Three-phase PWM signal generating device for inverters

Abstract: In an inverter in which six switching elements (13u-13z) of an inverter main circuit (10) is controlled to be turned on and off under a switching pattern in which a voltage space vector composed of adjacent two of six fundamental voltage vectors out of phase from one another by an electrical angle of π/3 and a zero vector figures a circular locus so that a three-phase substantially sinusoidal voltage is obtained, a three-phase PWM signal generating device includes a circuit (33) in which two switching patterns of the same kind are formed before and after the formation of the switching pattern corresponding to the zero vector, respectively, a circuit (42) in which the switching pattern corresponding to the zero vector is held for half of its holding time when the voltage space vector passes an intermediate phase position between the the two adjacent fundamental voltage vectors, and a circuit (45) in which when the magnitude of the command voltage vector exceeds the region of a circular locus figured by the fundamental voltage vector, the command voltage vector is converted to a new command voltage vector so as to correspond to the region.

Description: This invention relates generally to an inverter device having a plurality of switching elements turned on and off so that a dc voltage is converted to a three-phase ac voltage, and more particularly to a pulse width modulation (PWM) signal generating device for the inverter device provided for controlling on and off operations of each switching element in accordance with a time ratio so that a substantially sinusoidal output voltage is obtained. One conventional pulse width modulation (PWM) signal generating circuit in an inverter device will be described. FIG. 7 schematically illustrates an arrangement of an inverter main circuit 10 of the inverter device. Six switching elements 13u, 13v, 13w, 13x, 13y and 13z are bridge-connected between main circuit bus lines 11 and 12 as well known in the art. Since either of the upper and the lower switching elements of each of the three arms is turned on, these three switches turned on are referred to as reference symbols Sa, Sb and Sc respectively. The number of switching patterns is thus obtained as 2 3 =8. Each phase voltage is ±V/2 with respect to a virtual neutral point. Where voltage space vectors correspond to three-phase instantaneous voltages in consideration of phase differences among the phases, each of the switching elements Sa, Sb, Sc is represented as "1" when the positive switching element 13u, 13v or 13w in each phase is turned on. Each of the switching elements Sa, Sb, Sc is represented as "0" when the negative switching elements 13x, 13y or 13z in each phase is turned on. Accordingly, each switching pattern can be represented by substituting "1" or "0" for each of Sa, Sb, Sc. As shown by solid lines in FIG. 8, these switching patterns can be represented as six fundamental voltage vectors out of phase from one another by an electrical angle of 2π/6 and two zero vectors (0, 0, 0) and (1, 1, 1). FIG. 9 illustrates the three-phase PWM signal generating device for on-off control of the switching elements in the above-described inverter device. Phase command value classification means 14 classifies a phase command value * into unit regions obtained by dividing an electrical angle of 2π/6 into twelve equal parts, for example and calculates a lead angle in the unit region into which the phase command value is classified. The result of classification and the calculated lead angle are produced as 4-bit information. Switching pattern determining means 15 is provided for determining the switching patterns corresponding to two kinds of fundamental voltage vectors (voltage space vectors) nearest to the unit region to which the phase command value * classified by the phase command value classification means 14 belongs and the switching pattern corresponding to the zero vector. The fundamental voltage vectors are out of phase from each other by an electrical angle of π/3. The switching pattern determining means 15 further determines a sequence that these determined switching patterns are delivered to the base terminals of the switching elements 13u-13z. Generally, two kinds of fundamental voltage vectors of the phases nearest to the phase command value * as exemplified in FIG. 8 and the vectors (1, 0, 0), (1, 1, 0) and the zero vector are specified as the switching patterns in the example of FIG. 8. One of two kinds of zero vectors (1, 1, 1) and (0, 0, 0) is selected so that the number of the switching operations becomes the smallest in consideration of the previous switching patterns. The voltage space vectors which can be outputted must lie inside a hexagon formed by linking the points of six fundamental voltage vectors in FIG. 8 where the voltage space vectors each having a predetermined magnitude and phase are outputted by controlling the fundamental voltage vectors out of phase from each other by an electrical angle of 2π/6 and the zero vector in accordance with the time ratio. Accordingly, the region of the modulation to the sinusoidal waves performed by the above-mentioned control of the fundamental voltage vectors and zero vector is limited to the inside of an inscribed circle of the hexagon. Consequently, polar coordinates can be employed to realize optional voltage space vectors in the limit region and the region of an electrical angle of π/6 can be described in consideration of the symmetry. FIG. 