JPS6248476B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is based on pulse width modulation (hereinafter referred to as PWM).
Related to inverter control devices, especially those that control the inverter's output frequency and output voltage.
This invention relates to a control device for a pulse width modulation inverter that is used in a VVVF inverter and can supply stable output. [Conventional technology] As a conventional PWM inverter modulation device,
For example, by using the fact that the average value of the output pulse train P obtained by comparing a sine wave S and a triangular wave T as shown in Fig. Modulators for obtaining an alternating voltage output are well known. In the case of the modulation method of this example, the predetermined frequency is obtained as the variable frequency by controlling the frequency of the sine wave S, and the width of the output pulse is controlled by adjusting the peak value of the triangular wave T. A predetermined output voltage can be obtained through control. In other words, if the peak value of the triangular wave T is increased, the output pulse width becomes narrower and the output voltage becomes smaller;
If the peak value of is made smaller, the output pulse width becomes wider and the output voltage becomes higher. When such a PWM inverter is used as a power source for speed control of an induction motor, it is controlled so that the output voltage increases almost in proportion to the frequency, that is, the voltage/frequency is controlled to be constant (V/= constant). A device that does this is used. in this case,
In a PWM inverter, the modulation frequency (the frequency of the triangular wave in the case of Figure 1) is made as high as possible, and the number of pulses included in a half cycle of the output voltage is increased.
It is desirable to limit the magnitude of harmonic currents.
Within the range allowed by the minimum on/off time width determined by the commutation period of the switching element,
As the number of pulses included in a half cycle of the output voltage increases, the pulsation (peak to peak) of the output current increases.
is small, and the harmonic content can be kept low. On the other hand, however, as a control element for a thyristor used in the main circuit of a PWM inverter, it is not desirable to increase the number of pulses to an extremely large extent in order to increase switching loss. Furthermore, a control element such as a thyristor is not an ideal switch, but has a certain commutation period, typically 100 to 200 μs (the first
(corresponding to the slit width A in the figure), the larger the number of pulses included in the half cycle, the smaller the maximum fundamental wave amplitude of the output voltage that the inverter can output. That is, from the viewpoint of increasing the utilization rate of the voltage of the inverter, it is desirable to reduce the number of pulses included in a half cycle. In this way, since there are mutually contradictory conditions,
The operating frequency of the inverter, in other words, the ideal number of pulses that differs for each half-cycle time width (period) of the output voltage (i.e., a range in which current pulsation is not large and switching loss and voltage utilization are satisfactory) exists. For this reason, conventionally, the entire operating frequency of an inverter is divided into multiple frequency bands, and between each frequency range, the number of pulses included in a half cycle of the output voltage is kept constant, but the number of pulses is changed sequentially for each frequency band. is switching. Specifically, the output voltage can be adjusted by synchronizing the modulation frequency (carrier) and output frequency (fundamental wave) of the PWM inverter, and keeping the ratio of both frequencies at a constant integer ratio within a predetermined frequency range. The number of pulses included in a half cycle is kept constant and the number of pulses included in the half cycle is sequentially switched as the output frequency increases or decreases. For example, at low frequency, use 9 pulses as shown in Figure 1, and then as the frequency increases, from 3 pulses to 1 pulse.
A method is adopted that sequentially switches to pulse. [Problems to be solved by the invention] However, in a method in which the number of pulses is switched by detecting the output frequency, output voltage, etc. at which the commutation period at that number of pulses reaches its limit, the pulse number is not affected by input voltage fluctuations. The current pulsation after switching the number of pulses may be large, and the harmonic content may become large. Further, when switching from multiple pulses (for example, 3 pulses) to 1 pulse, the voltage jump becomes excessive, which may cause excessive current and excessive torque in the induction motor of the load. An object of the present invention is to reduce the increase in output current pulsation caused by switching the number of pulses in a pulse width modulation inverter that changes the number of pulses included in a half cycle of the output voltage in stages, and to reduce jumps in the output voltage. It is to reduce. [Means for Solving the Problems] The present invention provides for sequentially increasing or decreasing the output voltage by half as the output frequency of an inverter that receives a DC voltage as input and generates a controlled output frequency and output voltage or a signal corresponding thereto increases or decreases. A PWM inverter that performs control to switch the number of pulses included in a cycle is characterized by comprising means for adjusting a reference value for determining the point at which the number of pulses is switched in accordance with the DC input voltage. In one embodiment of the invention, the inverter supplies power to an induction motor, and a plurality of different reference values for switching the number of pulses are set for the rotational speed of the induction motor. Therefore, each time the induction motor speeds up to a predetermined speed, the number of pulses is reduced. Here, the reference value is configured to change depending on the DC power supply voltage of the inverter. [Function] The function of the present invention configured as described above will be explained with reference to FIGS. 2 and 3. Figure 2 shows the inverter output voltage V _INV characteristics with V/=constant control, and shows the maximum frequency and maximum voltage that can be output when the DC power supply voltage is normal V, low V _L or high V _H. This is what is shown. Taking as an example the switching of the number of pulses from 9 pulses to 3 pulses, the frequency and voltage that can be output with 9 pulses differ as follows depending on the DC input voltage.

