JPS638709B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an output current control method of a frequency exchange device that supplies variable frequency AC power to a load such as an AC machine.

FIG. 1 is a schematic configuration diagram showing an outline of this type of frequency conversion device, and FIG. 2 is a waveform diagram showing waveforms of various parts of FIG. 1.

A and B in FIG. 1 represent one phase of a controlled rectifier circuit consisting of a thyristor bridge, and three sets of these are used to construct a power converter. Thyristor bridge A supplies a positive polarity current to the load L, and thyristor bridge B supplies a negative polarity current, but these selections are determined by the current command value of each phase as shown in Figure 2a. This is done by issuing a thyristor selection signal as shown in c in the figure, depending on the polarity of i ^* . In addition, the PC is the current command value i ^* and the DC current transformer.
This is a control circuit that controls the phase of thyristor groups A and B by comparing the current value i detected by CT.

Here, the load L becomes a light load and the current command value i ^*
When it reaches a low level region, the influence of offset and noise of the current command calculator (not shown), as well as the positive and negative side thyristor selection signal generators (not shown)
Due to the hysteresis characteristic of the thyristor, the positive and negative thyristor groups may not be switched at the originally desired time, or the current command value i ^* and the actual value i may not match. In addition, in a region where the current command value is at a low level, there are periods t _p in which the actual current value i is zero and periods t _s in which it is not, as shown in FIG. 2b, that is, an intermittent phenomenon occurs in the current. It is known that the gain of the control system changes depending on the degree of this interruption. Therefore, in a small current region where the degree of current intermittent is large, a speed control system having a current control system as a minor loop may become unstable. For this reason, an intermittent current compensation method is known that compensates the gain of the current control system according to the degree of intermittent current, but even with this method, the amount of compensation increases in the small current region, so it is difficult to perform good compensation. The current situation is that it is difficult.

Therefore, it is an object of the present invention to provide a method for controlling the output current of a frequency converter, which eliminates the above-mentioned drawbacks in a region where the output current of the frequency converter is at a low level and can perform stable control. It is something.

A feature of the present invention is that in an output current control method of a frequency converter that supplies variable frequency AC power to a load, in a region where the output current is at a low level and the degree of intermittent power is large, a predetermined amount of power is applied to the load. The present invention is characterized in that a power factor changing means is provided to reduce the power factor by supplying a reactive current, and the output current level of the converter is maintained at a predetermined value by the means.

Before explaining embodiments of the present invention, the principle thereof will be explained.

Now, if we consider an AC motor as the load of a frequency converter, its output P is the line voltage v _l and the line current
When i _l is the motor power factor, COS is the motor power factor, and η is the efficiency, it can be expressed as P=√3v _l・i _l・COS・η (1). Therefore, assuming that P, v _l and η are constant in equation (1), it can be seen that if the power factor COS changes, the magnitude of the line current i _l also changes.
By the way, when the output current or load current of the converter becomes small, various problems such as those mentioned above occur, so in order to prevent the output current or the current flowing through the load from becoming small, it is possible to reduce the power factor COS. This is what I am trying to do.

Embodiments of the present invention will be described below with reference to the drawings. Fig. 3 is a block diagram showing an embodiment of the present invention, Fig. 4 is a characteristic explanatory diagram of the embodiment of Fig. 3, and Fig. 5 is a circuit configuration showing an embodiment of the power factor changing signal generator of Fig. 3. It is a diagram.

In Fig. 3, i ₂ ^* represents the torque current component command value of the armature current in the AC motor, and i ₁ ^* also represents the magnetization current component command value of the armature current, and these are used to control the current flowing through the AC machine. will be held. The power factor change signal generator 10 emits a signal for changing the power factor of the alternating current machine by superimposing a predetermined reactive current component described below on the magnetizing current component command value i ₁ ^* . .

For example, for a synchronous motor, the vector control method is a control method that allows it to be handled as if it were a DC machine even though the supplied power is AC (here, the specific details are not directly related to this invention, so (explanation is omitted), the electric motor is generally operated with a power factor of 1, so i ₁ ^* = 0,
Therefore, the level of i ₂ ^* becomes the actual armature current level.

Therefore, the motor is normally controlled using a power factor of 1, and when the armature current is at a very low level, the power factor can be reduced by setting an appropriate value for i ₁ ^* , that is, by passing the excitation current through the load. This increases the current flowing to the load, that is, the output current of the converter, to prevent interruptions in the current.

