Power unit of variable speed drive based on rectified voltage control

Abstract

The invention relates to a method for controlling a variable speed drive, the variable speed drive comprising, for each phase of a plurality of phases, Ni low voltage power cells connected in series, i being the phase index. The method comprises repeating P iterations, comprising: activating (201) at least one cell of one or more phases and deactivating other power cells of the variable speed drive, wherein at least one activated cell is selected based on a predefined activation control depending on the iteration index; receiving (205) at least one output voltage of the variable speed drive for at least one phase at the terminals of the electrical device; furthermore, the method comprises, at the end of the P iterations, determining (206) rectified voltage values ​​at the output terminals of the respective rectifier stages of the power cells of the variable speed drive based on the measured output voltages of the variable speed drive, and storing the obtained rectified voltages, which are respectively associated with the power cells of the variable speed drive.

Description

Technical Field

[0001] The present invention relates to the management of variable speed drives responsible for supplying power to electrical equipment, such as electric motors. Background Art

[0002] According to one power topology, a variable speed drive provides a high voltage by connecting a certain number of low voltage converters in series (these converters are called power cells). The control of these power cells allows the provision of multiple voltage levels or multi-level voltages, each power cell adding a voltage, allowing a continuous voltage level to be achieved.

[0003] According to European standards, low voltage is understood to mean voltages between 0 and 1000 V for AC and between 0 and 1500 V for DC. High voltage is understood to mean voltages above 1000 V for AC and above 1500 V for DC.

[0004] For example, the variable speed drive may include N power cells, N being greater than or equal to 2. When the variable speed drive provides three-phase power, it may include 3*N power cells, N power cells being dedicated to each of the three phases.

[0005] In multi-stage variable speed drive topologies, only motor voltage measurements are available to reduce product costs, rather than a single rectified voltage or DC bus voltage DC. These rectified voltages correspond to the voltages at the output of the rectifier bridges (mainly diode bridges or thyristor bridges) at the input of each power unit of the variable speed drive.

[0006] In the above multi-level architecture, the output voltage of each power cell is generated by applying a duty cycle to the rectified voltage of the power cell. Therefore, the rectified voltage or DC bus voltage of each power cell is necessary in order to calculate the duty cycle to be applied to achieve the target output voltage on the motor.

[0007] According to the prior art solution, a fixed rectified voltage common to all power units is considered, which is based on the theoretical value of the DC bus in an ideal situation.

[0008] However, this solution leads to errors in the output voltage applied to the motor, because in practice the rectified voltages of the power cells differ from each other. This is due to various disturbances that exist and affect the level of the rectified voltage of each power cell (the voltage at the input of the variable speed drive, transformer, etc.).

[0009] This error can be compensated by motor voltage measurement, but this requires an accurate motor voltage sensor so that the corrector can constrain the voltage generated at the output of the variable speed drive. Therefore, the voltage reference can be composed of two terms: the direct term and the correction derived from the corrector.

[0010] Using the knowledge of the rectified voltage, the larger the error magnitude in the direct term, the more the corrector needs to do to suppress that error. Therefore, the voltage generation will depend on the corrector dynamics and affect the overall control performance.

[0011] Furthermore, additional sensors may be provided to provide diagnostics for the power unit. For example, a temperature probe may be provided, for example, on a transformer upstream of the variable speed drive. However, no diagnostics are provided for the input stage of the variable speed drive.

[0012] Therefore, there is a need to improve the control of the output voltage delivered by the variable speed drive to the motor by suppressing various disturbances of the power conversion system (variable gain of the transformer, downtime of the switching action of the switches of the power unit).

[0013] The present invention overcomes the above-mentioned disadvantages. Summary of the invention

[0014] A first aspect of the present invention relates to a method for determining a rectified voltage of a power cell of a variable speed drive responsible for supplying power to an electrical device, the variable speed drive comprising Ni low voltage power cells connected in series for each phase in a plurality of phases, N being greater than or equal to 2, i being a phase index. The method comprises the following operations:

[0015] Repeat the following for P iterations, where P is a predefined integer greater than or equal to 2:

[0016] - activating at least one unit of one or more phases and deactivating other power units of the variable speed drive, wherein the at least one activated unit is selected based on a predefined activation control depending on an iteration index;

[0017] - receiving at least one output voltage of the variable speed drive for at least one phase at terminals of the electrical device;

[0018] The method further comprises, at the end of P iterations:

[0019] Determining the rectified voltage value of the output end of each rectifier stage of the power unit of the variable speed drive according to the measured output voltage of the variable speed drive;

[0020] The determined rectified voltage values ​​are stored, which are respectively associated with a power unit of the variable speed drive.

