RU2572097C1 - Asynchronous motor start-up method - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: asynchronous motor start-up method consists in delivery of power supply to the motor stator windings via additional active resistance, which value is generated according to the law

where R_s is active resistance of the stator windings; p is a number of pole pairs; U_su, U_sv are components in axes u and v of voltage vector at the stator windings; Ψ_su, Ψ_sv are components in axes u and v of the stator flux linkage vector; ω_n is rotation velocity of the coordinate system; Ψ² _s is amplitude square value of the stator flux linkage vector; M is value of electromagnetic torque; M_n is desired value of electromagnetic torque calculated as per the specified Kloss's formula.

EFFECT: reduced pulsation of the motor electromagnetic torque and current at its start-up provided that dynamic overvoltage is limited at semiconductor switches switching delivery of voltage to the stator windings to the closed electric circuit.

5 dwg

Description

The invention relates to electrical engineering and can be used to launch electric drives of scraper, belt conveyors and other machines with electric drives based on asynchronous electric motors.

It is a well-known method of direct start of an asynchronous electric motor (HELL), according to which the full voltage of the supply network is supplied to the stator windings. (Kopylov I.P. Mathematical modeling of electrical machines. - M.: Higher school, 2001. - 327 p.).

Its disadvantage is the appearance during start-up of significant pulsations of the electromagnetic moment and currents of the electric motor, leading to the appearance of dynamic forces in the structural elements of the electric motor itself, as well as in the mechanical transmission devices of the electric drives (Fig. 1).

In order to reduce the negative effects of direct start of an induction motor on the electric and mechanical components of the electric drive, soft starters or soft starters are used. There is a wide variety of soft starter circuits, the dominant ideology of which is based on the use of semiconductor voltage regulators in the supply voltage circuit, made, as a rule, on the basis of power semiconductor switches (thyristors, transistors, etc.). By applying a control signal to a power semiconductor switch, a supply voltage is supplied to the stator windings of the asynchronous electric motor. By stopping the supply of a control signal, or by supplying another signal (depending on the design of the power semiconductor switch), the voltage supply to the stator windings of the induction motor is stopped. Changing the gap of the closed and open state of the key, they achieve a smooth change in the effective voltage value (Kovchin S.A., Sabinin Yu.A. Electric Drive Theory: Textbook for High Schools. - St. Petersburg: Energoatomizdat, 2000. - 496 s). In most cases, the dependence of the change in the amplitude value of the supply voltage on the start-up time is linear or exponential (in the second case, this is due to the fact that the device that sets the intensity of the increase in the supply voltage is a capacitor)

The disadvantages of this method include the still remaining pulsations of the electromagnetic moment (Fig. 2), although they significantly reduced the amplitude of the oscillations compared to the direct start of the induction motor (Fig. 1), but at the same time they significantly increased in duration.

The closest in technical essence to the claimed is a method of starting an induction motor, according to which, initially, a voltage with an amplitude equal to the amplitude of the mains voltage is applied to the stator windings of the electric motor, and then sequentially in time, after a certain period of time from the beginning of the voltage supply, equal to π / 3 email hail., (with a mains frequency of 50 Hz - 0.0033 s.), the electric motor is disconnected from the mains and put into dynamic braking mode for the same time, π / 3 e. deg., (at a network frequency of 50 Hz - 0.0033 s.), after which voltage is again applied to the stator windings with an amplitude equal to the amplitude of the mains voltage (Fig. 3) (RF patent No. 2235410, IPC Н02Р 1/26, publ. 27.08 .2004).

The specified method of starting allows you to practically eliminate fluctuations in the electrical and mechanical parts of the electric drive, built on the basis of an asynchronous electric motor, and without increasing the duration of the start-up of the asynchronous electric motor.

Its disadvantages include the occurrence of significant overvoltages during the start-up phase of the dynamic braking dynamic start-up phase, since a short circuit of the stator windings actually occurs, which negatively affects the reliability characteristics of the semiconductor switches that form the on-off mode of the supply voltage of the stator circuit.

The technical result consists in reducing the ripple of the electromagnetic moment and the currents of the electric motor when it is started, provided that the dynamic overvoltage on the semiconductor switches is limited, switching the supply of voltage to the stator windings in a closed electric circuit.