〔Example〕

An embodiment of the PWM inverter modulation device of the present invention will be described with reference to FIG. In Figure 4, the input end of PWM inverter 1 is connected to a DC power supply V, and the output end is connected to an induction motor 1M as a load.
is connected. The square wave output of the oscillator 2 is converted into signals having a phase difference of 120° from each other by a three-phase divider 3, and is passed through a sine wave generator 4 to produce a three-phase sine wave voltage. Further, the square wave output of the oscillator 2 is generated by the triangular wave generator 5, which is composed of triangular wave generator elements 1P to 3P, and becomes 1 pulse, 3 pulses, and 9 pulses, respectively, for one cycle of the sine wave. Generates a triangular wave. Each triangular wave is applied to a multiplier 6, and its magnitude is adjusted as described later.
When the sine wave and the triangular wave are applied to the first comparator 7, for example, in the case of 9 pulses, an output pulse train whose average value becomes a sine waveform is obtained as shown in FIG. By controlling the inverter 1, a substantially sinusoidal three-phase AC output can be obtained. In order to control the magnitude of the three-phase AC output, a current detection circuit 8 is connected to the output terminal of the PWM inverter 1 to detect the load current, and a second comparator 9 compares the load current with the current command signal, and calculates the deviation. The signal is applied to the multiplier 6 to change the magnitude of the triangular wave, thereby controlling the output current of the PWM inverter 1 as described above. Constant current control of the induction motor 1M is performed as described above. Also
A desired output frequency of the PWM inverter 1 can be obtained by controlling the frequency of the output signal of the sine wave generator 4. The output frequency is
The rotational frequency of the induction motor is always controlled to a value obtained by adding a constant slip frequency to the desired rotational frequency. That is, by performing the constant current/constant slip frequency control as described above, the voltage/frequency of the PWM inverter 1 is controlled to a substantially constant value. Grounded-emitter transistors Tr 3 to Tr ₁ are connected to the output circuits of the triangular wave generator elements 1P to 9P, respectively, and control the ON/OFF state of the transistors Tr ₃ to Tr ₁ to supply the output to the multiplier 6 _. A selection of the number of pulses is made. In one embodiment of the invention, the voltage detector 10
is connected to the input end of the PWM inverter 1 to detect the input voltage, and its detection output is used as the reference value circuit power supply for switching the number of pulses. TG is a speed generator coupled to the induction motor IM to generate an output voltage signal proportional to the rotational frequency of the induction motor IM, and the output voltage signal is supplied to the third comparator 11.
The third comparator 11 includes transistors Tr3 to _Tr3 , respectively.
Three comparator elements 1 that control on/off of Tr ₁
PT, 3PT, and 9PT, the reference voltage value for switching the number of pulses is compared with a voltage proportional to the rotation frequency (output voltage signal of the speed generator TG), and the output voltage signal of the speed generator TG is set to the reference voltage. When the value exceeds the value, the transistors Tr ₁ to _{Tr 3} are turned on,
Controls off and switches the required number of pulses.
In other words, when the rotational speed of the induction motor is low, the voltage proportional to the rotational speed is lower than any reference value, each comparator element is zero, and this passes through the exclusive OR circuit EOR and the negation circuit NOT. By turning off transistor Tr ₁ , transistor