[0021] The term "motor voltage" is used hereinafter to denote the voltage at the terminals of an electrical device powered by a variable speed drive.

[0022] According to one embodiment, the integer P and the activation control may be predefined in the form of a matrix comprising P rows or P columns per phase, representing the activation control, and being equal in rank to the number of power cells of the variable speed drive.

[0023] Therefore, all rectified voltage values ​​can be accessed by minimizing the number of measurements.

[0024] Furthermore, for each of the three phases, the variable speed drive comprises N power cells, and for each iteration of index t, t being between 1 and P:

[0025] m_t = KALL_t × VBALL;

[0026] Wherein, KALL_t represents a matrix having 3 rows and 3*N columns, including [K1-K2 K0; K0 K2-K3; -K1K0 K3], where K1, K2 and K3 are activation controls of the three phases respectively, and are vectors of size N using binary values, the first value corresponds to activation, and the second value corresponds to deactivation, where K0 is a zero vector of size N,

[0027] Wherein, VBALL is a column vector of length 3*N composed of the following [VB1; VB2; VB3], where VB1, VB2 and VB3 are vectors of magnitude N of the rectified voltages of the power cells of the three corresponding phases;

[0028] Wherein M_t is a vector of three voltages measured at the terminals of the electrical device in a given iteration, the three measured voltages corresponding to the three output voltages of the variable speed drive (the three measured voltages can be determined from the three output voltages of the variable speed drive).

[0029] According to one embodiment, P is equal to the number of power cells of the variable speed drive, and in each iteration a single power cell may be activated, the power cells activated for two different iterations being separate.

[0030] Such an embodiment allows to easily obtain each of the rectified voltages of the power cells.

[0031] Therefore, the method according to the invention allows obtaining the rectified voltage of the power unit without adding additional sensors.

[0032] According to one embodiment, the method may further include, during a current phase of powering the electrical device, for each power cell, adjusting a command for controlling the power cell based on a rectified voltage associated with the power cell.

[0033] Thus, this embodiment allows the generation of a target motor voltage requiring little or even no correction. Thus, better dynamics are allowed in the generation of the target motor voltage.

[0034] Furthermore, adjusting the command for controlling the power cell includes determining a duty cycle of the power cell based on a rectified voltage associated with the power cell.

[0035] According to one embodiment, the method may further comprise analyzing the obtained rectified voltage with the aim of detecting deviations from nominal operation.

[0036] Therefore, failures in the power stage can be anticipated and preventive maintenance can be performed.

[0037] According to one embodiment, the method may be repeated at a plurality of separate moments in time, and the analysis of the rectified voltage may include determining a variation trend of the rectified voltage of the power unit and comparing the trend with nominal operation.

[0038] Therefore, failures in the power stage can be anticipated and preventive maintenance can be performed.

[0039] According to one embodiment, the method may be initiated after a stoppage of the electrical device.

[0040] In this way, the method does not interrupt the operation of the electrical equipment.

[0041] Furthermore, the method may be automatically initiated upon detection of a stoppage of the electrical device.

[0042] Therefore, no operator intervention is required. In addition, the method can be automatically started at a given frequency when the electrical equipment stops.

[0043] Alternatively, the method may be initiated manually after stopping of the electrical equipment.

[0044] Thus, the operator can perform a check on the operation of the power unit.

[0045] A second aspect of the present invention relates to a computer program executable by a processor, comprising instructions for carrying out the steps of the method according to the first aspect of the present invention when the instructions are executed by the processor.