The specified technical result is achieved by the fact that in the method of starting the induction motor, including changing the active resistance of the stator windings and, as a result, changing the supply voltage of the stator windings, according to the invention, the value of the active resistance connected in series in the stator winding circuit is formed in accordance with the target function of the electromagnetic moment in the form of a static mechanical characteristic and is calculated by the following formula:

where R _s is the active resistance of the stator windings;

p is the number of pole pairs;

U _su , U _sv - components along the u and v axes of the voltage vector on the stator windings;

Ψ _su , Ψ _sv - components along the u and v axes of the stator flux linkage vector;

Ψ ² _s is the square of the amplitude value of the stator flux linkage vector;

ω _n is the speed of rotation of the coordinate system;

M - values of the electromagnetic moment;

M _n is the desired value of the electromagnetic moment calculated by the refined Kloss formula.

In this case, the voltage directly on the stator windings will change according to FIG. 4, and the dynamic characteristics of starting an induction motor show the best quality in terms of the amplitude and duration of the oscillations.

The objective function that sets the value of the dynamic electromagnetic moment M _n should be taken to be the value calculated by the refined Kloss formula (Kopylov I.P. Mathematical modeling of electrical machines. - M.: Higher school, 2001. - 327 p.).

The invention is illustrated by drawings, where in FIG. 1 shows the start-up by direct connection to the network; in FIG. 2 shows the starting process using a semiconductor voltage regulator (soft starter); in FIG. Figure 3 shows the start-up by the method of sequential switching through π / 3 email. deg., (at a network frequency of 50 Hz - 0.0033 s) with a mode of short-term dynamic braking; in FIG. 4 shows the main characteristics of the starting process of an induction motor using the proposed method; in FIG. 5 shows a defining function that determines the formation of the electromagnetic moment in accordance with the angular velocity of rotation of the rotor (having the form of a static mechanical characteristic), and a dynamic mechanical characteristic showing the value of the electromagnetic moment corresponding to the angular velocity of rotation of the rotor formed as a result of controlled starting by the claimed method.

Designs of induction motors are designed in such a way as to provide sufficient starting torque and minimize the value of the starting current. For this design, the windings of the rotors of asynchronous electric motors are made of a special configuration (deep-groove, two-cell). Despite these design developments, the starting current remains quite large. A significant drawback is also the significant magnitude of the amplitude of the periodic component of the electromagnetic moment. In FIG. diagrams showing the start of an induction motor by direct connection to the network are shown. When using semiconductor voltage regulators, it is possible to significantly reduce the amplitude of the periodic component of the electromagnetic moment, FIG. 2, but at the same time, the duration of the oscillations of the electromagnetic moment remains high enough, which especially negatively affects the mechanical part of the asynchronous electric motor and gearbox during a prolonged heavy start under a significant load. The oscillation process is initially present in the asynchronous electromechanical converter, as a result of the interaction of the stator and rotor flux link vectors. For an effective universal solution to the integral reduction of system oscillations, it is necessary to shift the flux link vectors during the start-up process, and the method for starting sequential switching through π / 3 e-mail. deg., (at a network frequency of 50 Hz - 0.0033 s) with the short-term dynamic braking mode successfully solves this problem (Fig. 3). However, the transfer of the electromechanical converter to the electrodynamic braking mode essentially forms a short-circuit short circuit, which negatively affects the reliability of the semiconductor switches that form the on-off mode of the supply voltage of the stator circuit.

In FIG. 4 shows the process of starting an induction motor of the claimed method.

To get an idea of the process of starting the electric motor, we consider it as a task of controlling the state of an induction motor in order to ensure maximum proximity during the start-up of dynamic and static mechanical characteristics, taking into account the possibility of active influence on the process by changing the value of the total active resistance in the stator circuit (R _s ).

