System For Phase Shifting Inverters To Obtain A Variable Modulated Waveform

Abstract

A a. c. motor drive system for effecting fundamental voltage modulation by phase shifting inverters coupled in parallel, thereby providing a gradual voltage change between a minimum pulse width modulated waveform and an unmodulated waveform to overcome surges experienced in the transition therebetween. A voltage programmer produces a control signal predicated on vehicle speed so as to follow a preselected profile computed to control the degree of inverter phase shift so that the phase shift modulated fundamental precisely matches the pulse width modulated fundamental at one extreme of the phase shift region of the voltage curve and precisely matches the unmodulated fundamental at the other extreme of the phase shift region.

Description

BACKGROUND OF THE INVENTION

In a. c. motor drive systems, switching means are normally employed to convert a d. c. supply source into three waveforms symmetrically displaced in phase by 120 electrical degrees for sequentially energizing the three phase load windings comprising the motor field. This results in a rotating field flux which is inductively linked with the rotor to effect rotation of the rotor. In order to increase motor speed, it is necessary to increase the frequency of the rotating motor field flux by operating the switching means more rapidly. With the advancement of solid state technology, power inverters have been developed utilizing thyristors as the switching elements to obtain the fast control necessary in making the a. c. motor drive suitable for use in high speed systems such as railway transit vehicles, which have historically relied upon d. c. motors to provide tractive power. In one such inverter drive system, inverter means is utilized to produce a basic unmodulated line-to-line voltage waveform in a six-step configuration. By employing a reactor to inductively couple two or more of these inverters in phase displaced relationship between the power source and the motor load windings, a more suitable twelve-step waveform may be provided to reduce the harmonics affecting motor operation. In providing suitable motor operation over wide speed ranges, a "constant horsepower" mode of operation is employed at a preselected base speed, in which mode the unmodulated waveform produced by the system inverter is suitable for providing motor voltage at maximum value. However, at speeds below base speed, a "constant torque" mode of operation is employed, which requires that the motor voltage be varied in direct proportion to motor speed. To accomplish this, the basic unmodulated line-to-line waveform is provided with one or more notches, the width of which notches may be narrowed as speed increases to increase the net voltage at the motor, by employing the technique of pulse width modulation.

One of the difficulties encountered in employing a pulse width modulated waveform to control power to the motor in "constant torque" mode of operation is that of making the transition to the unmodulated waveform produced in "constant horsepower" mode of operation without an accompanying disruption of the voltage level. Because of the inherent limitation of the thyristor switching time, although extremely fast, at the base speed at which it is desired to change from constant torque to constant horsepower mode of operation, there exists a minimum notch width. Thus, the width of the single notch comprising the final pulse width modulated waveform cannot be reduced beyond this minimum value to bring the voltage up to a value which approaches the full voltage produced by the unmodulated waveform. Experience has shown that where a discrete change in the voltage level occurs during this transition a noticeable lurch in the motor results, which is discomforting to the passengers of a transit vehicle, and detrimental to the vehicle's gears and drive train. In addition, and perhaps more importantly, such a sudden voltage change results in the thyristor network of the inverter being required to momentarily support a heavy current surge in excess of that which the thyristors can safely commutate or turn off. This can cause a malfunction leading to a shut down. Also, any damage to the inverter thyristors, of course, is extremely costly.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide an a. c. motor drive system capable of effecting transition from a pulse width modulated voltage waveform to an unmodulated waveform without creating a disruption of the motor operation due to a distinct or discrete voltage change accompanying such transition, as heretofore experienced in attempting to reach maximum voltage.

It is another object of the invention to provide a smooth voltage transition from a modulated waveform, such as may be obtained by pulse width modulation, to an unmodulated waveform by employing an intermediate waveform capable of being modulated by phase shifting a pair of inverters producing the voltage waveforms.

It is still another object of the invention to phase shift inverters to obtain a modulated voltage waveform having a completely variable voltage component capable of being precisely matched to another voltage component.

