US1999376A - Speed and position regulating - Google Patents

Description

April 3U, 1935. H. M. STOLLER SPEED AND POSITION REGULATING Filed April 8. 1930 2 Sheets-Shem 1 //v VEN TOR H WM STOLLE A T TOR/V5 Y April 3w, 1%5.

H. M. STOLLER SPEED AND POSITIQN REGULATING 2 Sheets-Sheet 2 Filed April 8, 1950 I I i 0 I i I 9H I I l H H l /NI/E/V7'0H H M. STOLLEH W.

ATTDRN ntented Apr. 30, 1935 {i NITD STATE OFFICE SPEED POSITION REGHJILATING Application April 8, 1930, Serial No. 342,564

clai This invention relates f to apparatusuor and methods of controlling the movement of an element and particularly to an improved system for regulating the movement of the scanning and 5 image synthesizing devices of a television system.

An object of the invention is to provide a relatively simple and inexpensive high precision, synchronizing and phase controlling system which is suitable for use in a television system, for example.

Another object is to reduce or substantially prevent oscillations or irregularities in the movement of a movable element. and, when such a movable element is employed in a television system, to thereby prevent objectionable wabbling or instability of the television image.

In accordance with a feature of the invention, means are provided for reducing or retarding the shifting of flux from one portion of a magnetic circuit to another portion without retarding changes in the total flux'in the magnetic circuit.

In accordance with another feature of the invention, irregularities or oscillations in the movement of an element, such as a scanning device for a television system, are reduced or substantially prevented by providing both electrical and mechanical oscillation preventing devices in the driving system forthe movable element.

In accordance with another feature of the invention, the phase relation of two or more movable elements is controlled, for the purpose of framing an image in a television system, for example, by temporarily increasing or decreasing the frequency of an alternating electromotive force which controls the speed of one of the movable elements for obtaining an approximate phase adjustment and subsequently changing the phase of the electromotive force for obtaining an accurate phase adjustment.

It is now well known that the production of high quality television images requires that the rotating scanning elements at the transmitting and receiving stations be maintained accurately in synchronism. It has been found necessary in certain cases to synchronize the scanning elements at the two stations with such a high degree of accuracy that they never depart from a desired phase relation by more than a small fraction of a mechanical degree.

Television images have heretofore been produced by a system in which independent high precision crystal controlled oscillators are employed for controlling the driving elements for the scanning devices at different stations, respectively, without the use of synchronizing control between the stations. Such terminal equipment is relatively expensive, however, and its use is probably not justified in certain cases as, for example, in systems in'which the stations are separated by a relatively short distance. 5

A television system employing synchronizing control between the stations has been heretofore successfully employed. In this system current produced by a high frequency generator which is mechanically coupled to a direct current motor and a scanning disc at one station is transmitted to another station for energizing a synchronous motor which is mechanically coupled to a direct current motor and a scanning disc. Such high frequency synchronous motors are expensive and inemcient and such a synchronous system requires considerable attention in starting. Moreover, a high frequency motor developing a considerable amount of power produces an objectionable high frequency noise due to the vibrations of the motor laminations.

The synchronizing system of the present invention is relatively simple and inexpensive and requires but little attention in operation. In accordance with a specific embodiment of the invention herein shown and described for the purpose of illustration, independent direct current motors each having a regulating field winding are provided for driving the scanning discs at different stations, respectively. A pilot generator associated with each motor produces an alternating electromotive force, the frequency of which is controlled in accordance with the speed of the motor and may be of the order of a thousand cycles for example. An alternating current generator which may be a vacuum tube oscillator, for example, produces a current of substantially constant frequency, which under normal'operating conditions is substantially the same as that of the electromotive force produced by the pilot generator. Current from this constant frequency source is supplied to all of the stations of the system for controlling the current in the regulating field -Winding of each direct current motor at each station and, therefore, the speedof the motor in accordance with the phase relation between the electromotive force derived from the'pilot generator associated with the motor and the electromotive force derived from' the independent constant frequency source. For this purpose there is provided at each station a motor speed control circuit comprising a pair of three-electrode detector vacuum tubes connected in pushpull relation and three three-electrode regulator vacuum tubes, the control electrodes of which are connected in parallel and the anodes of which are connected through a three-phase transformer to the regulating winding of the direct current motor. The pilot generator supplies energy to the anode circuit of the detector vacuum tubes while a potential derived from the independent source of alternating current is impressed upon the control electrode circuit of the detector vacuum tubes. The amplitude of the anode current in a portion of the anode circuit common to both detector tubes varies in accordance with the phase relation of the instantaneous values of the electromotive forces supplied to the anode and control electrode circuits, respectively, and an electromotive force which varies in accordance with this anode current is'impressed upon the control electrodes of the regulator vacuum tubes, thereby controlling the current in the regulating field winding of the motor. The motor at each station is thus maintained at a substantially constant speed in accordance with the frequency of the current produced by the independent alternating current generator and the motors at different stations are thus maintained in synchronism.