If you use the well-known description of the status of an induction motor

, $$L_r^{\text{'}}$$

- переходные индуктивности статора и ротора, k_r, k_s - коэффициенты электромагнитной связи, p - число пар полюсов, ω - геометрическая угловая скорость вращения ротора электродвигателя, Ψ_s, Ψ_r с индексами u, v - составляющие потокосцеплений статора и ротора по осям системы координат, U_s - с индексами координатной системы - составляющие напряжения статора.according to (Kovach K., Ratz I. Transients in alternating current machines. - M. - L .: Gosenergoizdat, 1963. - 744 p.), the angular rotational speed of the rotor should be considered as an argument or disturbing effect (7). Parameters starting with R and indices s, r are the active resistances of the windings of the stators and rotors HELL, $$L_s^{\text{'}\text{'}}$$

, $$L_r^{\text{'}\text{'}}$$

 - transient inductances of the stator and rotor, k_r, k_s are the electromagnetic coupling coefficients, p is the number of pole pairs, ω is the geometric angular velocity of rotation of the rotor of the electric motor, Ψ_s, Ψ_r with indices u, v are the components of the stator and rotor flux linkages along the axes of the coordinate system, U_s - with indices of the coordinate system - components of the stator voltage.

We formulate the problem of controlling an asynchronous electric motor as the problem of minimizing a certain functional written in integral form and expressing the goal of control:

where M _n , Μ - the necessary and current values of the electromagnetic moment of blood pressure.

The necessary value of the electromagnetic moment can be set, for example, by an expression that determines the natural mechanical characteristic of an asynchronous electric motor according to the updated Kloss formula.

The condition for the introduction of control from the stator requires either the presence of information about the state of the stator in the integrand, or the presence of the controls, or both together. Otherwise, one should expect analytical unsolvability of the problem.

The solution can be obtained on the basis of sufficient conditions for Krotov’s absolute minimum (Krotov V.F. Methods and problems of optimal control [Text]: / V.F. Krotov, V.I. Gurman // - M .: Nauka, 1973. - 446 s. .).

Given that the optimal control problem is considered mathematically solved to the end if the control is found in the function of the state parameters of the object (L. Pontryagin, Mathematical theory of optimal processes [Text]: / L. S. Pontryagin, V. G. Boltyansky, R. V. Gamkrelidze, EF Mishchenko // -4th ed. - M .: Nauka, 1983. - 392 s), we obtain a method for forming the total value of the active resistance of the stator circuit of an induction motor at start-up

where R _s is the active resistance of the stator windings;

p is the number of pole pairs;

U _su , U _sv - components along the u and v axes of the voltage vector on the stator windings;

Ψ _su , Ψ _sv - components along the u and v axes of the stator flux linkage vector;

Ψ ² _s is the square of the amplitude value of the stator flux linkage vector;

ω _n is the speed of rotation of the coordinate system;

M - values of the electromagnetic moment;

M _n - the desired value of the electromagnetic moment, calculated by the refined Kloss formula (Fig. 5).

Thus, by forming the value of active resistance in the start-up process in the above way, one can obtain the result shown in Fig. 4 using an example of an electric motor of the VRP160M4 type.

The method of starting an induction motor is implemented as follows: active resistance is included in the stator windings of the electric motor, the value of which can be changed by sequentially switching on or off the active resistance segments with semiconductor switches in accordance with the formula

The inventive method of starting an induction motor allows:

- improve the operational characteristics of the electric motor and the electric drive as a whole;

- increase the efficiency of the electric motor;

- increase the reliability of the electric motor;

- reduce dynamic loads on a mechanical transmission device;

- limit overvoltage on the semiconductor switches switching the stator circuit.

Claims (1)

A method for starting an induction motor, including changing the active resistance of the stator windings and, as a result, changing the supply voltage of the stator windings, characterized in that the value of the active resistance connected in series in the stator winding circuit is formed in accordance with the target function of the electromagnetic moment in the form of a static mechanical characteristic and is calculated by the following formula:

where R _s is the active resistance of the stator windings;
p is the number of pole pairs;
U _su , U _sν - components along the axes u and ν of the voltage vector on the stator windings;
Ψ _su , Ψ _sν - components of the stator flux link vector along the u and ν axes;
Ψ ² _s is the square of the amplitude value of the stator flux linkage vector;
ω _n is the speed of rotation of the coordinate system;
M - values of the electromagnetic moment;
M _n is the desired value of the electromagnetic moment calculated by the refined Kloss formula.

Images