Abstract:
A voltage inverter device has a ROM which stores digital values in advance. The digital value is proportional to a voltage-time integration value of an AC output voltage waveform which corresponds to each of a plurality of basic electrical angle intervals. The digital values stored in the ROM are sequentially read out to be compared with time data supplied by a time counter. Comparison results correspond to specific electrical angles of the AC output voltage waveform. The results are combined by a logical circuit so as to obtain a control signal for generating an AC output voltage of desired waveform.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an inverter device and, more particularly, to a voltage inverter device which has a simple, highly reliable and high-performance control circuit. 
     Generally, except for special cases, a voltage inverter preferably has an output voltage of a sinusoidal waveform. Various types of waveform converting means have been proposed in order to obtain a sinusoidal output from a DC voltage source. For example, an inverter is known which generates a sinusoidal pulsewidth modulated signal. 
     FIG. 1A shows a basic signal for obtaining a timing signal for turning on/off a main circuit switch of an inverter of this type. A reference voltage signal S 1  of a sinusoidal waveform is compared with a carrier signal T of a triangular waveform to obtain timing or trigger signals (FIG. 1B and 1C) for triggering positive and negative switches, respectively, of a main circuit switch. The ON/OFF state of the main circuit switch is controlled by these signals, so that a pulsewidth modulated output voltage signal which is shown in FIG. 1D is obtained. The modulated voltage signal thus obtained may have a sinusoidal waveform indicated by a dotted line in FIG. 1D. In this manner, the sinusoidal voltage signal S 2  corresponding to the reference voltage signal S 1  is obtained. 
     In conventional sinusoidal modulation methods, a sinusoidal voltage signal is generally compared with a triangular carrier signal. However, this requires a multiplier to obtain a sinusoidal reference voltage signal by multiplying a unit sinusoidal wave by a value corresponding to a predetermined voltage. Furthermore, when the frequency of the triangular carrier wave is kept constant, a ratio of the carrier frequency to the output frequency decreases with an increase in the output frequency. As a result, a beat frequency significantly affects the output frequency. In order to suppress the beat frequency and to obtain a symmetrical output waveform, it is necessary to synchronize the output frequency and the carrier frequency. In order to obtain a symmetrical output waveform over a wide frequency range, it is still necessary to synchronize the output frequency and the carrier frequency. With a conventional inverter adopting such a conventional sinusoidal modulation method as described above, the configuration of the control circuit is complicated. In addition, in order to obtain the timing signal which turns the main circuit switch on/off, the conventional inverter requires an analog circuit comprising an operational amplifier circuit and the like for comparing the triangular wave and the sinusoidal wave. The conventional inverter is therefore likely to be adversely affected by a noise signal and so on. As a result, it has been difficult to manufacture an inverter having high performance and high reliability. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a voltage inverter device of improved performance and reliability which uses a control circuit of simple configuration. 
     According to the present invention, a period of an AC output voltage waveform is equally divided by a first predetermined number into a plurality of first electrical angle intervals. Each of the first electrical intervals is also equally divided by a second predetermined number into a plurality of second electrical angle intervals. A digital value which is proportional to an integrated value over time of the AC output voltage waveform for each second electrical angle interval of a first electrical angle interval is determined in advance. The digital value is stored in memory means in correspondence with each of the second electrical angle intervals in advance. 
     A control signal corresponding to the AC output voltage waveform of each of the first electrical angle intervals is obtained by combining the digital values stored in the memory means. More specifically, the digital values stored are sequentially read out from the memory means by address counter means to be compared with timing data supplied from a time counter. An output from the address counter means is further counted by another counter means. Every time the count reaches a predetermined value, a signal corresponding to each of the first electrical angle intervals is sequentially produced. The comparison output results are combined by a logical circuit for each period of the AC output voltage waveform on the basis of the signals corresponding to the first electrical angle intervals so as to obtain signals for turning the main circuit switch on/off. 
