JPS61251477A - Power converter - Google Patents

Abstract

PURPOSE:To eliminate the arrival of an output current at an overcurrent by continuously smoothly shifting the switching of a pulse pattern to the frequency of a next pulse pattern without varying the frequency of a carrier in a steep state. CONSTITUTION:When an output frequency reference f* arrives at a pulse pattern switching point f1*, a switching point detector 5 operates. If a switching point of 9 pulses to 3 pulses is assumed, for example, to be f1*, the output of a pulse pattern generator 6 varies at its value from 9 to 3. However, the value does not momentarily vary by a rate generator 7, but is gradually shifted at the determined time by a rate generator 7. Similarly, it is similarly switched at f2* of next switching point. Thus, the pulse pattern is switched smoothly to eliminate the arrival of the output current at an overcurrent state.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention can improve the efficiency, reliability, and performance of a power conversion device that performs PWM control (pulse width modulation control) using an improved control method. In particular, in a PWM control method that determines the energization width of the main switching element of a power conversion device by comparing carrier waves and modulated waves having a plurality of different frequency patterns, the stability when switching the carrier wave frequency pattern, The present invention also relates to a power conversion device that has improved reliability.

[Technical background and problems of the invention]

2. Description of the Related Art Power conversion devices that output variable alternating voltage and variable frequency are widely used as variable speed drive power sources for alternating current motors. Variable voltage and variable frequency power supplies include voltage type inverters and magnetic current type inverters.

There are devices called thyristor motors and cycloconverters, but among these, voltage source inverters are voltage controlled power conversion devices and are widely used as AC power supplies because they operate as ideal voltage sources. I'm tired.

A voltage source inverter uses a method of varying the DC voltage input to the inverter as a means of varying its output voltage (so-called PAM control).

Keep the DC voltage constant (2?ki), then set the output voltage to pulse (
A typical example is PWM control (pulse width modulation control) in which the output voltage is varied on average by dividing the pulse into two and varying the width of the pulse. However, since PAM control requires a variable DC 4i source, and PWM control can control the output current to a sine wave depending on the control method, PWM control is generally used.

As a method of PWM control, there is a method in which a pulse SP is created by the polarity of the difference between the carrier wave 8c and the modulated wave SM, as shown in Figure 5 (;
A method using a sine wave for M is called a triangular wave comparison sine wave PWM @ control and will be explained below. This triangular wave comparison sine wave PWM control is generally widely used because it can be realized with a relatively simple circuit configuration and because the output magnetic current is a sine wave C2.

In the PWM control shown in FIG. 5C-2, the frequency of the carrier wave S is three times the frequency of the modulated wave SM, and this will be referred to as 3-pulse mode. Hereinafter, similarly, the frequency of carrier wave Sc such as 9 pulse mote, 15 pulse mote etc. is modulated wave SM.
is given as a multiple of the frequency of Furthermore, when the frequency ratio between the carrier wave Sc and the modulated wave SM always maintains a constant value, this will be referred to as synchronous PWM control.

By the way, when the output pulse train, that is, the output voltage, is divided into sine wave frequency components, it is divided into fundamental wave components and harmonic components (by Fourier series expansion), and the higher the number of pulses, the smaller the lower harmonics. It is clear that higher harmonics become larger. This means that in an inductive load, the filtering effect becomes large for high-order harmonics, so the flowing current follows the fundamental wave and becomes a sine wave, and the ripple caused by the harmonics is small. When the number of inverse 1-pulses is small, low-order harmonics become large, resulting in a sinusoidal "magnetic current" containing many ripples.

Further, in order to vary the output voltage, the pulse width of the output can be varied by changing the ratio of the peak values of the carrier wave and the modulated wave. Hereinafter, this ratio of peak values will be referred to as a modulation rate.

To summarize the above, the greater the number of output pulses, the more a sine wave current output with less ripple current can be obtained in an inductive load. This means that in order to reduce the ripple current, the frequency of the carrier wave must be increased.