[0046] A third aspect of the present invention relates to a device for controlling a variable speed drive responsible for providing three-phase power to an electrical device, the variable speed drive comprising Ni low voltage power cells connected in series for each phase in a plurality of phases, N being greater than or equal to 2, i being a phase index. The control device comprises:

[0047] The processor is capable of controlling, through an output interface, the repetition of the following P iterations, where P is a predefined integer greater than or equal to 2:

[0048] activating at least one unit of one or more phases and deactivating other power units of the variable speed drive, wherein the at least one activated unit is selected based on a predefined activation control depending on an iteration index;

[0049] receiving at least two output voltages of the variable speed drive for two phases at terminals of the electrical device via an input interface;

[0050] In addition, the processor can determine the rectified voltage value at the output end of each rectifier stage of the power unit of the variable speed drive according to the measured output voltage of the variable speed drive at the end of P iterations,

[0051] The memory stores the determined rectified voltage values, which are respectively associated with the power units of the variable speed drive.

[0052] A fourth aspect of the present invention relates to a variable speed drive responsible for providing three-phase power to electrical equipment, the variable speed drive comprising N low-voltage power units connected in series for each of the three phases, N being greater than or equal to 2, and the variable speed drive comprising a control device according to the third aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] By way of example only, embodiments of the present invention will be described with reference to the accompanying drawings, in which:

[0054] Figure 1 A system for controlling power supply to an electrical device according to an embodiment of the present invention is shown;

[0055] Figure 2 is a diagram showing method steps according to one embodiment of the present invention;

[0056] Figure 3 The structure of a control device according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0057] Figure 1 A system for supplying power to an electrical device powered by a variable three-phase power supply, such as an electric motor 100 (e.g., an induction motor), is shown. Such a motor is given as an illustration, but the invention should not be limited to this single example, and the invention can be applied to any electrical device powered by a variable speed drive including a plurality of power cells.

[0058] The variable speed drive includes a transformer 111 that receives a variable three-phase power supply from a power source (mains) 110. The transformer 111 may be a multi-winding transformer capable of delivering a three-phase voltage to a plurality of power cells, as described below.

[0059] The variable speed drive 102 according to the present invention may include a power stage including one or more low voltage power cells 101. Figure 1 In the example shown, the electric motor receives a three-phase power supply, and the variable speed drive 102 includes 3*N power cells, where N power cells are dedicated to each phase, and N is greater than or equal to 2.

[0060] refer to Figure 1 , a system with 3*N power cells is shown. However, the present invention is equally applicable to a system with 3*(N+1) power cells, with one power cell being retained per phase in case one of the active power cells fails. Such an example is given for illustrative purposes only. Consider the example of a three-phase power supply, but the present invention is also applicable to a system with N or N+1 power cells. The present invention is also applicable to a system with N+2 (or more than N+2) power cells (or 3*(N+2) power cells for a three-phase power supply), wherein there are two replacement power cells per phase. In addition, the present invention can be applied to situations other than three. In addition, the number of power cells may be different per phase, in which case each phase includes a number Ni of power cells, i being the phase index. By way of illustration only, the following description considers the case of a three-phase power supply, wherein each phase has the same number of N power cells.

[0061] Each power unit 101 receives a three-phase power supply from the secondary of the transformer 111 at its input end, and may include a rectifier ( Figure 1 The rectifier is capable of rectifying the received three-phase power supply to provide a DC voltage. The DC rectified voltage obtained for each power unit 101 is also referred to as a DC bus voltage, DC or bus voltage. The rectifier may include a diode bridge, a thyristor bridge or any other known system for rectifying a voltage.

[0062] At the output of the rectifier, each power cell 101 may include a capacitor capable of storing electrical energy and a module for generating a pulse width modulation PWM (pulse width modulation) signal. This generation module may include an H-bridge, including four switches controlled in pairs. An electronic power system using this chopping voltage principle applies a voltage proportional to one or more rectified voltages to the motor 100 per phase. On average, the applied ratio corresponds to the ratio between the target voltage at the output of the power cell and the rectified reference voltage (defined below). The operation of the H-bridge is well known and will not be further described in this application.

[0063] The switches of the H-bridge may be IGBT (Insulated Gate Bipolar Transistor) type transistors, which have the advantage of being able to switch quickly.

[0064] The switches of the power unit 101 are controlled by a control unit 103 of the power unit 101 .

[0065] The system further comprises a control device 120 capable of controlling the operation of the power cell 101 of the variable speed drive 102 in order to control the power supply to the motor 100. To this end, the control device 120 can control the control unit 103 of the power cell 101. Furthermore, the control device 120 can control switches 104, allowing a subset of the N power cells for each phase to be connected in series. As a variant, these switches are controlled by the control device 120 via the control unit 103.

