JPH0491695A - Method of inhibiting current vibration of inverter drive induction motor - Google Patents

Abstract

PURPOSE:To inhibit load current vibrations by properly reducing the detecting arithmetic value of a vibrational current component in motor input currents transmitted as a signal for correcting subtraction to an output-voltage command value with the increase of the effective value or means value of motor input currents. CONSTITUTION:A current arithmetic circuit 8, which receives an output from a rectifier circuit 5, detects the mean value of motor input currents used as a value displaying the steady-state load state of an inverter 2 and adds proper time characteristics to a detecting value, is provided. A limiter 9 uses outputs taking both positive and negative polarity of a conversion arithmetic circuit 7 as inputs, the outputs are limited at a specified value at an output limit level to inputs having magnitude of specified values or more, and outputs are increased by an output from the current arithmetic circuit 8, to which the output limit level is input as the bias signal, while the output limit level is changed automatically so as to lower the limit level. A subtracter 10 subtracts the output value of said limiter 9 from an output-voltage command value to the inverter 2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for suppressing input current oscillations that occur depending on operating conditions of an induction motor driven and controlled by a voltage source inverter.

[Prior Art] A conventional method for suppressing current vibration of an induction motor driven and controlled by this type of voltage source inverter includes negative feedback correction of the inverter output voltage using a proportional value of an oscillating current component in the input current of the motor. Methods are known in which the control is performed, or the generation of vibrations in the input current of the motor is accompanied by overexcitation of the motor within its permissible range, or alternatively, the reduction of the switching frequency of the inverter.

[Problem to be solved by the invention]

However, regarding each of the conventional current oscillation suppression methods as described above, first, in the negative feedback correction control of the inverter output voltage using the oscillating current component, the oscillation state of the motor input current that generally becomes noticeable as the load of the inverter is reduced. However, there is a risk that the correction amount for the inverter output voltage is insufficient in the low load range and becomes excessive when the load increases, inhibiting the elimination and stabilization of the vibration state. Deterioration of the operating characteristics of the electric motor or the inverter is unavoidable due to reduced switching frequency operation.

In view of the above, an object of the present invention is to provide a current oscillation suppressing method that can easily and inexpensively improve load characteristics related to suppressing load current oscillations of an inverter-driven induction motor as described above.

[Means to solve the problem]

In order to achieve the above object, in the current vibration suppression method for an inverter-driven induction motor of the present invention, an oscillating current component in an input current of an induction motor driven and controlled by a voltage source inverter is negatively fed back to reduce the output voltage of the inverter. In current vibration suppression control of an inverter-driven induction motor that corrects and controls, a current calculation that detects the effective value or average value of the motor input current input via a current transformer and gives a predetermined time characteristic to the detected value. a circuit, and a limiter whose input signal is the detection calculation signal of the oscillating current component and whose output limit level is made variable such that it becomes smaller as the output signal of the current calculation circuit increases, and the output signal of the limiter is A subtraction correction signal is used for the output voltage command signal of the inverter, and a subtraction correction amount by the oscillating current component is set for the output voltage of the inverter at a predetermined time according to the load characteristics as well as an increase in the effective value or average value of the motor input current. The characteristics shall be small.

[Function] In the drive system of an induction motor using a voltage source inverter, the output voltage (V) of the inverter versus the output frequency (f)
Regarding the characteristics, an unstable region may be observed in which the amplitude of the output current of the inverter and therefore the input current of the motor oscillates in the range near the constant V/f ratio characteristic line, and this state is caused by the load on the inverter. It is well known that this phenomenon becomes more noticeable as the

The inverter output current in the above-mentioned oscillating state is obtained by superimposing an oscillating current component having a plurality of frequency components and amplitude components on a steady current corresponding to the operating state, and the power between the inverter and the motor is Regarding giving and receiving,
During a period when the oscillating current component is in the same direction as the steady current, the power supply from the inverter to the induction motor is increased by the oscillating current component, and similarly during a period when the oscillating current component is in the opposite direction, the oscillating current Power regeneration from the electric motor is performed using the components.

Therefore, vibration suppression of the inverter output current and hence the motor input current is made possible by controlling the fluctuations in the power transfer between the inverter and the induction motor as described above due to the oscillating current component. That is, by controlling the inverter output voltage, the voltage difference between the output voltage and the induced voltage when the induction motor operates as an induction generator is suppressed, or by controlling the inverter output frequency, the output frequency and the voltage difference between the motor and the output voltage are suppressed. This is made possible by suppressing the frequency difference from the frequency corresponding to the actual rotational speed using appropriate time characteristics corresponding to the fluctuation period of the oscillating current component.