10 shows enlarged fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector. In order that the voltage space vector corresponding to a command voltage vector including the phase command value * and voltage command value V* is outputted, it is obvious that the following expression needs to be satisfied by the geometric analysis shown in FIG. 10 : Vsin(π/6-) : Vsin(π/6+) : 1-V{sin(π/6-)+sin(π/6+)} =t 1 : t 2 : t 0 where t 1 , t 2 and t 0 are output times of the fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector and  is a lead angle of the phase command value * in the unit region to which the phase command value * belongs. Holding time calculating means 16 is provided for obtaining the output times t 1 , t 2 as shown in FIG. 9. More specifically, data of the lead angle  is supplied to ROM tables 17 and 18 from the phase command classification means 14 so that the value of sin(π/6+) corresponding to the supplied lead angle is obtained. The holding time t 1 of the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is obtained by multiplying one control period T SW by the voltage command value V* and further by the value of sin(π/6-). In the same way, the holding time t 2 of the switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is obtained by multiplying one control period T SW by the voltage command value V* and further by the value of sin(π/6+). The holding time t 0 of the switching pattern corresponding to the zero vector is obtained by subtracting these holding periods t 1 and t 2 from the one control period T SW . One control period refers to a period in which the switching patterns corresponding to the two kinds of fundamental voltage vectors and the zero vector respectively are sequentially produced once. Timing means 19 comprises a presettable counter 20, a switch 21 and a D-type flip flop 22. The presettable counter 20 has a data input terminal DATA to which data of each of the holding times t 0 , t 1 , t 2 is inputted in accordance with the switching of the switch 21. The presettable counter 20 also has a clock terminal CK to which clock signals fck are inputted. The switch 21 is switched to an output terminal of the holding time corresponding to the switching pattern determined by the switching pattern determining means 15 when timing of each of the holding times supplied to the presettable counter 20 is completed and then, the subsequent holding time data is inputted to the data input terminal DATA. The flip flop 22 holds the formation state of the corresponding switching pattern until the timing of each holding time is completed, whereby the switching elements 13u-13z of the inverter device are desirably controlled to be turned on and off. FIG. 11 illustrates a locus of a vector ψ representative of a magnetic flux induced in a three-phase induction motor M as a three-phase load when an output of the inverter main circuit 10 controlled by the above-described PWM signal generating device is supplied to the motor M. Reference symbol ψ' designates a mean locus and figures a circular orbit. The magnetic flux vector ψ is represented as the time integral of the voltage space vector. Since each fundamental voltage vector has a predetermined value, the direction of the magnetic flux vector ψ is the same as that of each fundamental voltage vector and the magnitude of the vector ψ is proportional to the holding time of each fundamental voltage vector. A manner of determining the output sequence of the switching patterns by the switching pattern determining means 15 will now be described. The output holding times t 1 , t 2 of the switching pattern corresponding to the two fundamental voltage vectors determined by the switching pattern determining means 15 are calculated by the holding time calculating circuit 16. The output sequence of the switching patterns is so determined that the zero vector is interposed between longer vectors, that is, the zero vector is positioned at the start or the end of the vector with the longer holding time in one control period T SW . Generally, since the holding time of the fundamental voltage vector with the phase nearest to the phase command value * becomes longer because of the method of calculating the holding time, the switching pattern corresponding to the fundamental voltage vector with the longer holding time can be determined at the stage of the classification by the phase command value classification means 14. FIG.11 shows the case where the phase command value * is in the condition shown in FIG. 10. The phase command value * when t 1 <t 2 is nearest to the fundamental voltage vector (1,1, 0) where the holding times of the switching patterns corresponding to the fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector are represented as t 1 , t 2 , t 0 , respectively. Accordingly, the switch 21 is switched so that the sequence of ...t 1 , t 2 , t 0 , t 0 , t 2 , t 1 ... is repeated. The above example will be described in more detail. When the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for the time t 1 in the previous control period, the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for the time t 1 in the following control period and then, the switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is held for the time t 2 and last, the switching pattern corresponding to the zero vector (1, 1, 1) is held for the time t 0 . One of problems to be solved in the above-described inverter device is that further improvement in the degree of approximation of the voltage waveform to