JPH04372599A - Output current limiting method for inverter - Google Patents

Abstract

PURPOSE:To set the rotational speed of an induction motor quickly by prohibiting output current limit control even if the output current exceeds a set value during increase control of output frequency toward a target value after droppage of the output current below the set value in the output frequency decrease control. CONSTITUTION:In output frequency decrease control through a frequency regulating circuit 4 upon occurrence of excessive increase of output current Im, increase control of output frequency toward a target value fs after droppage of the output current Im below a set value Is is sustained, even if the output current Im exceeds the set value Is through reacceleration of a motor 3, until an output frequency command value fo reaches a target value fs. When the command value fo reaches the target value fs, increase control is stopped and transition is made to steady operation. Consequently, control set time can be shortened.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for limiting the output current of an inverter for driving an induction motor during constant speed operation when the load suddenly changes.

0002

2. Description of the Related Art As a conventional method for limiting the output current of this type of inverter, if the output current Im becomes larger than its limiting current setting value Is, the output frequency f of the inverter
The load induction motor is decelerated by reduction control of o and the inverter output voltage Vo, which is usually linked in a proportional relationship with the fo, to make the current Im less than the set value Is, and further this output current reduction state When the frequency fo
is controlled to increase again with the target value fs, and thus the value of the output frequency before the increase fluctuation of the current Im occurs, that is, the current around the set value Is in order to make the current Im equal to or less than the set value Is. A device is known that repeats control to increase or decrease the frequency fo in response to changes in Im.

0003

However, in the conventional method as described above, after the inverter output current Im is reduced to below the limit current setting value Is by the limit control, the load is reduced by inverting and increasing the inverter output frequency fo. When the induction motor accelerates, if the current Im becomes larger than the set value Is again due to the acceleration, the frequency fo is again reduced and the motor is decelerated again. Frequency fo and the current Im
This means that each set value is approached by repeating increases and decreases through frequency control as described above, and therefore, the required settling time has to be long. In addition, this long settling time has been a major factor that impedes smooth operation when the motor is subject to a large and often sudden change in load.

In view of the above, it is an object of the present invention to provide an inverter output current limiting method that is capable of reducing the inverter output current to a limit value or less when the inverter output current increases excessively, and quickly stabilizing the inverter output frequency and therefore the induction motor rotation speed. This is the purpose.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the inverter output current limiting method of the present invention is provided such that the output current of the inverter that controls the speed of an induction motor is larger than its limiting current setting value. In the inverter output current limiting method, which reduces the output frequency of the inverter and the output voltage linked to the output frequency to make the output current below the set value, when an excessive increase in the output current occurs. , during the subsequent increase control of the output frequency to its target value after the output current is made below the set value by the reduction control of the output frequency, the Even if the output current exceeds the set value again, the output current is not limited by reducing the output frequency until the output frequency reaches the target value.

[0006]

[Operation] The output current Im of the inverter that controls the speed of the induction motor can be increased or decreased by accelerating or decelerating the motor by increasing or decreasing the output frequency fo of the inverter. The same holds true regardless of whether the characteristic is a constant torque characteristic or a constant torque characteristic. Note that in the above-mentioned inverter, the output voltage Vo is usually Vo/f with respect to the output frequency fo.
They are interlocked and controlled with a constant o ratio.

Therefore, when the current Im becomes larger than the current limit setting value Is due to an increase in the load on the motor, the current limit control is performed using a current deviation ΔI (Δ
The frequency fo is repeatedly increased and decreased according to the positive or negative polarity of I=Is-Im), and the current Im and the frequency fo are finally equal to the set value Is and the output frequency target, respectively. It will settle to a steady value that does not exceed the value fs, and the required settling time will become longer as the inertia time constant of the overall rotating system, the overall time constant of the control system, and the current deviation increase, respectively. .

However, the increasing fluctuations in the motor load, and thus the increasing fluctuations in the current Im not caused by control, end in a time comparable to or less than the settling time and return to their respective steady values. If so, ΔI following the first output frequency reduction control performed when the current deviation ΔI becomes negative
>0, fo→fs During control, even if the current Im increases again as the motor accelerates and the current deviation ΔI reverses to negative again, as long as fo≦fs, the frequency fo will not be reduced and the frequency fo will not be reduced. Even if the control to increase the frequency fo to its target value fs (fo→fs) is continued, the current Im itself naturally decreases due to the fluctuation pattern of the motor load as described above, so the negative state of ΔI continues excessively. Therefore, it is possible to continue the fo→fs control without repeating the increase and decrease, and the settling time is also shortened compared to the time of the repetitive control.

