JPS6159062B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a three-phase inverter (hereinafter referred to as
This relates to control devices for PWM inverters (abbreviated as PWM inverters).

Figure 1 shows an outline of a PWM inverter.

In Fig. 1, S is a DC power supply, M is an AC motor, and Th1 to 6 are switches that can be turned on and off. Th1 to 6 perform an inverter operation to convert DC power to AC power, and the AC motor M
to drive.

FIG. 2 shows the inverter output voltage waveform or current waveform when the inverter shown in FIG. 1 is operated as a 120° energizing inverter without PWM operation. Whether this output waveform is a voltage waveform or a current waveform is determined by the DC power source S in Figure 1.
In the case of a DC voltage source, the voltage waveform will be as shown in FIG. 2, and in the case of a DC current source, the current waveform will be as shown in FIG. The operation of this 120° energizing inverter is a well-known fact, so a detailed explanation will be omitted.

FIG. 3 shows the control signal (on signal) waveform applied to each switch and the inverter output waveform when the inverter shown in FIG. 1 is operated as a PWM inverter. FIG. 3 shows one cycle of inverter operation (conducting angle 360°). Of these, focusing on Th1, the ON signal is always given from 0° to 60° during the energizing period, and the period from 60° to 120° during the energizing period is Th1 and
A period in which the power is turned on and off alternately by Th2 (PWM period), a period in which it is always off during the energization period from 120° to 300°, and a period in which the power is turned on and off alternately by Th3 and Th1 during the energization period from 300° to 360°. This is a repeated period (PWM period). Below Th
The same thing is done from Th2 to Th6 with a phase difference of 60°. The waveform obtained as the inverter output by these control signals is e _u-0 (or i
_u ), e _v-0 (or i _v ), and e _w-0 (or i _w ). The three output waveforms are applied to the U, V, and W terminals of the motor as three-phase alternating current having a phase difference of 120 degrees. The details of the PWM period in which Th1 and Th2 are alternately turned on and off are as follows. That is, after the energization period from 0° to 600° in which Th1 is always turned on ends, it continues to be turned on for a further short period. After this, Th1 is turned off for an energization angle _α1 , and Th2 is turned on for the same period. Next, Th1 is turned on for the same conduction angle _α1 , and Th2 is turned off. Finally, Th1 turns off,
Th2 turns on and the PWM period ends. Here, the period in which Th1 turns off and the period in which it turns on must be at the same conduction angle α ₁ , and the switch from off to on occurs at the middle of the conduction period of 60° and 120°, that is, at the point of 90°. Two things are necessary. By satisfying the above two conditions, the waveforms of Th1 and Th2 can be the same waveform (phase difference is 120°).
The other two switches Th2 and Th3, Th3 and Th
1.Th4 and Th5, Th5 and Th6, Th6 and Th4
The same is true for the PWM period. Furthermore, although the case where the number of separated pulses of conduction angle α ₁ due to PWM operation is one has been described here, the same holds true when two or more pulses are generated.

FIG. 4 shows the control signal waveform when the number of pulses to be separated is changed by performing PWM operation. A is the control signal waveform during normal 120° current conduction operation without PWM operation. B is 120 above
゜This is the control signal waveform when the pulses are separated one by one by the energization angle _α1 from the energization period. The explanation in this case is as described above, but the two conditions mentioned above can be generally described as follows.

(1) The conduction angle of the separated pulse and the interval between the two separated pulses become equal.

(2) The above two equal conduction angles are always 30° and
They are arranged symmetrically with respect to the 150° point, and their on and off states are reversed.

C is a control signal waveform obtained when two pulses at energization angles α ₁ and α ₂ are separated from the 120° energization period, respectively. D is 3 pulses,
E is a control signal waveform when four pulses are separated, and the relationship between the on period and off period during the BWM period can be summarized as follows by combining these.

() During the PWM period of 60°, the conduction angle α is arranged symmetrically with respect to the central point of 30°. (i.e. PWM
If N pulses are generated during the period, α
₁ , α ₂ , ... α _N , α _N , ... α ₂ , α ₁ and
PWM periods are separated. ) () On and off are repeated alternately during the divided period.

The above are the characteristics when PWM operation is performed by a 120° energizing inverter, and clearly under the above two conditions, the energizing angle is 120° when the on-periods are totaled, regardless of the number of separated pulses. Further, due to the above two conditions, the six control signal waveforms Th1 to Th6 have the same waveform, and the three-phase output waveforms also have mutually equal and symmetrical waveforms. The conduction angle α
₁ ... It is known that by appropriately determining α _N it is possible to eliminate components of a specific order among the harmonic components included in the output waveform. For example, if α ₁ is appropriately determined in the control signal waveform of B, it is possible to make the 5th harmonic of the output waveform zero, and the output waveform contains the fundamental wave component and components higher than the 7th harmonic. Become. If α ₁ and α ₂ are appropriately determined in the control signal waveform of C, the fifth output waveform,
It becomes possible to reduce the seventh high frequency to zero, and the output waveform includes a fundamental wave component and components higher than the eleventh harmonic. For this reason, performing PWM operation with a 120° energizing inverter is effective in reducing torque ripple, especially at low frequency output.