For the purpose of framing the image at a receiving station there is provided a manually operable phase or frequency changing device connected between the source of electromotive force from the independent alternating current generator and the motor control circuit. By rotating the device in a desired direction, the frequency of the electromotive force supplied to the control electrodes of the detector vacuum tubes is temporarily increased or decreased so that the image is thereby brought approximately into frame. For accurately framing the image, the device is rotated through a fraction of a revolution thus Shifting the phase of the electromotive force supplied to the control electrodes of the detector vacuum tubes accordingly.

During the development of the motor control system, it was found necessary to provide means for reducing or substantially preventing hunting which would produce irregularities or oscillations in the movement of the scanning devices at each station and thus result in instability or wabbling of the television image. The problem of preventing oscillations in the movement of a rotatable element driven by a motor becomes more difiicult of solution the greater the precision of speed regulation desired and, in the case of control systems of the synchronous type, the greater the moment of inertia of the load connected to the motor. In the present system the moment of inertia in the scanning disc employed is large with respect to that of the motor armature. It was found that the tendency of the motor to hunt or oscillate was due to a change in the distribution of the field flux in a magnetic circuit of the motor field or, in effect, to the flux shifting back and forth across each pole face. To reduce the tendency of the flux to shift in such a manner and the resulting oscillations in the movement of the motor armature and still permit sudden variations in the total flux at each pole, the poles are each provided with a plurality of slots in which are positioned a plurality of coaxially arranged short-circuiting conducting windings, preferably insulated from each other and from the magnetic material of the field structure, the axis of. which is displaced by approximately electrical degrees with respect to the axes of the two poles with which each set of windings is associated.

.While such an arrangement resulted in a great reduction in the oscillations of the motor armature it was found necessary, in order to obtain the desired high quality of image production, to provide a hydraulically damped coupling element for connecting the motor armature to the scanning disc to suppress oscillations which might otherwise be present even when employing the short-circuited windings for the pole pieces to which reference has just been made. It was also found that a strong series field on the motor assisted in securing stability of the television image and it was necessary in fact to employ all three expedients to secure satisfactory performance of the system in producing high quality television images.

Fig. 1 of the drawings is a diagrammatic showing of a two-way television system employing a synchronizing arrangement in accordance with the present invention.

Fig. 2 is a detailed diagrammatic showing of the motor and its associated speed control apparatus for driving each scanning device of the television system shown in Fig. 1.

Fig. 3 is a perspective view showing the arrangement of the oscillation reducing windings associated with the field structure of the motor for driving each scanning device of the system shown in Fig. l.

Fig. 4"is a front elevational view of a hydrauliccoupling device for connecting each scanning device of the system shown in Fig. 1 with its driving motor.

Fig. 5 is a sectional side elevational view of thepencilof light from the source II is directedthrough the apertures l2 of the disc in succession upon the field of view defined by the opening in the screen l3 and including the subject ll. Elemental areas of the field of view are thus illuminated in succession along successive parallel lines. Light reflected from the subject I 4 impinges upon one or more light sensitive cells l5 and the varying current produced by the action of the light sensitive cells, after being amplified by the vacuum tube amplifier I6, is transmitted to the distant station B through a transformer l1 and a transmission channel l8. At the station B the received image current is transmitted through a transformer l8,

amplified by a vacuum tube amplifier 20 and impressed upon a light producing or controlling device such as the glow discharge lamp 2|. The scanning or image synthesizing disc 22, similar to the scanning disc ID at station A and driven in synchronism and in phase therewith, scans in succession the elemental areas along successive parallel lines of an electrode of the glow discharge lamps 2| thus in effect illuminating the eleby observing the opening in the screen 23.