Another object of the invention is that of shifting the phase of a pair of parallel coupled inverters in opposite directions by a like number of electrical degrees from a quiescent reference point or a pair of quiescent points so that the phase of the resultant voltage component following the phase shift coincides with the reference point of the component prior to the phase shift.

In accordance with the objects of the invention, there is provided in a preferred embodiment of the invention an a. c. motor drive system in which a pair of common loads comprising the stator windings of a squirrel cage type induction motor are parallel connected to the phase outputs of first and second inverters via reactor means. The coupled inverters are normally operated .+-. 15.degree. out of phase from a neutral reference, thereby producing a 12-step phase-to-neutral waveform substantially free of low order harmonics.

In a "constant torque" mode of motor operation which is employed between speeds from zero to a preselected base speed at which a "constant horsepower" mode of operation occurs, the motor voltage is adjusted to increase linearly with speed by employing a pulse width modulation technique, in which one or more notches are formed in the voltage waveform and progressively reduced in width as motor frequency increases. Because of the switching response limitations of the thyristors comprising the first and second inverters, it has been found necessary to vary the ratio of notches with speed in decreasing order from zero to base speed. A leading and a lagging carrier generator each provide a carrier signal in the form of a triangular-shaped wave whose frequency varies with motor speed, but at different rates selected in accordance with a plurality of different speed ranges to thereby establish the desired number of notches in the waveform produced. These phase displaced carrier signals are compared to a reference voltage signal programmed to follow a profile generated by a voltage programmer so as to vary in a preselected relationship with speed, thereby producing a pair of phase displaced, square-shaped waveforms having notches which vary in width dependent upon the frequency of the carrier and amplitude of the reference signals. These variable waveforms together with a speed responsive, six-phase output from a ring counter provide the digital input signals to a modulator logic network whose otputs generate the thyristor firing pattern to drive the inverters and in turn produce the desired pulse width modulated waveforms from which the fundamental motor voltage is determined.

At base speed where "constant horsepower" mode of operation begins, an unmodulated waveform is produced to obtain full fundamental voltage. Because of the voltage difference between the full voltage at base speed and the maximum voltage capable of being produced by the final pulse width modulated waveform, an inverter phase shift technique of modulating voltage is employed following pulse width modulation to bring the voltage up to full value in a smooth manner. This is accomplished at a predetermined speed of the motor by reason of the reference voltage signal generated by the voltage programmer, when compared with the carrier signal, producing the variable notched waveforms from which the modulator logic determines the thyristor firing pattern desired. At the predetermined motor speed at which phase shift modulation is initiated, the phase shift modulated waveform is made to precisely match the final pulse width modulated waveform in both fundamental voltage and phase. In approaching base speed, the inverter phase shift is gradually brough back toward the normal .+-. 15.degree. displacement, so as to match the voltage component of the unmodulated waveform produced in "constant horsepower" mode, thus effecting, in conjunction with pulse width modulation, a smooth and continuous voltage transition from zero voltage at zero speed to full voltage at base speed.

A better understanding of the invention will be realized from the following more detailed description when taken with the accompanying drawings in which:

FIG. 1 is a block diagram of the system comprising the invention;

FIG. 2 is a graph of unmodulated waveforms produced in generating full fundamental voltage;

FIG. 3 shows the relationship of the carrier waves generated by the carrier generator and the d. c. reference voltage signal generated by the voltage programmer for a particular speed and ratio frequency;

FIG. 4 shows the signal waveforms obtained by comparison of the carrier and voltage reference signals of FIG. 3;

FIG. 5 is a graph showing the desired motor voltage versus frequency and the several speed ranges in which the different techniques of voltage control are employed;

FIG. 6 shows the profile of the reference voltage signal generated by the voltage programmer with respect to speed;

FIG. 7 is a graph showing the waveforms produced in obtaining a modulated fundamental waveform by the pulse width modulation technique;

FIG. 8 shows the final 12-step fundamental waveforms of FIGS. 2 and 7 superimposed one on the other to readily gain an appreciation of the voltage difference therebetween;

FIG. 9 is a graph showing the waveforms generated in obtaining a modulated fundamental waveform by an inverter phase shift technique; and

FIGS. 10 and 11 are vector diagrams from which the desired degree of inverter phase shift may be determined in matching one fundamental with another.

