RU2694892C1 - Method for operation in synchronous mode of frequency-controlled asynchronous motors with phase rotor - Google Patents

Abstract

FIELD: electrical equipment.

SUBSTANCE: invention relates to the field of electrical equipment and can be used in the frequency-controlled electric drives. Method of operation in synchronous mode of frequency-controlled asynchronous motor with phase rotor, equipped with control and adjustment system, is characterized by that when sliding, close to nominal, at moment of stator current passing through one of its phases through zero its control and adjustment system includes both phases of two-phase winding of rotor to voltage of direct current source. Moment of time when one of the phases passes through zero is determined by the slip S, which is close to the nominal value and equal to S=1/(8⋅γ) (at γ= 1; 2; 3; …), and frequency of voltage ω_P rotor windings for both phases is determined at this sliding angle α_P=ω_P⋅t=π⋅k/4 (with k=1; 5; 9; …). Values of currents in phases of rotor winding in synchronous mode are set equal to effective (effective) value of currents in these windings in asynchronous mode.

EFFECT: technical result consists in improvement of engine efficiency by elimination of temperature deformations of rotor winding and short-term occurrence of its vibrations, increase of power factor, absence of reactive currents in windings of rotor and stator due to setting of current values in phases of rotor winding in synchronous mode equal to effective (effective) value of currents in these windings in asynchronous mode.

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Description

The invention relates to electrical engineering and can be used in variable frequency drives.

When using automated electric drives with frequency control of AC motors without additional speed sensors, problems arise in maintaining speed and protecting the motor and converter at loads higher than nominal. The difficulties of controlling asynchronous motors are characteristic of multi-motor drives, when two or more motors are powered by a single converter.

To assess the novelty of the claimed solution, we consider a number of well-known technical means of a similar purpose, characterized by a set of features similar to the claimed device.

A device for regulating the frequency of rotation of an asynchronous electric motor according to the patent of the Russian Federation No. 2635663, containing a three-phase electric motor with a bridge circuit of power switches, a control unit for power switches, a resistor and a power source, is known, characterized in that a low-power DC motor is built into the device circuit, which is rigidly fixed a permanent magnet, and around the electromotor radially placed dual Hall sensors, located at equal distances from each other, and in each pair of sensors installed They are located one above the other with a 180 ° turn relative to each other and are electrically connected to the power switches, and the DC motor is connected through a resistor to a power source.

A device for controlling an asynchronous motor according to the patent of the Russian Federation No. 2599529, containing an asynchronous motor and a frequency converter and voltage converter, whose control inputs are connected respectively to the outputs of the first and second adders, the first input of the first adder is connected to the source of the reference signal, and the second input to the output of the first static nonlinear converter, the first input of the second adder is connected to the functional converter, the input of which is connected to the source of the reference signal, and the second input - to the output static converter, as well as current sensors of the stator winding of the engine, the outputs of which are connected to the first and second inputs of the functional current converter, characterized in that the first output of the functional current converter, corresponding to positive feedback on the stator current, is connected to the input of the aperiodic link of the first order, output which is connected to the input of a static converter and to the input of the first static nonlinear converter.

Dynamic link, which corrects positive feedback on the stator current of the induction motor, provides the technical result of the utility model, as it increases the voltage amplitude on the motor stator to such values that, at high loads (up to one and a half times the nominal motor torque), provides an error in maintaining the rotation speed of the induction motor not higher than 2-3%, while maintaining the stability of the electric drive system with positive feedback.

Solved invention technical problem is as follows. Frequency-controlled asynchronous motors with a phase rotor have been widely used abroad in electric machine cascades for many years, beginning in the 20s of the last century [1], [2]; The variable frequency voltage source for powering the rotor winding from the side of the slip rings was AC collector generators with an electric motor drive. In the 90s of the last century, the structure of these cascades underwent a significant simplification: commutator generators with such a drive were replaced by static frequency converters with autonomous voltage inverters. By changing the amplitude and frequency of the voltage on the contact rings with such a converter, the rotation speed of the motor (slip S) is controlled in operating conditions. Accordingly, the scope of cascade circuits has expanded. They are used to drive grinding units ("mills") in the mining industry, to drive numerous turbomechanisms (main ventilation fans, pumps, compressors, etc.), in the building industry (in the production of cement, silicate brick, asphalt concrete, lime, gypsum). Their use is promising for driving grinding units and in various branches of the chemical industry: in the production of raw materials for paints and varnishes and a number of other products.

All of these mechanisms are characterized by a continuous production cycle, high energy consumption and operate mainly at a nominal speed, therefore their energy characteristics are essential. Therefore, among a number of problems put forward by practice in the operation of such engines is the problem of ensuring their competitiveness: increasing their efficiency and power factor.

The objective of the invention is to create a method of operation in a synchronous mode of a frequency-controlled asynchronous motor with a phase-rotor, providing an increase in power factor, the absence of reactive currents in the rotor and stator windings and, consequently, an increase in engine efficiency.