The main switching element of the power conversion device has an upper limit on the number of times of switching, that is, the switching frequency C, due to its performance. For example, the main switching element is GTO (
When a turn-off thyristor is used, the switching loss that occurs when the GTO is turned on and off poses a major problem for the temperature rating of the device and the efficiency of the entire device. Since switching loss is proportional to the switching frequency, it is necessary to reduce the switching frequency as much as possible. Since the switching frequency in the synchronous PWM control method is given by the product of the modulation wave frequency and the carrier wave frequency, it is necessary to reduce the carrier wave frequency and the number of pulses in order to reduce switching loss.

Therefore, in conventional power conversion equipment, the switching frequency is prevented from exceeding the upper limit by switching the pulse pattern depending on the output frequency band, and the ripple magnetic current that can be tolerated is reduced by reducing the number of pulses. By reducing the number of pulses to a certain value, the efficiency of the device due to switching losses was increased as much as possible.

On the other hand, when starting an AC motor with a voltage source inverter, the induced voltage of the AC motor is not established, so the load impedance seen from the voltage source inverter output is very small. To start the AC motor within the rated capacity of the converter while suppressing the P
With WM control, it was necessary to control the starting current by increasing the frequency of the carrier wave and chopping at a high frequency.

In other words, at startup, the carrier wave is set to a high frequency.

After startup, the carrier wave was set to a low frequency to reduce switching loss. As shown in FIG. 6, when looking at each pulse pattern, the difference in the number of pulses to be switched next becomes large. In FIG. 6, fc is a carrier wave frequency, I is an output frequency (modulated wave frequency), and the carrier wave changes in a step manner when switching in each pulse pattern.

Therefore, at the moment of switching, the harmonic voltage changes stepwise C2 even though the fundamental current is the same, so a large current flows with the harmonic current accompanied by an overwave phenomenon. If the main switching element of a power conversion device has a limit on the current that can be cut off during commutation, such as a GTO, the rated capacity of the device must be reduced in order to take into account the overcurrent that occurs when switching pulse patterns. However, the converter capacity could not be fully utilized.

[Purpose of the invention]

The present invention has been made to solve the above problems, and an object of the present invention is to provide a power conversion device having the following characteristics.

+1> By performing high-frequency chopping on the AC motor using PWM control, the current at startup can be suppressed and the startup torque can be secured.

(2) After startup, to reduce the switching loss of the main switching element (=, the switching frequency can be reduced and the efficiency of the device can be improved.

(3) If the above (1) and (2) are realized, the frequency difference between the carrier waves when switching each pulse pattern becomes large. A power conversion device with economical capacity can be provided.

[Summary of the invention]

To achieve the above objective, in a power converter using a PWM control method that obtains pulses by comparing a carrier wave and a modulated wave, when the output frequency of the converter is low, that is, immediately after startup, the frequency of the carrier wave is set to high. , the pulse pattern is such that as the output frequency of the converter increases and the frequency of the carrier wave decreases, the switching of the pulse pattern is continuous until the frequency of the next pulse pattern without changing the frequency of the carrier wave in steps. A smooth transition can be achieved by C2.

[Embodiments of the invention]

The configuration of an embodiment of the present invention will be explained with reference to FIG. The signal generated by the speed reference generator 1 is input to a voltage reference generator 2 serving as a reference for the output voltage and a frequency reference generator 3 serving as a reference for the output frequency. The output of the frequency reference generator 3 is input to a unit sine wave generator 4 which generates a unit modulated wave S'M, and a unit modulated wave 8- is obtained. The voltage reference v%, which is the output of the voltage reference generator 2, is multiplied by the unit modulated wave S4 to obtain the modulated wave SM.

With the frequency reference 1+ as input, the switching point detector 5 supplies the switching point to the pulse pattern generator 6 in accordance with the frequency pattern of the carrier wave Sc. The pulse pattern generator 6 is
carrier wave S in each pulse pattern. A predetermined value is output so that the frequency of is twice the frequency reference f+. That is, in a three-pulse pattern in which the frequency of the carrier wave Sc is three times the frequency of the modulated wave Syo, K,
= 3.

The pulse pattern signal passes through a rate generator 7 before being multiplied by a frequency reference 7'. This rate generator 7
dK acts to prevent the input signal C2 from outputting a signal that exceeds a predetermined time change, that is, dK
C2 acts so that P/dt becomes a constant value 62 regardless of the input signal.