For this purpose, the torque current component command value i ₂ ^* of the armature current is introduced to the input of the power factor change signal generator 10, and this i ₂ ^* generates the power factor change signal △i ₁ ^* . The magnetizing current component command value i ₁ ^* is added to the magnetizing current component command value (i 1 *)' to obtain a new magnetizing current component command value (i ₁ ^* )'. In this case, depending on the polarity of the power factor change signal △i ₁ ^* , the motor power factor can be made to be lagging or leading, but by setting the lagging power factor, the field current can be changed to approximately △i ₁ ^*
The excitation current can be reduced by the excitation current.
Furthermore, since only a small amount of reactive component is supplied in the low level region of the output current of the frequency converter, that is, the armature current, the influence on the system is small.

Generally, when driving an alternating current machine, if the peak value of the armature current of each phase is i _p , it is expressed as i _p =√( ₁ ^* ) ² + ( ₂ ^* ) ² ...(2) , i ₁ ^* and i ₂ ^* respectively represent the above-mentioned command values), the armature of the alternating current machine has a power factor of 1.
When controlling with magnetizing current component command value
Since i ₁ ^* is zero, i _p =i ₂ ^* according to the above equation (2). Therefore, the relationship between i ₂ ^* and i _p is, for example, the fourth
It is expressed as shown in the figure, and in particular where the armature current is small, the characteristics are as shown by the dotted line. In the region shown by the dotted line, the current discontinuities occur as described above, which is undesirable. Therefore, in the present invention, as shown by the solid line in the same figure, i _p is set to a constant value i _k in the low current region. Make it so that
Then, the magnetizing current component command value (i ₁ ^* )′ in this case becomes (i ₁ ^* )′ instead of i ₁ ^* in equation (2).
, and if i _p is set as i _k , it can be obtained as (i ₁ ^* )′ = √ ( _k ) ² − ( ₂ ^* ) ² .

In other words, without making the magnetizing current component value i ₁ ^* zero,
A significant value is selected such that the vector composite value of the component value and the torque current component command value becomes a constant value i _k . As a result, the power factor change signal Δi ₁ ^* can be expressed as follows.

△i ₁ ^* =√( _k ) ² − ( ₂ ^* ) ² [However, (i ₂ ^* ) ² (i _k ) ² ] △i ₁ ^* = 0 [However, (i ₂ ^* ) ² 〓(i _k ) ² ] Next, regarding an example of a power factor changing signal generator,
This will be explained with reference to FIG.

Since the above power factor change signal Δi ₁ ^* is generated based on the torque current component command value i ₂ ^* , the above current i ₂ ^* is input to the power factor change signal generator 10.
This input signal is converted into (i ₂ ^* ) ² by the multiplier 101
is calculated and provided to the adder 103. A potentiometer 102 is connected to the other side of the adder 103.
_{Since the} ^{signal −} (i _k ) ^{2 set} by be done. Note that since the adder 103 has polarity in only one direction due to the diode D, its output becomes zero in the region of (i ₂ ^* ) ² 〓(i _k ) ² . The calculated value (i _k ) ² - (i ₂ ^* ) ² is calculated as √ ( _k ) ² - ( ₂ ^* ) ² by the square root calculator 104, and this is output as the power factor change signal △i ₁ ^*. be done.

Note that in the area of (i ₂ ^* ) ² 〓(i _k ) ² , the adder 103
Since the output from the power factor is zero, the power factor change signal is also zero.

As described above, according to the present invention, by lowering the power factor of the load and increasing the levels of the current command value and actual value in the low load current region, the thyristor groups in the frequency converter can be switched in the forward or reverse direction. This has the advantage of being able to perform accurate control with desired characteristics, and eliminating current intermittent phenomena, making it possible to reduce gain fluctuations in the control system and ensuring stable control.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a frequency converter, FIG. 2 is a waveform diagram showing waveforms of each part of FIG. 1, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG. 4 is an embodiment of the invention. FIG. 5 is a circuit diagram showing an embodiment of the power factor changing signal generator of FIG. 3. Symbol explanation, L...Load, PC...Control circuit, CT
...DC current transformer, A, B...Thyristor group, 10
... Power factor change signal generator, 101 ... Multiplier, 1
02... Potentiometer, 103... Adder,
104...Square root calculator, D...Diode.

Claims (1)

[Claims] 1. In a control method that performs high power factor operation with the output current of a frequency converter that supplies variable frequency AC power to a load substantially only as an effective component, in a low level region of the effective output current, a predetermined value is applied to the load. A method for controlling the output current of a frequency converter, characterized in that the output current level of the converter is maintained within a range where there is almost no interruption by supplying a large amount of reactive current to reduce its power factor. .