Turn on Tr ₂ and Tr ₃ to perform 9-pulse control.
When the induction motor IM accelerates and the voltage proportional to the rotational frequency becomes larger than the reference value of the comparator element 9PT that switches from 9 pulses to 3 pulses, the output of the comparator element 9PT becomes 1, and the transistor
Tr ₁ and Tr ₃ are turned on, transistor Tr ₂ is turned off, and control is performed using three pulses. When the induction motor IM is further accelerated, the output of the comparator element 3PT, which switches from 3 pulses to 1 pulse, becomes 1, transistors Tr ₁ and Tr ₂ are turned on, and transistor Tr ₃ is turned off, controlling with 1 pulse. will be carried out. In this embodiment, as mentioned above, the power supply for the reference value circuit for switching the number of pulses is supplied with a voltage proportional to the input voltage of PWM inverter 1, so even if the input voltage fluctuates, it will respond accordingly. As a result, the reference value also varies, and the number of pulses is always switched at the optimal time shown in FIG. 2, so that current pulsation can be reduced. Also,
The voltage jump when switching the number of pulses to one pulse can be reduced, and the induction motor has the feature of not producing excessive current or excessive torque. In the embodiment of the present invention, the pulse switching is performed using a voltage proportional to the rotational frequency of the induction motor as a signal corresponding to the inverter output frequency. However, the pulse number switching is performed using the output voltage of the PWM inverter 1. It is clear that the same effect can be obtained. In this case, the fourth
As shown by the two-dot chain line in the figure, the second voltage detector 12
is connected to the output end of the inverter 1, and its output is supplied to each comparator element 1PT, 3PT, 9PT of the comparator 11. In the embodiment of the present invention, switching from 9 pulses to 3 pulses and switching from 3 pulses to 1 pulse have been described, but it is not necessary to limit the number of pulses to 9 pulses or the like. As is clear from the above description, according to the PWM inverter of the present invention, it is possible to reduce the resulting current pulsations, reduce voltage jumps when switching the number of pulses, and supply stable AC output to the load.

[Brief explanation of the drawing]

Figure 1 shows an example of the modulation method of the PWM inverter to which the present invention is applied, and is an operational waveform diagram when comparing a sine wave and a triangular wave. Figure 2 is an explanatory diagram of the operation when switching the number of pulses of the inverter according to the present invention. , FIG. 3 is an operating waveform diagram according to an embodiment of the present invention, and FIG. 4 is a circuit diagram of an embodiment of a PWM inverter control device according to the present invention. 1...PWM inverter, 10...1st voltage detector, 11...3rd comparator, 12...2nd voltage detector, TG...speed generator, 1PT, 3PT, 9PT
... Comparator element, EOR ... Exclusive OR circuit,
NOT...Negation circuit.

Claims (1)

[Claims] 1. An inverter that converts direct current to alternating current, and in order to control the output voltage and output frequency of this inverter, the number of pulses included in a half cycle of the inverter output voltage is sequentially switched in relation to the inverter's output frequency or a signal equivalent to it. A pulse width modulation inverter comprising means for adjusting a reference value related to the frequency or a signal corresponding to the frequency for switching the number of pulses according to a DC power supply voltage of the inverter. Inverter control device.