The essence of the claimed technical solution is expressed in the following set of essential features, sufficient to solve the technical problem indicated by the applicant and to obtain a technical result provided by the useful model.

According to the invention, the method of operation in a synchronous mode of a variable frequency asynchronous motor with a phase rotor, equipped with a control and regulation system, is characterized by the fact that when sliding, close to nominal, when one of the phases passes through the stator current, its control and regulation system includes both phase of the two-phase winding of the rotor to the voltage of the DC source.

In addition, the claimed technical solution is characterized by the presence of a number of additional optional features, namely:

- the time moment when current passes through one of the phases through zero is determined by sliding S close to the nominal and equal to S = 1 / (8⋅γ) (with γ = 1; 2; 3; ...), and the frequency of the voltage ω _{P of the} rotor winding for both phases are determined at this slip by the angle α _Р = ω _Р ⋅t = π⋅k / 4 (with k = 1; 5; 9; ...);

- the values of the currents in the phases of the rotor winding in the synchronous mode are set equal to the current (effective) value of the currents in these windings in the asynchronous mode.

The stated set of essential features ensures the achievement of a direct technical result, which is that setting the value of currents in the phases of the rotor winding in a synchronous mode equal to the current (effective) value of the currents in these windings in an asynchronous mode eliminates the temperature deformations of the rotor winding and a short-term appearance its vibrations, which ultimately leads to an increase in power factor, the absence of reactive currents in the rotor and stator windings and, therefore, increase engine efficiency.

The claimed method is implemented as follows.

Consider for this purpose the operating modes of the two-phase winding of the rotor at nominal slip S, for example, at 0 <S≤0.02.

Let us assume that the origin of the phase angles coincides with the phase A of the rotor winding. Then the currents in both phases have the form [3] - [5]:

Here I _{AMP, P} - the amplitude of the current in the rotor winding; ω _P is the frequency of this current;

t is time.

The time shift by the phase angle π / 2 is performed by the frequency converter in the rotor circuit.

In the nominal mode (with slip 0 <S≤0.02), the effective (effective) values of the currents in the windings of both phases are equal: I _EFF = I _{EFF, A} = I _{EFF, B} = (100 / √2) ≈70.7% ; here for 100% taken: I _AMP = 100%.

Let us now consider the mode of operation of such a rotor winding at nominal slip (0 <S≤0.02). At a certain point in time t, the instantaneous values of the currents in phases A (i _A ) and B (i _B ) are the same: i _A = i _B. This time t corresponds to the (1) corner:

In these modes, the instantaneous values of currents in two phases are as follows:

i _{A, P} = i _{B, P} = I _AMP , _P ⋅ 0.707 = 70.7%.

Modern control and regulation systems have the ability to fix this angle: α = ω _P ⋅t = π⋅k / 4.

Note the feature of the currents of the two-phase system: in synchronous mode (S = 0, ω _P = 0) taking into account (2) the long values of the currents i _{A, P} and i _B , _P are equal to the effective (effective) value of the currents in these phases (A, B ) in asynchronous mode. Therefore, in synchronous mode (S = 0, ω _P = 0), the overheating of the windings of both phases (A, B) due to these values of the currents i _{A, P} and i _B , _P is the same and almost equal to the overheating in asynchronous mode.

This eliminates the temperature deformations of the rotor winding and the short-term appearance of its vibrations when changing the operating mode of a frequency-controlled asynchronous motor.

It should be noted that the rotor winding of an asynchronous motor with a phase-rotor is usually three-phase, however, in order to reduce the number of contact rings, simplify the design of the brush apparatus, increase engine reliability and competitiveness for variable-frequency drives, it can also be two-phase; Unlike the three-phase two-layer six-zone stator winding design, the two-phase four-zone rotor winding has a winding coefficient (with the same pitch reduction) somewhat smaller (approximately 1.06-1.07 times). Accordingly, the length of the active steel of the stator and rotor is determined for the considered design of the machine by the winding coefficient of its rotor [6].

In addition to the problem of ensuring synchronous operation of the machine by supplying constant currents of the same amplitude to the phases of the rotor winding, according to (2) there is also the problem of ensuring this synchronous mode in parallel with the network. It is assumed that at the moment of supplying the currents to the rotor winding phases, according to (2), the amplitudes of the transient currents (including their aperiodic components) in the three phases of the stator winding should be minimal (“network synchronization problem”). To solve this problem, we write the expression for the MDS stator and rotor windings.

For stator winding MDS:

Similarly, for winding the MDS rotor:

Here ω _ST is the circular frequency of the stator current; τ - pole division; x - coordinate along the circumference of the stator (in diameter D _CP , the average between the diameter of the rotor and the stator).

From relation (3), we obtain the linear velocity V _CT of the MDS rotation and the stator reaction field (relative to the stator): V _CT = dx / dt = ω _CT ⋅ τ / π = (ω _CT / p) D _CP / 2.