     In the present invention, basic digital signals are combined according to predetermined rules so as to turn the main circuit switch on/off. This improves the performance and reliability of the inverter device. Furthermore, the configuration of the inverter device of the present invention may be simplified since a digital control circuit is used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of example and to make the description clearer, reference is made to the accompanying drawings in which: 
     FIGS. 1A through 1D are timing charts for explaining the operation of a conventional voltage inverter device; 
     FIGS. 2A and 2B are views for explaining control of voltage-time integration and comparison; 
     FIGS. 3A and 3B are schematic block diagrams showing a three-phase voltage inverter according to a first embodiment of the present invention; 
     FIG. 4 is a circuit diagram of a distributing circuit shown in FIG. 3A; 
     FIGS. 5A through 5M are timing charts for explaining the mode of operation of a b-scale of ring counter and drivers; 
     FIG. 6 is a graph corresponding to a part of TABLE 1; 
     FIG. 7 is a graph corresponding to TABLE 2; 
     FIGS. 8A and 8B are views for explaining data stored in a ROM shown in FIG. 3A; 
     FIGS. 9A through 9R, FIGS. 10A through 10F, and FIGS. 11A through 11C are timing charts for explaining the operation of the three-phase voltage inverter according to the first embodiment of the present invention; 
     FIG. 12 is a schematic block diagram showing the configuration of a three-phase voltage inverter according to a second embodiment of the present invention; and 
     FIG. 13 is a schematic block diagram showing the configuration of a three-phase voltage inverter according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A voltage inverter device according to a first embodiment of the present invention will be described below with reference to the accompanying drawings. First, control of voltage-time integration and comparison which is the basic principle of operation of the present invention will be described. 
     An instantaneous value e 0  an AC voltage having a sinusoidal waveform is given by; ##EQU1## where: 
     V 0  is the effective value of the AC voltage 
     f 0  is the frequency 
     A voltage-time integration value S of an electrical angle θ 2  (where t=θ/2πf 0 ) obtained from an electrical angle θ 1  (t=0) of e 0  is given by: ##EQU2## If V 0  and f 0  have a constant ratio K, that is, a voltage to frequency ratio (to be referred to as a V/f ratio hereinafter) of an AC voltage of sinusoidal waveform is kept constant, the voltage-time integration value S of it is given by; ##EQU3## As may be apparent from equation (3), the voltage-time integration value S can be obtained only as a function of the electric angle θ and independently of either voltage or frequency. 
     FIG. 2A shows one cycle of a waveform having a high voltage V H  and a high frequency f H  at a constant V/f ratio. FIG. 2B shows one cycle of a waveform having a low voltage V L  and a low frequency f L  at the same constant V/f ratio. It is seen from equation (3) that hatched areas S H  and S L  in FIGS. 2A and 2B, which correspond to the voltage-time integration values over a given electrical angle θ, are equal to each other. 
     As a result, a desired AC output voltage having a sinusoidal waveform and a predetermined V/f ratio can be obtained by turning the main circuit switch on/off in accordance with a predetermined voltage-time integration value corresponding to the electrical angle θ of the desired output waveform. 
     The following description will be made on the basis of the above principle of operation. FIGS. 3A and 3B are schematic block diagrams of a three-phase voltage inverter according to a first embodiment of the present invention. 
     A frequency setting circuit 10 sets a desired frequency of the AC output voltage. It produces a DC voltage which has a frequency proportional to the frequency of the output AC voltage. A transition speed controller 12 is connected to the frequency setting circuit 10 and sets timings for increasing and decreasing the frequency of the AC output voltage so that the transition speed of the load may be regulated. More specifically, the output from the transition speed controller 12 is a DC voltage. The level of the DC voltage changes at predetermined timings until it coincides with a level proportional to the preset output voltage from the frequency setting circuit 10. A voltage/frequency converter 14 (to be referred to as a V/F converter hereinafter) is connected to the transition speed controller 12, and produces pulses having a frequency proportional to the DC output voltage therefrom. The frequency of the pulses from the V/F converter 14 becomes proportional to that set by