US2178432A - Method of and apparatus for producing a variable alternating current - Google Patents

Description

I Oct. 31, 1939. w PETERSEN 2,178,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheet l Inventor: Waldemar Peters en,

His Attorneg. I

Oct. 31, 1939. w, PETERSEN 2,178,432 METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed'Aug. 12, 1938 8 Sheets-Sheet 2 His Attorney,

w. PETERSEN 8.432

Filed Aug. 12, 1938 8 Sheets-Sheet 3 [\I I A Inventor Waldemar Petersen,

b His Attorn e Oct. 31, 1939.

METHOD OF AND APPARATUS PRODUCING A VARIABLE ALTERNATING CURRENT 0d. 31-, 1939. w. PETERSEN METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT 8 Sheets-Sheet 4 Filed Aug. 12, 1938 Inventor: Waldemar Petersen,

His ttor-neg.

1939- w. PETERSEN 7 3 METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheeti- Inventor: Waldemar" Petersen,

Oct. 31, 1939. w, PETER N 2,118,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheet 6 Fig. IO.

Inventor: Waldemar- Petersen,

His Attorneg.

0a. 31, 1939. w. PETERSEN 2,178,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheet 7 Inventor: Waldemar" Petersen,

Hi 5 At tor'ne Oct. 31, 1939. v w. PETERSEN ,178.62

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheet 8 4 4! Fig l4. 1 I 42 44 DDEIUDU UUDUDEI DDEIDUU C C b Inventor:

1 His ttorneg.

Patented Oct. 31, 1939 METHOD AND APPARATUS FOR PRODUC- ING A VARIABLE ALTERNATING CUR- RENT Waldemar Petersen, Berlin, Germany, assignor to General Electric Company, a corporation of New York Application August 12, 1938, Serial No. 224,439 In Germany June 1, 1937 10. Claims.

My invention relates to the method of and the apparatus for producing a variable alternating current and more particularly to frequency changing systems operated fromavariable ampli- 5 tude or frequency alternating current or from two alternating currents both of which are higher in frequency than the output current.

In certain instances such as motor drives for Pilger mills it is desirable to obtain a variable speed or variable torque drive and for this purpose there may be produced a variable frequency or variable amplitude alternating current which is supplied to the motor in order to obtain the desired speed torque characteristic. Particularly 1b where it is desired to transmit energy from a higher frequency alternating current circuit to a lower frequency alternating current circuit by means of frequency changers it will be found that frequency changers of the type utilizing electric :10 valve converting system will have improved operation characteristics if these valve arrangements are energized from a source of variable frequency alternating current. In accordance with my invention such variable frequency alternating currents are obtained by adding together the voltages obtained from two alternating currents of different frequency. When these voltages are utilized to supply an electric valve frequency changing arrangement the output frequency will "so be an alternating current having a frequency equal to half the difference between the two alternating current supply frequencies.

It is an object of my invention to provide a method of and an apparatus for producing a variable frequency alternating current which will be reliable and simple in operation.

Another object of my invention is to provide an improved frequency changer apparatus operating to transmit energy betwen two or more 0 alternating current circuits.

Tne novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organiza- 45 tion and methodof operation, together with further objects and advantages thereof, will be better understood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1 to 6 illustrate graphically the method of producing variable frequency alternating currents; Fig. 7 shows a circuit arrangement by means of which my method may be carried out; Fig. 8 shows an electric valve frequency changing apparatus operating 55 in accordance with my invention; Fig. 9 shows a further modification of the arrangement shown in Fig. 8; Figs. 10 and 11 are further examples of frequency changing apparatus embodying the principles of my invention; Fig. 12 graphically illustrates the application of my invention to frequency changing apparatus utilizing zero or neutral point anodes; and Figs. 13 and 14 show further frequency changing apparatus to which my invention has been applied.