In a similar manner elemental areas of the subject 24 are illuminated in succession by light from source 25 due to the rotation of the apertured scanning disc 26. The image current produced by the action of the photoelectric cell 49 in response to light reflected from the successively illuminated elemental areas of the subject 23, after being amplified by the vacuum tube amplifier 2?, is transmitted through transformer 28 and over transmission line 29 to station A where \the image current is transmitted through transformer 33, amplified by amplifier 3 I, and impressed upon the glow discharge lamp 32. The rotation of the scanning disc 33 causes the elemental areas of the image field defined by the opening in the screen 33 to be illuminated in succession in cora speed control circuit 37! for controlling the speed of the motor. Alternating current of substantially constant frequency is produced by the vacuum tube oscillator 33 located at station A for controlling the operation of both speed control circuits 31! at that station and current from this source is also transmitted over line 33 to station B for controlling the operation of both speed control circuits 3'! at that station. ,The motors 35 at both stations are thus maintained in synchronism. The vacuum tube oscillator 39 located at station B is provided for use instead of the oscillator 38 in case of an interruption in the operation of the oscillator 33. The devices 30 are provided for bringing the receiving scanning disc 22 into phase with the transmitting scanning disc i3 and the receiving disc 33 into phase with the disc 25. By rotating the handle ii of the device 33 in one direction or the other, the frequency of the electromotive force from source 33 supplied to the motor control circuit with which the device Wis associated is temporarily increased or decreasedfor the purpose of bringing the image approximately into frame. The image may then bebrought accurately into frame by adjusting the position .of the handle M and thereby adjusting the phase of the electromotive force from source 33 supplied to the motor control circuit 31.

A driving motor 35 with it's associated speed control circuit is shown in detail in Fig. 2. The motor 35 is a four-pole, compound wound, direct current motor having a series field winding 59, a shunt field winding 5i and an auxiliary, regulating field winding 52, all of the windings being cumulative. The motor frame may be made from a standard 36 tooth stator punching by cutting out three teeth per pole, thus forming four polar areas 53 each having six teeth 53. Alternating current pilot generator 55 of the inductor type is mechanically coupled to the motor 35. This generator comprises a rotor 56 and a stator 53 on which is mounted an exciting winding 53 and a generating winding 59. The windings 59, 5i and 58 are energized from a source of direct current connectedto the terminals 69. The winding 59 supplies alternating current to the primary winding of transformer ii. The outer 'thousandths' of a watt.

and 93 rectify the voltage impressed on the anterminals of the secondary winding of this transformer are connected to the anodes of the pushpull detector vacuum tubes 62 and 63 and the mid-terminal of this winding is connected to the cathodes of these vacuum tubes through a coupling resistance element 65. supplies alternating current which is normally of the same frequency as that of the current produced by the pilot generator 55 to the primary winding of transformer 64 either directly, where the controlled motor drives atransmitting scanning disc, or through a frequency and phase changing device 39, where the controlled motor drives a receiving scanning or image synthesizing disc. The outer terminals of the secondary winding of the transformer 69 are connected to the control electrodes of vacuum tubes 62 and 53, respectively. The cathodes of the vacuum tubes 52 and 63 are connected to the mid-point of the secondary winding of transformer 54 through resistance element 83 and the source of direct current connected to the terminal 50 so as to give a negative bias to the control electrodes of the vacuum tubes 62 and 53.

One terminal of the resistance element 65 is connected'to the control electrodes, connected in parallel, of the three regulator vacuum tubes 56, 51 and 63, while the other terminal of the resistance element is connected to the cathodes of these regulator vacuum tubes. The anodes of vacuum tubes 55, 61 and 53 are connected to diiferent windings, respectively, of the star connected secondary windings of transformer 53. The common terminal of these windings is connected to one terminal of the regulating field winding 52, the other terminal of which is connccted through resistance element it to the cathodes ofvacuum tubes 65, 57! and 33. Anode potential is supplied to the anodes of these tubes through transformer 69, the star or delta connected primary of which is energized by low frequency alternating current derived from slip rings of the motor 35 connected to commutator bars 120 electrical degrees apart. The switch 99 is provided for disconnecting the primary windings of transformer 39 from the slip rings of motor 35 while it is being started. Windings are also provided on the transformer 59 for supplying heating current to the cathodes of vacuum tubes 52, 63, 56, MI and B8.