the frequency setting circuit 10. An address counter 16 is connected to the V/F converter 14 and counts output pulses therefrom. The address counter 16 comprises a binary counter. A NAND gate 18 is connected to the address counter 16 and determines boundaries between a given electrical angle 60° and the next electrical angle 60° of the AC output voltage. Every time the address counter 16 counts 8 pulses, the output signal from the NAND gate 18 rises from low level to high level at the leading edge of the eighth pulse. A 6-scale of ring counter 20 is connected to the NAND gate 18 and counts positive-going pulses therefrom. A read-only memory (to be referred to as a ROM hereinafter) 22 is connected to the address counter 16. An address in the ROM 22 is assigned in correspondence with the count of the address counter 16 so that data at that address can be read out. A comparator 24 is connected to upper-4 bit output terminals of the ROM 22 to receive a comparison input A 1  therefrom. Another comparator 26 is connected to lower-4 bit output terminals of the ROM 22 to receive a comparison output A 2  therefrom. A time counter 28 is connected to an oscillator 30 so as to count a pulse of a predetermined frequency produced thereby for counting time. The counter 28 comprises a binary counter. A one-shot circuit or a reset counter 32 is connected to the V/F converter 14 so as to generate a reset signal in synchronism with the positive-going pulse signal which is supplied to the address counter 16. The reset signal clears the time counter 28 so that the time counter 28 will be ready to start counting a pulse from the oscillator 30. In other words, the time counter 28 performs counting for every address in the ROM 22. The time counter 28 is connected to the comparators 24 and 26 for supplying comparison outputs B 1  and B 2 , respectively, thereto. The comparator 24 and 26 compare the comparison inputs A 1  and A 2 , with B 1  and B 2 , respectively. If the output value of the ROM 22 is greater than or equal to the count of the time counter 28, that is, whether the value of the comparison input A is greater than or equal to the value of the comparison input B, the comparator 24 and 26 respectively produce signals of high level through a comparison output terminal (A≧B) thereof, and signals of low level through another comparison output terminal (A&lt;B) thereof. On the other hand, if the output value of the ROM 22 is smaller than the count of the time counter 28, that is, if the value of the comparison input A is smaller than the value of the comparison input B, the comparator 24 and 26 produces signals of low level through the comparison output terminal (A≧B) thereof and of high level through the comparison output terminal (A&lt;B) thereof, respectively. An AND gate 34 is connected to the comparators 24 and 26 so as to detect signals of high level from their comparison output terminals (A&lt;B). In response to a detection signal from the AND gate 34, the time counter 28 stops counting up pulses. This prevents the time counter 28 from restarting counting from 0. A distributing circuit 36 is connected to the 6-scale of ring counter 20 and the comparators 24 and 26. The 6-scale of ring counter 20 supplies an output signal which is shifted every 60° of electrical angle to the distributing circuit 36. The comparators 24 and 26 supply comparison output signals to the distributing circuit 36. The distributing circuit 36 combines the input signals to generate a distributing signal. FIG. 4 is a circuit diagram of the distributing circuit 36. Referring to FIG. 4, the distributing circuit 36 includes: AND gates 36 1  to 36 12  for receiving outputs Q 1  to Q 6  from the 6-scale of ring counter 20 and comparison output signals from the comparators 24 and 26; OR gates 36 13  to 36 15  for receiving outputs from the AND gates 36 1  to 36 12  and outputs Q 2 , Q 4  and Q 6  from the 6-scale of ring counter 20 and for producing distributing signals; and inverter gates 36 16  to 36 18  for inverting the distributing signals. Drivers 38 1  to 38 6  are connected to the distributing circuit 36 so as to switch transistors T 1  to T 6  connected thereto under a predetermined condition. The transistors T 1  to T 6  are bridge-connected, thereby constituting a main circuit switch 40, as shown in FIG. 3B. Diodes D 1  to D 6  are connected in parallel between the collectors and emitters of the transistors T 1  to T 6 , respectively. A DC voltage source 42 is connected to the main circuit switch 40 in order to supply a DC voltage Ed. The main circuit switch 40 pulse-width modulates the DC voltage Ed and supplies a three-phase DC voltage of sinusoidal waveform to an induction motor 44. 
     The transistors T 1  to T 6  are turned on/off such that they have relationships with each other as shown in TABLE 1. 