Similarly, the linear velocity V _{P of the} rotation of the MDS and the rotor field (relative to the rotor):

V _{P, OTH} = dx / dt = (ω _P / p) ⋅ D _CP / 2.

The linear speed of rotation of the rotor relative to the stator is equal to:

V _BP = ω _BP ⋅ D _CP / 2 = (ω _CT / p) (1-S) D _CP / 2.

Therefore, the absolute speed of rotation of the MDS and the rotor field (relative to the stator):

MDS and the field of the rotor and stator in the gap in operating modes (asynchronous, synchronous) are mutually fixed in the air gap [5], so that V _{P, ABS} = V _CT . From this equality, after a series of transformations, we obtain the calculated equation:

According to equation (2), it was obtained that the angle α _P should be equal to: α _P = ω _P ⋅ t = π⋅k / 4 (for k = 1; 5; 9; ...). From (6), it follows that condition (2) turns out to be valid for certain angles α _CT = ω _CT ⋅t and slip S or angular speeds of rotation of the rotor ω _BP = (1-S) ω _CT / p.

We define the angle α _CT = ω _CT ⋅t required for solving the synchronization problem, taking into account (2) and (6).

To do this, we write the expressions for currents in three phases (A, B, C) of the stator winding:

In (7) it is assumed that the origin of the phase angles coincides with the phase A of the stator winding. Note that for phase A, when α α _CT = ω _CT ⋅t = 2⋅π⋅γ (with γ = 1; 2; 3; ...), the aperiodic component of the transient current is zero [3], so that the transient current is minimal and equal to its periodic component. The same holds for phases B and C of a three-phase stator winding operating in parallel with a three-phase network.

Therefore, to solve the synchronization problem, the inclusion of the rotor winding on the voltage of the DC source according to (2) is advisable to perform at an angle:

Let us determine at which slides the inclusion according to (2) and synchronization according to (8) takes place. From (6) we obtain that, when both conditions (2) and (8) are fulfilled, the slip S must satisfy the relation:

Example. At 10≤γ≤15 we have a series of slips close to the nominal (0 <S≤0.02) and of practical interest: S = 0.0125; S = 0,0114; S = 0.0104; S = 0.0096; S = 0.0089; S = 0.0083.

Thus, both conditions (2) and (8) of operation in the synchronous mode of variable frequency asynchronous motors with a phase-rotor are satisfied.

The process of synchronization with the network can be formulated taking into account (2), (8), (9) in the form of the following algorithm (sequence of operations):

- in the nominal asynchronous mode, the slip S is changed by several percent upwards (or downwards) according to (9) by changing the frequency of the converter in the rotor circuit;

- when slip reaches S according to (9) at the moment of passing the phase A current through zero according to (8), both phases of the rotor winding are turned on for a constant voltage according to (2).

The claimed method provides an increase in the power factor of the frequency-controlled asynchronous motor with a phase-rotor, the absence of reactive currents in the rotor and stator windings and, consequently, an increase in the efficiency of the engine.

Literature

1. Walker Miles. The Control of the Speed and P.F. of Induction Motors. London: Pittman. 1924.

2. Dreyfus L. Collector cascades. M .: - ONTI, 1934.

3. Demirchyan K.S., Neuman L.R., Korovkin N.V., Chechurin V.L. Theoretical Foundations of Electrical Engineering (in three volumes), M. - S. - Petersburg: Peter Publishing House, 2004.

4. Boguslawsky I .; Korovkin N .; Hayakawa M. Large A.C. Machines. Theory and Investigation Methods of Stats and Rotor Meshes incl. Operation with Nonlinear Loads: Springer. 2016. - 549 p.

5. Ed. Kopylova I.P. Design of electric cars. M .: Energy. 1980

6. Slezhanovsky OV, Datskovsky L.Kh., Kuznetsov I.S. and others. Systems of the subordinate regulation of AC electric drives with valve converters. M., Energoatomizdat, 1983

7. Onishchenko, G. B., Khvatov, SV. Electric drives on systems of the valve cascade and dual-feed motor. Encyclopedia of mechanical engineering. Volume IV-2. Electric drive Hydro and vibrating instruments. Kn.1. Electric drive., M., Mashinostroenie, 2012.

Claims (3)

1. A method of operation in a synchronous mode of a frequency-controlled asynchronous motor with a phase rotor, characterized in that when sliding, close to nominal, when the stator current passes through one of the phases through zero, its control and regulation system includes both phases of the two-phase rotor winding for voltage DC source. 2. The method according to claim 1, characterized in that the point in time during the passage of a current of one of the phases through zero is determined by the slip S, close to the nominal and equal to S = 1 / (8⋅γ) (with γ = 1; 2; 3; ….), And the frequency of the voltage ω _{p of the} rotor winding for both phases is determined by this slip angle α _p = ω _p ⋅ t = π⋅k / 4 (with k = 1; 5; 9; ...). 3. The method according to any one of paragraphs. 1, 2, characterized in that the values of currents in the phases of the rotor winding in synchronous mode is set equal to the current value of the currents in these windings in asynchronous mode.