the sinusoidal wave by the PWM control has been desired. Therefore, an object of the present invention is to provide a PWM signal generating device for an inverter device wherein the improvement in the degree of approximation of the inverter output voltage waveform to the sinusoidal waveform can be enhanced substantially over the whole PWM control range. In the present invention, a three-phase PWM signal generating device for an inverter device comprising an inverter main circuit including six bridge-connected switching elements which can be turned on and off in predetermined patterns so that a dc voltage supplied from an external source is converted to a three-phase ac approximately sinusoidal voltage, the three-phase PWM signal generating device connected to the inverter main circuit for generating switching pattern signals for turning on and off the switching elements in order to provide six kinds of switching patterns corresponding to six fundamental voltage vectors out of phase from one another by an electrical angle of 2Π/6, respectively, and switching patterns corresponding to a zero vector corresponding to an "on" or "off" state of substantially all the switching elements so that the inverter main circuit generates an approximately sinusoidal output voltage waveform, the three-phase PWM signal generating device comprising phase command value classification means for classifying a phase command value, pertaining to a voltage command value, into one of a plurality of unit regions calculating a lead angle in the one unit region into which the phase command value is classified, and generating corresponding output information representative of the result of the classification and the lead angle, wherein the phase command value classification means obtains the plurality of unit regions being obtained by dividing an electrical angle of 2Π into a plurality of equal regions, switching pattern determining means supplied with the output information of the phase command value classification means for setting the switching patterns corresponding to two kinds of fundamental voltage vectors corresponding to the one unit region to which the phase command value is classified and the zero vector, the fundamental voltage vectors being out of phase from each other by an electrical angle of Π/3, the switching pattern setting means further setting an output sequence of the set switching patterns, holding time calculating means supplied with the output information of the phase command value classification means for calculating a holding time of each of the switching patterns corresponding to the two kinds of fundamental voltage vectors and the zero vector, based on the lead angle of the phase command value belonging to the one unit region and the supplied voltage command value, and generating signals representative of the calculated holding times, and timing means supplied with the signals representative of the calculated holding times and connected to the switching pattern determining means for holding a formation state of each set switching pattern during the holding time of each switching pattern calculated by the holding time calculating means, characterized in that the switching pattern determining means inserts the switching pattern corresponding to the zero vector into each of the two kinds of switching patterns once during formation of each of the two kinds of switching patterns, and that the holding times of the switching patterns corresponding to the inserted two zero vectors are substantially equal to each other, respectively. In accordance with the above-described arrangement, the switching pattern corresponding to the zero vector wherein the output voltage of the inverter main circuit is decreased substantially to zero is inserted into each of the two fundamental voltage vectors per control period. Consequently, since the on and off control of the switching elements can be performed accurately, the voltage ripple and the torque ripple can be reduced in the range to the maximum PWM controlled voltage. The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a block diagram of the three-phase PWM signal generating device of a first embodiment of the invention; FIG. 2 is a vector diagram for explaining the operation of the three-phase PWM signal generating device; FIG. 3 shows a locus of the magnetic flux vector in the first embodiment; FIG. 4 is a block diagram of the three-phase PWM signal generating device of a second embodiment of the invention; FIG. 5 is a block diagram of the three-phase PWM signal generating device of a third embodiment of the invention; FIG. 6 is a block diagram of the three-phase PWM signal generating device of a fourth embodiment of the invention; FIG. 7 is a connection diagram of a three-phase inverter main circuit; FIG. 8 is a diagram of fundamental voltage vectors for explaining the operation of generating the three-phase PWM signals; FIG. 9 is a block diagram of a conventional three-phase PWM signal generating device; FIG. 10 is a vector diagram for explaining the sinusoidal waveform approximating operation of the three-phase PWM signal generating device in FIG. 9; FIG. 11 shows a locus of the magnetic flux vector obtained from the sinusoidal waveform approximation by the conventional device. A first embodiment of the three-phase PWM signal generating device in accordance with the present invention will be described with reference to FIGS. 1 to 3 of the accompanying drawings. The same parts are labeled by the identical reference