CONVERTER CONTROL DEVICE

Description

[0001] The present invention relates to a control device for an inverter arranged between a power supply and an electrical machine to convert electrical current, generally direct, supplied by the power supply into an alternating current usable by the machine.

[0002] Inverters include switches associated with the phase wires of the electrical machine. Their switching between opening and closing is synchronized to transform the direct current into an approximately sinusoidal current with the desired phase shift in each phase. In these processes, known as pulse-width modulation (PWM), an important parameter to consider is the frequency of the current switching pulses. Ideally, to obtain a smooth torque to the electrical machine and to maximize efficiency, the current in each phase should be as sinusoidal as possible, which is achieved by increasing the inverter's switching frequency. However, increasing the switching frequency also comes at the expense of the inverter's efficiency, and quite significantly so.Therefore, an optimal switching frequency (often defined as the overall efficiency of the system including the inverter and the electric machine) is generally sought to best reconcile these conflicting requirements. However, this optimal frequency varies depending on the operating parameters of the electric machine, and in particular its rotational speed ω and its load, which can be correlated to the current I of the supply. Methods for adjusting the inverter's switching frequency, based on one or the other of these operating parameters, have already been proposed. Examples include US patent 8456115 B2, which proposes controlling an electric motor with a variable switching frequency chosen according to the motor's operating point over two speed ranges, and US patent 9024557 B2, which proposes controlling an inverter with a variable switching frequency across the entire speed or load range of the motor.We will also mention EP 1 850 465 A2, EP 2 533 417 A1, EP 2 161 828 A1, US 2017 / 133961 A1, EP 2 237 401 A1 which propose to control the inverter with a variable switching frequency as disclosed in the preamble of claim 1.

[0003] The invention is disclosed in the attached independent claim 1. It is based on the observation that such methods are insufficient to guarantee the proper functioning of the system under all circumstances. An improved inverter control device is therefore proposed.

[0004] In general, the invention relates to a control device for an inverter belonging to a system comprising in particular an electrical machine and an electrical supply providing an electrical current to the electrical machine via the inverter, the control device being designed to apply a switching frequency to the electrical current by the inverter which varies according to operating parameters of the electrical machine in order to obtain optimal operation, characterized in that it comprises a card, indicating for each value of the parameters at least one extreme permissible frequency as a function of at least one other parameter, which is a parameter for the proper functioning of an element of the system.

[0005] The invention is therefore based on the recognition of parameters other than the operating parameters of the machine to choose the switching frequency, possibly foregoing the frequency which would ensure the optimal performance of the system, if it is found that this would cause it to go into a state of malfunction likely to damage or degrade it in any way.

[0006] The map can be established in advance and used at each system restart, or on the contrary online, that is to say during system service, at each startup of the system.

[0007] The operating parameters of the electric machine used in relation to the process improved by the invention can be the classic parameters of rotational speed of the machine and intensity of the electric supply current.

[0008] The map may have been created by any suitable method, including numerical analysis or simulations, particularly on a computer.

[0009] The maximum permissible frequency can depend on a plurality of parameters as described above, each of which becomes predominant for a respective portion of the possible values ​​of the operating parameters. The maximum frequency function can then be considered as a synthesis, or a concatenation, of the functions of each of the other parameters considered.

[0010] In many cases, the extreme frequency will be a minimum frequency. This will often be the case, in particular, with systems that typically include a capacitor input filter between the power supply and the inverter. The operating parameter could then be the RMS current reaching the capacitor or the rate of voltage change across its terminals. In other possible embodiments of the invention, the parameter could be noise emitted by the electrical machine, a torque distortion (irregularity) in the electrical machine, a total harmonic distortion (THD) of the current flowing through the electrical machine, or the absolute amplitude of this distortion.

[0011] The invention will now be described in detail by means of the following figures, which illustrate its context and represent an embodiment thereof, given for purely illustrative purposes: there figure 1 represents the system including the inverter; the figures 2a et 2b represent direct current switching processes at different frequencies; the figure 3 illustrates loss curves in the inverter; the figure 4 represents the inverter's control device; and the figure 5 illustrates curves for obtaining a limiting frequency point.

[0012] We present the figure 1 The diagram shows the classic schematic of a three-phase inverter system 1 connected to an electrical machine 2. A voltage source Ve (a battery, for example) is an electrical power supply 3 that provides a continuous supply current ie. A filtering stage 4 limits the disturbances generated by the inverter 1 and stabilizes the bus 5 carrying the supply current ie. Switches 6 on the inverter 1, controlled in a complementary manner, allow variable voltage pulses to be applied to the electrical machine 2, as illustrated in the diagrams. figures 2a et 2b On the figure 2a , the switching frequency f of the supply current ie by inverter 1 is significantly higher than on the figure 2b It is then observed that the harmonic content (distortions) of the sinusoidal current of phase a, Ia, supplied to the electrical machine 2, is much lower. However, the increase in the switching frequency f comes at the expense of the efficiency of the inverter 1: losses are indeed incurred at each switching of the switches 6. The figure 3 This illustrates the losses of inverter 1 as a function of the amplitude I0 of the phase current in the electrical machine and the switching frequency f. Conduction losses represent the irreducible portion of the losses in inverter 1. The efficiency of inverter 1 therefore decreases significantly with increasing switching frequency f. A trade-off must therefore be made between increasing the switching frequency f to limit the occurrence of detrimental phenomena such as aging of the power supply 3, excessive stress on the filtering stage 4, and system stability, and decreasing it to increase the efficiency of inverter 1. A known partial solution is the continuous adaptation of the switching frequency f during operation: a variable switching frequency f is made, for example, according to the operating conditions of the electrical machine 2, or according to the amplitude I0 of the phase current, as previously mentioned.

