RU2410813C1 - Method to provide for operability of three-phase asynchronous electric drive - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention refers to the sphere of electric engineering and may be used in asynchronous electric drives. Phase windings of stator of three-phase asynchronous electric motor are supplied from frequency converter operating in the mode of current source, which is realised by control of phase currents, at the same time settings for phase currents of motor are received from outlet of speed controller, to inlets of which a signal of rotation frequency setting is sent, as well as current frequency of rotation of motor shaft received from speed sensor. Simultaneously analysis of phase currents of motor is carried at each interval of pulse-width modulation, and in case a failure is detected on one of motor phases, current frequency is changed in two remaining phases k times according to previously introduced non-linear dependence of coefficient k from specified frequency of motor shaft rotation, and instantaneous values of phase currents of asynchronous motor are adjusted.

EFFECT: provides for operability of three-phase asynchronous motor in case of emergency disconnection of one of phases with preservation of motor shaft rotation frequency relative to three-phase mode of operation.

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Description

The invention relates to electrical engineering and can be used in widely adjustable asynchronous electric drives.

A known method of ensuring the survivability of five or seven-phase asynchronous motors with vector control from a frequency converter (Disturbance-free operation of a multiphase current-regulated motor drive with an opened phase / Fu Jen-Ren, Lipo Thomas A. // IEEE Trans. Ind. Appl . - 1994. - 30, No. 5. - C.1267-1274), in which, when one of the phases is broken in the frequency converter, the appropriate combination of currents is set to maintain the motor torque without ripples, while the connection wire to the neutral is excluded, since a zero sequence of currents, unlike a 3-phase induction motor, will essential.

The disadvantage of this method is the limited scope for multiphase induction motors, which does not allow the use of the proposed method with a three-phase motor design.

There is a method of ensuring the survivability of a three-phase asynchronous motor for driving an escalator (JP Patent No. 3293289, IPC B66B 23/02, publ. 24.12.1991), in which performance is restored when the power part of the frequency converter is faulty: rectifier - inverter. Survivability is provided by diagnostics, followed by disconnecting the motor power from the frequency converter and switching it directly to a three-phase AC network.

The disadvantage of this method is to reduce the uniformity of movement of the escalator when the frequency converter is switched off due to an abrupt decrease or increase in the frequency of rotation of the motor shaft depending on the degree of congestion of the escalator and, as a result, the limited applicability of this method, as well as the inability to operate this electric drive when the motor phase breaks directly supplying the stator winding .

A known method of ensuring the survivability of an asynchronous electric drive (Fault mode single-phase operation of a variable frequency induction motor drive and improvement of pulsating torque characteristics / Kastha Debaprasad, Bose Bimal K. // IEEE Trans. Ind. Electron. - 1994. - 41, No. 4. - C.426-433), in which, when the phase is broken or disconnected due to a malfunction, voltage harmonics are introduced to neutralize the second and other harmonics of the torque ripples and shift the moment ripple frequencies to a higher spectrum range, which makes it possible to compensate for the moment ripples beyond account of the moment of inertia of the engine. To control the electric drive in two-phase mode, the optimal voltage / frequency ratio U / f is introduced into the frequency converter to obtain the maximum torque / current ratio and reduce torque ripples, which ensures acceptable operation with the motor shaft speed control.

The disadvantage of this method is the impossibility of applying the proposed technical solution in the case of a deeply adjustable electric drive, especially in the low speed range.

A known method of controlling and ensuring the survivability of a three-phase asynchronous motor (RF patent No. 2326480, IPC ⁶ H02H 7/09, publ. 10.06.2008), selected as a prototype, which consists in the fact that the phase windings of the motor stator are powered from a three-phase frequency converter operating in the mode of a current source, which is realized by regulation of phase currents. The tasks for the phase currents of the motor are obtained from the output of the speed controller, the inputs of which supply a signal for setting the speed of the motor and the current frequency of rotation of the motor shaft, obtained from the speed sensor, at the same time continuously analyze the phase currents of the motor on each interval of pulse-width modulation, and in the case failure detection in one of the phases of the motor corrects the instantaneous values of currents in the two remaining phases of the motor for the leading phase of the current vector:

for the lagging phase of the current vector:

where I _ω is the amplitude value of the generated phase currents, A;

σ is the number of binary bits of the output code of the digital signal obtained by integrating the signal from the speed sensor;

a, b, c - three bits of failure of each phase;

,

,

,

- инверсные значения a, b, c, d;

,

,

,

- inverse values of a, b, c, d;

α is the value of the current binary code of the digital signal obtained by integrating the signal from the speed sensor;

d is the value of a bit calculated by a logical expression: d = a∪b∪c (∪ is a logical OR operation); if d = 1, then this indicates a failure in one of the engine phases.