Referring to Fig. 1 of the drawings I have shown therein a graphical representation of how the superimposition of two sine waves of approximately like frequency will produce a resultant wave whose frequency is the arithmetical mean of the sum of the frequencies of the two alternating waves. This resultant wave indicated as Eab in Fig. 1 has periodic variations in amplitude which are commonly called beats. As is well known, two resultant beat frequencies" are obtained by such superimposition of two sine waves having the same amplitudes and slightly different frequencies, namely, one having a frequency equal to the difference between the original frequencies and the other having a frequency equal to the sum of the original frequencies. Since a beat cycle is defined as one complete rise and fall in amplitude, Fig. 1 discloses one beat cycle of a frequency equal to the difference between the two frequencies of the superimposed waves which is quite obvious from a study of Fig. 2a.. The locus of the amplitudes of the resultant curve Eab describes an envelope Eu Figs. 1 and 4, which varies in sinusoidal form if one considers alternate positive and negative loci of the amplitudes of successive beat cycles and the frequency of this sinusoidal envelope is equal to half the difference between the frequencies of the two alternating current waves which produce the resultant wave Eab. Where the envelope is of sinusoidal form it is to be understood that this resultant envelope is obtained from the superimposition of two sinusoidal curves having equal amplitude. Mathematically the values of such curves or voltages are added in accordancewith the following formula:

Eab=2A sin Trt(fa+fb) -cos 1rt(fafb) :ZA sin obtained-by superimposing Eb on Ea, w is the circuit frequency in radians per second, 1 is the frequency in cycles per second and A is the amplitude of the voltages E9. and Eb. It is observed that the resulting wave Eab contains a product of two trigonometric functions. Considering the coefficient of the cosine term in the above derived equations, it is observed that the value of this term becomes zero at successive values of t differing by values of or one-half of the beat frequency and the amplitude varies between the value 2A and 0. Therefore the alternating wave Eu thus obtained contains two beat cycles for each cycle of Eu as is quite obvious from Fig. 3 since a beat cycle extends from To to T1 while a complete cycle of the alternating wave Eu extends from To to T2,. It Will be seen that the frequency of the envelope Eu is considerably less than either of the frequencies of the alternating waves Ea and Eb. This method of lowering the frequency has the disadvantage that a number of gaps exist in the energy flow. This is apparent from Fig. 1 by observing the envelope Eu of the resultant voltage Eab. Thus there are energy gaps during the time T1, T3, T5, etc. relative to the upper limit of the curve Eu. Where it is necessary to deliver a large energy output, such wave shapes would therefore be undesirable. It therefore, would be necessary to supply additional energy during these periods and such energy supply would also operate at a frequency equal to one-half of the sum of the frequencies of the supply circuit and at the same time a beat would result having a frequency equal to the difference of the two supply frequencies.

In accordance with my invention it is possible to reduce the frequency by combining two alternating current waves and yet at the same time maintain a substantially continuous energy flow. This is accomplished by superimposing the alternating voltage or current waves of a plurality of polyphase angularly symmetrical systems having a like direction of rotation and which Waves have approximately like amplitude and similar wave shape. The degree of continuity in the energy flow with reference to the envelope Eu may be increased by increasing the number of phases used. In the instance of three phases of sine wave form the values are added in accordance with the following mathematical formulae:

where Ea, E's. and E"& are the instantaneous volt- E"b are instantaneous phase voltages of another three-phase alternating curent voltage of a slightly different frequency and a, a and a, b, b and b" indicate the phase displacement in radians of the individual phase voltages compared with a starting point which is assumed to be zero. By superimposing Ea and Eb, E's and Eb, and Ea and E"b the following equations are obtained:

cos o F 1 pa n Let Then - Figs. 2A to 20. Figs. 2A to 20 differ from each other in that the voltages Ea, E'a, E".. and the voltages Eb, E'b, E"b are displaced 120 with respect to each other. The addition of these two voltages produces a resultant voltage Eab, E'ab, and E"ab, each of which has a similar envelope Eu as is apparent from Figs. 2A to 20. In Fig. 3 there is shown the manner in which the three phase voltages are combined. From this figure it willbe apparent that at the time T0, T1, T2, etc., the voltages Ea and Eb have an instantaneous value which is zero with the result that the resultant voltage Eab is zero. At the same time the remaining voltages E'a and Eb and E"a and E"b have instantaneous values which are of equal value in opposite phase so that the resultant values E'ab Or E"ab at the time T0, T1, T2, etc., are likewise equal to zero. These conditions can also be verified by reference to the mathematical Equation (2), which give the standards for the reduction of frequency in the cosine function. This function is identical for all three phases Eab, E' s and E"ab when the dfferences of the phase angles a and b is zero or of equal value for all phases. From this it follows that the displacement of the individual voltages which are added together in multiphase must be the same in each system with reference to the actual frequency scale.

