This invention relates to a method for controlling an electrical motor of the asynchronous type and to a control system for implementing said method for controlling said electrical motor.
In a conventional control scheme of an electrical motor of the asynchronous type, a control law is executed by a processing unit and receives in input a speed reference (or pulsation reference) and a flux reference. As a function of these two items of information and of measurements of values on the motor (current and/or speed), the processing unit determines a voltage reference to be applied to the electrical motor. From this voltage reference, the processing unit determines the control voltages to be applied to each output phase connected to the motor. In a known manner, these voltages are applied to the motor using an electronic power architecture.
As a general rule, this architecture is connected to an electrical network supplying an alternating electrical voltage. The architecture includes an AC/DC rectifier, a DC power supply bus connected to the rectifier and a voltage inverter of the DC/AC type. The voltage inverter is controlled, for example, in Pulse Width Modulation. Such an inverter delivers to the motor a sequence of pulses of fixed amplitude, positive or negative and modulated in width, according to a voltage control law.
The maximum voltage delivered to the electrical motor cannot exceed the voltage available at the DC power supply bus.
In this control scheme, it is known that when a speed reference greater than a certain threshold (generally near the rated speed of the motor) and a constant flux reference are imposed in input, the voltage calculated as a function of these references can be incompatible with the voltage available on the network or with a limitation of voltage chosen for the motor to be powered. In such a situation, in order to be able to apply this speed reference to the electrical motor and thus to obtain a voltage that the inverter can generate, the processing unit must of necessity reduce the flux value and thus enter into a “flux reduction” zone.
An approach to this problem was first made in U.S. Pat. No. 5,204,607, which proposes to correct the flux reference when the determined voltage reference becomes greater than the voltage that the inverter can supply. For this purpose, the system makes a comparison between the voltage reference and the maximum voltage that the inverter can supply. The calculated difference is used to determine a correction value to be applied to the flux reference. The flux reference is corrected as long as the voltage reference for the inverter is less than or equal to the maximum voltage that the inverter can supply. The flux reference and the real flux are therefore always in agreement, which prevents any flux reduction. However, in this system, the current references are not corrected dynamically, and it cannot therefore be guaranteed the same stability properties within the voltage limitation zone and outside this zone.
U.S. Pat. No. 7,221,117 proposes another solution that uses a model of the electrical motor and that makes it possible to adjust the flux in voltage limitation and hence to ensure stability of the control of the electrical motor in voltage limitation. However, in this latter method, the fact of reaching the voltage limitation during flux reduction causes transient dynamic disturbances of the mechanical values of the electrical motor and in particular of the torque of the electrical motor.
The aim of the invention is therefore to propose a control method implemented in the processing unit that makes it possible to resolve the problems of dynamic disturbances linked with the passage into the “flux reduction” region.
This aim is achieved by a method for controlling an asynchronous electrical motor, implemented in a processing unit, said processing unit being associated with a power converter connected by output phases to said electrical motor and disposed to execute a control law for the purpose of determining the voltages to be applied to said electrical motor based on a speed reference and a flux reference applied in input, said method including an identification phase, which consists in:
According to a particularity, the speed trajectory follows a profile as a staircase, each step of which corresponds to a distinct value to be applied. According to a first embodiment, for each value taken by the speed reference, the method consists in:
In this first embodiment, the method consists in determining the intersection between the constant formed by said threshold value and the curve of variation of the motor voltage obtained when the flux reference varies at a given speed reference.
According to a second embodiment, for each value taken by the speed reference, it consists in:
According to another particularity of the invention, the method includes an operating phase that follows the identification phase and in which each flux value stored in conjunction with each speed reference during the identification phase can be used to adjust the flux in real time when the control law of the electrical motor is executed.
The invention also relates to a control system for an electrical motor comprising a processing unit, said processing unit being associated with a power converter connected by output phases to said electrical motor and disposed to apply variable voltages to said electrical motor while executing a control law, said system, during an identification phase, executes:
According to a particularity of the system, the speed trajectory generated by the module for generating a trajectory follows a profile as a staircase, each step of which corresponds to a distinct value to be applied.
According to a first embodiment, for each value taken by the speed reference, the system executes:
According to a particularity of this first embodiment, said module for determining the flux reference value is disposed to determine the intersection between the constant formed by said threshold value and the curve of variation of the motor voltage obtained when the flux reference varies at a given speed reference.
According to a second embodiment, for each value taken by the speed reference, the system fixes the flux reference at a determined value and executes:
According to a particularity, during an operating phase that follows the identification phase, it is disposed to adjust the flux in real time when the control law of the electrical motor is executed on the basis of flux values stored in conjunction with the speed references during the identification phase.
Other characteristics and advantages will appear in the detailed description that follows in the light of the attached drawings, in which:
The invention described below applies to controlling an asynchronous (induction) motor, preferably with a three-phase power supply. It is implemented in a conventional control scheme of the vectorial or scalar type, in open loop, that is to say without any return of a speed measurement at the electrical motor, or in closed loop, that is to say with a return of a speed measurement at the electrical motor.
