the same relative degree to the action of a current, not simultaneously, but after different intervals of time.
In another Tesla motor, however, the torque, instead of being solely the result of a time difference in the magnetic periods or phases of the poles or attractive parts to whatever cause due, is produced by an angular displacement of the parts which, though movable with respect to one another, are magnetized simultaneously, or approximately so, by the same currents. This principle of operation has been embodied practically in a motor in which the necessary angular displacement between the points of greatest magnetic attraction in the two elements of the motor—the armature and field—is obtained by the direction of the lamination of the magnetic cores of the elements.
Fig. 62 is a side view of such a motor with a portion of its armature core exposed. Fig. 63 is an end or edge view of thesame. Fig. 64 is a central cross-section of the same, the armature being shown mainly in elevation.
Fig. 62, 63, 64.Fig. 62.Fig. 63.Fig. 64.
LetA Adesignate two plates built up of thin sections or laminæ of soft iron insulated more or less from one another and held together by boltsaand secured to a baseB. The inner faces of these plates contain recesses or grooves in which a coil or coilsDare secured obliquely to the direction of the laminations. Within the coilsDis a discE, preferably composed of a spirally-wound iron wire or ribbon or a series of concentric rings and mounted on a shaftF, having bearings in the platesA A. Such a device when acted upon by an alternating current is capable of rotation and constitutes a motor, the operation of which may be explained in the following manner: A current or current-impulse traversing the coilsDtends to magnetize the coresA AandE, all of which are within the influence of the field of the coils. The poles thus established would naturally lie in the same line at right angles to the coilsD, but in the platesAthey are deflected by reason of the direction of the laminations, and appear at or near the extremities of these plates. In the disc, however, where these conditions are not present, the poles or points of greatest attraction are on a line at right angles to the plane of the coils; hence there will be a torque established by this angular displacement of the poles or magnetic lines, which starts the disc in rotation, the magnetic lines of the armature and field tending toward a position of parallelism. This rotation is continued and maintained by the reversals of the current in coilsD D, which change alternately the polarity of the field-coresA A. This rotary tendency or effect will be greatlyincreased by winding the disc with conductorsG, closed upon themselves and having a radial direction, whereby the magnetic intensity of the poles of the disc will be greatly increased by the energizing effect of the currents induced in the coilsGby the alternating currents in coilsD.
The cores of the disc and field may or may not be of different magnetic susceptibility—that is to say, they may both be of the same kind of iron, so as to be magnetized at approximately the same instant by the coilsD; or one may be of soft iron and the other of hard, in order that a certain time may elapse between the periods of their magnetization. In either case rotation will be produced; but unless the disc is provided with the closed energizing coils it is desirable that the above-described difference of magnetic susceptibility be utilized to assist in its rotation.
The cores of the field and armature may be made in various ways, as will be well understood, it being only requisite that the laminations in each be in such direction as to secure the necessary angular displacement of the points of greatest attraction. Moreover, since the disc may be considered as made up of an infinite number of radial arms, it is obvious that what is true of a disc holds for many other forms of armature.
As has been pointed out elsewhere, the lag or retardation of the phases of an alternating current is directly proportional to the self-induction and inversely proportional to the resistance of the circuit through which the current flows. Hence, in order to secure the proper differences of phase between the two motor-circuits, it is desirable to make the self-induction in one much higher and the resistance much lower than the self-induction and resistance, respectively, in the other. At the same time the magnetic quantities of the two poles or sets of poles which the two circuits produce should be approximately equal. These requirements have led Mr. Tesla to the invention of a motor having the following general characteristics: The coils which are included in that energizing circuit which is to have the higher self-induction are made of coarse wire, or a conductor of relatively low resistance, and with the greatest possible length or number of turns. In the other set of coils a comparatively few turns of finer wire are used, or a wire of higher resistance. Furthermore, in order to approximate the magnetic quantities of the poles excited by these coils, Mr. Tesla employs in the self-induction circuit cores much longer than those in the other or resistance circuit.
Fig. 65 is a part sectional view of the motor at right angles to the shaft. Fig. 66 is a diagram of the field circuits.