[0013] Ideally, the switching frequency f can be determined, for example, by minimizing an arbitrary cost. Typically, this is the overall efficiency of the system (other cost parameters can be chosen without departing from the scope of the invention).

[0014] We are then looking for a diet ω and a current I given : f ω I tel que η ω I = max η f ω I where η is the efficiency of the system including inverter 1 and electric machine 2.

[0015] The compromise considered is that increasing the switching frequency f improves the efficiency of the electrical machine 2 while decreasing it improves the efficiency of the inverter 1.

[0016] Traditional methods, however, do not guarantee satisfactory operation of the system. figure 1 as described so far. Good efficiency of the electrical machine 2 can be accompanied by unacceptable torque distortions (irregularities) or high noise in the electrical machine 2, which may indicate the onset of damage. Sudden current variations through the filter stage 4 can also damage it, and especially a capacitor 8 used to absorb oscillations from high-frequency current surges and relieve the load on the power supply 3, since the capacitor 8 can itself be damaged if it is not sized with sufficient capacitance. Other criteria for system malfunction, leading or not to possible damage, can be considered.

[0017] The inverter is equipped with a control device 9 for the switches 6, the construction of which will now be detailed by means of the figure 4 .

[0018] It comprises a main stage 10, which ensures the application of the method for finding optimal efficiency, or another method for selecting the switching frequency f in accordance with the foregoing, as a function, for example, of the intensity I of the supply current le and the rotational speed ω of the electric machine 2, which are provided to it by any sensors. It also comprises, characteristic of the invention, a downstream stage 11, which could be called a digital filtering stage, and which verifies whether the switching frequency f proposed by the main stage 10 conforms to one or more criteria for proper system operation.

[0019] The downstream stage 11 indicates at least one limit value of the switching frequency f, according to a specification defined at the outset, for any value of the operating parameters (I, ω, etc.) exploited by the main stage 10. The idea is to translate the constraints of good operation of the system by expressing them as permissible limits (most often minimum) of the switching frequency f.

[0020] For example, choosing a maximum noise level for the electrical machine, in dB, can translate into a constraint on the minimum switching frequency f. We saw earlier that the harmonic content of the currents in the electrical machine 2 has an impact on the torque and therefore on the noise. Similarly, a maximum torque distortion criterion can also translate into a minimum switching frequency limit.

[0021] Similarly, the filtering stage 4 at the input of inverter 1 is directly affected by the switching frequency f, because the input capacitor 8 smooths out current surges. Two limitations are imposed on this capacitor 8: the dissipated energy (function of ic eff< ) 2< , which is the amplitude of effective current at its terminals) which, if it is too high, will lead to its destruction; the dV / dt (rate of change of voltage) undergone by the capacitor 8, which can also lead to its destruction.

[0022] These two operating limits can also be translated into minimum switching frequencies, which will also depend on the operating point of the electric motor.

[0023] In summary, the constraints on the minimum switching frequency could be, possibly in descending order of importance: the dV / dt experienced by capacitor 8; the energy dissipated by capacitor 8; the maximum noise level of electrical machine 2; the maximum torque deformation criterion of electrical machine 2; the harmonic distortion rate of the current flowing in electrical machine; the absolute amplitude of harmonic distortion of the current flowing in electrical machine 2.

[0024] The preparation of the downstream stage 11 of the control device 9 can be carried out as follows: 1. At least one operating constraint dependent on the switching frequency f is determined, for example, based on specifications for the electrical machine 2 and / or the inverter 1 and / or the filtering stage 4; 2. This constraint is translated into an extreme switching frequency f for a set of operating points of the electrical machine (defined by the phase current I and the rotational speed). ω of the electrical machine); this set of points forms a map of the extreme switching frequency f as a function of I and ω This step can be implemented analytically or by numerical simulations for different switching frequencies f; 3. this card is registered in the control system 9 of the inverter 1.