Referring now to FIG. 1 of the drawings, there is shown for purposes of best illustrating the present invention, an a. c. motor drive system suitable for use in powering a railway type transit vehicle. The system includes a plurality of polyphase induction type a. c. traction motors only one of which is shown, as represented by the dotted block 10. The induction motor includes two sets of load windings W1, W2, W3 and W4, W5, W6 forming the motor stator and a conventional cast aluminum squirrel cage rotor R. The respective load windings may be preferably arranged in a conventional Y configuration, with each set of windings being mechanically displaced by 30 electrical .degree., as shown, to offset the normal .+-. 15.degree. phase displacement of a pair of three-phase inverter devices 11 and 12. These inverters are preferably of a solid state design which employs thyristors in a manner similar to the type disclosed in U. S. Pat. No. 3,207,974, issued Sept. 12, 1965 to W. McMurray entitled "Inverter Circuits." Since a complete description is provided in the aforementioned McMurray patent, no further explanation is deemed necessary except to point out that a thyristor bridge network of each inverter operates to convert a suitable d. c. supply source at conductor 13, which represents either a third rail or overhead power line along a transit system right of way into six-step, phase displaced outputs R, S, T of inverters 11 and U. V, W of inverter 12. The advantage of using thyristors, as is well known, lies in the fast switching times obtained in commutating the d. c. supply. This is especially critical in motor drives employing pulse width modulation to vary voltage, as will hereinafter be apparent. The firing signals controlling the state of the thyristor switches in the inverter bridge network are supplied via inputs A, B, C of inverter 11 and inputs A', B', C' of inverter 12. These firing signals are represented in FIG. 2 by waveforms A, B, C and A', B', C', which vary in a step fashion between zero and the V.D.C. line voltage, turning the appropriate thyristors "ON" when the voltage is high and "OFF"when voltage is low. The thyristor bridge networks thus generate the line-to-line voltage at outputs R, S, T and U, V, W of inverters 11 and 12, respectively for connection to the load windings from the d. c. supply at conduit 13. These line-to-line waveforms alternate between positive and negative polarities for 120 electrical .degree. each, being at zero potential for 60.degree. during each polarity transition, as shown in FIG. 2. It is noted that only waveforms R and U in FIG. 2 are shown, since the other output waveforms of inverters 11 and 12 are of identical shape, but phase displaced by 120.degree. for symmetry. Waveforms R and U represent corresponding outputs of the inverters 11 and 12 and are seen to be phase displaced by 30 electrical .degree., as are the waveforms of the other corresponding inverter outputs, to provide the normal .+-. 15.degree. inverter phase displacement. The normal .+-. 15.degree. phase shift between inverters 11 and 12 is thus brough about by the thyristor firing signals at inputs A, B and C being 30 electrical .degree. out of phase relative to corresponding signals at inputs A', B' and C', as shown by the waveform displacements in FIG. 2.