In operation, the pilot generator 35 delivers approximately one watt of power at 300 vol s, 1275 cycles to the plate circuits of the vacuum tubes 52 and 63. The power required from oscll- I later 33 for controlling the control electrode circuits of vacuum tubes 52 and 63 is only a few The detector tubes 62 ode circuits thus producing a unidirectional potential drop across the terminals of the coupling resistance element 65, the magnitude ofwhich is dependent upon the phase relation of the electromotive force from pilot generator 55 impressed upon the anode circuit and the electromotive force from oscillator 38 impressed upon the control electrode circuit of each of the detector vacuum tubes, the amplitudes of these alternating voltages being constant. the control electrode voltages are in phase so that the control electrode of each tube is positive at the same time that the anode of the tube The oscillator 38 If the anode and is positive then the anode current and, therefore, I

When the anode and control electrode voltages are 180 out of phase so that the control elecshould lag the voltage impressed on the anode by about 90. As the voltage drop across the coupling resistance 65, varies, the unidirectional electromotive force impressed upon the control electrodes of regulator vacuum tubes 66, 61 and 68 will vary accordingly and corresponding variations will be produced in the anode current of these tubes and, therefore, in the current in the regulating field winding 52 of motor 35. If, after the motor has been running at a constant speed, i s speed should tend to increase due to an increase in the line voltage applied to the terminals 60, for example, the frequency of the current produced in the generating field winding 59 of the pilot generator 55 will tend to increase, thus causing an increase in the phase displacement between the voltages applied to the control electrodes and anodes respectively, of the detector vacuum tubes 62 and 63. This will result in a decrease in the root-mean-square value of anode current flowing through resistance element 65 and a corresponding decrease in the negative potential applied to the control electrodes of the regulating vacuum tubes 66, 61 and 68. The anode current of the tubes 66, 61 and 68 and the current through the regulating field winding 52 will thus increase and prevent the speed of the motor 35 from increasing. When changes in line voltage, for example, tend to decrease the speed of the motor below the operating speed, the reverse facts are true, that is, the frequency of the current produced by the pilot generator tends to decrease, thus causing the phase displacement between the voltages applied to the control electrode and anode, respectively, of each of vacuum tubes 62 and 63 to decrease and causing the control electrodes of regulating vacuum tubes 66, 61 and 68 to become 'more'negative. This will result in decreased anode current for the regulating vacuum tubes and decreased current through the regulating field winding 52, thus preventing the motor 35 from decreasing in speed. As long as'the frequency of the current produced by the oscillator 38 remains constant, any tendency for the motor to increase or decrease in speed is checked by a change in the current through the regulating field winding of the motor, thus maintaining the motor at a constant speed. While the frequency of the current produced by the oscillator 38 is substantially constant, having a frequency precision of the order of one part in a thousand, slight variations in the frequency of the current produced by the oscillator affect each of the speed control circuits 31 simultaneously so that the synchronization and phase relationship of the different scanning elements are'not affected. When the frequency of the current produced by. the oscillator 38 increases, the phase displacement between the electromotive forces applied to the anode and control electrode of each of the vacuum tubes 62 and 63 decreases, thus causing anode current of increased amplitude to flow through the resistance element 65.

The resulting increased negative potential impressed upon the control electrodes of the regulating vacuum tubes 66, 61 and 68 causes a decrease in the anode current flowing in the regulating field winding of the motor 52 and the speed of the motor increases until the frequency of the current produced by the pilot generator 55 is equal to the frequency of the current produced by the oscillator 38. When the frequency of the current produced by the oscillator 38 decreases, the motor 35 decreases in speed until the frequencies of the currents produced by the oscillator 38 and the generator 55, respectively, are equal.

