JPS60174070A - Carrier control system in ac converter circuit - Google Patents

Abstract

PURPOSE:To smoothly perform a PWM control by switching in an asynchronous type at a low speed and in a synchronous type at a high speed. CONSTITUTION:When a speed command VCMD increases from the operating state using an asynchronous carrier and arrives at the predetermined rotating speed, the output of a speed discriminator 22 becomes from low speed 0 to high speed 1, and 1 is applied to an AND circuit 25. On the other hand, a coincidence detector 21 detects the coincidence of the synchronous carrier with the asynchronous carrier, and when the coincidence of the frequency and the coincidence of the phase are satisfied, 1 signal is applied to the circuit 25. When an output is obtained at the AND circuit 25, a reset input is applied to a flip- flop 26, a switching element 28 is OFF, and a switching element 27 becomes ON. As a result, the synchronous carrier is used as the carrier.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a carrier wave control method for performing pulse-N modulation (PWM) control in an AC inverter circuit.

(Prior Art) In order to generate a sine wave voltage in an AC inverter circuit, a PWM control method is adopted in which the waveform of a command wave is compared with a carrier wave such as a triangular wave and converted into a pulse width. This PWM control method includes a synchronous method in which the number of carrier waves is always constant with respect to the waveform of the command wave, and an asynchronous method in which the number of carrier waves is constant regardless of the waveform of the command wave. Figure 1 shows an outline of these methods, and shows a waveform diagram of the synchronization method.
As shown in Figure 11, the period TCMD of the command wave FCMD
The period TC of the carrier wave FC is TCMD = TCXP (P = 10 in the C17 example), and the number of output pulses P is constant regardless of the frequency of the command wave, that is, the number of pulses is P = TCMD/TC. be done. In other words, the carrier wave FC is changed in synchronization with the command wave FCMD.

FIG. 2 shows a waveform diagram of the asynchronous method, and the asynchronous method will be explained based on this diagram. According to this method, the carrier wave FC is always constant regardless of the frequency of the command wave FCMD, and in the case of a large capacity such as an inverter, it is generally several KH2 due to the heat generation of the switching elements.

(Problems with the prior art) According to the above-mentioned synchronization method, the carrier wave frequency increases in proportion to the command wave frequency, and since there is a limitation on the switching frequency, the command wave and the carrier wave cannot be switched as shown in Fig. 3. Make sure the switching frequency is within a certain range. It was inevitable that the circuit configuration would become complicated due to necessity. Also, there was a problem of crashing when switching to low speeds.

On the other hand, according to the asynchronous method, the carrier wave FC is constant. Therefore, the number of output pulses P changes depending on the change in the command wave FCMD, so the -+ waveform of the command wave FCMD is at most 01 when the output pulse 1jQP is an odd number and an even number.
．． 5 pulses deviation, no problem with Tq<<TCMD, but T
When the ratio between C and TCMD becomes smaller, vibrations due to the influence of the O15 pulse shift begin to appear, causing the problem that the operation of equipment that serves as a load on the inverter becomes unstable.

(Objective of the Invention) As mentioned above, both the synchronous method and the asynchronous method have advantages and disadvantages, and in order to solve these problems, the object of the present invention is to combine these methods to control carrier waves that can perform asynchronous PWM control at low speeds. The goal is to provide a method.

(Summary of the Invention) The present invention utilizes a pulse width control method (PWM) that converts a command wave that determines the output frequency of an inverter circuit into a switching pulse width by comparing it with a carrier wave that determines the switching frequency and wave number of a switching element. In this system, a synchronous carrier wave and an asynchronous carrier wave are generated as carrier waves, and a speed command is set so that the speed range below a predetermined speed is a boundary, the asynchronous method is used, and the other speed range is a synchronous method. It is configured to switch between a synchronous carrier wave and an asynchronous carrier wave depending on the value.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 4 is a block diagram of an embodiment of the present invention, 11 is a voltage/frequency converter, and the input speed command VCM
12 is a counter that converts to a frequency proportional to D, and a read-only memory (hereinafter referred to as RO-M) that determines the output frequency of the voltage/frequency converter 11 and outputs a signal corresponding to that frequency, which will be described later. 13 is a ROM, which supplies command waves FCMD in the PWM control system.
A sine wave serving as a sine wave and a triangular wave serving as a synchronous carrier wave FC are stored in memory in association with each other, and output a sine wave (U phase, ■ phase, W phase) and a synchronous carrier wave according to the signal from the counter 12, 14 is a carrier wave switching circuit, and ROM13
The synchronous carrier wave from the synchronous carrier wave, the asynchronous carrier wave generated separately from the speed command VCMD, and the speed command VCMD are input, the synchronous carrier wave and the asynchronous carrier wave are switched according to the value of the speed command VCMD, and one of the carrier waves is output. This is a pulse width modulation circuit, into which a sine wave and a carrier wave serving as the command wave FCM-D are input, and is a well-known PW circuit for supplying a switching signal to an in/outside switching element (not shown).
It is an M circuit.