The mathematical Formula (2) also gives an indication as to the manner in which the resultant envelopes Eu are displaced in phase relation with respect to each other in multiphase systems. The factor 11 which occurs in Equation (2) indicates the difference in phase in radians between the individual voltages participating in the voltage formation as compared with the initial value. If measures are taken so that the values a and b differ from each other by certain predetermined angular amount, that is, in contradistinction to the previous requirements of equality, then envelopes will be obtained which have a different phase position. Thus, for example, if it were desired to obtain a system having envelopes corresponding to a three-phase system one would displace one of the two producing voltages by 180 and then the phase displacement of the envelope would be 90. Fig. 4 illustrates graphically how this occurs. If the voltages represented by the curves Eao' which comprise the individual voltages Ea, E'a, E"a, are combined with the voltages of .the curves Ebo which comprise the individual voltages Es, E's, E"b, an envelope Eu corresponding to the fourth curve in this figure would be obtained. If now the voltages similar to the curves E are combined with voltages shown by the curves Eco which are 180 out of phase with respect to the curves Ebo then the envelope E'u forming the last portion of Fig. 4 would be obtained which envelope has a phase displacement of 90 with respect to the envelope Eu. The same results would follow if instead of displacing the voltages Ebo by 180 as shown by Eco, the voltage E80 were displaced by the same amount. It will be apparent to those skilled in the art that this illustration is not limited to the three-phase example just described but can be applied equally well to other multiphase arrangements.

While for the purposes of illustration up to this point it has been shown that sinusoidal voltages may be added by these methods, it is also possible to combine multiphase voltages having non-sinusoidal wave shapes. These non-sinusoidal wave shapes when added, however, will produce similar shaped non-sinusoidal envelopes.

The only limitation to be observed is that the higher harmonics should not exhibit any phase displacement relative to thefundamental wave with respect to the passage through zero of the latter wave and that both forces of energy should "have mutually equal mean symmetrical wave shapes. This is adequately illustrated by the graphical representation of Figs. 5 and 6 which show the possibilities of adding non-sinusoidal v wave shapes such as triangular waves and trapea a); b b wherein the function 1 (wt) fluctuates with the circuit frequency w between the values +1 and 1; A is the amplitude which is the same for I both alternating current voltages. In comparison to the sinusoidal form this formula is developed into a Fourier series the coefficients of which (a for a system with a frequency, of wt, b for a system with a frequency of 10b) assume such values that Ea and Eb for different time instants are given values corresponding to those of the triangular or trapezoidal shape. Such a phase is represented by the following equation:

This simple equation .is' obtained by assuming that the higher harmonics do not exhibit any phase displacement relative to the fundamental wave. If a series is set up for each of the two producing functions or alternating current sources, then these will differ only in the circuit frequencies 10a and wt and in the phase angles a and s but not in the values of the Fourier coefiicients an to an and be to bu. From this it is apparent that it is necessary to have two wave shapes which are similar with the exception of their phase displacement. If now two such waves are added, which waves are produced by the individual currents or voltages E9. and Eb, then the following formula'is obtained:

+2.13 sin wm m) cos wd+ 4)+ If the first and last members of theresulting voltages Eab to E'Hm are combined, then there will be obtained voltages which have cosine functions with the frequency wa and its multiples as a limit and which exhibits an associated phase angle of d and its multiples. The combination of the individual limits then take place} in the same manner as in the case of the funda inental functions Ea and Eb. The intermediate quantities, the sine functions, indicate the course of the voltages between the envelopes. As has been stated, curve forms similar to the fundamental wave shapes are only obtained when both fundamental components or. voltages have similar forms, since. otherwise the Fourier coeflicients or to a and In to ba would differ from each other and it would not be possible to add in the individual harmonic components, or a zero value frequency change may be obtained which will have a wave shape satisfactory for all purposes, without the use of additional or auxiliary energy supply forces or energy storage devices, by adding together two polyphase voltages of different frequencies of any one of a large number of symmetrical wave shapes.