An inductive reactor 14 couples the inverters 11 and 12 in parallel, preferably by connecting the line-to-line voltage of one inverter via inductors wound on a three-legged magnetic core on which similar inductors associated with the line-to-line voltage of the other inverter are wound. The purpose of this unique winding configuration in parallel coupling the inverters is to cancel unwanted low order harmonics without requiring an excessively large reactor, as fully explained in copending U. S. Pat. application identified by Ser. No. 187,974, filed Oct. 12, 1971, by Udo H. Meier and entitled "Inverters Paralleled With Reactor." For an understanding of the present invention, however, it is only necessary to understand that inverters 11 and 12 are operated .+-. 15 electrical .degree. out of phase with respect to neutral and that they are coupled in parallel to obtain a 12-step phase-to-neutral voltage waveform, as represented in FIG. 2 by waveform VW1. Of course, identical phase-to-neutral waveforms (not shown) are generated in accordance with the waveforms effective at the other inverter outputs S, T, V and W. These phase-to-neutral waveforms are 120.degree.out of phase, thereby energizing the motor windings in a symmetrical fashion so as to produce an apparent rotating field. This particular techique of parallel coupling the inverters to obtain the low harmonic phase-to-neutral voltage waveforms is known as interdigitation. The resultant average fundamental voltage produced by this waveform is represented graphically by a quasi-sinusoidal wave X superimposed on waveform VW1 and having a frequency equal to the frequency at which the inverter is driven and having an RMS value proportional to the area under the waveform VW1. In calculating the fundamental voltage from waveform VW1, maximum and minimum values are assigned to the high and low voltage levels of waveforms A, B, C and A', B', C', and the formula V= 1/6 [2A-B-C + .sqroot.3 (A' - B')] is employed at each point where any one of the A, B, C, A', B'or C' waveforms change state to determine the actual voltage effective at that particular step. It is to be noted, of course, that this voltage waveform is unmodulated, and is effective during a predetermined range of motor speed, as hereinafter explained.

The system further includes a speed sensor device, represented by block 15 as a tachometer generator, associated with the shaft of rotor R of induction motor 10. The tachometer generates an analog voltage representative of the motor speed, which of course can also be assumed to be vehicle speed. This speed signal is connected via conduit 16 to a voltage to frequency converter 17. Also connected to converter 17 via conduit 18 is a slip command signal generated within the system regulator 19. The slip command signal represents desired torque to be produced by motor 11 and is summed at the voltage to frequency converter 17 with the speed signal effective at conduit 16 to produce an analog voltage signal representative of the frequency at which the motor field flux must rotate in order to maintain a slip frequency corresponding to the slip command signal. It is understood of course that the slip frequency expresses the relationship between the rotating motor field flux and rotating rotor R in terms of their different rotating frequencies.

The analog frequency voltage signal resulting from the summation of the slip and speed signals is converted to a digital clock signal by the voltage to frequency converter, and is connected via conduit 20 to a frequency divider network 21. The frequency divider comprises counter means capable of dividing the input frequency into a plurality of different output frequencies which are an integral ratio of the input frequency. One of these outputs is connected via conduit 22 to drive a 12-step ring counter 23 so as to produce six symmetrical, phase-displaced outputs, as indicated by reference numeral 24, having a frequency which establishes the fundamental frequency at which the inverters are operated. This basic six-phase output waveform is connected to the modulator logic network 25, which provides at the outputs A, B, C and A', B', C' the firing signals controlling the inverter thyristors, which in turn generate the line-to-neutral voltage, represented by waveforms R AND U, at the desired fundamental frequency. Modulator logic network 25 includes conventional integrated circuits which form the necessary logic to provide the proper thyristor firing pattern by utilizing gates, matrixes, amplifiers and the like. Since any programmer skilled in digital logic design could, by employing Boolean algebra and the Mahoney Mapping technique, devise a software program which could be reduced into appropriate hardward circuits, an explanation of the details of such hardware comprising logic network 25 will not be undertaken.

In addition to the ring counter input signal 22, the frequency divider network 21 also produces a plurality of outputs 26, each of which are a different integral ratio of the input frequency signal at conduit 20. These different frequency clock signals are connected to a range switch 27, which is also subject to the speed signal effective at conduit 16. Range switch 27 operates to gate one of the integral ratio input signals 26 to output 28, establishing thereon a fundamental frequency clock signal in accordance with the level of the speed signal. Range switch 27 also generates a plurality of digital outputs 29, successive ones of which become energized as the speed signal increases.