FIG. 5 is a block diagram showing the internal circuit of the carrier wave switching circuit 14 in FIG. It is comprised of a speed discrimination circuit 22 that discriminates a value, outputs a high speed/low speed signal, and has hysteresis characteristics, a NOT circuit 23, AND circuits 24 and 25, a flip-flop circuit 26, and switching elements 27 and 28.

Here, since the speed discrimination circuit 22 discriminates the value of the speed command VCMD and has a hysteresis characteristic, it is simply constituted by, for example, a Schmitt circuit. The coincidence detection circuit 21 outputs a coincidence signal when it detects coincidence in frequency and phase between the synchronous carrier wave and the asynchronous carrier wave, and its internal circuit has various circuit configurations to satisfy the detection conditions. Conceivable. An example of this circuit configuration is shown in FIG.

In FIG. 6, a frequency-number configuration circuit 31 detects whether the frequencies of the synchronous carrier wave and the asynchronous carrier wave match. Increasing and decreasing zero points of the synchronous carrier wave are detected by an increasing zero point detection circuit 32 and a decreasing zero point detection circuit 33, respectively. Similarly, for the asynchronous carrier wave, the increasing zero point detection circuit 34,
A decreasing zero point detection circuit 35 detects each zero point. The outputs of the increasing zero point detection circuits for both carriers are fed to an AND circuit 36, and the outputs of the decreasing zero point detecting circuits are fed to an AND circuit 37. The outputs of these AND circuits 36 and 37 are given to the AND circuit 39 via the OR circuit 38, and when the AND condition with the output of the frequency-number calculation circuit 31 is satisfied, the output as the -number calculation circuit 21 is occurs. Note that the detection accuracy of each zero point detection circuit 32, 33, 34, 35 and frequency-number configuration circuit 31 may be made rough within a range that does not cause any problem as an inverter/event device.

Next, the operation of the embodiment shown in FIGS. 4 and 5 will be explained with reference to the characteristic curve diagram shown in FIG. In the following explanation, P
This is performed on the assumption that an induction motor is driven by the output of an inverter that is WM controlled. In Fig. 7, when the rotational speed of the induction motor reaches 150 rpm, in the range lower than the rotational speed, the carrier wave frequency is constant (2KH2), and in the range higher than the rotational speed, the carrier wave frequency increases in proportion to the rotational speed. becomes. That is, an asynchronous carrier wave is used as a carrier wave for PWM control in a range lower than a predetermined rotation speed of 1500 rpm, and a synchronous carrier wave is used as a carrier wave in a higher range. Switching between the synchronous carrier wave and the asynchronous carrier wave is performed based on the output of speed discrimination having hysteresis characteristics and the output of coincidence detection of both carrier waves.

Now, when starting up the inverter circuit, first the speed command V
CMD is applied to the voltage/frequency converter 11, the output frequency of which is determined by a counter, and a signal corresponding to the frequency is supplied to the ROM 13. The ROM 13 stores a sine wave and a synchronous carrier wave in association with each other, and outputs the sine wave and synchronous carrier wave according to the signal from the counter 12, that is, the speed command VCMD. Since the speed command VCMD at the time of startup of the inverter circuit starts from a small value, the output of the speed discrimination port 22 is a low speed "0°" at the time of startup, and the output of the coincidence detection circuit 21 is is set, the flip-flop circuit 26 is set, the synchronous carrier switching element 27 is turned off, and the asynchronous carrier wave is sent out as a carrier wave. Note that it is preferable that the flip-flop circuit 26 has a circuit configuration that automatically sets the flip-flop circuit 26 to a set state when the inverter circuit is started.

