METHOD FOR CONTROLLING AN ASYNCHRONOUS ELECTRICAL MOTOR

Abstract

A method for controlling an asynchronous electrical motor, implemented in a processing unit associated with a power converter connected to the electrical motor, the method including an identification phase, which includes generating a speed trajectory in input of a control law of the motor in order to make the speed reference take several determined successive values, for each value taken by the speed reference, determining the voltage at the terminals of the electrical motor, for each value taken by the speed reference, determining and storing the flux value for which the voltage at the terminals of the electrical motor is equal to a determined threshold value.

Claims

1. 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 (a, b, c) to said electrical motor and disposed to execute a control law for the purpose of determining voltages to be applied to said electrical motor based on a speed reference and a flux reference applied in input, comprising an identification phase, said method comprising: generating a speed trajectory in input of the control law in order to make the speed reference take several determined successive values, for each value taken by the speed reference, determining the voltage at the terminals of the electrical motor, for each value taken by the speed reference, determining and storing the flux value for which the voltage at the terminals of the electrical motor is equal to a determined threshold value.

2. The control method according to claim 1, wherein the speed trajectory follows a profile as a staircase, each step of which corresponds to a distinct value to be applied.

3. The control method according to claim 1, wherein for each value taken by the speed reference, said method comprises: making the flux reference vary between a minimum value and a determining the curve of variation of the motor voltage obtained when the flux reference varies, determining the flux reference value for which the motor voltage is equal to said threshold value.

4. The control method according to claim 3, wherein 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.

5. The control method according to claim 1, wherein for each value taken by the speed reference, said method comprises: fixing the flux reference at a determined value, determining the motor voltage obtained as a function of said speed reference value and said flux reference, both applied in input, determining the voltage difference between the motor voltage obtained and said threshold value, correcting the flux reference value applied in input until the motor voltage is equal to said threshold value, storing the flux reference value obtained when the motor voltage is equal to said threshold value.

6. The control method according to claim 1, comprising an operating phase that follows the identification phase and wherein 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.

7. 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, wherein, during an identification phase, it said system comprising: a module for generating a speed trajectory in input of the control law so as to make the speed reference take several determined successive values, for each value taken by the speed reference, a module for determining the voltage at the terminals of the electrical motor, for each value taken by the speed reference, a module for determining the flux value for which the voltage at the terminals of the electrical motor is equal to a determined threshold value, and a module for storing said determined flux value.

8. A system according to claim 7, wherein 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.

9. A system according to claim 7, wherein for each value taken by the speed reference, it said system comprises: a module for generating a flux reference trajectory between a minimum value and a maximum value, a module for determining a curve of variation of the motor voltage when the flux reference varies, a module for determining the flux reference value for which the motor voltage is equal to said threshold value.

10. The system according to claim 9, wherein 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.

11. The system according to claim 5, wherein for each value taken by the speed reference, said system fixes the flux reference at a determined value and comprises: a module for determining the motor voltage obtained as a function of said speed reference value and said flux reference value, both applied in input, a module for determining the voltage difference between the motor voltage obtained and said threshold value, a module for determining a correction to be applied to the flux reference value applied in input until the motor voltage is equal to said threshold value, a module for storing the flux reference value obtained when the motor voltage is equal to said threshold value.

12. The system according to claim 7, wherein during an operating phase that follows the identification phase, said system 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.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0041] Other characteristics and advantages will appear in the detailed description that follows in the light of the attached drawings, in which:

[0042] FIG. 1 shows the diagram of a conventional variable speed drive including the control system of the invention.

[0043] FIG. 2 shows a synoptic view illustrating the functioning principle of the control method of the invention, according to a first embodiment.

[0044] FIGS. 3A to 3D show curves illustrating the functioning principle of the invention according to the first embodiment.

[0045] FIG. 4 shows a synoptic view illustrating the functioning principle of the control method of the invention, according to a second embodiment.

[0046] FIG. 5 shows a curve connecting the flux reference to the speed reference at a maximum voltage.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

[0047] 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.