The phase shifting devices 40 are provided for bringing the receiving scanning or image synthesizing disc 22 into a desired phase relation with respect to the sending scanning disc l0 and so also the receiving disc 33 into a desired phase relation with respect to the sending scanning disc 26 for the purpose of framing the image. Each device 40 comprises two stationary windings the axes of which are at right angles with respect to each other. These windings are connected in series with each other and with the source of constant frequency currentsupplied from oscillator 38. A condenser 82 is connected in shunt with respect to one of the windings 80. The movable winding 83 is adapted to be rotated by manually turning the handle 4| attached to shaft 84 so that, as it is rotated in one direction or another, its axis may be made to coincide with the axes of the windings 88 and 8| in succession. Due to the condenser 82 being shunted across one of the windings, the currents in the windings 88 and BI, respectively, are 96 out of phase and, when the alternating electromotive force from source 38 is impressed upon these windings, a rotating magnetic field is set up. As the winding 83 is rotated in one direction or the other by turning the handle 4|, the frequency of the electromotive forceimpressed upon the primary winding of transformer 64 will be the sum or difference of the frequency of the current produced by the oscillator and the frequency of rotation of the winding 83, depending on the direction of rotation. The speed of the motor 35 may thus be temporarily increased or decreased until the image is brought approximately into frame. The image may be accurately framed by carefully adjusting the position of the handle 4| for shifting the phase of the electromotive force supplied to transformer 64 with respect to the phase of the electromotive force supplied to the device 40 from oscillator 38.

One of the difficulties encountered during the development of the control system was "hunting" of the controlled motor, that is, irregularities or oscillations in the movement of the motor armature. The hunting appeared to be due to changes in the distribution of the field flux in the magnetic circuits of the motor, that is, due to the field flux shifting back and forth from one portion of each pole face to another portion. At any instant the shift in flux was probably in the same angular direction at each pole face. It has heretofore been proposed to employ damping wind ings for the field structure of a motor for reducing the tendency of the motor to hunt, but such arrangements are not suitable for the motor of the present invention because they tend to suppress or retard variations in the total flux through the pole faces. As illustrated in Fig. 3, the field structure of the motor 35 is provided with windings which reduce the rate of shifting'of the flux from one portion of a pole face to another without retarding changes in the total flux at each' pole face. It is necessary that the total flux be permitted to change rapidly so that the motor speed may be effectively controlled in accordance with changes in the current through the regulating field winding 52. Each of the four poles is provided with six teeth as shown and a plurality of short-circuited windings, preferably of.

enamel and elements of insulating material are preferably positioned in the slots. The windings of each set have a common axis which is displaced by approximately 90 electrical degrees from the axes of the adjacent poles of Op osite sign with which the windings are associated. The winding 85 extends from one pole to the adjacent pole through the slots between the teeth- 54l nearest each other. The winding 86 is plositioned in the next adjacent slots and the winding M is positioned in the central slots of the poles. A total of four sets of windings are employed for interlinking the poles. The central slot of each pole holds two windings such as the windings 871 and 88, for example. It will be noted that, if the field fiux is distributed over. the pole face with uniform flux density, there will be no interlinkage between the fiux and the oscillation reducing windings 85, 86 and 87, since such lines of flux as enter each winding on one pole face are neutralized by equal lines of flux of opposite polarity which leave the winding on the adjacent pole face area included within the winding. Suppose, however, that at a certain instant the flux tends to shift alongeach pole face in the direction indicated by the arrow, that is, toward the right-hand portion of the poles shown in Fig. 3. This increase in fiux density in a portion of one pole and the corresponding decrease in flux density in an adjacent portion of another pole of opposite sign will produce an interlinkage of fiux between the field fiux and the oscillation reducing windings 85, 86 and 81, thereby generating electromotive forces and hence currents in these windings. The direction of the resulting current will be such as to cause a field to be set up which will oppose the change-in fiux density at the portions of the poles which are interlinked by the windings 85, 86 and 8i. When the flux tends to shift in the opposite direction the current which is caused to fiow in the coils 85, 8t and 817 will be in the opposite direction and the shift in flux will likewise be-opposed. When the total flux at each pole is increased or decreased without an accompanying changein the vented, but changes in total flux will not be retarded.