Outputs 28 and 29 are connected in parallel to a pair of carrier generators 30 and 31, each of which comprise an integrator network for converting the fundamental frequency clock signal 28 into a carrier signal constant, as the ratio frequencies change. The respective carrier signals are connected by conduits 32 and 33 to comparators 34 and 35, each being subject to a d. c. reference voltage via conduit 36, the level of which voltage may be automatically or selectively adjusted, as hereinafter explained. FIG. 3 shows one specific level at which the d. c. reference voltage represented by line 37 may be assumed to lie. When the amplitude of this d. c. reference signal exceeds the amplitude of the carrier signal, comparators 34 and 35 produce an output 38 and 39, respectively. These comparator outputs 38 and 39 are represented in FIG. 4 by correspondingly numbered waveforms in which notches 40 are shown having a width determined by the point of intersection of the reference line 37 with waveforms 32 and 33 of FIG. 3. It is apparent now that as the d. c. reference voltage increases and decreases, the level of line 37 rises and falls accordingly so as to effect a variation in the width of notches 40, which may in fact disappear entirely at very low levels of the reference voltage to produce an unnotched waveform 38 and 39.

Comparator outputs 38 and 39 are connected as inputs to the modulator logic 25 to set up the logic therein in conjunction with inputs 24 to generate a thyristor firing waveform at outputs A, B, C and A', B', C' which will result in the inverters 11 and 12 varying the fundamental motor voltage in accordance with the frequency at which the thyristor firing waveform is running.

The aforementioned portion of the control system comprising devices 17, 21, 23, 25, 27, 30, 31, 34 and 35 shown in block form and hereinafter referred to as the modulator network is shown and fully described in U. S. Pat. No. 3,611,086, filed Jan. 14, 1970 by Boris Mokrytzki and Dennis L. Szymanski, entitled "Integral Carrier Ratio Inverter." A cursory review of this patent will show that the modulator network of the present invention differs only in that it employs not one but two carrier generators and two comparators as well as a six-phase ring counter instead of a three-phase counter. Accordingly, it is not believed necessary to provide any detailed description of the components comprising the above mentioned devices of the modulator network. The basic operation remains the same as described in U.S. Pat. No. 3,611,086, with the additional duplicate circuitry being necessary in the present invention only to accommodate the requirement of the two parallel inverters 11 and 12. Of course, the logic of the modulator 25 is also expanded to accommodate the additional input data.

Referring now to FIG. 5 showing a plot of the desired motor voltage versus frequency, it will be seen that motor 10 operates in a "constant torque" mode at frequencies between 0-60 hertz. Within this range, the upper limit of which is considered base speed, voltage is varied with frequency, as shown by power curve 41. Above base speed, motor 10 is operated in a "constant horsepower" mode, wherein the voltage is held constant at a maximum value, with increasing frequency, as shown by curve 42. It has been found that the voltage during "constant torque" mode of operation can be effectively controlled to follow curve 41 by a pulse width modulation technique, such as that afforded by the aforementioned modulator network of the present invention.

In accelerating a transit vehicle from zero, for example, a voltage programmer 43, subject to the speed signal effective on conduit 16 generates the voltage reference signal via conduit 36 in accordance with the function V.sub.ref /V.sub.speed =1. FIG. 6 shows a profile of the programmed voltage reference signal 37, which jumps to a maximum value at 45 hertz and follows a different course between 45 and 60 hertz in accordance with a new function. In accelerating up to 45 hertz therefore, the amplitude of the d. c. reference signal 37 rises linearly to a predetermined value, thereby intersecting the carrier waves 32 and 33 at different points. This results in the reference voltage signal 37 exceeding the carrier signals 32 and 33 for progressively shorter periods as the speed increases. As clearly explained in U.S. Pat. No. 3,611,086 and only mentioned at this point as a matter of interest, the ratio selected frequency signal 28 varies the frequency of the carrier wave at different rates, as the speed increases through different preselected ranges, to assure the maximum switching rate of the thyristors comprising the inverters 11 and 12 is not exceeded by reason of the progressively shorter periods of the notches 40 in waveforms 38 and 39 of FIG. 4.

