US2333502A - Single phase to polyphase converter - Google Patents

Description

Nov. 2, 1943.

P. B. WICKHAM SINGLE PHASE TO POLYPHASE CONVERTERS F i led March 24, 1941 4-3- 250 was INVENTOR. Par/(gm B. Walk/11am wag Patented Nov. 2, 1943 Parker B. Wlckham, Delavan,

Wis.,. assignor, by

. mesne assignments, to George W. Borg Corporation, Chicago, 111., a corporation of Delaware Application March 24, 1941, Serial No. 384,785

11 Claims. (Cl. 172-238) The present invention relates in general to single phase to ,DOlyphase converters, and the object of the inventio'n'is the production of a new and improved apparatus of this character. More in particular, the invention deals with the utilization of a variable frequency source of single phase alternating current having a negligible power output to obtain, by suitable conversion and amplification, a variable frequency quarter phase or three phase output of several kilowatts or more with good voltage regulation and suitable for use in running a polyphase motor at a variable speed. It may bestated, moreover, that the invention not only provides a power output of variable frequency,'but affords means for adjusting the frequency to any desired value within the limits of the equipment.

The lnvention'has application to numerous industrial problems where an alternating current motor with accurately adjustable speed is required, and solves these problems in an economical and satisfactory manner.

The invention and various features thereof will be described more fully hereinafter, reference being made to the accompanying drawing, which shows by means of a conventional type of circuit diagram sufiicient of the apparatus and circuit connections to enable the invention to be explained and understood.

Referring to the drawing, the rectangles to 5, inclusive, represent the component parts of a beat frequencyoscillato'r, which comprisesthe also includes a resistance I5, but in circuits 2 and I3 the corresponding elements are a condenser |1 (circuit I2) and an inductance l9 (circuit I3). The capacity and inductance values At the next circuit stage two tubes and 3| are employed, one for each phase of the output from the preceding stage. These tubes may be of the Type 6F6G. The plate circuit of tube 20 is coupled to the grid of tube 30 by means of a condenser 25, the grid being provided with 8.

- grid leak 28. The plate circuits of the tubes 2| radio frequency oscillators l and 3, a mixer stage 2, and filter and amplifier stages 4 and 5. The construction and operation of a beat frequency oscillator are well known and hence the circuits are not shown in detail. It will sufilce to say that the outputs of the two radio frequency oscillators and 3 are combined at the mixer stage 2, to produce a beat frequency which is equal to the difierence between the oscillator frequencies. The output from the mixer stage also includescomponents having the oscillator frequencies and a component having a frequency equal to the sum of the oscillator frequencies,

jjbut these undesired components are eliminated at the filter stage 4, which is arranged to pass only the beat frequency component. The filter output is delivered to the amplifier stage 5, having a grounded output circuit Ill.

The output circuit l0 may be regarded as having three parallel branches l2, and |'3. These branch circuits include resistances l4, |6, and I8, respectively, which may have a value of about 100,000 ohms each. The branch circuit and 22 are coupled to the gridof tube 3| by means of a. transformer 23"and potentiometer 24 The plate circuit of the tube 30 includes the primary winding of the transformer 34, while the plate circuit of the tube 3| includes the primary winding of the transformer 35. The secondary windings of these two transformers are wired in accordance with the familiar Scott connection to provide a three phase output circuit comprising'the conductors 36, 31, and 38. This arrangement will be readily understood, although it may be stated that the tap 83 is at the midpoint of the secondary winding, while the tap 84 is an 86.6 per cent tap, it being assumed that the two secondary windings have the same number of turns.

The remainder of the circuit drawing shows a two stage three phase amplifier. The first stage comprises the three tubes 40, 4|, and 42, which are preferably Type '845 tubes. These are filament type tubes, having their filaments connected in parallel and supplied with heating current by the transformer 43. The grids of the tubes 40, 4|, and" are controlled over the three conductors 36, 31, and 38; respectively, to which they are directly connected. A common .grid return is provided by the grounded tap 35,

which should be located one-third of the distance from'tap 84 to the upper end of the secondary The last or power stage comprises six Type 805 power tubes connected in pairs to provide three push-pull power amplifiers, one for each of the three phases. These are also filament type tubes. A transformer 56 supplies heating current to tubes 50 and another transformer 51 supplies heating current to tubes 52 and 53, .while transformer 58 supplies heating current to tubes 54 and 55. The plate circuit of tube 40 is coupled to the grids of tubes 50 and 5| by means of a transformer 46. Similarly, the plate circuit of tube 1| is coupled to the grids of tubes 52 and 53 by the transformer 41, while the plate circuit of tube 42 is coupled to the grids of tubes 54 and 55 by'means of transformer 48. The center points of the secondary windings of these three transformers are grounded to complete the grid circuits of the tubes.