numerals in FIG. 1 showing the three-phase PWM signal generating device 36 for the on and off control of the switching elements 13u-13z of the inverter main circuit in FIG. 7 as in FIG. 9. Phase command value classification means 30 in the embodiment classifies the supplied phase command value * into any one of unit regions obtained by dividing an electrical angle of 2π into six equal regions. The result of classification by the phase command value classification means 30 is delivered as 3-bit information. Further, the phase command value classification means 30 calculates and delivers a lead angle  in the classified unit region. With these operations of the phase command value classification means 30, the following equation (2) as a transformation of the above-mentioned equation (1) is set in ROM tables 31 and 32: V*sin( π 3 -) : V*sin : 1-V* sin( π 3 -)+sin  = t 1 : t 2 : t 0 . FIG. 2 is a view similar to FIG. 10 showing a basis of the equation (2). Switching pattern determining means 33 is arranged so that switching patterns corresponding to two kinds of fundamental voltage vectors corresponding to the unit region classified by the phase command value classification means 30 and the switching pattern corresponding to the zero vector are delivered in a sequence of (t 1 , t 0 , t 1 ), (t 2 , t 0 , t 2 ), expressing those switching patterns using the respective holding times thereof. Referring to FIG. 3, the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for time t 1 and then, the switching pattern corresponding to the zero vector (0, 0,0) is held for time t 0 . Subsequently, the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is again held for time t 1 and then, the switching pattern corresponding to the fundamental voltage vector (1,1,0) is held for time t 2 . Subsequently, the switching pattern corresponding to the zero vector (1, 1, 1) is held for the time t 0 and then, the switching pattern corresponding t 0 the fundamental voltage vector (1,1,0) is again held for the time t 2 . The switching pattern corresponding to the zero vector is always inserted once while the switching pattern corresponding to the fundamental voltage vector is delivered twice. In this case the switching pattern determining means 33 selects the zero vector (0,0,0) not the zero vector (1,1,1) in the movement from the fundamental voltage vector to the zero vector. This is because only one switching operation is needed when the switching pattern corresponding to the fundamental voltage vector (1,0,0) is transferred to the switching pattern corresponding to the zero vector (0,0,0), whereby the switching loss is reduced to the minimum so that efficient operation is performed. In accordance with the above-described embodiment, the switching pattern corresponding to the zero vector is inserted into each of the switching patterns corresponding to the two kinds of fundamental voltage vectors which form one control cycle. Consequently, the zero vector which is produced at one time in the prior art is dispersed to the two times. The approximation of the inverter output voltage waveform can be improved as the result of dispersion of the zero vector. The holding times of the zero vectors dispersively produced at the two times are substantially equal to each other or set at substantially the same value t 0 . Consequently, even when the circuit arrangement has a relatively low resolution, a wider control range of the holding time of the zero vector can be ensured, and accordingly, the PWM-controlled maximum voltage can be increased. FIG. 4 illustrates a second embodiment wherein the invention is applied to an inverter of the type that the voltage-to-frequency ratio is controlled so as to be constant. The reference numeral 36 in FIG. 4 designates a three-phase PWM signal generating device such as shown in FIG. 1. When supplied with a frequency command value f*, a voltage command value V* corresponding to the supplied frequency command value f* is delivered to the three-phase PWM signal generating device 36 by a function generator 34 and the phase command value * is delivered to the three-phase PWM signal generating device 36 by a phase signal generator 35 in the same manner as in the first embodiment. - FIG. 5 illustrates a third embodiment wherein the invention is applied to an inverter of the current follow-up type. When a detection current value i detected by a current detector 37 provided at the output side of the inverter main circuit 10 having the same configuration as shown in FIG. 18 and a current command value i* are supplied to a conversion circuit 38, these values are converted to the voltage command value V* and the phase command value *, both of which command values are supplied to the three-phase PWM signal generating device 36. Since a voltage space vector having an optional voltage and phase can be realized in a time which is twice the control period T SW , the present invention may be applied to high-speed current control when high switching frequency elements are employed. FIG. 6 illustrates a fourth embodiment. The fourth embodiment differs from the first embodiment in that the holding time calculating means 40 is composed into a function table by the use of a ROM. In this case even when an 8-bit voltage comment value V* and a 7-bit lead angle  are inputted and 10 bits are assigned to each of the holding times t 0 , t 1 , t 2 to be outputted, the memory capacity required of the ROM is 960 kilobits, which shows that the ROM can be realized by one memory IC of 1 megabit.