[0025] Inverter 1 will then be controlled in the usual way, but with a variable limitation of the switching frequency f, governed by the digital card expressing an extreme frequency value for each value of the parameters I or ω, taken into account in the main process. The switching frequency f before any limitation can be obtained in the main stage 10 by any method known to those skilled in the art. The explicit calculation of the switching frequency f to be applied can then become the solution of: f ω I tel que η ω I = max η f ω I f ω I ≥ f min ω I if all the limits are minimum frequencies; a third line is added as follows: f ( ω, I ) ≤ f max ( ω, I ) if a maximum frequency f max exists.

[0026] This explicit calculation of the switching frequency f to be applied is therefore performed during system operation, while the other steps, relating to the creation of the circuit board, were carried out beforehand during a system calibration phase. This calibration can be permanent, meaning it is not necessarily repeated at each system restart, since it was completed by writing the circuit board to memory. However, it is specific to the system and may need to be redone if the electrical machine 2 or the filtering stage 4 is changed, for example. The circuit board can be created using analytical methods or simulations, among others. In other embodiments, the calibration could instead be performed in operation, after each system startup, using analytical methods, for example.

[0027] An example is given by means of the figure 5 This example concerns the criterion of the energy dissipated by capacitor 8. Depending on the capacitance of capacitor 8, a threshold current ic< eff< flowing through capacitor 8 is defined. The ordinate axis is the RMS value of the current ic< eff< and the abscissa axis, the switching frequency f. In the example, the permissible current threshold is set at 10A.

[0028] We simulate the electrical machine 2 and the inverter 1. For each simulation, we vary the switching frequency f, and we simulate the current ic eff< flowing through the capacitor.

[0029] The simulated value points can be bounded by envelopes 12 and 13. The extreme switching frequency adopted is at the intersection of the permissible current threshold and the simulation curve (here, the upper envelope 13 will be chosen). The minimum switching frequency fmin will therefore be, depending on the chosen threshold value of 10 A, approximately 7 kHz for the corresponding values ​​of the parameters I and ω. It suffices to repeat the procedure for other pairs ( I,ω ) to obtain this limit fmin(ω, I) according to this criterion relating to the energy dissipated in the capacitor 8 for all operating points.

[0030] The extreme switching frequency can be, depending on the case, a minimum or a maximum frequency. Processes incorporating both a minimum and a maximum frequency are possible. If there are several criteria for determining the minimum (or maximum) frequency, the limit that satisfies all the criteria will be retained for each value of the operating parameters I and ω; that is, the highest limiting frequency in the case of a minimum frequency. The map is then a concatenation of the limiting values ​​for each selected criterion.

Claims

1. Device (9) for controlling an inverter (1) belonging to a system comprising in particular an electric machine (2) and a power supply (3) supplying an electric current to the electric machine (2) via the inverter (1), the control device (9) being designed to apply a switching frequency (f) to the electric current through the inverter (1) which varies as a function of operating parameters (I, ω) of the electric machine (2), in order to achieve optimum operation by minimizing a cost of the overall efficiency of the system such that the switching frequency (f) satisfies the following equation: η(ω, I) = max(η(f, ω, I)), where η is the overall efficiency of the system and I, ω are operating parameters of the electric machine, characterized in that it comprises a chart (11) indicating for each value of the parameters (I, ω) at least one permissible extreme frequency as a function of at least one other parameter which is a parameter of proper operation of an element of the system, and in that said other parameters supplied by the chart are parameters other than the operating parameters of the electric machine and which are chosen from among a voltage change rate dV dt across the terminals of a capacitor (8) of a capacitor-type (8) input filter (4) between the power supply (3) and the inverter (1), the energy dissipated by a capacitor-type (8) input filter (4) between the power supply (3) and the inverter (1), an RMS current intensity (iceff) flowing through the capacitor (8) of a capacitor-type (8) input filter (4) between the power supply (3) and the inverter (1), a maximum noise level of the electric machine, a maximum torque deformation of the electric machine, a harmonic distortion rate of the current flowing through the electric machine (2), a total harmonic distortion of the current flowing through the electric machine, an absolute amplitude of harmonic distortion of the electric current flowing through the electric machine.

2. Device for controlling an inverter according to Claim 1, characterized in that said chart is obtained beforehand.

3. Device for controlling an inverter according to Claim 1, characterized in that said chart is obtained during operation after the system has been started up.

4. Device for controlling an inverter according to Claim 1, 2 or 3, characterized in that the operating parameters of the electric machine are a rotational speed (ω) of the electric machine (2) and an intensity (I) of the electric current.

5. Device for controlling an inverter according to any one of Claims 1 to 4, characterized in that the chart has been established through numerical analyses and / or simulations.

6. Device for controlling an inverter according to any one of Claims 1 to 5, characterized in that the permissible extreme frequency is a function of a plurality of said other parameters, each of which is predominant for a respective part of possible values of the operating parameters.

7. Device for controlling an inverter according to any one of Claims 1 to 6, characterized in that the permissible extreme frequency is a minimum frequency (fmin).

8. Device for controlling an inverter according to any one of Claims 1 to 7, characterized in that the system comprises a capacitor-type (8) input filter (4) between the power supply (3) and the inverter (1).