The present invention suppresses vibrations in the input current of the load induction motor by controlling the output voltage of the inverter as described above, and uses the effective value or average value of the motor input current as a value indicating the steady load state of the inverter. a current calculation circuit that detects the detected value and adds an appropriate time characteristic corresponding to the fluctuation period of the oscillating current component to the detected value to generate an output signal thereof;
The output response to an input signal having both positive and negative polarities is defined by a linear characteristic consisting of a linear proportional section and an output limiting section with a constant output, and the limiting level of the output limiting section is increased by an external bias signal. Then, a separately prepared detection calculation value of the oscillating current component in the motor input current is used as the input signal of the limiter, and the output signal of the current calculation circuit is The output limit level is varied as a bias signal of the limiter, and the inverter output voltage is commanded by setting the direction of current conduction to be positive when power is being supplied from the inverter to the load induction motor. The circuit is configured to subtract the output signal value of the limiter from the value.

By doing as described above, when vibration occurs in the input current of the load induction motor of the inverter, the output voltage of the inverter is corrected to step down during the power operation period (power supply period) of the inverter, and conversely during the regenerative operation period. is boosted and corrected, and the correction amount decreases as the load on the inverter increases, thereby preventing overcorrection, and as a result, current oscillations in the input current of the load induction motor as described above are suppressed.

〔Example〕

Embodiments of the present invention will be described below with reference to the circuit diagram shown in FIG.

In Fig. 1, 1 is an AC power supply, 2 is an inverter, 3 is an induction motor, 4 is a current transformer whose primary current is the input current of the motor, and 5 is a rectifier that performs full-wave rectification of the output of the current transformer. 6 is an oscillating current component detection circuit that receives the output of the rectifying circuit and detects the oscillating component in the motor input current; 7 receives the output of the detection circuit and converts the signal by multiplying it by a predetermined conversion coefficient. A conversion calculation circuit 8 receives the output of the rectifier circuit 5, detects the average value of the motor input current to be used as a value indicating the steady load state of the inverter 2, and adds appropriate time characteristics to the detected value. This is a current calculation circuit. Further, reference numeral 9 denotes a limiter, which receives the output of the conversion calculation circuit 7 having both positive and negative polarities as an input, and limits its output to a constant value at an output limit level when the input is larger than a predetermined value. The output limit level is automatically changed by the output of the current calculation circuit, which is input as a bias signal, so that as the output increases, the limit level is lowered. Furthermore, 10 is a subtracter, which subtracts the output value of the limiter 9 from the output voltage command value for the inverter 2.

With the above-mentioned circuit configuration, when performing correction control on the inverter output voltage command value as described above, the correction amount becomes smaller as the load of the inverter increases, and the above-mentioned correction amount that originally tends to disappear as the load increases. Smooth current oscillation suppression control can be performed in accordance with the current oscillation phenomenon.

[Effects of the Invention] According to the present invention, regarding suppression of current oscillation in the input current of an induction motor driven and controlled by a voltage source inverter, the motor input current given as a subtraction correction signal to the output voltage command value of the inverter By appropriately reducing the detected calculated value of the oscillating current component in the inverter as the effective value or average value of the motor input current increases, load current oscillations can be smoothly suppressed over the entire load fluctuation range of the inverter, In addition, in a normal inverter control device, most of the above functions can be realized by software processing, and there is no need to install a dedicated circuit, making it possible to inexpensively and easily improve the functions of the inverter device.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. ■... AC power supply, 2... Inverter, 3... Induction motor, 4... Current transformer, 5... Rectifier circuit, 6...
Oscillating current component detection circuit, 7... Conversion calculation circuit, 8...
-Current calculation circuit, 9...IJ mitter, 1o...subtractor.

Claims (1)

[Claims] 1) In current oscillation suppression control of an inverter-driven induction motor in which the output voltage of the inverter is corrected and controlled by negative feedback of the oscillating current component in the input current of the induction motor that is drive-controlled by a voltage source inverter, a current calculation circuit that detects an effective value or an average value of the inputted motor input current and gives a predetermined time characteristic to the detected value; and a current calculation circuit that uses the detection calculation signal of the oscillating current component as its input signal and its output limit level. a limiter that is variable such that it becomes smaller as the output signal of the current calculation circuit increases;
The output signal of the limiter is used as a subtraction correction signal for the output voltage command signal of the inverter, and the amount of subtraction correction by the oscillating current component for the output voltage of the inverter is determined by increasing the effective value or average value of the motor input current and the load characteristics. 1. A method for suppressing current vibration in an inverter-driven induction motor, characterized in that the current vibration is reduced at a predetermined time characteristic according to the current vibration.