RU2362264C1 - Method of controlling alternating current drive - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering, particularly to systems for controlling alternating current drives with frequency converters. In the method of controlling an alternating current drive, the inverter is controlled by a relay regulator, at the input of which given and the measured values of phase currents at the output of the frequency converter are compared. Commutation of the inverter switches takes place if the difference between these values is greater than a threshold level, when current frequency at the output of the inverter is less than 1/3 of the supply frequency. In the region of instantaneous values of phase currents of the stator, less than √3/2 of the given amplitude values, a smooth current component is formed at the output of the frequency converter by toggling the switch of the inverter. In the region of instantaneous values of phase currents of the stator greater than √3/2 of the given amplitude value, the inverter switches in the corresponding phases are left closed until the instantaneous value of phase current at the output of the frequency converter drops to √3/2 of the given amplitude value, and a smooth current component is formed at the output of the frequency converter by regulating the switch-on point of the rectifier switches. This reduces the number of commutations of the inverter switches and as a result, reduces energy loss in the electric drive.

EFFECT: reduced energy loss and improvement of harmonic composition of current in windings of an alternating current synchronous motor.

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Description

The invention relates to electrical engineering, and in particular to control systems for AC electric drives based on asynchronous and synchronous motors with frequency converters (IF), and can be used when operating at reduced engine speeds.

A known method of controlling an AC electric drive by generating currents at the IF outputs with a first rectifier connected in series, a smoothing reactor and an inverter made on fully controllable valves, in which the rectified current at the rectifier output connected by the AC voltage clamps to the source (supply network) is regulated, and switch the valves of the inverter connected by clamps of alternating voltage to the motor windings using pulse-width modulation ( MI) and according to the desired phase current distribution (current spatial position of the imaging vector). High-frequency components of the inverter output currents are closed through a three-phase capacitor bank connected to the motor windings [1].

The disadvantages of this known solution include a reduced power factor, a high content of harmonics in the currents and a high switching frequency of the inverter keys, thereby increasing the loss in the inverter. Closest to the proposed one is a method of controlling an AC electric drive by generating currents at the IF outputs with a rectifier connected in series, made on fully controlled valves, a smoothing reactor and an inverter made on fully controlled valves, where the rectifier valves on fully controlled valves are also switched depending from the given amplitude of the smooth component currents in its AC wires and separately from the rectified current. And the inverter keys are switched using PWM and in accordance with the required distribution of phase currents (spatial position of the imaging current vector). The high-frequency components of the inverter output currents are closed through a three-phase capacitor bank connected to the motor windings [2].

The disadvantages of this known solution include the high frequency of switching the keys of the inverter and, consequently, increased losses in the electric drive.

The proposed control method solves the problem of reducing the switching frequency of the inverter keys, as well as improving the harmonic composition of the current in the windings of the synchronous AC motor at reduced speeds.

от заданного амплитудного значения, переключая ключи инвертора, формируют гладкую составляющею тока на выходе преобразователя частоты, а в области мгновенных значений фазных токов статора больших, чем

от заданного амплитудного значения, ключи инвертора в соответствующих фазах оставляют включенными до тех пор, пока мгновенное значение фазного тока на выходе преобразователя частоты не снизится до уровня

от заданного амплитудного значения, и формируют гладкую составляющую тока на выходе преобразователя частоты путем регулирования момента включения ключей выпрямителя.This problem is solved in that the control method of the AC electric drive is carried out by forming currents at the outputs of the frequency converter supplying the electric motor and made with an adjustable rectifier and inverter, on fully controlled valves connected to each other through a smoothing reactor, in which the rectified current is regulated depending on the required amplitude of the smooth component currents at the output of the frequency converter, switch the valves of the rectifier and inverter depending on Additions for the distribution of currents in AC wires and filter the high-frequency components of the currents at the output of the frequency converter, the signals for switching the inverter keys are supplied from the relay current regulator, at the input of which the set and measured values of the phase currents at the output of the frequency converter are compared, the inverter keys are switched at a difference these values are greater than the threshold level, at a frequency of a smooth component of the current at the output of the frequency converter, less than 1/3 of the frequency of the supply network, in Asti instantaneous stator phase current values less than

from a given amplitude value, by switching the keys of the inverter, they form a smooth component of the current at the output of the frequency converter, and in the region of instantaneous values of the stator phase currents are greater than

from a given amplitude value, the inverter keys in the corresponding phases are left turned on until the instantaneous value of the phase current at the output of the frequency converter decreases to the level

from a given amplitude value, and form a smooth component of the current at the output of the frequency converter by adjusting the moment of switching on the rectifier keys.