     
                       TABLE 1______________________________________  IntervalsTransistors    A       B       C     D     E     F______________________________________ T.sub.1  S.sub.1 (θ)             ON      S.sub.2 (θ)                           ##STR1##                                 OFF                                       ##STR2## T.sub.2     ##STR3##             OFF                     ##STR4##                           S.sub.1 (θ)                                 ON    S.sub.2 (θ) T.sub.3  OFF             ##STR5##                     S.sub.1 (θ)                           ON    S.sub.2 (θ)                                       ##STR6## T.sub.4  ON      S.sub.2 (θ)                     ##STR7##                           OFF                                 ##STR8##                                       S.sub.1 (θ) T.sub.5  S.sub.2 (θ)             ##STR9##                     OFF                           ##STR10##                                 S.sub.1 (θ)                                       ON T.sub.6     ##STR11##             S.sub.1 (θ)                     ON    S.sub.2 (θ)                                 ##STR12##                                       OFF______________________________________ 
    
     FIG. 5A shows intervals A to F shown in TABLE 1. FIGS. 5B through 5G are timing charts of the output Q 1  to Q 6  from the senary ring counter 20, respectively, for the intervals A to F. FIGS. 5H through 5M show signals supplied to the drivers 38 1  to 38 6  according to the timings shown in FIGS. 5B through 5G. These signals correspond to the contents of TABLE 1. 
     FIG. 6 shows a graph corresponding to a part of the contents of TABLE 1. In TABLE 1, symbols A to F represent control intervals obtained by dividing the output period by every electrical angle of 60°. Symbols &#34;ON&#34; and &#34;OFF&#34; represent that a transistor is set at the ON state and the OFF state, respectively. A voltage-time integration value S 1  (θ) is given by: ##EQU4## and S 1  (θ) indicates an inverted value of S 1  (θ). Another voltage-time integration value S 2  (θ) is given by: ##EQU5## and S 2  (θ) indicates an inverted value of S 2  (θ). 
     The on/off control of the transistors T 1  to T 6  will now be described with reference to interval A. Assume that the transistor T 4  is ON and is connected to a negative terminal of the DC power source 42. The transistor T 3  is OFF. The transistor T 1  is turned on/off such that the output therefrom corresponds to a voltage-time integration value S 1  (θ) with respect to an electrical angle θ. This sets a line-to-line voltage V U-V  between U and V phases shown in FIG. 3B at √2f 0  Ksin(θ+30°). The transistor T 2  is turned on/off such that the output thereof corresponds to S 1  (θ). The transistor T 5  is turned on/off such that the output thereof corresponds to a voltage-time integration value S 2  (θ) with respect to the electrical angle θ. These on/off control operations keep a line-to-line voltage V W-V  between W and V phases at √2f 0  Kcosθ. As a result, a line-to-line voltage V V-W  between V and W phases is kept at √2f 0  Kcosθ. The transistor T 6  is turned on/off such that the output thereof corresponds to S 2  (θ). As a result, a line-to-line voltage V W-U  between W and U phases is kept at: ##EQU6## A similar explanation can be made for other intervals B to F. The line-to-line voltages for these intervals A to F are shown in TABLE 2. 
     
                                           TABLE 2__________________________________________________________________________Line-to-line voltageIntervalsV.sub.U-V   V.sub.V-W   V.sub.W-U__________________________________________________________________________ A ##STR13##             ##STR14##                         ##STR15##   B ##STR16##             ##STR17##                         ##STR18##   C ##STR19##             ##STR20##                         ##STR21##   D ##STR22##             ##STR23##                         ##STR24##   E ##STR25##             ##STR26##                         ##STR27##   F ##STR28##             ##STR29##                         ##STR30##__________________________________________________________________________ 
    
     It is seen from a graph in FIG. 7 corresponding to TABLE 2 that an AC voltage of sinusoidal waveform which is proportional to the frequency can be obtained at an inverter output terminal. 
     The data of S 1  (θ) and S 2  (θ) are stored in the ROM 22, which comprises at least 8 bits. The following explanation is made under an assumption that an 8-bit ROM 22 is used. Output data of the ROM 22 is divided into upper 4 bits and lower 4 bits. The upper 4 bits store data relating to S 1  (θ) and the lower 4 bits store data relating to S 2  (θ). S 1  (θ) or S 2  (θ) are not directly stored in the ROM 22. A value which is proportional to a voltage-time integration value for each of the divided intervals is obtained by equally dividing the electrical angle of 60° by a predetermined number (eight in the first embodiment of the present invention) as shown in FIG. 8A and 8B. The increase in S 1  (θ) and S 2  (θ) corresponding to each of the eightdivided intervals is indicated by hatched areas, respectively. A value which is proportional to the increase in S 1  (θ) or S 2  (θ) is used as data. S 1  (θ) or S 2  (θ) may alternatively be stored as data. However, in that case, the memory capacity of the ROM 22 must be increased and the signal processing becomes complicated. Therefore, it is preferable to use the proportional value as the data. TABLE 3 shows data obtained in a case where the period of the electrical angle of 60° is equally divided into eight intervals. Digits of hexadecimal data are stored in addresses 0 to 7, respectively, of the ROM 22. 
