RU2253179C2 - Method for limiting induction-motor starting current - Google Patents

Abstract

FIELD: power electronics.

SUBSTANCE: proposed method includes correction of comparator current reactive component by means of parallel-connected correction converter using three-phase circuit arrangement with dual-operation diodes and storage reactor used as load. Bridge diodes are supposed to be controlled by vertical method involving two-fold connection during each period of supply mains affording independent two-coordinate regulation of converter current and its phase shift relative to voltage. For proportional regulation of mentioned coordinates use can be made of proposed mathematical expressions of functional plotter characteristics at input of diode pulse-phase control system.

EFFECT: provision for generating reactive component of corrector current.

1 cl, 2 dwg

Description

The invention relates to power electronics, in particular to semiconductor reactive power compensators, one of the applications of which may be to reduce the mains current when starting asynchronous short-circuited electric motors by excluding the reactive component from the composition of this current.

It is known that the direct start of asynchronous motors (AM) is accompanied by a short-term inrush current of the stator windings, many times exceeding the nominal level. This feature creates significant difficulties in the operation of blood pressure, as it leads to drawdowns of the mains voltage, violating the normal power supply of the drive and other electricity consumers. The limitation of the inrush current of the AM by trivial methods is in most cases unacceptable, since the current decrease in the active component of the current of the stator windings leads to a decrease in the starting torque, overload capacity, energy performance and speed of the drive. Under these conditions, the method of parallel compensation of reactive power, which, as you know, consists in the fact that the need for reactive power of the motor is provided not by the network, but by a parallel compensator, which leads to a decrease the total current of the network, that is, to unload the network from the reactive component of the current AM. The well-known start-up compensating devices of asynchronous electric drives are based on discretely switched capacitor banks (see, for example, Dzhendubaev A.Z.-R. "Capacitorial limitation of the starting current of an asynchronous squirrel-cage motor." - Proceedings of the Russian Academy of Sciences. Energetics, No. 5, 2001, p. .144-149).

However, the use of devices of this kind in dynamic modes is difficult due to the impossibility of smooth regulation of reactive power. Attempts to carry out with their help the so-called multi-stage start-up of BP are accompanied, as noted in the indicated work, by the appearance of resonance modes, overvoltage, etc. In this regard, controlled semiconductor compensators may turn out to be more effective, out of the total number of which compensation transformers (KP) should be recognized as the most energy-consuming and simple. These devices are performed, as a rule, according to a three-phase bridge rectification circuit, which is connected to the stator windings of the AD by AC clamps, and contains an inductive element in the form of a storage choke in the DC circuit. The elementary database of the converters are dual-operation valves, for example transistor IGVT modules with diodes connected in series to block the reverse voltage. The inclusion of two-stage valves in the range of leading control angles α ≤ 0 leads to the appearance of a rectangular capacitive reactive current at the mains input of the control unit, the main harmonic of which has a leading phase shift relative to the mains voltage φ ₁ ≤ 0. The effective value of the reactive component of this current I _{КР is} found in in the form of the product of two coordinates: i _K1 - the effective value of the fundamental harmonic of the current and sinφ ₁ - the phase parameter, which depends trigonometrically on the phase shift of the fundamental current ki relative to voltage, according to the expression

The task of parallel compensation is to form a reactive component of the compensator current, which, if the amplitudes are equal, would be in antiphase to the reactive component of the current of the stator windings

what can be achieved by automatic control of the specified coordinates, providing separate maintenance of equalities

а второе включение - с опережающим относительно указанного момента углом управления, по модулю равным

и одновременным запиранием ранее работающих вентилей, на основе вертикального принципа путем включения вентилей в моменты равенства управляющих сигналов:

- при первом включении и

- при втором включении и периодически изменяющихся синхронно с сетью опорных сигналов косинусоидальной формы с вершинами, совпадающими с указанными точками отсчета углов управления.However, with conventional pulse-phase valve control, such two-coordinate control is impossible, since when the control angle α changes, both of the indicated current parameters simultaneously change. In addition, the short circuit mode, in which the compensation rectifier actually operates, significantly limits the range of possible deviations of the control angle from its initial value α = -π / 2. The possibility of separate independent regulation of the current and its phase at the mains input of the gearbox in a wide range appears when the valves are turned on twice during the period of the mains voltage (see, for example, AS No. 436430). The closest technical solution is contained in the invention (see "Method for controlling power at the network input of a three-phase valve converter." // SN Sidorov. RF Patent No. 2167484. Published in BI No. 14, 2001). This method provides autonomous proportional control of the active and reactive components of the current at the network input of a three-phase bridge converter when switching on each pair of bridge valves twice during the network period, when the first switching on is performed with the control angle lagging behind the positive half-wave of the line voltage

and the second inclusion - with an angle of control ahead of the specified moment, modulo equal

and simultaneous locking of previously operating valves, based on the vertical principle by turning on the valves at the moments of equal control signals:

