SU558366A1 - A method for automatically controlling a variable frequency drive and a device for implementing this method - Google Patents

Description

In order to simplify, the proposed method of automatic voltage regulation of a static frequency converter feeding an asynchronous motor with non-sinusoidal voltage in the intermittent current mode according to the law of constancy of the magnetic flux of the motor requires the first voltage and current harmonic, and the device for its implementation does not contain a decisive element for calculating the EMF of the magnetizing circuit.

The proposed method differs from the well-known fact that in the feedback circuit of a closed-loop frequency control system, an electrical signal is used, which is measured at no-current intervals directly at the stator winding terminals of the motor and is emf proportional to this flux at a given frequency.

In the short circuit mode, the flux from higher harmonics in the magnetizing circuit is small and can be neglected. Thus, the resulting magnetic flux is represented by the first harmonic. It induces an emf in the stator windings, which, at current-free time intervals, is close to the sinusoid shown in FIG. 1. This EMF is proportional to the resulting magnetic flux, measured in the Nestokovy interval, is formed into a continuous curve, and fed to the feedback circuit to adjust the voltage of the static frequency converter according to the law of maintaining the magnetic flux constant.

Thus, we directly obtain the magnitude of the EMF of the magnetizing circuit without any filters for extracting the first harmonic of the voltage and current and without the need to have a solver for calculating the EMF.

FIG. Figure 2 shows a block diagram of a regulator for carrying out the proposed method of controlling the voltage of a static frequency converter feeding an asynchronous motor with non-sinusoidal voltage in the discontinuous current mode.

The reference frequency is determined by the fsT signal, which is supplied from the master oscillator 1 to the control system 2 by the static frequency converter 3 and to the functional converter frequency-voltage 4. At the output of the converter 4, a signal is taken proportional to the emf value of the magnetizing circuit of the asynchronous motor 5. The control system 2 provides control of the frequency / and angle a of opening the valves of the converter 3. The output voltage t / 2 (see Fig. 1) of the converter 3, consisting of the emf of the magnetizing coil round sections and sinusoids mains voltage t / i, takes into e, scheme 6. There also receives signal from sensor 7 / (FIG. 2) of the presence of current in the phase

motor stator to determine the boundaries of the current-free interval during which the EMF is measured. The device 6, which forms a continuous EMF curve based on its value in the measurement areas, can be implemented depending on the specific type of frequency converter 3. In FIG. 3 is a schematic diagram.

regulator.

Receiving a signal proportional to the average value of the emf of the resulting magnetic flux, which is necessary to control the voltage of the converter

Frequencies in accordance with the law of constancy of flow, is carried out using three (by the number of phases) identical electronic circuits 8, 9, 10.

Consider the work of one electronic circuit. The voltage from the phase of the induction motor I is fed to the isolation transformer 12, rectified by a single-phase bridge rectifier 13, and through the switch 14 is fed to the memory capacitor 15.

Key 14 is closed when there is no current in this phase of the motor and is open when there is current in it. The presence of current in the phase of the motor is monitored by the sensor 16. The voltage on the memory capacitor 15 follows

voltage on the motor phase in the current-free time interval, and equal to the EMF of the magnetizing circuit. In the presence of current in this phase, the voltage on the capacitor 15 is maintained at a level equal to

voltages at the final moment of the dead time. The memorization of this level occurs due to the opening of the key 14 when current appears in the phase of the motor. By appropriately selecting the capacitance of the capacitors 15 and the size of the discharge resistors 17, a negligibly small decrease in the level of the memorized voltage can be achieved with accurate reproduction of the EMF measurement section. Thus, at the output of the emitter follower 18, a voltage is formed consisting of half-waves formed by half-sine EMF, which are distorted in the current flow sections by horizontal "memory steps" (Fig. 1). The average value of the half-wave with a high degree of accuracy is equal to the average value of the half-sine EMF, since the horizontal segments have a small extent, and, moreover, their presence in the upstream segment,

where they cut out the voltage pads, are compensated to a large extent by the presence of the same steps in the downstream area, when the same voltage pads are added. Weekends

The signals from the emitter followers 18 are summed in adder 19, from which a signal proportional to the average value of the real emf of the magnetizing circuit is fed to a comparison element 20, where a signal of a given magnitude of the emf of the magnetizing circuit at a given frequency / gr is compared with the actual emf signal. The difference of these sticks affects the control system of the 21 cyclo-converter, which determines the opening angle of the thyristors. With a change in the frequency of the fsr reference through the block 22, the proportional signal of the given emf value of the magnetizing circuit e- is also changed in proportion. Thus, in this static voltage regulator, which stitches an induction motor with a non-sinusoidal voltage in the discontinuous current mode, the constant magnetic field current across the entire frequency range is eliminated.

Claims (3)

1. A method of automatically controlling a frequency-controlled electric drive containing a static frequency converter supplying an induction motor with a non-sinusoidal voltage in the discontinuous current mode by generating an output voltage of the converter using the error signal representing the difference of the feedback signal and a signal proportional to the input frequency I distinguish HUI in order to simplify the measurement of the feedback signal during the current-free interval in order to form a feedback signal. eiie directly with the terminals of the stator, and during the subsequent current flow interval form a voltage corresponding measured emomu. 2. A device for carrying out the method according to claim 1, comprising a measuring unit consisting of an adder, emitter repeaters and storage capacitors, each of which is connected via a control key to a rectifier, whose input is connected to one of the stator windings via a control transformer, and the control key input is connected to the current sensor of the corresponding phase of the converter. Sources of information taken into account in the examination: 1. Authors' certificate of the USSR № 103057, M. Kl.2 N 02P 7/58, 1965 (prototype). 2. Authors' testimony of the USSR LR 375744, M. Kl.2 n 02R 7/42, 1974. 3. Authors' testimony of the USSR LR 3775652, M. Kl.2 n 02R 5/40, 1974. 1D V ,, i. and / iTtrn rr