In the continuation of the description, motor voltage Um is understood to be the amplitude of the voltage reference vector having the two components ud_ref and uq_ref . Furthermore, passing from the rotating identifier (d, q) to the three-phase identifier a, b, c, which corresponds to the physical identifier of the controlled electrical motor, is known. We therefore also have:
Where ua, ub, uc are the instantaneous values of the voltages applied to each output phase and θ is the angle of phase difference applied among voltages applied between the, output phases.
The control method of the invention is implemented in a control system that includes a processing unit UC. The processing unit UC includes at least one microprocessor and a memory. This control system is associated with a variable speed drive designed to control an electrical motor. It will be able in particular to be integrated with said variable speed drive.
In a known manner, the variable speed drive includes, as a general rule;
In a non-limitative manner, the invention will be described for a control law of the U/F scalar type in open loop. It must be understood that the method described below will be identical regardless of the control law used.
In a known manner with reference to
The processing unit UC, determines, from a voltage calculator module M1, a forward voltage reference ud_ref yet and a voltage reference uq_ref in phase quadrature.
Other modules can, of course, be implemented by the processing unit, but these will not be described in this application.
This control law is implemented during an operating phase, that is to say during normal functioning of the electrical motor M controlled by the variable speed drive.
The invention relates to a control method that includes an identification phase, preferably conducted prior to said operating phase. This identification phase aims to determine, for different pulsations ωs applied in input, the flux values for which the motor voltage is equal to a determined threshold value. This threshold value will preferably be linked to the limit voltage value Ulim that the variable speed drive can supply. This threshold value, designated Umax, is chosen lower than the limit voltage value Ulim. This threshold value is preferably stored by the processing unit and, for example, is equal to:
Umax=0.95×Ulim
During normal functioning of the electrical motor M, the identified flux values will make it possible to adjust in real time, if necessary, the value of the flux reference
In other words, it is a question of constructing a curve profile connecting the amplitude of the flux reference at the pulsation ωs to a determined threshold voltage, for example, equal to the threshold value defined above. Such a profile is shown in
The demonstration that follows makes it possible to show that a variation of the flux has an effect on the motor voltage.
The electrical equations of an asynchronous motor in the rotating identifier d,q, according to a standard model, are as follows (notation in complex form):
Where:
The Electrical Parameters are:
As already explained the voltage supplied by the variable speed drive to the electrical motor cannot exceed a limit value Ulim, and so the amplitude of the motor voltage is subjected to the following constraint:
|us|<Ulim
In equilibrium, the motor voltage is:
us=(Rs+jLfωs)is+jωsφ
The amplitude of the motor voltage Um=|us| increases with the flux module |φ| or the pulsation ωs.
The identification phase for the flux values can be implemented according to different embodiments.
In these two embodiments, the identification phase consists in scanning a whole pulsation range following a pulsation trajectory. The processing unit UC executes a pulsation trajectory, module M5 in order to make the pulsation ωs take several successive values. As shown on
For each of the values thus taken by the pulsation ωs, the processing unit UC will determine the flux value |φ| for which the motor voltage Um is equal to the predefined threshold value, that is to say, equal to the value Umax.
With reference to
The processing unit implements these different steps for the N values taken by the pulsation, over the whole range, such as:
ωmin<ωs<ωmax
With reference to
In this second embodiment, the different steps are also implemented by the processing unit for the N values taken by the speed reference over the whole range, such as:
ωmin<ωs<ωmax
On conclusion of the identification phase, implemented according to one or other of the embodiments described above, N torques of (ωi, |
At the end of the identification phase, the values obtained of (ωi, |
During normal functioning, the flux profile can be implemented in different forms:
In which:
The method described above will be valid regardless of the control law used, this method consisting in a general manner in forming N torques of (ωi, |
The solution of the invention thus offers many advantages, listed below:
Number | Date | Country | Kind |
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16 52955 | Apr 2016 | FR | national |
Number | Name | Date | Kind |
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5204607 | Hugel | Apr 1993 | A |
7221117 | Jadot et al. | May 2007 | B2 |
9197152 | Jebai et al. | Nov 2015 | B2 |
20060208689 | Jadot et al. | Sep 2006 | A1 |
20120068639 | Sejimo | Mar 2012 | A1 |
Number | Date | Country |
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2 804 310 | Nov 2014 | EP |
Entry |
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U.S. Appl. No. 15/176,735, filed Jun. 8, 2016, 2017/0005597 A1, Al Kassem Jebai et al. |
U.S. Appl. No. 15/275,927, filed Sep. 26, 2016, Al Kassem Jebai et al. |
French Preliminary Search Report dated Dec. 7, 2016 in French Application 16 52955, filed on Apr. 5, 2016 ( with English Translation of Categories of Cited Document s and Written Opinion). |
Number | Date | Country | |
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20170288587 A1 | Oct 2017 | US |