[0066] The power unit 101 may receive a control signal from the control device 120 , based on which the power unit 101 may control the switching action of the switches of the H-bridge.

[0067] Therefore, the three-phase voltage or motor voltage supplied to the motor 100 is the PWM output voltage (at Figure 1 V cell ) is obtained by summing. Switch 104 allows the power cell 101 to be "bypassed" without controlling the switches of the H-bridge of the power cell. However, such switch 104 is optional, because the power cell 101 can be deactivated by controlling the H-bridge of the power cell 101, so that the power cell 101 has a zero duty cycle and therefore has a zero output voltage from the power cell.

[0068] The system according to the invention further comprises a measuring device 130 for measuring the three-phase voltage supplied to the motor. The measuring device 130 is capable of transmitting the measured motor voltage(s) to the control device 120.

[0069] The control device 120 according to the invention is configured to control the various power cells 101 and the bypass switches directly or indirectly (directly or via the control unit 103). In particular, according to the invention, the control device 120 is capable of controlling the power cells 101 or the bypass switches 104 so that some cells do not provide voltage, so that for example the motor voltage is only provided by one power cell. Thus, in an embodiment where a single power cell is activated at a time, the measured motor voltage allows the output voltage of the selected power cell to be acquired, as explained in detail later, and therefore allows the DC bus voltage to be acquired based on the duty cycle applied to the power cell.

[0070] Figure 2 is a diagram illustrating method steps according to one embodiment of the present invention.

[0071] In step 200, the method according to the present invention is implemented. This implementation may be caused by the stopping of the motor 100. For example, after the motor 100 stops, the method is automatically or manually triggered via the human-machine interface of the control device 120.

[0072] In step 201, the control device 120 controls the switches 104 or H-bridges of the various cells in order to activate a subset of the power cells of the variable speed drive 102, for example, only the first cell 101 of the first phase. 1,1 The operation of an embodiment in which multiple power cells are activated simultaneously will be described later. Figure 2 It is described within the framework of an embodiment based on sequential activation of power cells (activating a single power cell per iteration).

[0073] Meanwhile, in step 202, the control device 120 determines a voltage value to be applied to the variable speed drive 102. The voltage to be applied may be direct, or calculated by a current regulator, for example, allowing the output current of the variable speed drive flowing through the motor 100 to be controlled.

[0074] In step 203, the control device 120 determines the power to be applied to the activated power unit 101 1,1 The duty cycle of the bypass switch 104 is set to generate a target output voltage V0 to be applied to the motor 100. For other inactive units, the bypass switch 104 is closed or the duty cycle is zero.

[0075] In step 204 , the variable speed drive 102 is powered and the power unit 101 is controlled according to steps 202 and 203 .

[0076] Then, in step 205, the motor voltage is measured, for example, by a sensor, and then the motor voltage is received by the control device 120. The motor voltage is received by the first power unit 101. 1,1 supply.

[0077] In step 206, the control device 120 determines whether all power units 101 have been activated successively. If this is the case, the method moves to step 207. More generally, the number of iterations P is predefined, P being greater than or equal to 2. Thus, in step 207, the iteration index can be compared with P. If the iteration index is equal to P, the method moves to step 207.

[0078] Otherwise, by activating the next power unit (e.g., power unit 101 1,1 After power unit 101 1,2), or in the general case to be described later, a subsequent plurality of power units are activated, and the method returns to step 201 by deactivating all other power units 101. In an alternative embodiment, the method returns to step 201, but also to step 202. In this alternative embodiment, steps 202 and 203 are implemented in a different way for each iteration of the method.

[0079] Based on the motor voltage measured in the subsequent step 205 , in a step 206 the control device 120 according to the invention derives the rectified voltage of the power unit.

[0080] To determine the rectified voltage of the power unit, control device 120 takes into account the measured motor voltage and the already applied pulse duty factor.

[0081] It is noteworthy that in an embodiment where the method is iterated 3*N times by activating a single power cell each time (when the variable speed drive includes N power cells for each of the three phases), the rectified voltage of the power cell activated during a given iteration can be determined at the end of the given iteration (rather than at the end of all iterations).

[0082] At the end of the determination of the rectified voltage of the power cell 101 of the variable speed drive 102 , the method moves to step 208 .