Claims: A three-phase PWM signal generating device for an inverter device comprising an inverter main circuit (10) including six bridge-connected switching elements (13u-13z) which can be turned on and off in predetermined patterns so that a dc voltage supplied from an external source is converted to a three-phase ac approximately sinusoidal voltage, the three-phase PWM signal generating device connected to the inverter main circuit (10) for generating switching pattern signals for turning on and off the switching elements (13u-13z) in order to provide six kinds of switching patterns corresponding to six fundamental voltage vectors out of phase from one another by an electrical angle of 2Π/6, respectively, and switching patterns corresponding to a zero vector corresponding to an "on" or "off" state of substantially all the switching elements (13u-13z) so that the inverter main circuit (10) generates an approximately sinusoidal output voltage waveform, the three-phase PWM signal generating device comprising phase command value classification means (30) for classifying a phase command value (*), pertaining to a voltage command value (V*), into one of a plurality of unit regions, calculating a lead angle in the one unit region into which the phase command value is classified, and generating corresponding output information representative of the result of the classification and the lead angle, wherein the phase command value classification means obtains the plurality of unit regions by dividing an electrical angle of 2Π into a plurality of equal regions, switching pattern determining means (33) supplied with the output information of the phase command value classification means (30) for setting the switching patterns corresponding to two kinds of fundamental voltage vectors corresponding to the one unit region to which the phase command value is classified and the zero vector, the fundamental voltage vectors being out of phase from each other by an electrical angle of Π/3, the switching pattern determining means (33) further setting an output sequence of the set switching patterns, holding time calculating means (16) supplied with the output information of the phase command value classification means (30) for calculating a holding time of each of the switching patterns corresponding to the two kinds of fundamental voltage vectors and the zero vector, based on the lead angle of the phase command vaue (*) belonging to the one unit region and the supplied voltage command value (V*), and generating signals representative of the calculated holding times, and timing means (19) supplied with the signals representative of the calculated holding times and connected to the switching pattern determining means (33) for holding a formation state of each set switching pattern during the holding time of each switching pattern calculated by the holding time calculating means, characterized in that the switching pattern determining means (33) inserts the switching pattern corresponding to the zero vector into each of the two kinds of switching patterns once during formation of each of the two kinds of switching patterns and that the holding times of the switching patterns corresponding to the inserted two zero vectors are substantially equal to each other, respectively. A three-phase PWM signal generating device according to claim 1, characterized in that the switching pattern determining means (33) forms two switching patterns of the same kind before and after the formation of the switching pattern corresponding to the zero vector, respectively. A three-phase PWM signal generating device according to claim 1, characterized in that the switching pattern determining means (33) forms the switching pattern corresponding to the zero vector by inverting one of three switching elements corresponding to each switching pattern into which the switching pattern corresponding to the zero vector is inserted.

Inventors: HIROSE TATSUYA, HORIGUCHI TAKI, MOCHIKAWA HIROSHI, HIROSE, TATSUYA, HORIGUCHI, TAKI, MOCHIKAWA, HIROSHI, HIROSE, TATSUYA, MIE WORKS K.K. TOSHIBA, HORIGUCHI, TAKI, MIE WORKS K.K. TOSHIBA, MOCHIKAWA, HIROSHI, MIE WORKS K.K. TOSHIBA
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H02M7
IPC codes: H02M7