In Fig. 66, letArepresent the coils in one motor circuit, and B those in the other. The circuitAis to have the higher self-induction. There are, therefore, used a long length or a large number of turns of coarse wire in forming the coils of this circuit. For the circuitB, a smaller conductor is employed, or a conductor of a higher resistance than copper, such as German silver or iron, and the coils are wound with fewer turns. In applying these coils to a motor, Mr. Tesla builds up a field-magnet of platesC, of iron and steel, secured together in the usual mannerby boltsD. Each plate is formed with four (more or less) long coresE, around which is a space to receive the coil and an equal number of short projectionsFto receive the coils of the resistance-circuit. The plates are generally annular in shape, having an open space in the centre for receiving the armatureG, which Mr. Tesla prefers to wind with closed coils. An alternating current divided between the two circuits is retarded as to its phases in the circuitAto a much greater extent than in the circuitB. By reason of the relative sizes and disposition of the cores and coils the magnetic effect of the polesEandFupon the armature closely approximate.
Fig. 65, 66.Fig. 65.Fig. 66.
An important result secured by the construction shown here is that these coils which are designed to have the higher self-induction are almost completely surrounded by iron, and that the retardation is thus very materially increased.
Let it be assumed that the energy as represented in the magnetism in the field of a given rotating field motor is ninety and that of the armature ten. The sum of these quantities, which represents the total energy expended in driving the motor, is one hundred; but, assuming that the motor be so constructed that the energy in the field is represented by fifty, and that in the armature by fifty, the sum is still one hundred; but while in the first instance the product is nine hundred, in the second it is two thousand five hundred, and as the energy developed is in proportion to these products it is clear that those motors are the most efficient—other things being equal—in which the magnetic energies developed in the armature and field are equal. These results Mr. Tesla obtains by using the same amount of copper or ampere turns in both elements when the cores of both are equal, or approximately so, and the same current energizes both; or in cases where the currents in one element are induced to those of the other he uses in the induced coils an excess of copper over that in the primary element or conductor.
Fig. 67.Fig. 67.
The conventional figure of a motor here introduced, Fig. 67, will give an idea of the solution furnished by Mr. Tesla for the specific problem. Referring to the drawing,Ais the field-magnet,Bthe armature,Cthe field coils, andDthe armature-coils of the motor.
Generally speaking, if the mass of the cores of armature and field be equal, the amount of copper or ampere turns of the energizing coils on both should also be equal; but these conditions will be modified in different forms of machine. It will be understood that these results are most advantageous when existing under the conditions presented where the motor is running with its normal load, a point to be well borne in mind.
In this form of motor, Mr. Tesla's object is to design and build machines wherein the maxima of the magnetic effects of the armature and field will more nearly coincide than in some of the types previously under consideration. These types are: First, motors having two or more energizing circuits of the same electrical character, and in the operation of which the currents used differ primarily in phase; second, motors with a plurality of energizing circuits of different electrical character, in or by means of which the difference of phase is produced artificially, and, third, motors with a plurality of energizing circuits, the currents in one being induced from currents in another. Considering the structural and operative conditions of any one of them—as, for example, that first named—the armature which is mounted to rotate in obedience to the co-operative influence or action of the energizing circuits has coils wound upon it which are closed upon themselves and in which currents are induced by the energizing-currents with the object and result of energizing the armature-core; but under any such conditions as must exist in these motors, it is obvious that a certain time must elapse between the manifestations of an energizing current impulse in the field coils, and the corresponding magnetic state or phase in the armature established by the current induced thereby; consequently a given magnetic influence or effect in the field which is the direct result of a primary current impulse will have become more or less weakened or lost before the corresponding effect in the armature indirectly produced has reached its maximum. This is a condition unfavorable to efficient working in certain cases—as, for instance, when the progress of the resultant poles or points of maximum attraction is very great, or when a very high number of alternations is employed—for it is apparent that a strongertendency to rotation will be maintained if the maximum magnetic attractions or conditions in both armature and field coincide, the energy developed by a motor being measured by the product of the magnetic quantities of the armature and field.
To secure this coincidence of maximum magnetic effects, Mr. Tesla has devised various means, as explained below. Fig. 68 is a diagrammatic illustration of a Tesla motor system in which the alternating currents proceed from independent sources and differ primarily in phase.