The disadvantage of this method is the drop in the rotational speed of the motor shaft to a level of 30% relative to the rotational speed when operating in three-phase mode.

The objective of the invention is to ensure the survivability of a three-phase asynchronous electric drive during an emergency shutdown of one of the phases while maintaining the frequency of rotation of the motor shaft relative to the three-phase mode of operation.

The problem is solved due to the fact that in the method of ensuring the survivability of a three-phase asynchronous electric drive, as well as in the prototype, the phase windings of the motor stator are powered from a three-phase frequency converter operating in the current source mode, which is implemented by controlling phase currents, tasks for the phase currents of the motor are obtained from the output of the speed controller, to the inputs of which a signal for setting the speed of rotation and the current frequency of rotation of the motor shaft, obtained from the speed sensor, are continuously received clearly analyze the phase motor current in each interval of pulse-width modulation and in the case of failure detection in one motor phase corrected instantaneous values of the currents in the other two motor phases.

According to the invention, in case of failure detection in one of the engine phases, the current frequency is changed k times in the two remaining phases according to a previously introduced nonlinear dependence of the coefficient k on the given rotation speed of the motor shaft and the instantaneous values of the phase currents of the induction motor are adjusted for the leading phase of the current vector according to the expressions:

for the lagging phase of the current vector:

where I _ω is the amplitude value of the generated phase currents;

p is the number of pole pairs of the induction motor;

t ₃ - current direct reading of timer labels in three-phase mode;

t ₂ is the current countdown of timer labels in two-phase mode from the moment of emergency switching of the engine control structure;

f is the stator current frequency;

a, b, c - the value of the bits of the status word of the drive for failures (produced by the state sensors of the converter cells);

d is the value of a bit calculated by a logical expression: d = a∪b∪c, where ∪ is a logical OR operation, and if d = 1, then a conclusion is made about a failure in one of the engine phases;

,

,

,

- инверсные значения a, b, c, d.

,

,

,

- inverse values of a, b, c, d.

It is known that increasing the operating frequency of the stator current f of an induction motor leads to an increase in the operating power P of the motor with a fixed value of the moment M on the shaft of the induction motor. This follows from the expression: P = M · 2 · π · f / p. [Chilikin M.G. General course of electric drive: a training manual / M.G. Chilikin, A.S. Sandler. - 6th ed., Ext. and reslave. - M .: Energoizdat, 1981. - 576 p.] - S.101. In the event of failure of one of the engine phases, an increase in the frequency of the stator current by a factor of k will lead to an increase in the operating power of the engine and, accordingly, can compensate for the decrease in the frequency of rotation of the motor shaft. The coefficient k nonlinearly depends on a given frequency of rotation of the motor shaft ω _ass . Accordingly, it is necessary to introduce a nonlinear characteristic k = f (ω _ass ) into the functional converters in advance. Thus, by introducing a nonlinear characteristic k = f (ω _ass ) and using expressions for the instantaneous values of phase currents (1), (2), (3), (4), (5), (6), the proposed technical solution allows in case of failure of one of the phases of the electric drive, to ensure the conservation of a circular rotating field of the motor and to preserve the direction of rotation of the motor shaft with an extended control range, as well as to ensure a 100% probability of starting in a given direction when switching from a working three-phase operation mode to an emergency two-phase operation mode with injury to the working speed of the engine shaft.

Figure 1 presents a functional diagram of a microcontroller control system for a three-phase asynchronous motor.

Figure 2 presents the characteristic dependence of the coefficient k on a given frequency of rotation of the motor shaft ω _ass ^∗ in relative units, embedded in functional converters.