The resultant voltages obtained by the addition of a plurality of voltages of different frequencies to produce an envelope which varies periodically in accordance with this invention may be utilized for power purposes. A number of examples showing the manner in which such resultant voltages having an envelope of a relatively low frequency may be used will now be explained. The arrangement shown in Fig. 7 shows a simple form of frequency changer operated from an alternating current source or line l which has a frequency equal to fa which supplies energy to a transformer 2 having a star connected secondary winding 3. A suitable motor 4 is utilized to drive an alternating current generator 5 which has a frequency fb which is supplied to the primary winding of a transformer 6, the secondary winding 1 of which is connected in series with the secondary winding 3 of the transformer 2 and the resultant voltages are supplied to the alternating current cirwit 8, which has a frequency-which varies so as to have an envelope of a frequency fu- The voltages combined in the transformer secondaries 3 and 7 are similar to the graphical representations of voltages disclosed in Figures 2A to 2C and Fig. 3. If a three phase motor is now connected to the alternating current circuit 8 the torque thereof will be varied in accordance with the frequency difference between the two voltages fa and ft. The difference between the frequencies, however, is not apparent in the operation of the motor since the torque is proportional to the square of the voltage or to the square of the current. As will be apparent from formula 1 the motor will operate at a frequency which is equal to the arithmetical mean of the sum of the frequencies of the two supply voltages. If the moment of inertia of the motor is not/too great or if the motor is not connected to an extremely heavy load, then a change in speed will occur which i has a rhythm corresponding to the frequency difference between the two supply voltages. Such motors may be used with advantage to drive Pilger mills and any other drives which require similar movement as for instance drives for riddles and certain feedmovements in seam welding operation.

A further important field of use to which the idea of this invention may be applied is found in those instances where it is advantageous to supply 'single or multiphase low frequency systems from a multiphase system of higher frequency or where.

it is desired to transfer energy between two such systems. Since rotating machines have certain disadvantages static electric valve converting devices have been utilized. By means of my invention it'is now possible in a simple manner to; satisfy all the requirements of converter operation and asynchronous connection between alternating current systems. Heretofore the wave shape produced by electric valve converting frequency changing systems have had the disadvantage of producing wave shapes which were nonsinusoidal in form. Numerous methods have been su gested, such as tap transformers, for improving thelwave shape of such systems. The disadvantages of such additionalcomplications and apparatus may be obviated by the application of my to cycles.

invention to electric valve converting apparatus. In Fig. 3 reference is made to the possibility of reducing the frequency by producing a resultant wave having an envelope Eu. From this it is apparent that there is only the problem of converting such resultant waves into a pure alternating current wave. The resultant polyphase voltages will therefore be supplied to an electric valve converting apparatus in which one group of discharge devices will operate to produce a positive half wave and another group will operate to produce a negative half wave. Such an arrangement is disclosed in Fig. 8 wherein the polyphase alternating current supply line 9 is used to supply energy to a motor H) which is coupled to an alternating current generator II. It will be assumed that the supply line has a frequency equal to It and the output of the generator H has a frequency equal to fb. The generator I is connected to a transformer having a primary winding l2 and a plurality of groups of secondary windings I3 and H. The alternating current supply line 9 is also connected to a transformer having a primary winding l5 and a plurality of groups of secondary windings IS and. IT. The secondary winding l6 of the one transformer and the secondary winding |3 of the other transformer are connected in series and similarly the secondary winding ll of the one transformer and the secondary winding M of the other transformer are likewise connected in series. These two sets of series connected secondary windings of the transformers operate as two polyphase networks supplying voltages to two groups of arc discharge paths which may comprise a single cathode, multi-anode electric discharge device -|8. Any of the types of electric valves well known in the art may be utilized although it is preferable to use those having an anode and a cathode contained within an envelope filled with an ionizable medium. The cathode of the electric discharge path I 8 is connected through a suitable reactor I9 to the midpoint of the primary winding of a transformer 20 the secondary winding of which is connected to the alternating current load circuit 2|. One extremity of the primary winding of the transformer 20 is connected to the neutral point of the transformer secondary winding l6 and the other extremity of the primary winding of the transformer 20 is connected to the neutral point of the transformer secondary winding Thus when the voltages which result from'the voltage component. introduced by the transformer primary windings l2 and I5 are such as to produce positive resulting potentials in the network comprising the secondary windings 3 and 6 the various arc discharge paths associated therewith will becomeconductive to supply to the transformer 20 energy serving to produce one-half wave of the alternating current appearing in the output circuit 2|. If, for example, it is shown that the alternating current circuit 9 has a commercial frequency of 60 cycles and it is desired to supply 25 cycles to the alternating current output circuit 2|, the frequency of the generator I I will be equal Obviously of course it is immaterial as towhether the alternating current supply line or the generator. H is of a lower frequency since it is only necessary t o supply to the two transformer secondary networks ico'mprising the poly- A phase networks of the electric valve converting apparatus twp frequencies which differ by a sum equal to twice the frequency to be transmitted to the alternating current load circuit 2|. -If "desired, the alternating current supplied by the generator may be variable in frequency and hence nonconductive.