[0083] In step 208, the rectified voltage of the power cell of the variable speed drive is stored in the memory of the control device 120. The rectified voltage can be stored by replacing the previous value of the rectified voltage. As a variant, the rectified voltage is stored in association with the date of implementation of the iteration of the method according to the invention, allowing the variation of the rectified voltage of each power cell 101 of the variable speed drive 102 to be tracked.

[0084] In optional step 209, an analysis of the rectified voltage is performed. This analysis may include a comparison with the nominal rectified voltage and triggering a warning in the event of a deviation from one of the nominal rectified voltages. If the rectified voltages are stored in association with the respective dates, a trend of the rectified voltage can be determined for each power cell 101 and a warning can be generated based on this trend.

[0085] The generation of warnings allows preventing the implementation of maintenance.

[0086] Independently of steps 208 and 209 , the variable speed drive changes to an “activated” or “ready” mode in step 210 , allowing the electric motor 100 to start in step 211 .

[0087] After steps 209 and 210, in step 211, the control device 120, upon receiving the motor control, determines the target motor voltage and controls the variable speed drive 102 based on the target motor voltage and based on the previously determined rectified voltage. To this end, the control device 120 determines a corresponding duty cycle for the power unit 101. The generation of the target motor voltage is then more accurate, since the rectified voltage of the power unit is known and can be updated regularly (e.g., whenever the motor 100 is stopped).

[0088] According to some embodiments, such a variable speed drive also comprises a corrector which corrects the control of the input to the control device 120 based on the voltage measured on the motor 100. In this case, according to the invention, the role of such a corrector is reduced, allowing the dynamics of the generation of the output voltage applied to the motor 100 to be improved.

[0089] The method can then be repeated at a later time, for example after electric motor 100 has been stopped again in step 200 .

[0090] Alternatively, the method can be repeated at a given frequency, for example weekly, after the motor has stopped. In this case, a minimum interval of one week separates the execution of two iterations of the method.

[0091] refer to Figure 2 , a specific embodiment involving activating a single power cell in each iteration has been described. Such an embodiment is easy to implement, and once the motor voltage is measured, the rectified voltage of each power cell is easily obtained. More generally, the present invention may activate at least a subset of power cells in each iteration, step 201, provide one or more output voltages produced by the subset in step 204, and measure one or two motor voltages in step 205. This is because there is no need to measure more than two motor voltages, because the third motor voltage can be derived from the first and second motor voltages (see voltages U12, U23 and U31 described below).

[0092] The power cells to be activated in each iteration and the motor voltages to be measured in each iteration can be determined from a predefined matrix. Such a matrix is ​​predefined in order to ensure that at the end of all iterations, a sufficient number of equations have been obtained to determine the value of each unknown quantity of the system of equations (for example, 3*N rectified voltages of the power cells when the variable speed drive comprises 3*N power cells). To this end, the rank of the predefined matrix is ​​equal to the number of unknown quantities (or 3*N).

[0093] If i is the phase index (varying from 1 to 3) and k is the power cell index within the phase (varying from 1 to N in a power stage with 3*N power cells), then the power cell with phase i and index k:

[0094] Activated with a duty cycle r (between -1 and 1);

[0095] There is a rectified voltage VB(i,k), which constitutes the unknown quantity of the equation system;

[0096] Provide potential difference rx VB(i,k)

[0097] A column vector VBi having N components may be defined, each component having an index k in the value of the rectified voltage VB(i,k).

[0098] The combination of controls for activating the cells of the branch dedicated to the power stage of phase i can be represented by a line vector Ki with N values ​​(0: inactive power cell, 1: active power cell).

[0099] The potential difference Vi generated by the branch with index i is equal to the sum of the potential differences of all cells in the branch, or: Vi = Kix VBi

[0100] The above-mentioned measuring device 130 is capable of providing three-phase voltages between U12 = V1-V2, U23 = V2-V3 and U31 = V3-V1. As a variant, the measuring device 130 is designed to directly measure the voltages V1, V2 and V3.