Conventionally, control signals for PWM inverters have been generated by comparing one carrier wave with one reference value. An example is shown in FIG. One carrier wave CAR higher than the fundamental frequency and an arbitrary waveform (normal sine wave, trapezoidal wave, etc.) synchronized with the fundamental frequency
REF is compared with the control signal Th by a comparator.
Get 1.

However, with this method, 120
゜Synthesizing PWM control signals for energizing inverters involves various difficulties. First, in order to satisfy condition (), it is necessary to synchronize the carrier wave CAR and the reference value REF, and moreover, the two waveforms must intersect at 30° and 150°. Second, the carrier wave CAR needs to be an integral multiple of the frequency of the reference value REF, and when switching the number of pulses as shown in Figure 4, it is necessary to switch the frequency of the carrier wave CAR. . Thirdly, the first half of one cycle
Since the control mode changes during the PWM period, the period in which an ON signal is always issued, and the period in which an OFF signal is always issued, it is necessary to switch the control signal for each mode, or to change the carrier wave CAR and/or the reference value REF. be. In order to solve the above problems, the control circuit has to be complicated.

The purpose of the present invention is to eliminate the above-mentioned drawbacks.One purpose is to simplify control by comparing one triangular wave with one or several reference values in order to synthesize a control signal. Provided is a three-phase inverter control device that can control a three-phase inverter by generating a control signal that satisfies the above conditions using a circuit.

The present invention will be explained below with reference to the drawings.

FIG. 6 shows an embodiment of the present invention. In addition, Fig. 7 shows a comparison between one reference value REF1 and the triangular wave TR1, and Fig. 8 shows three reference values REF1 and REF.
2. The operating waveforms when comparing REF3 and triangular wave TR1 are shown. A symmetrical triangular wave TR1 having six times the fundamental frequency is generated by the triangular wave generating circuit 1 by inputting a signal 6f six times the fundamental frequency of the inverter. At this time, one period of the triangular wave TR1 has a conduction angle of 60 degrees, and the peak value is inversely proportional to the fundamental frequency. The comparator 2 compares the triangular wave TR1 with one or several reference values REF1 to REFN generated by one or several reference value generation circuits 3, and depending on the magnitude relationship, the comparator 2 outputs _C1 to CN. occurs. In this case, the reference value REF is preset to a constant value. Next, the output signals C1 to CN of the comparators and the signal 6f of six times the fundamental frequency of the inverter are input to a logic processing circuit, and a desired logic operation is performed to obtain a signal such as an output waveform L. This waveform L is repeated at a cycle of 60 degrees and is the basic signal for PWM operation. The triangular wave
Since TR1 has one period of 60 degrees and is symmetrical with respect to the central point of 30 degrees, the condition () that the control signal during the PWM period should satisfy is satisfied. Further, regarding the condition (), necessary calculations are performed in the logic processing circuit 4 so that the condition () is satisfied.
The six times the inverter fundamental frequency signal 6f is also input to a hexadecimal ring counter 5 in addition to the triangular wave generating circuit 1. The hexadecimal ring counter 5 outputs six outputs from RC1 to RC6 based on this input. The three outputs from RC1 to RC6 are 60 each.
The on-period of the switches Th1 to Th6 is determined by a signal having an on period of 300° and an off period of 300°, and a phase difference of 60°. Therefore, by changing the phase rotation direction of the hexadecimal ring counter 5, the rotation direction of the electric motor can be changed. Further, by stopping the phase rotation of the hexadecimal ring counter 5, it is also possible to apply direct current braking to the electric motor. The outputs RC1 to RC6 of the hexadecimal ring counter 5 and the output L of the logic processing circuit 4 become inputs to the next waveform synthesis circuit 6. In the waveform synthesis circuit 6, from the RC1
Each of up to RC6 and the above L are combined to output the target control signal waveforms Th1 to Th6. To explain the control signal Th1 in detail, the ON period of RC1 of 60° corresponds to the period in which the ON signal is always given for Th1, and the 60° before and after this corresponds to the above-mentioned period.
Corresponds to the PWM period. 60 just before the on period of RC1
In ゜, the above L becomes the output Th1 as it is. Also RC
Immediately after the ON period of 1, at 60°, the negation (NOT) of L becomes the output Th1. Therefore, these three
By combining the 60° period, the desired complete waveform of Th1 can be obtained.

Figure 8 shows an example when the reference value REF is increased to three, and the comparator output obtained by comparing the triangular wave TRI and the reference value REF1 is C1, and the triangular wave TRI
The comparator output obtained by comparing the triangular wave TRI and the reference value REF2 is C2, and the comparator output obtained by comparing the triangular wave TRI and the reference value REF3 is C3. With these three waveforms as input, the output L of the logic processing circuit 4 is as shown in FIG. Thereafter, the operation of the waveform synthesis circuit 6 is exactly the same as described above.