Now, the sine wave as the command wave FCMD and the asynchronous carrier wave are supplied to the PWM circuit 15, and the inverter is operated at a switching frequency determined by the frequency of the carrier wave and an output frequency determined by the frequency of the command wave (sine wave). . Note that the output voltage of the inverter is determined by the amplitude ratio of the command wave (sine wave) and the carrier wave, and a sine wave with a predetermined amplitude It] is output from the ROM 13.

From the operating state using this asynchronous carrier wave, the speed command V
When the CMD increases and reaches a predetermined rotational speed, the output of the speed determination circuit 22 changes from the low speed 0'' to the high speed 1'', and 11' is applied to the AND circuit 25. On the other hand, the minus number configuration circuit 21 detects coincidence between the synchronous carrier wave and the asynchronous carrier wave, and as shown in the block diagram shown in FIG. 6, when frequency coincidence and phase coincidence are satisfied, the AND circuit Apply the “°l” signal to 25. When an output is obtained to the AND circuit 25, a reset input is given to the flip-flop circuit 26, and the QQ terminal is "0"".
The mutual terminal becomes "1", the switching element 28 is turned off, and the switching element 27 is turned on. As a result, a synchronous carrier wave is used as a carrier wave.

The above-mentioned operation is performed when moving from a low-speed area to a high-speed area (1), but even when moving from a high-speed area to a low-speed area, switching between the synchronous method and the asynchronous method is performed smoothly.

Note that although the sine wave and the synchronous carrier wave are derived from the ROM, the present invention is not limited to this, and commonly used analog type ones can also be applied.

(Effects of the Invention) As explained above, according to the present invention, the output of the high-speed discrimination circuit for discriminating the frequency difference and the coincidence detection circuit for detecting coincidence between the synchronous carrier wave and the asynchronous carrier wave is lower than a predetermined frequency. By performing asynchronous PWM control in the frequency range and using synchronous PWM control in the range higher than a predetermined frequency, it is possible to compensate for the drawbacks of the synchronous method and the asynchronous method and perform smooth PWM control. In addition, it is possible to smoothly switch between the synchronous method and the asynchronous method.

[Brief explanation of drawings]

Fig. 1 is a waveform diagram of the synchronous method, Fig. 2 is a waveform diagram of the asynchronous method, Fig. 3 is a diagram showing the modulation ratio in the synchronous method, Fig. 4 is a block diagram of an embodiment of an undeveloped method, and Fig. 5 is a diagram showing the modulation ratio in the synchronous method. 6 is a diagram showing the configuration of the carrier switching circuit in the configuration shown in FIG. 4, FIG. 6 is a diagram showing the configuration of the coincidence detection circuit in the configuration shown in FIG. 5, and FIG. 7 is a characteristic curve diagram. In the figure, 11... Voltage/frequency converter, 12... Counter, 13... Read-only memory, 14... Carrier wave switching circuit, 15... Pulse width modulation circuit, 21...
・-Number output circuit, 22...Speed discrimination circuit, 23...
Not circuit, 24.25...AND circuit, 26...
Flip flop 0-, flip circuit, 27.28... switching element. Patent applicant Fanuc Co., Ltd. Agent Patent attorney Minoru Tsuji (1 other person) Calculator I Zubo 2 Illustrated Imanami Illustrator No. 4 No. 5 Fig. 7 Fig. 6 Fig. 7

Claims (1)

[Claims] In a pulse width modulation control method that compares a command wave that determines the output frequency of the inverter circuit with a carrier wave that determines the switching frequency of the switching element and converts it into a switching pulse width, the frequency and phase of the synchronous carrier wave and the asynchronous carrier wave match. At the same time, it is determined whether the speed command value is above or below a predetermined value, and based on these detection and discrimination results, the asynchronous carrier wave is used as a carrier wave in a speed range below a predetermined speed, and the asynchronous carrier wave is used as a carrier wave in a speed range above a predetermined speed. Here, we will discuss a carrier wave control method in an AC inverter circuit, which is characterized in that it is switched and used as a synchronous carrier wave.