от заданного амплитудного значения, с помощью инвертора, а в области мгновенных значений токов больших, чем

от заданного амплитудного значения, - с помощью управляемого выпрямителя. Это позволяет уменьшить количество пульсаций тока в обмотках статора и уменьшить потери энергии в электроприводе.In this method of controlling an AC motor in the region of low frequencies of rotation of the electric motor and in the region of low frequencies of current at the inverter output, a smooth component of the stator current is formed in the region of instantaneous currents less than

from a given amplitude value using an inverter, and in the region of instantaneous values of currents greater than

from a given amplitude value, - using a controlled rectifier. This allows you to reduce the number of ripple current in the stator windings and reduce energy loss in the drive.

Figure 1 shows a diagram of a device that implements the proposed method for controlling a synchronous motor, figure 2 is a timing diagram of the parameters obtained on the mathematical model for the option of constructing an AC electric drive based on a synchronous motor.

A device that implements the proposed method comprises an electric motor 1 connected to the inverter outputs with a rectifier 2 and an inverter 3, interconnected via a smoothing reactor 4. The inverter is made on fully controllable valves, for example, GTO thyristors. A capacitor bank 5 is connected to the inverter outputs (AC wires A, B, C). Current sensors 6 are installed at the inverter output. The inverter 7 control unit contains a current relay controller 8, at the input of which there is a comparison unit 9, to which the given and measured instantaneous values of the phase currents of the stator of the motor 1. The rectifier control unit 10 contains a rectified current regulator 11, at the input of which there is a comparison unit 12 connected to the output of the rectified current reference unit 13 and the rectified sensor 14. 14. The output of the controller 11 is connected to the input of the pulse forming unit 15, the output of which is connected to the control inputs of the valves of the rectifier 2.

The method of controlling an AC motor in this device is as follows.

The current regulator 11 generates a task for the control unit 15, which controls the keys of the rectifier 2. The current Id of the rectified circuit is set by the signal level of the Idset reference. The signal for setting the rectified current Idad is determined based on the signals for setting the instantaneous values of the phase currents of the stator of the motor 1 I _A , I _B , I _C supplied to the input of the relay controller of the stator current. The Idset signal is calculated according to the algorithm:

if | I _a |> | I _b | and | I _a |> | I _c |, then I _d = | I _a |;

if | I _b |> | I _a | and | I _b |> | I _c |, then I _d = | I _b |;

if | I _c |> | I _a | and | I _c |> | I _b |, then I _d = | I _c |.

Thus, the signal level Idad is equal to the maximum reference of the instantaneous phase current of motor 1 modulo one of the phases of block 13. If the instantaneous value of the reference current of phase “A” modulo is greater than the instantaneous values of the phase currents “B” and “C” modulo, then the instantaneous setting of the rectified current is equal to the instantaneous setting of the current of phase "A", if the instantaneous value of setting the current of phase "B" modulo is greater than the instantaneous values of setting the currents of phases "A" and "C" modulo, then the instantaneous setting of rectified current is equal to the instantaneous setting of phase current “B”, if the instantaneous value of setting the current of phase “C” modulo is greater than the instantaneous values of setting the currents of phases “A” and “B” modulo, then the instantaneous setting of the rectified current is equal to the instantaneous setting of the current of phase C.

The parameters of the current controller 11 are determined as follows. The voltage balance in the rectified network has the form:

U _d = 2E ₁ + (R _dΣ + pL _dΣ ) I _d ,

where E ₁ - EMF of one phase of the stator winding; R _dΣ is the total active resistance; L _dΣ is the total inductance reduced to a direct current circuit.

,

,

where T = L _dΣ / R _dΣ .

The transfer function of the object of regulation for the current regulator will be

where k _n is the transfer coefficient of the rectifier, T _µ is the time constant of the rectifier.