[0101] Corresponding to an iteration of the method (t can be a date or an iteration index of the method), in particular the iteration of step 205, the three measurements at time t can be associated in a column vector M_t equal to [U12; U23; U31], or: M_t = KALL_t × VBALL

[0102] where K0 is a zero vector of the same dimension as K1, K2, or K3,

[0103] And KALL_t represents a matrix with 3 rows and 3*N columns, which is composed as follows: [K1-K2 K0; K0 K2-K3; -K1K0 K3];

[0104] VBALL represents a column vector of length 3N, which is composed as follows: [VB1; VB2; VB3].

[0105] According to the invention, combinations K1 to K3 are selected by “stacking” or aggregating matrices KALL_t for times (iterations) t1, t2 to tP. u (u varies between 1 and P), so that the matrix KALL_t ALL is structured into 3*P rows and 3*N columns, KALL_t ALL It is a matrix of rank 3*N, so that the 3*N unknowns of the equation system (rectified voltage of the power unit) can be determined.

[0106] refer to Figure 2 The described embodiments are specific examples of this general embodiment.

[0107] This particular embodiment at each time t u Activates a single unit, where u is an index between 1 and 3*N.

[0108] When a cell is activated with an index “u-INT[(u-1) / N]*N” of a branch of a power stage “1+INT[(u-1) / N]”, where INT denotes a function of the integer part, the following result is obtained in the case where N=3 (given as an example, but this does not limit the scope of the invention):

[0109] At time t1, K1 = [1 0 0]; K2 = [0 0 0]; K3 = [0 0 0]. The matrix KALL_t1 is [1 0 0 0 0 0 0 0 0; 0 0 0 0 0 0 0 0 0; -1 0 0 0 0 0 0 0 0];

[0110] At time t2, K1 = [0 1 0]; K2 = [0 0 0]; K3 = [0 0 0]. The matrix KALL_t2 is [0 1 0 0 00 0 0 0; 0 0 0 0 0 0 0 0; 0-1 0 0 0 0 0 0 0].

[0111] By constructing each matrix KALL_tu, the first row consists of 0 except for the uth element (u is 1). It should be understood that according to the above rules, after 3×3 times, the first 9 rows are combined to form the identity matrix, which guarantees a rank of 9 (or the number of unknowns in the system).

[0112] Figure 3 The structure of the control device 120 according to one embodiment of the present invention is shown.

[0113] The control device 120 includes a processor 300, which is configured to communicate unidirectionally or bidirectionally with a memory 301 via one or more buses. The memory 301 is a memory such as a "random access memory" RAM type memory, or a "read only memory" ROM type memory, or any other type of memory (flash memory, EEPROM (erasable programmable read-only memory) etc.).

[0114] The memory 301 can permanently or temporarily store at least some of the data used and / or derived from the embodiments of the method according to the invention. In particular, the memory 301 can store rectified voltage values ​​of the power unit, optionally associated with corresponding dates.

[0115] The processor 300 is capable of executing steps for implementing the method according to the present invention (such as Figure 2 ) instructions.

[0116] The control device 120 may further comprise an input interface 302 and an output interface 303 in order to communicate with other entities of the system according to the invention.

[0117] In particular, input interface 302 can receive a speed command, and processor 300 can determine target motor voltages (one target motor voltage per phase) based on the speed command.

[0118] The output interface 303 is capable of providing control commands to the power cells 101 so that they generate a duty cycle, in particular via their H-bridges. To this end, the control commands may include commands for switching IGBT-type switches, such as partially (for each power cell, a command to switch only two IGBT switches from the top) or completely (four IGBT switches) switching the H-bridge. These switching commands can be calculated by the control device 120 by comparing a reference voltage with a triangular wave signal corresponding to each power cell. This technique is well known and will not be described in detail.

[0119] Although the invention has been described above with reference to specific embodiments, it is by no means limited to the forms described. The invention is limited only by what is defined in the claims and embodiments other than the above described may also be included within the scope of the claims.

[0120] Furthermore, although the embodiments have been described above as combinations of components and / or functions, it should be understood that alternative embodiments may be obtained by other combinations of components and / or functions without departing from the scope of the present invention.