In the above, the case where the reference value is 1 and 3 was explained as an example, but the reference value can be any number (N).
Even in this case, the operation of the circuit remains unchanged. The only difference is that the number of separated pulses is N.

Generally, the upper limit of the frequency at which an inverter can operate is determined by the turn-off time of a semiconductor switch, the commutation time of a commutation circuit, etc.

When performing PWM control, the switching frequency of the inverter's semiconductor switch will naturally rise above the fundamental frequency, so it is important to take care to perform PWM control within a range that does not exceed this upper limit frequency. From this point of view, an inverter that performs PWM control performs PWM with a large number of pulses when the fundamental frequency is low, and PWM with a small number of pulses when the fundamental frequency is high, thereby controlling the switching frequency of the semiconductor switch to be kept constant or almost constant. It is desirable to adopt a method.

Therefore, in the present invention, the switching frequency of the semiconductor switch can be kept constant or almost constant by performing the following PWM control depending on whether the fundamental frequency is high or low. In other words, by keeping the slope of the triangular wave constant,
The height of the triangular wave is shown in Figure 9a, 2 when the frequency is .
It is inversely proportional to the frequency as shown in FIG. 9b for f and FIG. 9c for 6f. If we take advantage of this feature and appropriately set the reference value to be compared with the triangular wave, as shown in Figure 10, a certain setting value and the triangular wave will intersect when the frequency is below a certain fixed value, and the inverter will This will modulate the output frequency. That is, in Figure 10, REF
1 crosses the triangular wave when the fundamental frequency of the inverter is 2f or less, and REF2 already crosses the triangular wave when the fundamental frequency of the inverter is 6f. For this reason, as the fundamental frequency decreases, the peak value of the triangular wave increases, the number of reference values that intersect increases, and the number of pulses increases. This is particularly suitable for low frequency torque ripple reduction. Conversely, when the fundamental frequency increases, the peak value decreases and the number of reference values crossed decreases. If there are no more reference values to cross, the inverter will operate as a normal 120° energizing inverter.

As explained above, according to the present invention, it is possible to generate a PWM control signal for a 120° current-carrying inverter using a simple circuit, and there is no need for a complicated switching circuit.
The number of pulses in the PWM waveform can be changed automatically according to the inverter's fundamental frequency, and it is also possible to shift continuously to the normal 120° energization mode.

[Brief explanation of the drawing]

Figure 1 is a schematic circuit diagram of a pulse width control inverter, Figure 2 is an operating waveform diagram of a 120° energizing inverter, Figure 3 is an operation waveform diagram of a pulse width control inverter, and Figure 4 is a diagram of a pulse width control inverter. Various control signal waveform diagrams, FIG. 5 is an operational waveform diagram of a conventional pulse width control inverter control circuit, FIG. 6 is a configuration diagram of an inverter control circuit showing an embodiment of the present invention, and FIGS. 7 to 8 is an operating waveform diagram of the control circuit shown in FIG. 6, and FIGS. 9 and 10 are diagrams showing the relationship between the fundamental frequency and the triangular wave and the reference value with which the triangular wave is compared, for explaining other embodiments of the present invention. It is a figure showing a relationship. Th1 to Th6...Semiconductor switch that can be turned on and off, S...DC voltage source or DC current source, M...
... Three-phase AC motor, U, V, W ... Terminals of three-phase AC motor M, REF ... Reference value, CAR ... Carrier wave, 1 ... Triangular wave generation circuit, 2 ... Comparator, 3 ... Reference value generation Circuit, 4...Logic processing circuit, 5...Hex ring counter, 6...Waveform synthesis circuit, ThI...Triangular wave, REF1 to REF3...Reference value, C1 to C3...Comparator output, L...Logic processing Circuit output, 6f... Six times the frequency signal of the inverter fundamental frequency, RC1 to RC6... Ring counter output.

Claims (1)

[Claims] 1. In a three-phase inverter control device that modulates the pulse width of an output waveform at a frequency higher than the fundamental frequency, a logic signal 6f having a frequency six times the fundamental frequency is input, and the peak value is varied in inverse proportion to the fundamental frequency. A triangular wave generation circuit that generates a triangular wave TRI, and one or several reference values REF1, REF
2...) and the triangular wave TRI and generates signals C1, C2... according to the magnitude relationship thereof, and output signals C1, C2... of this comparator.
and the logic signal 6f are input, and a desired logic signal L is obtained.
, a hexadecimal ring counter which receives the logic signal 6f as input, and a semiconductor switch which receives the output signals RC1 to RC6 of the hexadecimal ring counter and the logic signal L and constitutes the inverter.・Control signals Th1 to Th6 to turn off
A three-phase inverter control device equipped with a waveform synthesis circuit that outputs.