Figure 3 presents the timing diagrams of transients of stator currents: I _A , I _B , I _C , voltages: U _A , U _B , U _C and motor shaft rotation frequency ω when switching from operating 3-phase to emergency 2-phase mode excluding the operation of functional converters.

Figure 4 presents the timing diagrams of transients of the stator currents: I _A , I _B , I _C , voltages: U _A , U _B , U _C and motor shaft rotation frequency ω when switching from a working 3-phase to emergency 2-phase mode taking into account the work of functional converters.

The proposed method is implemented using the microcontroller control circuit of a three-phase asynchronous motor (Fig. 1), which contains an asynchronous motor 1 (HELL), which is connected via current sensors 2 (ДТ1), 3 (ДТ2), 4 (ДТ3) to converter cells 5 ( ПЯ1), 6 (ПЯ2) and 7 (ПЯ3) of a three-phase frequency converter, to which a diagnostic unit 8 (DB) is connected, connected to microcontroller 9 (MK). The outputs of the current sensors 2 (DT1), 3 (DT2), 4 (DT3) are connected to the microcontroller 9 (MK). A speed sensor 10 (DS) is installed on the shaft of the induction motor 1 (HELL), the output of which is connected to the microcontroller 9 (MK). The setpoint of the rotational speed of the motor shaft 11 (HF) is connected to the microcontroller 9 (MK), which is connected to the functional converters 12 (FP1), 13 (FP2), 14 (FP3), and they, in turn, are connected to the corresponding converter cells 5 (P1), 6 (P2), 7 (P3).

As an asynchronous motor 1 (AM), you can choose any three-phase asynchronous motor with isolated phases, i.e. the beginning and end of each stator winding are displayed in the terminal box, for example, ADM63V4U3. As current sensors 2 (DT1), 3 (DT2), 4 (DT3), standard sensors with galvanic isolation can be used, for example LEM modules of type LA 25-NP. Speed sensor 10 (DS) can be of any type with an analog or digital output, for example SG751 - SG753 [www.start-vector.com]. As microcontroller 9 (MK), a single-board microcontroller type AT89C2051 with a clock frequency of 4 MHz can be selected. As converter cells 5 (ПЯ1), 6 (ПЯ2), 7 (ПЯ3) of a three-phase frequency converter, an inverter made according to a bridge or half-bridge circuit is used. The speed controller 11 (ZHV) can be made in the form of a block that generates an analog or digital reference signal. An embodiment of the diagnostic unit 8 (DB) is described in Odnokopylov G.I. Transducer cell state sensor // IV International Scientific and Technical Conference "Microprocessor, analog and digital systems: design and circuitry, theory and application issues", Novocherkassk: Publishing House of SRSTU 2004. - 70 p. Microcontrollers of any type can be selected as functional converters 12 (FP1), 13 (FP2), 14 (FP3), for example, based on an ATMEGA 103 processor such as MS103.01.P [www.ssd.ru].

To verify the method of ensuring the survivability of a three-phase asynchronous electric drive, a mathematical model was used [RF certificate on registration of the computer No. 2007613578 “Program for calculating transients of an asynchronous electric drive in non-phase and emergency operation modes”]. Using this program, we constructed a nonlinear characteristic of the coefficient k from a given rotation frequency k = f (ω _ass ^∗ ) (Fig. 2). Each motor will have its own k value corresponding to ω _ass , at which the motor will operate stably, while the phase currents and speed will be close to the nominal. Based on this condition, the coefficients k are selected for various values of ω _ass . Next, the dependence k = f (ω _ass ), constructed, for example, at ten points, is approximated and introduced into functional converters 12 (FP1), 13 (FP2), 14 (FP3). In the process of operation of the electric drive, the signal from the speed controller 11 (HFV) enters the microcontroller 9 (MK). The phase currents of the motor, determined using current sensors 2 (DT1), 3 (DT2), 4 (DT3) and the rotational speed of the motor shaft, determined using the speed sensor 10 (DC), are supplied to the microcontroller 9 (MK), in which they produce assignments to the frequency f and the amplitude of the currents for each phase of the motor, which, in turn, pass through the functional converters 12 (FP1), 13 (FP2), 14 (FP3) to the corresponding converter cells 5 (ПЯ1), 6 (ПЯ2), 7 (P3). Diagnostic unit 8 (DB) analyzes the state of the converter cells 5 (ПЯ1), 6 (ПЯ2), 7 (ПЯ3) on each pulse-width modulation interval and generates three failure bits a, b, c that make up the drive status word that goes to the microcontroller 9 (MK). If a failure is detected either in one of the phases of the motor, or in one of the cells of the frequency converter in the microcontroller 9 (MC), a recovery algorithm is included in which the reference to the frequency of the stator current f is increased by a factor of k according to the characteristics inherent in the functional converters 12 (FP1 ), 13 (ФП2), 14 (ФП3), and also adjust the instantaneous values of the phase currents of the induction motor 1 (HELL) according to the expressions (1), (2), (3) or (4), (5), (6) .