[0048] In the continuation of the description, motor voltage U.sub.m is understood to be the amplitude of the voltage reference vector having the two components u.sub.d.sub._.sub.ref and u.sub.q.sub._.sub.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:

[00001]$u_a=U_m.Math.\cos \left(\theta \right)$$u_b=U_m.Math.\cos \left(\theta -\frac{2.Math.\pi }{3}\right)$$u_c=U_m.Math.\cos \left(\theta +\frac{2.Math.\pi }{3}\right)$

[0049] Where u.sub.a, u.sub.b, u.sub.c 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.

[0050] 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.

[0051] In a known manner, the variable speed drive includes, as a general rule; [0052] Input phases R, S, T connected to an electrical network supplying alternating voltage. [0053] An AC/DC rectifier 10, such as for example, a bridge of diodes designed to transform the alternating voltage supplied by the network to direct voltage, [0054] A DC power supply bus connected at output, of the rectifier and including two power supply lines L1, L2, between which the direct voltage is applied. [0055] At least one bus capacitor Cbus connected between the two bus power supply lines and designed to keep the direct voltage at a constant value, [0056] A DC/AC inverter 11 connected to the DC power supply bus and comprising several power transistors, of the IGBT type for example, controlled by the processing unit so as to apply the required voltages to the output phases connected to the electrical motor. The inverter 11 is controlled for example by traditional technologies of the PWM (Pulse Width Modulation) or DTC (Direct Torque Control) type. A control law executed by the processing unit UC makes it possible to determine the voltages to apply to the output phases. [0057] Output phases a, b, c intended to be connected to the electrical motor M to be controlled.

[0058] 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.

[0059] In a known manner with reference to FIGS. 2 and 4, a conventional U/f scalar control law, executed by the processing unit in order to control an asynchronous electrical motor in open loop, includes the following Main characteristics: [0060] A speed reference that corresponds to the electrical pulsation ω.sub.s of the currents to he injected at the stator, and a flux reference φ, are applied in input.

[0061] The processing unit UC, determines, from a voltage calculator module M1, a forward voltage reference u.sub.d.sub._.sub.ref yet and a voltage reference u.sub.q.sub._.sub.ref in phase quadrature. [0062] From the two voltage components, the processing unit UC executes a module M2 for limiting voltage to a determined value U.sub.max, function of the limit voltage U.sub.lim that the variable speed drive can supply to the electrical motor M. We will have, for example:

U.sub.max=0.95×U.sub.lim [0063] At output of the limitation module M2, a limited forward voltage reference ū.sub.d and a limited voltage reference ū.sub.q in phase quadrature are obtained, which are the basis for determining the voltages to be applied at output. [0064] From these two voltages, a module M3 applies a Park's transformation to determine the voltages u.sub.a, u.sub.b, u.sub.c to be applied to each output phase, [0065] The processing unit also executes a module M4 for determining the angle of phase difference θ.sub.s to be applied among the voltages u.sub.a, u.sub.b, u.sub.c to be applied to the motor based on the pulsation ω.sub.s introduced in input.

[0066] Other modules can, of course, be implemented by the processing unit, but these will not be described in this application.

[0067] 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.

[0068] 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 ω.sub.s 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 U.sub.lim that the variable speed drive can supply. This threshold value, designated U.sub.max, is chosen lower than the limit voltage value U.sub.lim. This threshold value is preferably stored by the processing unit and, for example, is equal to:

U.sub.max=0.95×U.sub.lim

[0069] 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 φ and thus avoid a voltage limitation.

[0070] In other words, it is a question of constructing a curve profile connecting the amplitude of the flux reference at the pulsation ω.sub.s to a determined threshold voltage, for example, equal to the threshold value defined above. Such a profile is shown in FIG. 5, in normal functioning, if the flux of the electrical motor remains at an amplitude that is situated below this curve, then the electrical motor M will not reach the voltage limitation.

[0071] The demonstration that follows makes it possible to show that a variation of the flux has an effect on the motor voltage.