Claims

1. A method for determining a rectified voltage of a power cell of a variable speed drive (102) responsible for supplying power to an electrical device (100), the variable speed drive comprising, for each phase of a plurality of phases, Ni low voltage power cells (101) connected in series, N being greater than or equal to 2, i being a phase index, characterized in that The method comprises the following operations: a) Repeat the following for P iterations, where P is a predefined integer greater than or equal to 2: - activating (201) at least one unit of one or more phases and deactivating other power units of the variable speed drive, wherein at least one activated unit is selected based on a predefined activation control depending on an iteration index; - receiving (205) at least one output voltage of the variable speed drive (102) for at least one phase at terminals of the electrical device (100); The method further comprises, at the end of P iterations: b) determining (206) a rectified voltage value at an output end of each rectifier stage of a power unit of the variable speed drive based on the measured output voltage of the variable speed drive; c) Storing the determined rectified voltage values, which are respectively associated with a power unit of the variable speed drive.

2. The method according to claim 1, wherein: The integer P and the activation control are predefined in the form of a matrix comprising P rows or P columns per phase, representing the activation control, and being equal in rank to the number of power cells of the variable speed drive.

3. The method according to claim 2, wherein: The variable speed drive comprises N power cells for each of the three phases, and for each iteration of index t, t being between 1 and P: M_t = KALL_t x VBALL; Wherein, KALL_t represents a matrix with 3 rows and 3*N columns including [K1-K2 K0; K0 K2-K3; -K1 K0 K3], wherein K1, K2 and K3 are activation controls of the three phases respectively and are vectors of size N using binary values, the first value corresponds to activation and the second value corresponds to deactivation, wherein K0 is a zero vector of size N, Wherein, VBALL is a column vector of length 3N consisting of the following [VB1; VB2; VB3], where VB1, VB2 and VB3 are vectors of magnitude N of the rectified voltages of the power cells of the three corresponding phases; Wherein, M_t is the vector of three voltages measured at the terminals of the electrical device in a given iteration, the measured motor voltages corresponding to the three output voltages of the variable speed drive.

4. A method according to any one of the preceding claims, wherein: P is equal to the number of power cells of the variable speed drive, and wherein a single power cell is activated in each iteration, the power cells activated for two different iterations being separate.

5. The method according to any one of claims 1 to 3, further comprising, during a current phase of powering the electrical device (100), for each power cell, adjusting a command for controlling the power cell based on a rectified voltage associated therewith.

6. The method according to claim 5, wherein: Adjusting a command for controlling the power cell (101) includes determining a duty cycle of the power cell (101) based on the rectified voltage associated with the power cell.

7. The method according to any one of claims 1 to 3, further comprising analyzing (209) the obtained rectified voltage in order to detect deviations from nominal operation.

8. The method according to any one of claims 1 to 3, wherein: The method is repeated at a plurality of separate moments in time, and wherein the analysis (209) of the rectified voltage comprises determining a variation trend of the rectified voltage of the power unit and comparing the trend with nominal operation.

9. The method according to any one of claims 1 to 3, which is started after stopping of the electrical device (100).

10. The method according to claim 9, wherein: The method is automatically initiated upon detection of a stop of the electrical device (100).

11. The method according to claim 9, wherein: The method is manually initiated after stopping of the electrical device (100).

12. A computer program executable by a processor (300), comprising instructions for implementing the steps of the method according to one of claims 1 to 11 when the instructions are executed by the processor.

13. A device for controlling a variable speed drive (102) responsible for supplying power to an electrical device (100), the variable speed drive comprising, for each phase of a plurality of phases, Ni voltage power units (101) connected in series, N being greater than or equal to 2, i being a phase index, characterized in that The device comprises: A processor (300) capable of controlling, through an output interface, the repetition of the following P iterations, where P is a predefined integer greater than or equal to 2: activating (201) at least one unit of one or more phases and deactivating other power units of the variable speed drive, wherein at least one activated unit is selected based on a predefined activation control depending on an iteration index; receiving at least two output voltages of the variable speed drive (102) for two phases at terminals of the electrical device (100) via an input interface (302); The processor (300) is also capable of determining the rectified voltage value at the output end of each rectifier stage of the power unit of the variable speed drive according to the measured output voltage of the variable speed drive at the end of P iterations, The memory (301) stores the determined rectified voltage values, which are respectively associated with the power units of the variable speed drive.

14. A variable speed drive for providing three-phase power to an electrical device, the variable speed drive comprising Ni low voltage power cells connected in series for each phase of a plurality of phases, wherein: N is greater than or equal to 2, i is a phase index, and the variable speed drive comprises the apparatus of claim 13.

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