the alternating current in the output circuit 2| will be variable in frequency. For this purpose I have schematically illustrated motor ID as of the wound rotor induction motor type havinga variable resistor llls connected to the rotor terminals I whereby variation of the speed of motor l may be obtained and hence also variation of the output frequency of generator H. The various arc discharge paths of the electric valves may be controlled by control grids. Any suitable control circuit well known in the art may be utilized in conjunction with the control electrodes or grids. For example, in Fig. 8 I have shown the well known control circuit described and illustrated in United States Letters Patent No. 1,408,118, grantedFebruary 28, 1922, to F. W. Meyer. The control electrodes or grids 66 for the discharge paths associated with the secondary windings l4 and H are energized with a positive potential from battery 61 through distributor 68 and resistances 69 and 10 every 180 electrical degrees with reference to a voltage cycle of the output frequency I; that is,

Il -f1, 2

During the other half cycle control electrodes or grids H for the discharge paths associated with secondary windings l3 and iii are energized with positive potentials from battery 12 through distributor 13 and resistances I4 and 15. A negative bias potential from batteries 16 and TI is impressed upon control electrodes 66 and II, respectively, during the alternate half cycles for which the associated discharge paths are to be maintained Distributors 68 and 13 may be driven by a synchronous motor 16 energized from a suitable source of supply 19 having a frequency equal to half the difference between frequencies fa and It. If the frequency changer illustrated in Fig. 8 is operating, synchronous motor 18 may be energized directly from the output circuit 21. It will be understood by those skilled in the art that any other control circuit well known in the art may be used where only a single frequency is utilized for controlling the arc discharge paths and this frequency corresponds to the alternating current output frequency. During one half cycle of the output frequency in control electrodes 66 are energized with positive potential and the three associated discharge paths become successively conductive twice during each half cycle as illus trated by Fig. 12. The current commutates from one discharge path to the other whenever the voltage impressed on the succeeding anode to become conductive is equal to or higher than the voltage impressed upon the anode of the discharge path which is conducting current. At the end of the half cycle the negative bias impressed upon control electrodes 66 by virtue of bias battery 16 maintains the associated discharge paths nonconductive and during the next half cycle the discharge paths associated with control electrodes H conduct current so as to produce a resultant output voltage of a frequency equal to half the difference between the two supply frequencies.

If desired the arrangement shown in Fig. 8 may be modified in accordance with that shown in Fig. 9 wherein similar parts have been given similar reference characters. In this instance, however, the motor I0 is of the synchronous type and the generator. i I has been replaced by a generator 22 which is of the double winding type. A set of windings of the generator is in series with a different set of secondary windings such as l6 and M, respectively, of the transformer the primary winding l5 of which is connected to the alternating current circuit 9. As shown here it would be preferable to connect the extremities of the output transformer 20 to the neutral points of the two groups of windings 23 and 24 of the machine or generator 22 in order to simplify the number of connections to the generator.

Still another system to which my invention may be applied is shown in Fig. 10. An alternating current supply circuit 25 supplies energy to a transformer having a primary winding 26 and a secondary winding 21. The circuit also supplies energy to a motor 28 which drives a generator 29 the output of which is connected to a transformer having a primary winding 30 and a secondary winding 3|. The alternating current supply circuit will be assumed to have a frequency equal to fa and the generator 29 will be assumed to have a frequency equal to it. The two secondary windings 21 and 3| of the transformers are connected in series, the one being connected in star relation so that the neutral point thereof is connected to one side of the alternating current load 32. remaining connections to the secondary windings 21 and 3| are each connected by a pair of oppositely connected valves such as 33, 34; 35, 3 6; 31, 38; to the other side of the alternating current load 32. For the purposes of illustration each of the valves 33 to 38 have been shown as comprising an anode, a cathode and a control grid contained within an envelope filled with an ionizable medium, but it will be understood that any other valves commonly known in the art may be utilized. Any suitable control circuit which is well known in the art may be used to render conductive these valves in proper sequence so as to transmit to the alternating current'load 32 alternating current energy having a frequency equal to one-half of the difference between the frequencies fa and is.

