UA125933C2 - Method to control an asynchronous electric drive - Google Patents

Abstract

The invention relates to electrical engineering, namely to electric drives and can be used in an adjustable asynchronous electric drive, especially with increased requirements for the speed control range and control quality. The method to control the asynchronous electric drive is to change the frequency and voltage of a frequency converter. The invention is novel in that the frequency of the frequency converter is additionally changed by positive feedback on a reactive current component of an induction motor. The technical result of the invention is the stability of the asynchronous motor without deteriorating the characteristics of the electric drive and thereby improving the quality of the asynchronous electric drive.

Description

The method belongs to the electric drive.

There is a known method of controlling an asynchronous electric drive, in which the frequency and voltage of the frequency converter (IF) are changed, to ensure stable operation of the asynchronous motor (AD), the transfer function of the electric drive is changed, for example, negative feedback is introduced into the frequency control channel by current or by its active component, which reduces the frequency when the load of the electric drive increases. (I.I. Zpstein

Automated alternating current electric drive. - M. Znergia, 1982). The specified change in the transmission function of the electric drive deteriorates its quality. Thus, the introduction of feedback on the current or on its active component deteriorates the quality of the speed regulation of the electric drive, because when working without a speed sensor, a static adjustment error is formed, and when working with a speed sensor, the dynamic accuracy and speed of regulation deteriorates.

What is new according to the invention is that a new operation is introduced - "the frequency of the frequency converter is additionally changed by positive feedback according to the reactive component of the asynchronous motor current." At the same time, the stability of AD is ensured without deterioration of the characteristics of the electric drive, thereby increasing the quality of the electric drive.

The cause-and-effect relationship between the set of features of the invention and the technical result is as follows: the introduction of the operation "additionally change the frequency of the frequency converter with positive feedback according to the reactive component of the current of the asynchronous motor" allows to ensure the stability of AD (especially at low frequencies within 0, 02-0.15 from the nominal value). At the same time, the quality of the electric drive does not deteriorate, as with the known method of control, because the reactive component of the current with the correct matching of the voltage and frequency of the inverter does not affect the transient processes in the electric drive, in particular, it does not affect the speed, the change of which is determined by the active component of the current of the asynchronous motor. Therefore, compared to the known method, the quality of the electric drive increases.

This is explained as follows.

On Fig. 1 are shown calculated on the mathematical model of the self-oscillation of the speed of the electric drive and the AD moment, reduced to one pair of poles, with a gradual increase in the set frequency of the drive from zero to 10 Hz (above this frequency, there are no self-oscillations).

The following parameters are presented:

Go output frequency of PU, Hz; . frequency of rotation of the rotor (speed of the electric drive), Hz;

M - electromagnetic moment AD, N m;

Their is the reactive component of the AD current relative to the output voltage of the IF, A.

The amplitude of the voltage Iv varied proportionally to the frequency I with the proportionality factor keov| - Who. With other dependencies, Ov - KO. ate

О0-Кк- 2-1 2 о I for example, , the result is similar.

It can be seen from the graphs that with a rather slow, within 20 s, uniform increase in the output frequency of the IF in the frequency range from 1 to 9 Gu, fluctuations in speed, electromagnetic moment and current take place.

If you introduce positive feedback on the reactive component of the current and additionally change the frequency of the IF in accordance with the change in the reactive component of the AD current, then the self-oscillations disappear (Fig. 2): the increase in the frequency Х occurs almost linearly, and the changes in the moment М and the current Х are insignificant .

Suppression of self-oscillations due to the effect on the frequency of positive feedback on the reactive component of the current is explained by the fact that this feedback dampens changes in the current phase relative to the voltage phase that occur during self-oscillations

Analogous calculations showed that the introduction of positive feedback on the active component of the current into the frequency control channel in accordance with the known method of controlling an asynchronous electric drive also suppresses self-oscillation. However, with this method, the frequency becomes dependent on the moment of load on the AD shaft, which reduces the rigidity of the mechanical characteristics of the electric drive and worsens its static and dynamic characteristics.

On Fig. 3, and the oscillogram of the current during self-oscillations in AD is given. With a constant oscillating process, the current can reach an emergency value. When introducing positive feedback on the reactive component of the current, self-oscillations disappear (Fig. 3, b).

The implementation of positive feedback based on the reactive component of the current is shown on the example of an electric drive with frequency control according to the functional scheme shown on

Fig. 4.

The diagram uses the notation:

VI. rectifier and IF inverter, respectively;

RF - frequency regulator, which combines the functions of changing the frequency of the AD when the load changes and the controlled intensity setter with the limitation of the rate of change of frequency when the current or voltage of the DC circuit exceeds the limit values;

Ф is a functional converter that implements a function proportional to the frequency Yrch, and inversely proportional to the voltage of the direct current circuit of the IF.

M - modulator, which modulates the output voltage of inverter I according to voltage and frequency signals;

PC - a coordinate converter that selects the maximum value of the current It; of its active and reactive components.

The active and reactive components of the current and its maximum value It are determined by the currents of the two phases of the IF s. It is appropriate to single out the variable component of the reactive component of the current and ... x-, which is important for the suppression of self-oscillations. K; is introduced into the frequency control channel.

It is also possible to start it in the common voltage and frequency control channel, while leaving the influence of the reactive component of the current on the frequency (dotted line in Fig. 4).

The active component of the current, multiplied by the Ku coefficient, is introduced into the IF voltage control channel and is used as positive feedback (IN-compensation) to stabilize the AD magnetic flux.

An additional change in the frequency of the IF when the reactive component of the AD current changes with positive feedback from the reactive component of the current introduced into the frequency control channel, in accordance with the above, prevents the occurrence of self-oscillations. At the same time, it is not necessary

To change the transmission function of the electric drive, for example, to introduce feedback that deteriorates the quality of regulation of the current or its active component. This improves the quality of the electric drive.

Claims (1)

FORMULA OF THE INVENTION The method of controlling an asynchronous electric drive, in which the frequency and voltage of the frequency converter are changed, which is characterized by the fact that the frequency of the frequency converter is additionally changed by positive feedback according to the reactive component of the current of the asynchronous motor.