- at the first inclusion and

- when turned on for the second time and periodically changing synchronously with the network, the reference signals are cosine in shape with vertices coinciding with the indicated reference points of the control angles.

- сигнал задания амплитудного значения тока компенсатора,

- сигнал задания фазы тока компенсатора. Для получения пропорциональной зависимости между задающими сигналами и указанными координатами реактивного тока компенсатора данные сигналы должны быть поданы на входы функционального построителя (ФП), на выходах которого формируются сигналы управления первым

и вторым

включениями вентиля на периоде сети. Точные уравнения характеристик "вход-выход" ФП

имеют следующий вид:As noted in (3), to solve the compensation problem by linear methods of the theory of automatic control, it is necessary to be able to proportionally control the magnitude of the current and its phase parameter at the output of the compensator. To do this, it is proposed to use the driving signals, the relative values of which are proportional to the specified coordinates:

- signal setting the amplitude value of the current compensator,

- signal to set the phase current of the compensator. To obtain a proportional relationship between the driving signals and the specified coordinates of the reactive current of the compensator, these signals must be fed to the inputs of the functional builder (FP), at the outputs of which control signals are generated first

and second

by switching on the valve during the network period. The exact equations of the input-output characteristics of the FP

have the following form:

These initial dependences are rather complicated, therefore, in practice they can be approximated, and the characteristics of the FPs can be interpolated, which will simplify the implementation of the FP by hardware or software.

Figure 1 shows the time diagrams of the rectified voltage U _d (v), as well as the voltage U _a (v) and current i _a (v) of phase A at the network input of the compensation converter, illustrating its operation with double switching on each pair of bridge valves for a period mains voltage; figure 2 shows a diagram of a device for limiting the starting current HELL.

When considering the principle of operation of the device, we will proceed from the assumption that the voltage in the network remains sinusoidal, and a perfectly smoothed rectified current Id = const flows in the load circuit of the gearbox.

According to the diagrams of figure 1, the operation of the gearbox with the selected control method is accompanied by the alternation of the following pairs of turn-on valves on the network period: 1.6; 4.3; 1,2; 5.4; 3.2; 5.6; 3.4; 1.6; 5.4; 1,2; 5.6; 3.2. It is seen that the operation of each pair occurs twice in the period, for example, valves 1, 2 are turned on for the first time at time v ₁ with the control angle α lagging behind the start of the positive half-wave of the linear voltage U _ac at their anodes (relative to the point of natural switching of the valve 1 of phase A) ₁ ≥ 0, and the second time at the moment v ₂ - with the control angle α ₂ ≤ 0 ahead of the indicated point. In this case, the rectified voltage at each ripple period π / 3 is formed from the sections of the out-of-phase line voltage U _ac U _ca , for a short time transferring the converter from rectifier mode to inverter mode and vice versa. The same drawing shows the voltage curves U _a (v) and current i _a (v) of phase A at the network input of the bridge. It can be seen that by a corresponding change in the control angles α ₁ , α _2, it is possible to independently control the average rectified voltage, and hence the current at the inlet of the converter, on the one hand, as well as the phase shift φ _{1 of the} fundamental harmonic of the phase current i _a1 (v) with respect to phase voltage U _a (v) - on the other hand. The latter means the possibility of an autonomous two-coordinate control of the reactive current of the compensation converter, significantly expanding the range of its smooth regulation due to the amplitude or phase shift.