The plate circuits of the tubes of the three power amplifiers include the primary windings of the transformers 60, 6 I, and 62, respectively. The secondary windings of these transformers are delta connected, and are provided with the three phase output leads 12, and I3.

Suitable sources of direct current are provided for the various plate circuits of the tubes. The plate supply for tubes 20, 2| 22, 30, and 3| may have a potential of 250 volts. The plate voltage at the tubes of the three phase amplifier is higher, preferably in the neighborhood of 1200 volts. These direct current sources are not shown in the drawing, which merely shows the two conductors 85 and 66, with appropriate labels, as is customary in such circuit drawings. It will be understood, however, that conductors 55 and 60 are connected to the positive poles of the 250- volt and 1200-volt direct current sources, respectively, and that the negative poles of these sources are grounded. I

The cathodes of the various tubes are of course grounded also, in order to complete the cathode plate circuits through the tubes. Thus in the case of the power tubes 50 to 55, inclusive, the cathodes or filaments are connected to ground by grounding the center taps of the secondary windings of the heating current transformers 58, 51, and 58. Where a grid bias is necessary or desirable, cathode resistors are provided. Thus the filaments of tubes 40. 4 and 42 are grounded through the resistor H, which is preferably shunted by a by-pass condenser 45. Tubes 20, 30, and 3| are provided with individual cathode resistors 25, 32, and 33, respectively, while tubes 2| and 22 have a common cathode resistor 21.

The tubes 20, 2|, 22, 30, and 3|.require suitable arrangements for supplying current for heating their cathodes. These circuits are well known, however, and are not shown in the drawing.

The apparatus and connections thereof having been briefly described, the operation of the complete circuit will now be explained.

The output frequency of the beat frequency oscillator may vary over a considerable range, from 60 cycles per second to 400 cycles per second, for example. Within this range the frequency is adjusted to any desired value by adjusting the frequency of one of the radio frequency oscillators. For this purpose the oscillator 3 may be provided with adjustable tuning means controlled by a dial 6, which is calibrated in cycles per second, the calibrations referring to the beat frequency output. It may be assumed that the oscillator is adjusted to some frequency near the middle of the frequency range, say 230 cycles per second.

The beat frequency oscillator accordingly main tains a. flow of single phase alternating current at 230 cycles per second in the output circuit Hi. It will be understood that no appreciable amount of power is furnished by the oscillator, its function being to supply control voltages of the desired frequency.

As previously explained, the output circuit It has three branch circuits l2, and lit. The three branch circuits are used for controlling the grids of tubes 20, 2|, and 22, respectively. The manner in which this takes place will be briefly explained.

Considering first the branch circuit I and tube 20, the resistance l5 in circuit H is also included in the grid circuit of the tube and accordingly alternating voltages are impressed on the grid,.

these voltages having a value which is determined by the voltage drop across the resistance |5. In other words, the potential across the resistance I5 is available and effective to control the grid. The circuit H has no inductive or capacitative reactance, and accordingly the voltages impressed on the grid are in phase with the applied voltage derived from the oscillator.

In the branch circuit l2 there is a condenser instead of a resistance such as |5. There is a voltage drop across the condenser which is im pressed on the grid of tube 2|, the value of the impressed voltage depending on the impedance of the condenser. The grid of tube 2| is therefore controlled similar to the way the grid of tube 20 is controlled. The voltages applied to the grid of tube 2| are, however, displaced in phase from the voltages applied to the grid of tube 20 by approximately ninety degrees. Explaining this briefly, it may be stated that the current in the branch circuit I2 is nearly in phase with the applied voltage, due to the fact that the capacitative reactance of the condenser is relatively small as compared to the value of the resistance IS. The voltage across the condenser, which is the voltage effective to control the grid of tube 2|, lags behind the current by ninety degrees, and accordingly is approximately ninety degrees out of phase with the voltage applied to the grid of tube 20.