In keeping with the aforementioned pulse width modulation technique of voltage variation, the modulator logic is conditioned by the notched waveforms 38 and 39 of comparators 34 and 35 to modify the basic six-phase signal pattern, from which the thyristor firing signals at the modulator logic outputs A, B, C and A', B', C' are generated. This modified firing pattern is illustrated in FIG. 7 by the notched waveforms A, B, C, A', B' and C'. The thyristor switching times limit the motor frequency. FIG. 7 shows the resultant fundamental voltage waveform VW1 effective at motor winding W1, it being understood that the other motor windings are likewise provided with identical waveforms having a phase displacement as explained relative to the waveforms of FIG. 2, except that notches are contained in the waveform. This notch rate will vary in accordance with the ratio of the carrier frequency signal 28 in effect. The waveform VW1 of FIG. 7 shows the minimum single notched waveform which is generated at a speed corresponding to the 45 hertz level shown in FIG. 5 and represents the maximum fundamental voltage capable of being produced by the pulse width modulation techinque. A quasi-sinusoidal wave Y is shown superimposed on waveform VW1 to graphically illustrate the fundamental voltage produced by the minimum notched, minimum width waveform VW1. As mentioned relative to the interdigitated waveform VW1 of FIG. 2, the precise shape of this intricate waveform VW1 of FIG. 7 can be calculated by employing the formula V= 1/6 [ 2A- B-C +.sqroot.3 (A' - B')] at each point where any one of the A, B, C, A', B', C' waveforms change state.

As previously mentioned, the unmodulated waveform of FIG. 2 is produced, beginning at base speed. It has been found from actual experience that the voltage transition between the pulse width modulated waveform of FIG. 7 and the unmodulated waveform of FIG. 2 produced in "constant horsepower" mode of operation is too extreme both in the sense of disrupting the motor to the extent that it causes passenger discomfort, and also because of the danger to the running gear of the vehicle and the likelihood of momentarily drawing high inrush currents which cannot be tolerated by the thyristors comprising inverters 11 and 12. FIG. 8 shows an overlay of the voltage waveforms of FIGS. 2 and 7, to graphically illustrate by the shaded area that this fundamental voltage difference can be quite substantial.

In order to avoid this extreme voltage transition, a phase shifted technique of voltage modulation is employed between 45 and 60 hertz, which entails operating the inverters 11 and 12 so as to be shifted in phase from a normal .+-. 15.degree. displacement toward a preselected phase displacement in excess of the normal displacement. The amount of phase shift is controlled by the voltage signal 37 generated by the voltage programmer 43 along the portion of the voltage profile shown in FIG. 6 generated between 45 and 60 hertz. In the present invention, it is desired to shift the phase displacement of the inverters to a point where the fundamental voltage of the resultant 12-step waveform produced precisely matches the fundamental voltage of waveform VW1 in FIG. 7, corresponding to the final pulse width modulated waveform, and thence bring the inverters back to their normal .+-. 15.degree. phase displacement to produce the maximum fundamental voltage, as represented by waveform VW1 in FIG. 2, so that a smooth transition into "constant horsepower" mode of operation may be realized.

It can be seen from FIG. 8 that the difference in area under the curves shown and represented by the shaded area is the voltage difference produced by the two waveforms VW1 of FIGS. 2 and 7. While this may be accurately calculated in accordance with the previously mentioned formulas, let it be assumed for the purpose of explanation that the voltage difference between the waveforms of FIGS. 2 and 7 is 13 percent. It will be seen from FIG. 5 that as the motor frequency exceeds a value of 45 hertz, a variable phase shift range of inverter operation occurs, as hereinafter explained, in order to generate a modulated waveform VW1 of FIG. 9. This waveform produces 13 percent less voltage than waveform VW1 of FIG. 2 and thus precisely matches the voltage produced by waveform VW1 of FIG. 7, so that there is no disruption of fundamental motor voltage at the transition between the pulse width modulated and variable phase shift regions of FIG. 5.