The timing diagrams of transients (Figs. 3, 4) show the operation of an MTF 311-6 brand induction motor with a rated load moment (M _n = 111N · m) on the shaft when switching from a working 3-phase to emergency 2-phase operation at failure in phase "A" at time equal to t = 0.3 s. Without taking into account the operation of functional converters (figure 3), the current in the failed phase ("A") became equal to zero. In the remaining phases "B" and "C" after the electromagnetic transient, the currents alternated, the amplitude of the currents increased by 1.5 times, and the phase shift between them became π / 3, which is determined by the restoration algorithm (2) and (3) to maintain a circular rotating field of the engine. The frequency of rotation of the motor shaft after a phase failure fell from 37 rad / s to 26 rad / s (30% relative to operation in three-phase mode). The engine continues to run, maintaining the rated load on the shaft.

Given the operation of functional converters 12 (FP1), 13 (FP2), 14 (FP3) (figure 4), the currents in the two remaining phases "B" and "C" remained the same as in figure 3, but with a frequency k times bigger. The frequency of rotation of the motor shaft ω after the electromechanical transition process remained at the level of the three-phase mode - 37 rad / s. For the MTF 311-6 engine: k = 1.4 at a given speed of rotation ω = 37 rad / s.

The proposed technical solution allows in the event of a failure in one of the phases of the electric drive (failure of the inverter power switch, phase failure of the motor) to ensure that the direction of rotation of the circular rotating field and the frequency of rotation of the motor shaft at a nominal level, as well as provide 100% probability of starting in a given direction with an extended control range and a specific reaction time of the microprocessor control system when switching the recovery algorithm from three-phase to two-phase mode, equal to the time interval of the pulse width modulation in the absence of additional hardware costs to support the recovery algorithm.

This technical solution imposes restrictions on the use of a three-phase asynchronous electric drive at high speeds. Changing the frequency of the stator current can lead to an increase in phase voltage, which is not permissible, therefore, the control range will be from the minimum speed to the speed corresponding to the rated voltage of the stator.

Claims (1)

A method of ensuring the survivability of a three-phase asynchronous electric drive, which consists in the fact that the phase windings of the motor stator are powered from a three-phase frequency converter operating in the mode of a current source, which is implemented by regulating the phase currents; rotational speeds and the current rotational speed of the motor shaft, obtained from the speed sensor, at the same time continuously analyze the phase currents of the motor on In each interval of pulse-width modulation and in the event of a failure in one of the motor phases, the instantaneous currents in the two remaining phases of the motor are corrected, characterized in that if a failure is detected in one of the motor phases, the frequency of the currents is changed in k two times by k times a previously introduced nonlinear dependence of the coefficient k on a given frequency of rotation of the motor shaft, and the instantaneous values of the phase currents of the induction motor are adjusted for the leading phase of the current vector according to the expressions:

;

;

,
for the lagging phase of the current vector:

;

;

,
where I _ω is the amplitude value of the generated phase currents;
p is the number of pole pairs of the induction motor;
t ₃ - current direct reading of timer labels in three-phase mode;
t ₂ is the current countdown of timer labels in two-phase mode from the moment of emergency switching of the engine control structure;
f is the stator current frequency;
a, b, c - the value of the bits of the status word of the drive for failures (produced by the state sensors of the converter cells);
d is the value of a bit calculated by a logical expression: d = a∪b∪c, where ∪ is a logical OR operation, and if d = 1, then a conclusion is made about a failure in one of the engine phases;

,

,

,

- inverse values of a, b, c, d.

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