In the branch circuit |3 there is an inductance l9 and the voltage drop across this inductance is impressed on the grid of tube 22. In circuit I! also the current is nearly in phase with the applied voltage, due to the large value of the resistance l8 as compared to the inductive reactance of coil l9. The voltage across the inductance I9, however, leads the current by ninety degrees, and accordingly the voltage impressed on the grid of tube 22 is approximately ninety degrees out of phase with the voltage impressed on the grid of tube 20. The grid voltage at tube 22 is leading, whereas the grid voltage at tube 2| is lagging, from which it will be seen that the grid voltages at tubes 2| and 22 are displaced in phase from each other by one hundred eighty degrees, ap-

proximately.

The control voltages impressed on the grid of tube 20 cause variations of plate current in tube 20 and the resulting variations in plate voltage at the tube are impressed on the grid of tube 30 through the condenser 25.

Similarly, the control voltages impressed on the grid of tube 2| bring about fluctuations of plate current in the tube, and since the plate circuit includes a part of the primary winding of the transformer 23, alternating currents are caused to flow in the secondary winding of the transformer and in the potentiometer resistance 24. There is a fall of potential across the reettembt to do sistance 24, and a. portion of this voltage is impressed on the grid of tube 3!, the value or the impressed grid voltage depending on the adjust-- ment of the potentiometer.

The grid voltages at tubes and 3i are ninety degrees out of phase with each other due to the described phase shift in the grid voltage at tube 25, which legs behind the voltage at the grid of tube 20 by ninety oegrees. This phase displacement is carried over to the grids of tubes es and .35, as will be obvious from the fact that phase shifts of one eighty oegrees such as ere produced by tubes couplings ceimot change e ninety degree phase difference.

Now it will be recalled that the grid voltage at tube leads the applied voltage by the some eugie that ti: voltage at tube 28 legs behind it; is,

c out; it will be appreciated o titious correct for push e supply i to the oriiuery winding" of These two tubes coope ".1 the welt imovm meouer in suopl e "ormer.

the grids two tubes plate coitus,

the ou of tub--- couplings e you, it is preferable not to so. standard trunsiormer used here, i

seconds e is largely in excess or? .s of the potentiometer voitezze is to the required value, the .ooteutiom udjustec'i so that "coits toe of tube Si is the as that imp e'i on the grid of tube As explained, the voltages the grids of tubes and iii ere out oi hose with each other by ninety degrees. The tubes end may ther iore he roger :i as a two phase or ousrter phase emplifier, which operates in at summer which will be ree understood and which requires no detailed explanation.

The two output Erorn tubes and Si is converted into u three phase output means oi the transformers 3:; and 3&3, the secondary wiurlings of which are connects in accordance with the known Scott connection, as previously mentioned. operation of this circuit orrangement also well understood.

In this manner there is produced a. three phase output in t 1e three phase circuit comgorisins, conductors 36, W, and 35, which is used to control the grids of tubes so, ll, and 82. these tubes constituting a three phase amplifier. Each tube operates to amplify one ehsse, and to deliver controlling voltages to the associated tubes in the power amplifier. Thus the tube 40 is controlled over conductor and by means of transformer supplies control voltages to the push-pull connected power amplifier comprising tubes and 5!. In a similar manner the tube 4! controls the power amolifier comprising tubes 52 and 53,

voltsg'es st tubes iii and 22 frequency is the the coat frequency oscillator,

complisherl while the tube 42 controls the power amplifier comprising the tubes 54 and 55.

In connection with the foregoing it may be remarked that the power amplifiers are class B amplifiers, and hence the grids of the tubes draw grid current, which means that the tubes at the preceding stage must supply a certain amount of power in order to properly control the grids at the power tubes. This is the reason i'or using the Type e45 tubes at the preceding stage. A Type 8&5 tube lies a power output of about thirty watts, which is ample for the purpose.

The outputs of the three power amplifiers are coupled by meuns of trsusformers so, 6i, and 32 to the three phase output circuit comprising condoctors H, 32, end '33, as previously stated.

,These conductors extend directly to s, motor and supply three phase power for coex sting it. The transformer win ore, of course, deeisned to give the riesiretl o out voituge.

s the outout ouehcy oi cycles per second.