The presented current diagrams help to obtain analytical dependences for the organization of two-coordinate pulse-phase control of the gearbox. By expanding the current curve i _a (v) in a Fourier series, we write the expressions for the cosine and sine coefficients in the first term of the series

что дает возможность записать расчетные зависимости для действующего значения основной гармоники тока на сетевом входе КПExpressions are presented in a system of relative units with a base value

which makes it possible to record the calculated dependences for the effective value of the main harmonic of the current at the power supply network input

phase shift of this harmonic relative to the mains phase voltage

as well as the active and reactive components of this current

After substituting (6) into (7), (8), (9), the reactive and active components of the phase current at the power supply network input are expressed

Introducing the concepts of driving signals for proportional control of the current at the network input of the gearbox and its phase parameter

we write the law of the autonomous two-coordinate regulation by the compensation converter in the following form:

Solving this system with respect to one of the trigonometric functions, we obtain

найти требуемый угол управления вентилями α ₁ при каждом первом включении на периоде сети. Практическая реализация импульсно-фазового управления вентилями осуществляется, как правило, на основе так называемого вертикального принципа. Рассматривая выражение (12) как уравнение точки встречи двух сигналов: управляющего сигнала

и представленного слева опорного периодического сигнала косинусоидальной формы с вершиной косинусоиды, совпадающей с точкой перехода вступающего в работу линейного напряжения сети через нуль, запишем уравнение регулировочной характеристики системы импульсно-фазового управления вентилями относительно первого входаThis equation allows, given the quantities

find the required valve control angle α ₁ at each first start-up on the network period. The practical implementation of pulse-phase valve control is carried out, as a rule, on the basis of the so-called vertical principle. Considering the expression (12) as the equation of the meeting point of two signals: the control signal

and the reference periodic signal of the cosine shape presented on the left with the peak of the cosine wave coinciding with the point of transition of the line voltage of the network that comes into operation through zero, we write the equation of the control characteristic of the pulse-phase valve control system relative to the first input

Similarly, the equation is found for the meeting point of the control and reference signals when the valves are turned on a second time during the period

а левую часть - в качестве периодического опорного сигнала косинусоидальной формы, выражение регулировочной характеристики этой системы относительно второго управляющего входа запишетсяTaking the right-hand side of (14) as the control signal at the second input of the pulse-phase control system

and the left part - as a periodic reference signal of a cosine shape, the expression of the regulatory characteristics of this system relative to the second control input is written

поступают на вход функционального построителя 11. На выходах функционального построителя формируются согласно выражениям (4), (5) управляющие сигналы

которые подаются далее на первый и второй входы системы импульсно-фазового управления 12. При надлежащем выполнении и настройке данное устройство может служить для ограничения пускового тока АД в соответствии с предлагаемым техническим решением.The compensation converter shown in FIG. 2 is made according to a three-phase bridge rectification circuit on transistor switches 1-6, which is connected in parallel with the stator windings of the induction motor 7, and in the DC circuit contains a storage choke 8. The control device contains a task unit 9, which, on the basis of information about the current of the stator windings of the AM, generates input signals, as well as a feedback block 10, similarly according to (9), generates signals proportional to the param frames on the network current manual input. The driving signals obtained by comparing the input signals and feedback signals

arrive at the input of the functional builder 11. At the outputs of the functional builder, control signals are generated according to expressions (4), (5)

which are then fed to the first and second inputs of the pulse-phase control system 12. With proper execution and configuration, this device can serve to limit the inrush current of the blood pressure in accordance with the proposed technical solution.

Claims (1)

The method of limiting the starting current of an induction motor by compensating the reactive component of this current using a compensation converter connected in parallel to the stator windings, performed according to a three-phase bridge rectification circuit using two-operation valves with an accumulating choke in the load circuit when controlling each pair of bridge valves with two turns on the network period when the first inclusion is carried out with a lag relative to the beginning of the positive half-wave of linear angle control

and the second inclusion - with an angle of control ahead of the specified moment, modulo equal

and simultaneously locking previously operated valves, on the basis of the vertical principle by turning on the valves at the moments of equal control signals

and periodically varying in synchronism with the mains voltage, reference signals of a cosine shape with vertices coinciding with the indicated reference points of the control angles, provided that the sinusoidality of the mains voltage and the smoothed current shape at the output of the compensation converter are preserved, characterized in that, the formation of the reactive component of the main current harmonic on the mains the input of the compensation Converter in antiphase to the reactive component of the current of the stator windings of the motor is carried out using yi input signals defining the function builder, one of which is proportional to the effective value of the fundamental harmonic of the phase current compensation converter

and the other is proportional to the sine of the phase angle between the specified current harmonic and the mains voltage

at the same time, control signals are received at the output of the functional builder for their subsequent supply to the inputs of the pulse-phase control system of the valves of the compensation converter, of the following form:

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