     
                       TABLE 3______________________________________           Data stored in ROMElectrical angle θ      Address    upper-4 bits                           lower-4 bits______________________________________0 ≦ θ &lt; 7.5      0          7         C7.5 ≦ θ &lt; 15      1          8         C15 ≦ θ &lt; 22.5      2          9         B22.5 ≦ θ &lt; 30      3          A         B30.0 ≦ θ &lt; 37.5      4          B         A37.5 ≦ θ &lt; 45      5          B         945 ≦ θ &lt; 52.5      6          C         852.5 ≦ θ &lt; 60      7          C         7______________________________________ 
    
     The mode of operation of the three-phase voltage inverter shown in FIGS. 3A and 3B, according to the first embodiment of the present invention, will now be described with reference to FIGS. 9A through 9R. Timing charts shown in FIGS. 9A through 9R relate to the interval A. FIG. 9A shows a case wherein the electrical angle of 60° of the interval A is divided into eight intervals. When the frequency setting circuit 10 sets the frequency of the output voltage, an output signal therefrom is supplied to the V/F inverter 14 through the transition speed controller 12. The V/F inverter 14 generates a pulse shown in FIG. 9B. The address counter 16 counts the pulse and produces the count shown in FIG. 9C. The count designates an address of the ROM 22. Then, the data (FIGS. 9D and 9E) at the designated address of the ROM 22 is transferred to the comparators 24 and 26, respectively. Meanwhile, a clock pulse shown in FIG. 9F is generated by the oscillator 30. The clock pulse is counted by the time counter 28 which then produces the count shown in FIG. 9G. The time counter 28 stops counting in response to the output signal from the AND gate 34 when the count becomes greater than a greater value between upper-4 bits and lower-4 bits of the ROM 22. The reset circuit 32 produces a pulse synchronous with the positive-going signal (FIG. 9H) from the V/F converter 14 and resets the time counter 28. The comparators 24 and 26 compares data from the ROM 22 and the time counter 28 to produce pulse signals shown in FIGS. 9I through 9L. The distributing circuit 36 transfers control signals SG 1  to SG 6  to the drivers 38 1  to 38 6  in accordance with the comparison signals from the comparators 24 and 26 and shift signals from the 6-scale of ring counter 20, respectively. The control signals SG 1  to SG 6  are amplified by the drivers 38 1  to 38 6  and are supplied to the corresponding transistors T 1  to T 6 . FIGS. 10A through 10F show drive signals DSG 1  to DSG 6  produced by the drivers 38 1  to 38 6  with respect to electrical angles from 0° to 360°. As a result, line-to-line output voltage waveforms as shown in FIGS. 11A through 11C are obtained. 
     FIG. 12 is a schematic block diagram showing the configuration of a three-phase inverter according to a second embodiment of the present invention. The same reference numerals are used to indicate the same parts as in FIGS. 3A and 3B. According to the second embodiment, the electrical angle of 60° is equally divided into sixteen intervals and the content of a 6-scale of ring counter 20 is shifted in response to a carry signal CR from an address counter 50. 
     FIG. 13 is a schematic block diagram showing a three-phase voltage inverter according to a third embodiment of the present invention. The same reference numerals are used to indicate the same parts as in FIGS. 3A and 3B. According to the third embodiment, the electrical angle of 60° is equally divided into four intervals. Every time an address counter 52 counts four pulses, a NAND gate 54 detects the count, generates a signal which rises from low level to high level, whereby the content of 6-scale of ring counter 20 is shifted. 
     It is to be understood that the present invention is not limited to these embodiments. In the first to third embodiments, the electrical angle is equally divided into eight, sixteen or four intervals, respectively. However, the electrical angle can be equally divided into any desired number of intervals. In that case, the address counter must be designed so as to be reset every time it counts pulses corresponding in number to the number of divided intervals. Furthermore, two or more of the above embodiments can be combined so that the number of intervals can be changed when the output frequency reaches a predetermined value. 
     Various other changes and modifications may be made without departing from the spirit and the scope of the present invention.