The degree of phase shift of the respective inverters 11 and 12 required to produce waveform VW1 of FIG. 9 may be determined graphically by vector diagrams, as shown in FIGS. 10 and 11, or may be accurately calculated for a more exact determination. For example, in FIG. 10 vector a is drawn at a .+-. 15.degree.angle from a component C whose length is to be determined graphically. By letting vector a equal a value of one, and closing the triangle by drawing vector b at a 15.degree. angle with component C, the length of the component C is found to have a value approximately 1.9 times the length of vector a. Since it is known that the voltage component of the phase shifted waveform VW1 of FIG. 9 must match that of waveform VW1 of FIG. 7, the component value 1.9 must be reduced by 13 percent, which is represented in FIG. 11 as having a value of 1.65 times the value of vector a. By setting off this distance along component C and plotting the point where the area struck by the assumed voltage vector radii intersect, the angle that the resultant vectors a and b make with the component C is found to be .+-. 30.degree.. Consequently, each inverter is phase shifted from .+-. 15.degree. to .+-. 30.degree. to effect a 60.degree. phase shift between inverters 10 and 11, as shown by waveforms A, B, C, of FIG. 9, which are displaced 60.degree. from corresponding waveforms A', B' and C'. By employing the formula V= 1/6 [2A-B-C +.sqroot.3 (A' - B')], the interdigitated waveform VW1 of FIG. 9 may be found whose fundamental voltage is that approximated by the quasi-sinusoidal wave Z. It will be further noted that this waveform VW1 of FIG. 9 produces a voltage level corresponding precisely to the voltage produced by waveform VW1 of FIG. 7.

In order to accomplish the desired .+-. 30.degree. phase shift of inverters 10 and 11, reference voltage signal 37 is generated by voltage programmer 43 to follow the profile shown in FIG. 6, thereby jumping to a maximum value when the speed signal at conduit 16 corresponds to a frequency of 45 hertz at the point where transition from pulse width modulation to phase shift modulation occurs. From this maximum value, reference voltage signal 37 varies as an inverse function of frequency until base speed is reached. It will thus be understood that the reference signal 37 intersects the carrier waves 36 in FIG. 3 at varying amplitudes, thereby varying the width of notches 40 forming waveforms 38 and 39, which represent the outputs of comparators 30 and 31, respectively.

It is apparent therefore that the variable input signals 38 and 39 together with the basic six-phase waveform indicated by reference numeral 24 provide the modulator logic with the necessary input variables from which a unique thyristor firing pattern is produced at outpus A, B, C, A', B', C' to generate the desired line to neutral voltage waveforms at the transition between the pulse width modulation and the phase shift modulation region of FIG. 5. As the motor speed increases, the voltage programmer adjusts the amplitude of voltage reference signal 7, accordingly, thereby gradually bringing the degree of inverter phase shift back toward the normal .+-. 15.degree. displacement at base speed. Being 180.degree. out of phase, waveforms 38 and 39 of FIG. 4 assure that the inverters 11 and 12 are phase shifted by equal amounts and in opposite directions, as illustrated in FIGS. 10 and 11, so that the voltage component resulting from a phase shift lies along the same line as the component prior to the phase shift throughout the entire phase shift region. Therefore, not only is a smooth and continuous change in voltage realized in the region between the expiration of pulse width modulation and base speed, but also the phase of the fundamental voltages at the motor windings is properly maintained to avoid poor motor action in the transition from pulse width modulation to phase shift modulation, as might otherwise occur.