[0009] The present invention aims at shortening the settling time of each of the above-mentioned specifications by continuing the control regardless of the polarity of the current deviation ΔI during the fo->fs control as described above. In addition, if the above-mentioned specifications can only be stabilized at a frequency lower than the lower limit frequency of the predetermined operating range due to an excessive and continuous increase in the motor load, measures such as stopping the operation according to a separately prepared protection program will be taken. Treatment is required.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the block circuit diagram of FIG. 1 and the time chart of FIG. 2. First, Figure 1
, 1 is an AC power supply, 2 is an inverter consisting of a rectifier, a rectified voltage smoother, and a DC/AC converter, and 3 is an induction motor that is variable speed controlled by the inverter and drives a variable load. be. Next, 5a is a current transformer for detecting the inverter output current, 5 is a current detection circuit which receives the output of the current transformer 5a and outputs the inverter output current Im at a calculation level, and 6 is a current comparison circuit. The current deviation ΔI (ΔI=Is−Im) is calculated from the current Im and its limiting current setting value Is, including its positive/negative polarity determination.

Reference numeral 4 denotes a frequency adjustment circuit which adjusts the frequency target value fs appropriately according to the load condition of the motor 3 determined from the positive/negative polarity and magnitude of the current deviation ΔI. The output frequency command value fo of the inverter 2 is calculated and outputted, and the deviation ΔI
When ΔI is negative, the fo is controlled to be smaller than the fs with an appropriate time gradient, and when the ΔI is positive, the command value fo is controlled to be smaller than the target value fs by an appropriate time. Increase control is performed using the gradient, but during the increase control, dfo/dt
>0, the current Im is increased again so that the ΔI
Even if the polarity of ΔI may be reversed from positive to negative, the fo increasing control is continued until the frequency fo reaches the target value fs regardless of the polarity reversal of ΔI. Note that the output frequency control in the illustrated inverter 2 is open-loop control, and does not have a negative feedback control system for the output frequency.

Next, diagrams (a), (b), and (c) in FIG. 2 show temporal changes in the load driven by the motor 3, the output current Im, and the output frequency command value fo, respectively. This shows that. Now, if the motor load rapidly increases and decreases as shown in Figure (A), in response to this rapid increase in load, the current Im also increases with a specific time constant as shown in Figure (B), and its limit The current setting value Is is exceeded. The polarity of the current deviation ΔI changes from positive to negative from the point in time when the current deviation is exceeded, and from this point in time when the polarity changes, the reduction control for the command value fo is started as shown in FIG. The steady-state operation period A ends and the command value f
Shifts to the reduction control period B of o. Next, the electric motor 3 is decelerated by the fo reduction control to reduce its output, and as a result, the current Im also decreases, and when it becomes less than the set value Is, the polarity of the current deviation ΔI is reversed from negative to positive. After a specific control time delay has elapsed, control to increase the command value fo toward the target value fs is started. That is, the fo reduction control period B ends and the fo increase control period C begins.
Move to.

Next, during the period C, a case may occur in which the current Im exceeds the set value Is due to re-acceleration of the electric motor 3, but this excess state applies to a fluctuating load as shown in Figure (A). If so, it will not continue for a long time due to the natural reduction of the current itself, so the increase control of the fo is uniformly continued without interruption until the fo reaches the target value fs, and the fo When fo becomes equal to fs, the increase control of fo is stopped and a transition is made to a steady operation period D similar to period A. Therefore, compared to the case where the fo reduction control is performed again when the Im>Is state occurs in the period C, the end point of the period C is earlier, and the control settling time can be shortened.

[0014]

[Effects of the Invention] According to the present invention, when the output current of an inverter that controls the speed of an induction motor becomes larger than its limiting current setting value, the output frequency of the inverter is linked to the output frequency. In the inverter output current limiting method of reducing the output voltage to make the output current below the set value, when an excessive increase in the output current occurs, the output frequency is reduced by controlling the output frequency to make the output current below the set value. During the subsequent increase control of the output frequency to its target value, even if the output current exceeds the set value again due to the reversal increase of the output frequency that has been decreased, the output By not controlling the output current by reducing the output frequency until the frequency reaches the target value, it is possible to improve responsiveness and settling time in the inverter output current limiting operation when the load of the motor repeatedly changes significantly and suddenly. It is possible to improve the smoothness of motor operation by shortening the time, and furthermore, the detection current for control may be an effective value instead of an instantaneous value for each phase, making the configuration of the control system simple and inexpensive. .

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing an embodiment of the present invention.

[Figure 2]
Time chart showing an embodiment of the present invention

[Explanation of symbols]

1 AC power supply 2 Inverter 3 Induction motor 4 Frequency adjustment circuit 5 Current detection circuit 5a Current transformer 6 Current comparison circuit

Claims (1)

[Claims] 1. When the output current of an inverter that controls the speed of an induction motor becomes larger than its current limit setting value, the output frequency of the inverter and the output voltage linked to the output frequency are reduced. In the method for limiting the output current of an inverter, when an excessive increase in the output current occurs, the output current is reduced to the set value or less by the output frequency reduction control, and then the output current is reduced to the set value or less. During control to increase the output frequency to its target value, even if the output current exceeds the set value again due to a reversal increase of the output frequency that has been lowered, the output frequency will not reach the target value. A method for limiting an output current of an inverter, characterized in that the output current is not limited until the output frequency is reduced.