[0072] 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):

[00002]$\hspace{1em}\{\begin{array}{c}\frac{d}{\mathrm{dt}}.Math.\phi =-\left[T_r^{-1}+j\left({\omega }_s-{\omega }_r\right)\right].Math.\phi +R_{\mathrm{req}}.Math.i_s\\ L_f.Math.\frac{{\mathrm{di}}_s}{\mathrm{dt}}=-\left(R_s+R_{\mathrm{req}}+{\mathrm{jL}}_f.Math.{\omega }_s\right).Math.i_s+\left(T_r^{-1}-j.Math..Math.{\omega }_r\right).Math.\phi +u_s\end{array}$

[0073] Where;

[00003]$R_{\mathrm{req}}=R_r.Math.\frac{L_m^2}{L_r^2},T_r=\frac{L_r}{R_r},L_f=L_s-\frac{L_m^2}{L_r}.Math..Math.\mathrm{et}.Math..Math.\Phi =\frac{L_m}{L_r}.Math.{\Phi }_r.$

The electrical parameters are: [0074] R.sub.s: stator resistance; [0075] R.sub.f: rotor resistance; [0076] L.sub.s: stator inductance; [0077] L.sub.r: rotor inductance; [0078] L.sub.m: mutual inductance between the stator and the rotor.

[0079] The variables are: [0080] φ.sub.rl : rotor flux. [0081] i.sub.s: stator current. [0082] u.sub.s: motor voltage. [0083] ω.sub.s: electrical pulsation. [0084] ω.sub.r: mechanical speed multiplied by the number of pairs of poles.

[0085] As already explained the voltage supplied by the variable speed drive to the electrical motor cannot exceed a limit value U.sub.lim, and so the amplitude of the motor voltage is subjected to the following constraint:

|u.sub.s|<U.sub.lim

[0086] In equilibrium, the motor voltage is:

u.sub.s=(R.sub.s+jL.sub.fω.sub.s)i.sub.s+jω.sub.sφ

[0087] The amplitude of the motor voltage U.sub.m=|u.sub.s| increases with the flux module |φ| or the pulsation ω.sub.s. FIG. 3D illustrates an example of the relationship between the voltage and the flux of an asynchronous motor at constant speed. From this, it is deduced that, in order to prevent the motor voltage U.sub.m from being higher than the limit voltage U.sub.lim with a constant speed it is sufficient to reduce the flux value.

[0088] The identification phase for the flux values can be implemented according to different embodiments. FIGS. 2 and 4 illustrate two distinct embodiments. On these two figures, the function blocks that allow the implementation of the identification phase are enclosed in the greyed area.

[0089] 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 ω.sub.s take several successive values. As shown on FIG. 3A, this is, for example, a staircase profile followed by the pulsation ω.sub.s, each step of the staircase representing a distinct value taken by the pulsation ω.sub.s, during a determined period.

[0090] For each of the values thus taken by the pulsation ω.sub.s, the processing unit UC will determine the flux value |φ| for which the motor voltage U.sub.m is equal to the predefined threshold value, that is to say, equal to the value U.sub.max.

[0091] With reference to FIG. 2, according to the first embodiment, the processing unit implements the following steps: [0092] For each pulsation value ω.sub.s=ω.sub.i (where i is between 1 and N) that follows the pulsation trajectory generated by the module M5 applied in input, the processing unit UC executes a flux trajectory module M 6 making it possible to vary the flux reference between two extreme values, that is to say between a minimum flux value and a maximum flux value. The profile of flux variation between these two extreme values and for each speed reference value is shown on FIG. 3B. We therefore have:

Φ.sub.min=|Φ|<Φ.sub.max [0093] For each pulsation value ω.sub.i and for each value taken by the flux reference, the processing unit UC determines the motor voltage U.sub.m. For this purpose, it executes the aforementioned voltage calculator module M1.

[0094] The processing unit UC thus obtains a voltage variation profile as a function of the flux reference applied in input, with constant pulsation. Such a profile is shown on FIG. 3D.

[0095] From the voltage variation profile thus obtained, the processing unit UC then determines for which flux reference value, applied in input, the motor voltage U.sub.m is equal to the defined threshold value, that is to say, to the value U.sub.max. For this purpose, it is just a question of executing a module M7 in order to determine the intersection between the constant defined by U.sub.max and the voltage variation curve. FIG. 3B therefore shows, for each value taken by the pulsation ω.sub.s=ω.sub.i, the flux reference value for which the motor voltage is equal to the threshold value U.sub.max. Any suitable method of determination can, of course, be envisaged. [0096] For each speed reference value, the processing unit UC stores (M10) the flux reference value for which the motor voltage is equal to the threshold value U.sub.max.

