Converter and method for driving an electric machine

Abstract

The present disclosure relates to a converter (101) for driving an electric machine whose stator windings are changeable to a low-speed configuration or a high-speed configuration with fewer series-connected turns than the low-speed configuration. The converter includes a converter stage (102) that supplies a stator voltage to the stator windings and a control system (103) that controls the stator windings to be in either the low-speed or high-speed configuration. The control system disables the converter stage during changes between the low-speed and high-speed configurations and limits the torque of the electric machine so that the torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration. Changing the number of series-connected turns in the stator windings changes the torque limit, thereby reducing unwanted transient currents.

Description

[Technical Field]

[0001] The present disclosure relates generally to converters for driving electric machines. More particularly, the present disclosure relates to converters for driving electric machines in which the stator windings are modifiable to have a low-speed configuration or a high-speed configuration with fewer series-connected turns than the low-speed configuration. Furthermore, the present disclosure relates to methods and computer programs for driving such electric machines. [Background technology]

[0002] In conventional designs of permanent magnet machines, the nominal rotational speed of the permanent magnet machine cannot be safely exceeded by more than approximately 40%. For example, if the nominal rotational speed is 2000 revolutions per minute (rpm), this means that the maximum recommended speed is approximately 2800 rpm. This limit is due to the back electromotive force (EMF) of the permanent magnet machine, which increases linearly with the rotational speed of the permanent magnet machine. During fault conditions, the alternating current (AC) system (e.g., a converter) connected to the permanent magnet machine cannot necessarily suppress the back EMF of the permanent magnet machine. Unsuppressed back EMF during overspeed and fault conditions such as those described above can cause overvoltage conditions that can damage the permanent magnet machine and / or the AC system connected to the permanent magnet machine.

[0003] The above technical problem is often solved by selecting a permanent magnet machine with a higher nominal rotational speed than otherwise required. However, this workaround limits the maximum peak torque achievable at a given peak current. Therefore, the AC system (e.g., converter) connected to the permanent magnet machine must be designed for a higher peak current. This further reduces the achievable steady-state torque achievable at a given steady-state current, and therefore the AC system must be further designed for a higher steady-state current.

[0004] Variable speed drives implemented with induction machines have their own challenges regarding the selection of the nominal rotational speed of the induction machine. The nominal rotational speed is the speed that can be achieved at the nominal stator voltage without field weakening, i.e., without reducing the breakdown torque of the induction machine. The higher this nominal rotational speed, the higher the stator current required to generate the required torque at a given magnetic flux (e.g., nominal magnetic flux) of the induction machine.

[0005] EP 3723276 describes an electric machine including a multi-phase winding in which the number of series-connected turns of the multi-phase winding can be changed using configuration switches connected to the multi-phase winding sections. The configuration switches can then be used to change the nominal rotational speed of the electric machine. However, converters (e.g., frequency converters) that drive such electric machines present challenges, especially when the winding configuration needs to be changed while the electric machine is operating. One challenge relates to the need to avoid very high transient currents during the winding configuration change. Summary of the Invention [Means for solving the problem]

[0006] The following presents a simplified summary in order to provide a basic understanding of some aspects of various embodiments. The summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention, nor is it intended to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of example embodiments.

[0007] According to the present invention, a new converter for driving an electric machine is provided in which the stator windings can be changed to have a low speed configuration or a high speed configuration with fewer turns connected in series than the low speed configuration. a converter stage configured to supply a stator voltage to the stator winding; a control system configured to control the stator windings to be in a low speed configuration or a high speed configuration and to control the converter stages to provide a stator voltage; Includes.

[0008] The control system is configured to limit the torque of the electric machine so that the torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration. The control system is configured to deactivate converter stages during the change between the low-speed and high-speed configurations and to execute the change. It should be noted that torque limiting occurs only when necessary, i.e., when the actual torque is so high that it is limited by the torque limit. Thus, when the actual torque is less than the torque limit, the torque limit is inactive. Increasing or decreasing the number of series-connected turns in the stator windings increases or decreases the torque limit, thereby reducing transient currents during the change in winding configuration.