While the short-circuited windings on the field structure of the motor greatly reduce irregularities in the movement of the motor armature, it

has been found that, even when employing these windings on the field structure, objectionable oscillations in the movement of the scanning discs still occur, and that these may be reduced by employing a hydraulic damping coupling element 36 of the type shown in Figs. 4 and 5 for coupling the scanning disc to the motor shaft. The motor shaft 98 is rigidly secured to the bridge structure 9i and the end portions of the bridge structure are secured to the bases of the flexible metallic bellows 92. The upper portions of the bellows are connected rigidly to the scanning disc 18. The

two bellows are connected by a fluid conduit 93 having a constricted portion or valve 94 the size of which may be controlled by adjusting the thumb nut 95. The system is preferably completely filled with oil or other suitable damping medium. The counterweight 86 is provided for balancing the system. It should be noted that the constricted portion of the fluid system is located at a point in the system which is farthest removed from the center of rotation. This prevents the trapping of air in the constricted portion which would interfere with the damping action of the de'- vice. The supporting structure 91 for the scanning disc i8 is mounted on ball bearings 98. The hydraulic coupling element is such that the scanning disc it may be moved in either direction from an equilibrium position by approximately five mechanical degrees.

It has been found that, if the system is disturbed by a large momentary change in load such as that due to the pressure of the hand against the scanning disc, the uniform movement of the scanning disc will be resumed after it has made approximately two oscillations, the oscillating f requency being of the order of two cycles per second. It was also found that a strong series field assists in securing stability of operation. Good results are obtained when the ampere turns of the series field winding, the shunt field winding and the regulating field winding are approximately equal under normal operating conditions. In actual operation it has been found that the normal fluctuations in line voltage of the commercial power supply for the motor are not of sufiicient magnitude to cause any objectionable instability of the received image.

While the best results are obtained when employing a relatively high series field, the special oscillation reducing winding for the field of the motor and the hydraulic coupling element, there is some advantage in using only one or a combination of two of these oscillation reducing means. The invention is particularly useful as applied to television systems because of the very high degree.

of precision required with respect to the synchronization and phase adjustment of the movable element but it may, of course, be adapted for use in other systems. While the specific embodiment of the invention herein shown and described isapplicable only to systems employing a direct current motor, obviously a similar arrangement may be employed for regulating an alternating current motor in which a saturating reactor element is employed in place of the regulating field winding of the direct current motor, as described in an article by the applicant published in the Transa magnetic circuit to another portion without retarding changes in total flux density in the magnetic circuit. For example, a winding in the form of a figure 8 with the overlapping portions insulated from each other may be associated with a single magnetic pole so that, when there is a shift in flux across the pole face, the current induced in the winding will flow in a clockwise direction in one portion of the circuit and in a counter-clockwise direction in another portion of the circuit, thus setting up a magnetic field which retards the shift in flux.

What is claimed is:

1. A television system comprising a transmitting station and a receiving station, a rotatable image scanning or synthesizing device at each of said stations, a motor for driving each of said image scanning or synthesizing devices, a source of alternating current common to said plurality of stations, independent means at each station for generating an alternating current, and means at each station under the joint control of current from said common source of alternating current and current from said local source of alternating current at one of said stations for controlling the speed of the motor at that station for maintaining current, the frequency of which is determined,

by the speed of the driving motor at that station, means at each station under the joint control of current from said common source of alternating current and current from said local source of alternating current at one of said stations for controlling the speed of the motor at that station for maintaining the image scanning and the image synthesizing at said stations in synchronism.

3. In a television system, a rotatable image scanning or synthesizing means, a driving motor for said rotatable means, anti-hunting windings on said motor the efiectiveness of which in reducing hunting is a function of the natural frequency of hunt of the rotating system, the moment of inertia of the rotatable means being such as to lower the natural frequency of hunt of the rotating system to such an extent that hunting would occur if the rotatable means'were rigidly coupled to the motor armature, and means for coupling the motor armature to the rotatable means of such a character as to prevent such a decrease in the natural frequency of hunt of the rotating system.

4. In a television system, a motor, windings associated with said motor for substantially preventing oscillations in the rotation of its armature, a rotatable image scanning or synthesizing device,

the moment of inertia of which is of such a mag nitude that said windings would be ineffective in reducing oscillations in the rotation of said motor armature if said rotatable means were rigidly coupled thereto, and means for non-rigidly coupling said rotatable means to said motor armature of such a character that said windings remain eflective in substantially preventing oscillations in the rotation of the motor armature when coupled to said rotatable means.

5. In a television system, a motor, damping windings associated with said motor to reduce its hunting action resulting from the shift of flux in the pole-pieces produced by the reaction of the armature thereon, a rotatable scanning or synthesizing element driven by said armature having such a large moment of inertia that if rigidly coupled to said armature the damping action of said windings would become ineflicient

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