It may be increase assumed new that it desired to speed motor. This oc= frequency to e value which will give the i sired mo r speed. The change in freuuericy is e I 1.2; the dial e at the radio ireoue iiioiia may be reset for on outvou secoucl, for example.

The choose in os or frequencaffect the proper o ion of opp-oletus; that is, it produces rely the intended result, which is to reuse cutout frequency of the power amplifier to it cycles oer second. it is noted in particular the output voltages of tubes so and PM rem in euuei, which insures equal voltages in the three phases of the power output circuit. clue to compensating action which st tubes 25 and 22, as will now be explained.

When the frequency of the cutout to is rcisecl, the iouceduhco of the ooucieuser W is corre eomiiugly the drop across this circuit element is which lowers the voltages impressed on the grid of tube 29. other Words, the voltage drop ucross the condenser and the voltages on the grid of tube 2! are inversely proportional to the irequency. At tube 21, therefore, the result of the increase in frequency is to decrease the some ofplete current fluctuations, which woulo, unless compensated for in some way, reduce the voltages irstlucecl in the secondary ding oi the transformer 23 and tine voltages impressed on the grid of tube 35.

The necessary compensation is provided by tube 22. When the frequency is raised, the eifec tive imzaedunce of the inductance i3 is increased '-proportionateiy, and the voltages impressed on the grid of tube ere increased. That is, the drop across the inductance it and the voltages impressed on the grid of tube 32 ere directly proportions! to the frequency. The effect of the increased frequency at tube 22, therefore, is to increase the range of plate current fluctuations, which tends to-increase the voltages induced in the secondary winding of transformer 23 and the voltages impressed on the grid of tube 3!.

The net result ofthe foregoing is that the changes in output at the tubes Hand 22 approximately cancel each other and the impressed voltages on the grid of tube 3i are maintained substantially unchanged in spite of the change in frequency. The greater part of the grid voltage at tube 3|, however, is due to the operation of tube 22; that is, the push-pull hookup of tubes 2| and 22 is now in unbalanced condition, with tube 22 supplying the greater part of the effective energy delivered to the transformer 23.

The change in frequency has no effect on the operation of tube 20, since the branch circuit H is made up substantially of pure resistance. The voltages delivered to the grid of tube 30, therefore, have a constant amplitude and since the voltages delivered to the grid of tube II are also maintained substantially constant in the manner explained, the voltage balance between the differentphases, both in the quarter phase section and in the three phase section, is not disturbed, and satisfactory operation of the equipment as a whole is maintained.

In case the frequency is lowered instead of raised, the necessary compensation is eflected in a manner similar to that described, as will be readily understood. If the frequency is lowered to 100 cycles per second, for example, the

impedance of the condenser i1 is increased, while that of the inductance I9 is decreased, with the result that the roles enacted by the tubes 2| and 22 are reversed, the former instead of the latter now supplying the greater part of the effective energy delivered to the transformer 23 and there utilized in generating grid control voltages for the tube 2|.

In view of all the foregoing, it will be seen that the frequency can be varied over a considerable range, affording a wide range of motor speed, the equipment operating satisfactorily without adjustment other than the adjustment of the dial 6 to change the frequency. The balanced condition of the equipment is maintained automatically by the compensating arrangement.

It will be understood that a two phase power amplifier can be substituted for the three phase amplifier shown herein if a two phase power output is desired. A two phase or three phase inverter could also be used in place of an amplifier.

It will be understood also that the invention permits of other modifications within the principles set forth, and that the embodiment specifically described herein is intended merely as an illustration of one way in which the invention may be carried out in practice. I do not therefore wish to be limited to the exact form of the invention as shown and described herein, but desire to include and have protected by letters patent all forms and modifications of my invention that come within the ,scope of the appended claims.

I claim:

1. In combination, a single phase circuit supplied with a variable frequency alternating current, a two phase circuit, means including said single phase circuit and a phase shifting device for supplying alternating current to said two phase circuit, and means for preventing the voltages in the two phases of the two phase circuit from changing relative to each other in response to a change in the frequency of the current in said single phase circuit.

2. In combination, a single phase circuit supplied with a variable frequency alternating current, a two phase circuit, means including said single phase circuit and a phase shifting device for supplying alternating current to said two phase circuit, and compensating means for automatically maintaining equal voltages in the two phases oi. the two phase circuit while the frequency of the single phase circuit is changed.