Then the transfer function of the PI current controller will be:

,

,

- постоянная интегрирования ПИ-регулятора.Where

- integration constant of the PI controller.

The relay inverter current controller 8 receives signals from the comparison unit 9 of the instantaneous values of the set and measured phase currents of the stator. The control signals come from the relay current controller to the keys of the inverter 3, implement the following control algorithm:

- when the difference between the set value of the phase current and the measured value of the phase current of the dead zone is exceeded, turn on the upper arm valve, turn off the lower arm valve of the corresponding phase;

- when the difference between the set value of the phase current and the measured value of the phase current below the dead zone is reduced, turn off the upper arm valve and turn on the lower arm valve of the corresponding phase.

The specified algorithm ensures that the amplitudes of the smooth components of the currents in the AC wires correspond to the specified values of this current with an error set by the deadband of the relay current controller 8.

Graphs of transients of the characteristics of the electric drive of figure 1 are presented in figure 2, which shows the given (smooth) and current values of the phase current I _A , I _B and I _C ; setting the control voltage of the phase keys “A, B and C”, U _{control A} ,

от заданного амплитудного значения, ключи инвертора в соответствующих фазах оставляют включенными, что соответствует промежутку от «а» до «б», и формируют гладкую составляющую тока на выходе преобразователя частоты путем регулирования момента включения ключей выпрямителя. При фазном токе меньшем, чем

от заданного амплитудного значения, гладкую составляющую тока на выходе преобразователя частоты формируют переключением ключей инвертора по законам релейного регулятора. В результате имеем систему с минимальным количеством коммутации ключей инвертораU _{control B} , U _{control C} , where “1” corresponds to the open state of the upper phase arm and the closed state of the lower phase arm, and “0” corresponds to the closed state of the upper phase arm and the open state of the lower phase arm; current value of the rectified current Id. From the graphs of figure 2 it is seen that when the phase current is greater than

from a given amplitude value, the inverter keys in the corresponding phases are left on, which corresponds to the interval from “a” to “b”, and form a smooth current component at the output of the frequency converter by adjusting the moment the rectifier switches are turned on. With a phase current less than

from a given amplitude value, the smooth component of the current at the output of the frequency converter is formed by switching the inverter keys according to the laws of the relay controller. As a result, we have a system with a minimum amount of inverter key switching

The proposed method for controlling an AC motor allows you to reduce the number of commutations of the inverter keys when controlling the frequency at the inverter output and, accordingly, the motor rotation speed, in the range of more than 1: 3, by controlling the instantaneous current values in the rectified circuit. Reducing the number of commutations of the inverter keys reduces the energy loss in the electric drive, allows you to increase the service life of the insulation of the motor stator winding.

Literature

1. RF patent No. 2074394, MKI G01P 3/46, 1995.

2. RF patent No. 2115218, MKI Н02Р 7/42, Weinger A.M. A method for controlling an AC electric drive, 10.07.1998.

Claims (1)

A method of controlling an AC electric drive by generating currents at the outputs of a frequency converter supplying an electric motor and made with an adjustable rectifier and inverter on fully controllable valves interconnected via a smoothing reactor, in which the rectified current is regulated depending on the required amplitude of the smooth components of the currents at the converter output frequency, switch the rectifier and inverter valves depending on the task for the distribution of currents in the wires of change of current and filter the high-frequency components of the currents at the output of the frequency converter, characterized in that the signals for switching the inverter keys are supplied from a relay current regulator, at the input of which the set and measured current values at the output of the frequency converter are compared, the inverter keys are switched at a difference of more threshold level, when the frequency of the smooth component of the current at the output of the frequency converter is less than 1/3 of the frequency of the supply network, in the region of instantaneous phase currents in a stator smaller than

from a given amplitude value, by switching the keys of the inverter, they form a smooth component of the current at the output of the frequency converter, and in the region of instantaneous values of the stator phase currents greater than

from a given amplitude value, the inverter keys in the corresponding phases are left turned on until the instantaneous value of the phase current at the output of the frequency converter decreases to the level

from a given amplitude value, and form a smooth component of the current at the output of the frequency converter by adjusting the moment of switching on the rectifier keys.

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