[0097] The processing unit implements these different steps for the N values taken by the pulsation, over the whole range, such as:

ω.sub.min<ω.sub.s<ω.sub.max

[0098] With reference to FIG. 4, according to the second embodiment, the processing unit UC implements the following steps: [0099] For each pulsation value ω.sub.s=ω.sub.i (where i is between 1 and N), applied as a reference, the processing unit UC applies in input a flux reference φ fixed at a determined value (for example, arbitrarily). [0100] As a function of the value of the pulsation ω.sub.i and the value of the flux reference φ, both applied in input, the processing unit UC determines the motor voltage U.sub.m in the voltage calculator module M1. [0101] The processing unit UC implements a module M8 for comparing the value of the calculated motor voltage U.sub.m and the threshold value U.sub.max. [0102] The difference between these two voltages is injected into a regulator M9 with integral proportional action in order to correct the flux reference value applied in input. [0103] The regulating loop is implemented until the motor voltage U.sub.m is at a value equal to the threshold value U.sub.max. [0104] When, equality is reached, the processing unit UC stores (M10) the corresponding flux reference value that is applied in input.

[0105] 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:

ω.sub.min<ω.sub.s<ω.sub.max

[0106] On conclusion of the identification phase, implemented according to one or other of the embodiments described above, N torques of (ω.sub.i, |Φ.sub.i|) are obtained such as i=1,2,3 . . . N. These values constitute a curve |Φ|=f(ω.sub.s) such as that shown on FIG. 5. If the flux of the electrical motor is below this curve, then the electrical motor does not reach the voltage limitation.

[0107] At the end of the identification phase, the values obtained of (ω.sub.i, |Φ.sub.i|) where i=1,2,3, . . . N are stored in the memory of the processing unit UC. These values are intended to be used when the motor is functioning, during the operating phase, in order to adjust the flux value in real time as a function of the pulsation requested in input, in order to avoid voltage limitation.

[0108] During nor al functioning, the flux profile can be implemented in different forms: [0109] From a stored table. The N torques of (ω.sub.i, |Φ.sub.i|) can be stored in the form of a table to which the processing unit UC refers in order to verify that the torque, which includes the speed reference and the flux reference, both applied in input, is situated well below the determined profile and in order to adjust in real time the value of the flux reference to the stored value if the requested pulsation ω.sub.s exceeds a certain threshold. [0110] From a rational parametric function where the parameters of this function are estimated from points already identified (by interpolation). In view of the profile of the curve shown on FIG. 5, the interpolation is conducted for example from a hyperbola, such as defined by the following expression:

[00004]$.Math.\stackrel{\_}{\Phi }.Math.=\{\begin{array}{cc}\frac{{\Phi }_n}{\alpha \left(.Math.\frac{{\omega }_s}{{\omega }_n}.Math.-x_0\right)+1}& \mathrm{if}.Math..Math..Math.\frac{{\omega }_s}{{\omega }_n}.Math.>x_0\\ {\Phi }_n& \mathrm{if}.Math..Math..Math.\frac{{\omega }_s}{{\omega }_n}.Math.\le x_0\end{array}$

[0111] In which: [0112] Φ.sub.n is the rated flux. [0113] ω.sub.n is the rated pulsation, [0114] α and x.sub.o are constants and are interpolated from data from the curve of FIG. 5.

[0115] 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 (ω.sub.i, |Φ.sub.i|) such as i=1,2,3 . . . N for which the motor voltage is equal to the threshold value defined by U.sub.max.

[0116] The solution of the invention thus offers many advantages, listed below:

[0117] It is easy to implement as it does not require additional means.

[0118] It can be implemented for controlling an asynchronous electrical motor, regardless of the type of control applied to this motor.

[0119] It is reliable and makes it possible with certainty to avoid functioning with voltage limitation in the event of flux reduction and hence to avoid transient dynamic disturbances of the mechanical values of the motor that could result therefrom.