Fig. 11 shows another embodiment similar to that shown in Fig. 8 and hence similar parts have been given similar reference characters. In this arrangement, however, the electric arc discharge paths have been arranged in two groups l8a and I81). Both the neutral points of the transformer secondary groups I6 and I! have been connected together to the midpoint of the primary winding of the transformer 20. The outer extremities of the transformer primary winding have each been connected to a different one of the cathodes of the discharge groups '89. and I813. This arrangement has the advantage over Fig. 8 of supplying a still more constant flow of energy to the alternating current circuit 2!.

Heretofore in electric valve frequency converting systems it has been common to control the activities of the various arc discharge paths by a control circuit having two control components one corresponding to the input frequency and the other corresponding to the output frequency. Such an arrangement operated to prevent certain arc discharge paths from becoming conductive so that only those anodes having the proper phase The voltages for production of the alternating current ing of the output transformer. Therefore, in the arrangement shown in Figs. 8, 9 and 11 it is only necessary to provide three are discharge paths for each polyphase network group since each of the arc discharge paths is rendered conductive twice during each half cycle of alternating current in the output circuit. Thus it will be seen that each discharge occurs over a considerable period thereby providing a good utilization of the transformer and yet the entire electric valve apparatus operates only intermittently thus providing good operation with reference to the necessity for dissipating the heat with the result that the danger of are back is materially reduced. The harmonic content of the electric valve converting apparatus by a three phase converter utilizing my invention is about equal to that of a six phase converter of the usual type.

If it is desired further to improve the shape of the low frequency wave, then this can be accomplished by utilizing a greater number of phases for example six phases. The output voltage'will therefore be composed of several partial voltages which have equal or different phase positions. It will be apparent from Fig. 3 that the addition of additional phases are similarly dependent upon whether one considers either the upper or the lower limits of the envelope Eu. Thus in a six phase rectifier one group of three phases will supply an envelope corresponding to U2. and the other group will supply an envelope corresponding to Up.

The systems disclosed in Figs. 7, 8, 9, l0 and 11 are capable of producing a constant frequency alternating current output since it is relatively easy to maintain the speed of these generators constant in view of the fact that the frequency of the output current fu is equal to from which it is apparent that any variation of the generator only produces half as great a variation in the output frequency. If it is desired to produce a variable frequency alternating current output it furthermore will be apparent that the arrangements disclosed in these figures provide a simple arrangement for accomplishing this. Such arrangements have numerous advantages over the arrangements in the prior art since at all output frequencies the power factor of the converter is relatively high. Such arrangements furthermore have the advantage of producing symmetrical alternating current outputs in contrast to the,

asymmetrical wave shape produced by ordinary electric valve translating means which have rectangular or trapezoidal alternating current wave shapes. While the frequency relation between the input frequency and the output .frequency is not an integral factor it is apparent that in the more simple systems the output waves are not symmetrical in shape. Symmetry in the conventional arrangements can only be approximated by utilizing a large number of'phases in a large number of arc discharge paths so that each wave in the output circuit is produced by a plurality of partial wave components supplied by the different electric discharge paths. From this it will be apparent that the arrangements disclosed in accordance with my invention have a relatively high power factor, relatively low harmonic components, and produce symmetrical alternating current wave outputs for both rigid frequency relation and variable frequency relation between the input and output circuits."

' winding 43.