3. In combination, a single phase circuit 811D- plied with a variable frequency alternating current, a two phase circuit, means including said single phase circuit and a phase shifting device for supplying alternating current to said two 'phase circuit, said means including condenser and transformer couplings for the two phases, respectively, means for adjusting the voltage in the transformer coupled phase, and means for automatically preventing changes in the adjusted voltage in response to changes in the frequency of said single phase circuit.

4.. In combination, a single phase circuit supplied with a variable frequency alternating current, a-three phase circuit, means including said single phase circuit, phase splitting devices, and a plurality of amplifiers for supplying power to said three phase circuit, adjusting means for equalizing the voltages in the three phases of the three phase circuit, and automatic compensating means for preventing a change in the frequency of the single phase circuit rrom changing said voltages.

5. In combination, a single phase circuit supplied with a variable frequency alternating current, a two phase circuit, means including said single phase circuit for supplying power to one phase of said two phase circuit, means including said single phase circuit and two phase shift-ins devices for supplying power to the second phase of said two phase circuit, and means whereby the said phase shifting devices cooperate to prevent frequency changes in the single phase circuit from changing the voltage in said second phase.

6. For use in a single phase to polyphase converter, a phase shifting arrangement comprising two space discharge devices, means including capacitative and inductive reactances for impressing lagging and leading control voltages on the grids of said devices, respectively, said means including a source of alternating current, and means for combining the outputs of said devices to energize a common output circuit, said last means including means for utilizing the opposite changes in said control voltages in response to changes in the frequency of said source for preventing voltage changes in said output circuit.

'1. For use in a single phase to polyphase converter, a phase shifting arrangement comprising two space discharge devices, two input circuits for said two devices, respectively. to which alternating voltages may be applied. means in one circuit for producing control voltages on the grid of the associated device which are inversely proportional in amplitude to th frequency of the applied voltages and which lag the same by ninety degrees, means in the other circuit for producing control voltages on the grid of the associated device which are directly proportional in amplitude to the frequency of the applied voltages and which lead the same by ninety degrees, an output circuit, and means including said discharge devices for applying control voltages to said output circuit which are ninety degrees out of phase with the said applied voltages and which have a constant amplitude independent of the frequency of said applied voltages.

8. In combination, a single phase circuit supplied with a variable frequency alternating current, said circuit having three branches, a resistance in the first branch, a condenser in the second branch, an inductance in the third branch, three space discharge devices, circuit connections for impressing the voltages developed across said resistance, condenser, and inductance on the grids of said tubes, respectively, a two phase circuit, means for supplying power to one phase from the discharge device controlled over said first branch, and means including a push-pull connected transformer whereby power is supplied to the second phase by the discharge devices which are controlled over the said second and third branches.

9. In combination, a single phase circuit supplied with a variable frequency alternating current, said circuit having three branches, a resistance in the first branch, a condenser in the second branch, an inductance in the third branch, three space discharge devices, circuit connections for impressing the voltages developed across said resistance, condenser, and inductance on the grids of said tubes, respectively, two additional space discharge devices, means for controlling the grid of the fourth discharge device by voltages supplied from the discharge device controlled over the first branch, means for controlling the grid of the fifth discharge device by voltages derived from push-pull operation of the two discharge devices controlled over the second and third branches, and a twophase circuit supplied with power from the fourth and fifth discharge devices.

10. In combination, a three phase power supply circuit, three power amplifiers for supplying power to the three phases of said circuit, respectively, a three phase amplifier for supplying grid voltages for said power amplifiers, means including a two phase amplifier and suitable transformer connections for supplying grid voltages to said three phase amplifier, a beat frequency oscillator, and means including said oscillator and phase shifting means for supplying grid voltages to said two phase amplifier.

11. In combination, a single phase circuit, means for supplying alternating current to said circuit, a two-phase circuit, means for supplying power to said two phase circuit from said single phase circuit, including reactance means for securing a phas displacement of approximately 90 degrees between the voltages in the two phases of said two phase circuit, means for varying the frequency of said alternating current, and means for preventing such frequency variation from creating a difierence in' the amplitudes of the voltages in the two phases of said two phase circuit.

' PARKER B. WICKHAM.

Images