Next, the operation of an embodiment according to the present invention C, which is constructed from FIG. 1C2, will be explained based on FIG. 2.

Output frequency reference f” is the pulse pattern switching point)/-
When 9 is reached, the switching point detector 5 is activated, for example, in this example, 9 is reached.
Switching point from pulse to 3 pulses! -, the output of the pulse pattern generator 6 changes in value from 9 to 3. However, due to the rate generator 7, the value does not instantly change to 9 or 53, but gradually changes at a predetermined time by the rate generator 7. Similarly is the next switching point! - Switching is also performed in the same way.

As described above, the frequency of the carrier wave Sc does not change stepwise, but is gradually switched to the next frequency with time. This means that the harmonic components of the output in each pulse pattern do not change instantaneously, but gradually change over a predetermined period of time.

As mentioned above, the effects of the present invention C: are as follows.

Since the frequency of the carrier wave changes continuously, the harmonic components included in the output voltage change continuously, and the harmonic magnetic current in the output current C2 also changes continuously (-changes with transient phenomena). The pulse pattern is smoothly switched without causing any overcurrent in the output current.

FIG. 1: Therefore, one embodiment of the present invention shown relates to pulse pattern switching in so-called synchronous PWM control. That is, it is based on a carrier wave frequency that is n times the output frequency, and is based on the fact that the ratio of the carrier wave frequency to the output frequency is always n times.

In addition to the above-mentioned synchronous type, there is an asynchronous type of PWM control system, and FIG. 3 shows an example in which the present invention is applied to an asynchronous PWM control type.

The synchronous/asynchronous switch 11 has a function in which a constant value is output up to a predetermined frequency reference I-, and when the frequency reference f" exceeds the switching point!-, the frequency reference!" is output as is. , the rate generator 6 is the carrier wave generator 8
The first stage (two are provided).

According to the above configuration, as shown in FIG. 4, the frequency N of the carrier wave does not change instantaneously but changes continuously over time even at the synchronous/asynchronous switching point fa'', and the output The harmonic current of is stable without any transient phenomenon.

〔Effect of the invention〕

From the above explanation, the pulse pattern of PWM control required for the four-force conversion device to secure the starting torque of the electric motor, and to perform efficient operation of the device and economical capacity selection using an efficient switching pattern is as follows. to make an invention (
- Since it is possible to switch more stably, it is possible to provide a magnetic force converting device having the characteristics described in C2 below.

1) When starting an AC motor, the starting magnetic current can be suppressed by high frequency chopping using PWM control, and starting torque can be ensured.

2) Since an efficient switching frequency can be determined depending on each operating frequency, the efficiency of the entire device can be improved.

3) In order to realize 1) and 2), it is necessary to switch the pulse patterns during operation, and by applying the present invention when performing this switching, stable switching can be performed.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment according to the present invention, and FIG.
3 is a diagram illustrating another embodiment of the present invention. FIG. 4 is a diagram illustrating the operation of FIG. 3. FIG. 5 is a diagram illustrating the triangular wave comparison PWM control method. , FIG. 6 is a diagram for explaining the state of conventional pulse pattern switching. 1... Speed reference generator 2... Voltage reference generator 3.
...Frequency reference generator 4...Unit sine wave generator 5.
...Switching point detector 6...Pulse pattern generator 7.
... Rate generator 8 ... Carrier wave generator 9 ... Comparator 10 ... Multiplier 11 ... Synchronous asynchronous switch (7317) Agent Patent attorney Kensuke Chika (and 1 others)
Figure 1 Figure 2 Figure 4 Figure 6

Claims (1)

[Claims] In a pulse width modulation control power converter that varies the output power by varying the energization width of the main switching element, when pulse width modulation is performed by comparing the modulated wave and carrier wave to perform pulse width modulation. , the frequency of the carrier wave is set to n_1 to n_2 times the frequency of the modulated wave.
When switching from the frequency of the carrier wave, which is set independently of the frequency of the modulated wave, to n_3 times the frequency of the modulated wave, or vice versa, the frequency of the carrier wave at the time of switching is changed to the frequency of the carrier wave at the time of switching. 1. A power conversion device characterized in that the frequency of a carrier wave to be switched to is continuously shifted at a predetermined time.