While my invention has been shown as applied to a frequency changing system operating between a polyphase alternating current source and a single phase load circuit, it is equally applicable to transmission of energy between two polyphase alternating current circuits. An example of such an arrangement is disclosed in Fig. 13 wherein an alternating current source 22 having a frequency fa and a second alternating current source 23 having a frequency fb supplies energy to an I electric valve converting system connected to the two phase-alternating current circuits 24 and 25; One of the alternating current supply circuits may be energized from a source of energy comprising a generator such as shown in Figs. '7 to 11. The alternating current source 22 is connected to energize a transformer having a primary winding 26 and two secondary windings 2'! and 28. The alternating current source'23 is connected to energize a transformer having a primary winding 29 and a plurality of secondary windings 30 to 33. Two groups of electric discharge valves 34 and 35 are provided to supply energy to the primary winding of the transformers 36 and 31 which are connected, respectively, to the output circuits 24 and 25. The secondary winding 2'! of the one transformer and the secondary windings 30 and 3| are connected together so that the winding 21 is in series with the winding 30 and supplies energy through the valve group 34 and the primary winding of the transformer 26 through'a suitable smoothing reactor 36. Winding 21 connected in series with the winding 3| supplies energy through the group of valves 35 through a suitable smoothing reactor 31 to the primary. winding of the transformer 21 The winding 28 in series with the' winding 32 supplies energy through the group of valves 34 and the smoothing reactor 36 to the primary winding of the transformer 26 'and the winding 28 in series with the winding 33 supplies energy through the group of valves 35 through the smoothing reactor 31 to the primary winding of the transformer 21. The alternating currents appearing across the circuits 24 and 25 are out of phase and ob- I viously if desired these may be connected to a T- cennected transformer to supply threephase current. It will be apparent to those skilled in the art that any of the arrangements disclosed hereconstant and the frequency of the other alternating current circuits such as 23 is variable the resultant frequency appearing-in either of the output circuits 24 or 25 will also be variable.

Still another manner of obtaining a variable frequency output would be to vary the frequency of both the input circuits since the frequency of the alternating current load circuit corresponds to a frequency which is half of the difference between the frequency of the input circuit. Thus in order to produce a variable frequency output current it -is only necessary to vary the frequency rela- 'tion between the two alternating current supply circuits.

In Fig. 14 there is shown an arrangement for transmitting energy between two three phase systems. Two alternating current supply lines 40 and 4| are'provided, one of which may be energized from a generator. The alternating current circuit 40 .is connected to a transformer having a primary winding 42 and asix phase secondary The alternating current circuit 4| is connected to a transformer having a primary winding 44 and a plurality of three phase secondary windings 45 to 50. Each of the outer extremities of the phase windings of the star-com nected seconudary transformer winding 43 is connected to the neutral point of a different one of the secondary transformer windings 45 to 5.

Each of the outer extremities of the secondary windings 45 to 50 is connected to two different electric valve groups. Six groups of electric valves to 56 each having six are discharge paths are provided and each group is connected through a suitable" smoothing reactor 5i to 52, respectively, to the outer extremities of a six phase star-connected primary winding 53 of a transformer the secondary winding 54 of which is connected to the alternating current output circuit 65. For the purposes of simplicity only one of each of the outer extremities of the six secondary transformer windings 45 to 50 has been shown connected to the two valve groups St to 55 since it will be apparent to those skilled in the art that the remaining transformer terminals are similarly connected to the valve groups 52, 55; 53, 54.

While my invention has been shown and described in connection with certain specific embodiments it will, of course, be understood that it is not to be limited thereto, since it is apparent that the principles herein disclosed are susceptible of numerous other applications, and modifications may be made in the circuit arrangements and in the instrumentalities employed without departing from the spirit and scope of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. The method of producing a lower frequency alternating current quantity from two alternating current quantities of higher frequency which comprises vectorially adding together said higher frequency alternating current quantities to produce a resultant alternating current quantity having a variable frequency and a variable amplitude, and converting said resultant quantity to produce an alternating current quantity corresponding to the envelope. of said resultant quantity.

2. The method of transmitting energy from a higher frequency alternating current circuit to a lower frequency alternating current circuit which comprises generating an alternating current having an amplitude and a wave shape similar to the amplitude and wave shape of said source of alternating current, the frequency of said generated alternatirfi current being greater than that of said lower frequency alternating current circuit, adding together vectorially said generated alternating current with energy from said higher frequency alternating current circuit, and converting said resultant alternating current to produce an alternating current having a frequency equal to that of said lower frequency alternating current circuit.

3. The method of producing a lower frequency alternating current quantity from two alternating current quantities having higher frequencies, similar wave shapes and similar amplitudes which comprises adding vectorially said alternating current quantities of higher frequency to produce a resultant alternating current quantity of variable frequency and variable amplitude, and converting said resultant value to produce an alternating current value having a wave shape corresponding to the envelope of said resultant quantity.