It is understood of course that increasing frequencies beyond base speed can result in "constant horsepower" mode of operation, in which the inverter phase displacement is maintained constant at .+-. 15.degree. so that maximum voltage is produced as the speed increases. It should also be understood that the reverse operation occurs in response to decreasing speeds so that in passing base speed and until pulse width modulation occurs at a frequency of 45 hertz, the phase shift modulation is effective with the inverter phase displacement changing with speed from .+-. 15 degrees to 30.degree.. In either case, the net result is that of passing from a pulse width modulated waveform to an unmodulated waveform or vice versa without experiencing an extreme voltage transition accompanied by a disruption in the motor operation.

Claims

Having now described the invention, what we claim as new and desire to secure by Letters Patent, is:

1. A control system for energizing an a. c. motor in accordance with a modulated load waveform being generated within a first preselected speed range and in accordance with an unmodulated load waveform being generated within a second preselected speed range, said system comprising:

a. multi-phase load windings forming the stator of said motor,

b. a source of d. c. voltage,

c. first and second inverter means each having a plurality of phase displaced outputs for connection with said load windings, said inverter means including switch means for periodically reversing the polarity of said d. c. voltage potential at said outputs to effect energization of said load windings, and

d. control means for operating said switch means within a first region of said first speed range to provide said modulated waveform by pulse width modulation, and within a second region of said first speed range to provide said modulated waveform by phase shifting said first and second inverter means from a predetermined phase angle effective within said first region to a phase angle which varies with the speed of said motor, thereby effecting a smooth voltage transition from said modulated waveform to said unmodulated waveform.

2. The system, as recited in claim 1, wherein said control means further comprises voltage programmer means for generating a reference signal which varies in accordance with the speed of said motor so as to follow a predetermined profile and thereby effect said phase shift of said inverter means to an extent sufficient to produce said phase shift modulated waveform having a voltage component equal to the voltage component of said pulse width modulated waveform at the transition between said first and second regions of said first speed range, and to provide said predetermined phase angle of said inverter means to effect said unmodulated waveform at the transition from said second region of said first speed range into said second speed range.

3. The system, as recited in claim 1, further characterized in that the degree of phase shift of said first and second inverter means is equal and of opposite sense relative one to the other so that the resultant position of the voltage component of said phase shift mdoulated waveform coincides with the position of the voltage component of said pulse width modulated waveform.

4. The system, as recited in claim 1, wherein said switch means includes solid state switch means.

5. The system, as recited in claim 2, wherein said first and second inverter means are each comprised of a three-phase, solid state bridge network consisting of said switch means.

6. The system, as recited in claim 5, wherein said control means further comprises motor speed responsive drive means for controlling the switching frequency of said bridge network.

7. The system, as recited in claim 6, wherein said control means further comprises:

a. said drive means comprising a ring counter having a plurality of outputs providing phase displaced digital control signals,

b. comparator means for providing first and second digital control signals in accordance with the level of said reference signal exceeding the level of a first and a second voltage signal of triangular waveshape having a frequency corresponding to the speed of said motor, and

c. logic means subject to said phase displaced digital control signals and to said first and second digital control signals for operating said switching means to produce said phase shift modulated waveform.

8. The system, as recited in claim 7, further characterized in that said first and second voltage signals of triangular waveshape are 180.degree. out of phase so that said switching means are operated to effect said phase shift of said first and second inverter means equally in opposite sense, thereby maintaining coincidence between the relative positions of the voltage component of said phase shift modulated waveforms and said pulse width modulated waveform.

9. The system, as recited in claim 7, wherein said control means further comprises means for integrating a pulse signal having a frequency corresponding to the speed of said motor to provide said first and second voltage signals of triangular waveshape.

10. The system, as recited in claim 1, wherein said multiphase load windings comprise first and second sets of three-phase load windings, one mechanically displaced relative to the other an amount corresponding to said predetermined phase angle of said inverter means.

11. The system, as recited in claim 10, further comprising reactor means for coupling the phase displaced outputs of said first and second inverter means with the windings of said first and second sets of load windings, respectively, to form said modulated and said unmodulated waveforms.