4. A method which comprises adding vectorially two alternatingcurrent quantities of different frequency but of similar amplitude and similar wave shape to produce a resultant alternating current quantity of variable frequency and variable amplitude, varying the frequency relation between said higher frequency alternating current quantities, and rectifying and converting said resultant alternating current quantity to produce a variable frequency alternating current quantity having a wave shape corresponding to the envelope of said resultant alternating current quantity.

5. An electric frequency changing system comprising a source of alternating current having a certain frequency, amplitude and wave shape, means energized from said source for producing an alternating current of different frequency and similar amplitude and wave shape, and means energized from said latter means and from said source for combining said alternating currents to produce a resultant alternating current of variable frequency and variable amplitude.

6. An electric valve frequency converting system comprising two sources of alternating current of different frequencies and of similar wave shape and similar amplitude, means for adding together said alternating currents, electric valve converting means energized from said latter means for producing an alternating current having a frequency equal to half of the difference between the frequencies of said first mentioned alternating currents.

7. In combination, an electric valve frequency changing apparatus including a plurality of arc discharge paths and two groups of serially connected polyphase inductive networks, an alternating current output circuit for said apparatus, said output circuit having a predetermined frequency, and means for supplying energy to said groups of polyphase connected networks, said energy comprising two alternating currents having similar wave shapes and like amplitudes and differing in frequency by an amount equal to twice the frequency of said alternating current output circuit.

8. The combination comprising an electric valve corr'erting apparatus including a plurality of arc discharge valves and two groups of serially connected polyphase inductive networks, means for energizing one of'said inductive networks from a source of polyphase alternating current having a certain frequency amplitude and wave shape, means for energizing the other of said inductive networks with alternating current of similar wave shape and amplitude and means for varying the frequency relation of the alternating currents supplied to said networks.

9. The combination comprising an electric valve frequency converting system including a plurality of arc discharge devices, two groups of polyphase inductive networks, said networks being connected in series relation, an output circult having a predetermined frequency, and means for energizing each of said inductive networks with alternating current having similar wave shapes and amplitudes, said alternating currents differing in frequency by an amount equal to twice the frequency of said alternating current output circuit.

10. An electric frequency changing system lit) comprising a source of alternating current having a predetermined frequency, amplitude and wave shape, means energized from said source for producing a second alternating current of difierent frequency and similar amplitude and wave shape, means energized from said latter means and from said source for vectorially combining said alternating currents of difierent frequencies to produce a resultant alternating current of variable frequency and variable amplitude, and electric valve frequency covering means energized by said resultant alternating currents for producing an alternating current having a frequency corresponding to the frequency of the envelope of said resultant alternating current.

WAIDEMAR PETERSEN.

Certificate of Correction Patent No. 2,178,432. October 31, 1939. WALDEMAR PETERSEN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 20, for the word system read systems; and second column, line 46, in the formula, for E sin W read E =A sin w i; page 2, second column, line 31, for 3A read 2A; page 3, first column, l1ne 18, for E,,, E,, read E, and E,,; and second column, line 44, for (3 +3 read ($w,,+8,,)+; page 5, first column, line 5, for resistor read resistance; page 6, second column, line 47, for cennected read connected; page 7, first column, line 5, for seconudary read secondary; and second column, line 59, claim 8, after amplitude insert a comma; page 8, second column, line 2, claim 10, for covering read converting; and that the said Letters Patent should be read with these corrections therein that the same may conform to the Word of the case in the Patent Office.

Signed and sealed this 12th day of December, A. D. 1939.

HENRY VAN ARSDALE,

Acting Commissioner of Patents,

Certificate of Correction Patent N 0. 2,178,432. October 31, 1939. WALDEMAR PETERSEN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 20, for the word system read systems; and second column, line 46, in the formula, for E sin W read E =A sin w h; page 2, second column, line 31, for 3A read 2.4; page 3, first column, lme 18, for E',,, E,, read E, and E";,; and second column, line 44, for (3 '+3 read (8w +3 page 5, first column, line 5, for resistor read resistance; page 6, second column, line 47, for connected read connected; page 7, first column, line 5, for seconudary read secondary; and second column, line 59, claim 8, after amplltndeli insert a comma; page 8, second column, line 2, claim 10, for covering read converting; and that the said Letters Patent should/be read with these corrections therein that the same may conform to the of the case in the Patent Office. Signed and sealed this 12th day 0f December, A. D. 1939.

HENRY VAN ARSDALE,

Acting Commissioner of Patents,

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