[0009] Advantageously, the configuration of the stator windings can be changed relatively quickly while the electric machine is operating, thereby resembling a gear change in a mechanical gearbox. In an advantageous embodiment, the stator windings are arranged so that they can be configured to provide a low-speed, high-torque configuration and a high-speed, low-torque configuration, i.e., achieving characteristics similar to a two-speed mechanical gearbox. For example, in low-speed traction operations or when starting from standstill, the electric machine can be operated in the low-speed configuration. When the speed increases, for example, to a speed value requiring field weakening, the high-speed configuration can be switched on, and the electric machine can be driven to a higher speed without field weakening. The nominal speed of the high-speed configuration can be, for example, two or three times the nominal speed of the low-speed configuration. Advantageously, the change in winding configuration can take, for example, 100 milliseconds or even less.

[0010] The present invention further provides a new method for driving an electric machine in which the stator windings can be changed to a low speed configuration or a high speed configuration with fewer turns connected in series than the low speed configuration. - controlling the stator windings to be in a low speed configuration or a high speed configuration; - controlling a converter stage to supply a stator voltage to the stator winding; - limiting the torque of the electric machine such that the torque limit is higher when the stator windings are in a low speed configuration than when the stator windings are in a high speed configuration; - deactivating a converter stage during a change between a slow and a fast configuration and performing the change; Includes.

[0011] According to the present invention there is further provided a novel computer program for driving an electric machine in which the stator windings are modifiable to have a low speed configuration or a high speed configuration with fewer turns connected in series than the low speed configuration. controlling the stator windings to be in a low speed configuration or a high speed configuration; controlling a converter stage to supply a stator voltage to the stator winding; limiting the torque of the electric machine such that the torque limit is higher when the stator windings are in a low speed configuration than when the stator windings are in a high speed configuration; - deactivating a converter stage during a change between a slow and fast configuration and performing the change; The programmable processing system includes computer executable instructions for controlling the programmable processing system to:

[0012] According to the present invention there is further provided a novel computer program product, which comprises a non-volatile computer readable medium (e.g. a compact disc "CD") on which is encoded a computer program according to the present invention.

[0013] Exemplary, non-limiting embodiments are set forth in the accompanying dependent claims.

[0014] Various exemplary, non-limiting embodiments as to their construction and method of operation, together with additional objects and advantages of the embodiments, will be best understood from the following description of certain exemplary, non-limiting embodiments when read in conjunction with the accompanying drawings.

[0015] The verbs "to comprise" and "to include" are used in this document as open-ended and do not exclude or require the presence of unrecited features.

[0016] Features recited in dependent claims are mutually freely combinable unless expressly stated otherwise.

[0017] Furthermore, the use of "a" or "an" (i.e., singular) throughout this document does not exclude the plural.

[0018] Exemplary non-limiting embodiments and advantages thereof are described in more detail below, by way of example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0019] [Figure 1a] 1 illustrates a converter according to an exemplary non-limiting embodiment for driving an electric machine in which the stator windings are modifiable to be in a low speed configuration or a high speed configuration with fewer turns connected in series than the low speed configuration. [Figure 1b] 10 illustrates torque limits implemented in a converter according to an exemplary non-limiting embodiment. [Figure 1c] 10 shows exemplary measurement results associated with accelerating the motor with full torque and changing from a low speed configuration of the stator windings to a high speed configuration of the stator windings. [Figure 2] 1 illustrates a flowchart of a method according to an exemplary non-limiting embodiment for driving an electric machine in which the stator windings are modifiable to a low-speed configuration or a high-speed configuration with fewer series-connected turns than the low-speed configuration. DETAILED DESCRIPTION OF THE INVENTION

[0020] The specific examples provided in the following description should not be construed as limiting the scope and / or applicability of the appended claims. The listings and groupings of examples provided in the description are not exhaustive unless expressly stated otherwise.

[0021] 1a illustrates a converter 101 according to an exemplary, non-limiting embodiment driving an electric machine 105 whose stator windings are modifiable to have a low-speed configuration or a high-speed configuration with fewer series-connected turns than the low-speed configuration. The electric machine 105 can be, for example, an induction machine, a permanent magnet synchronous machine, a permanent magnet brushless direct current (DC) machine, an electrically excited synchronous machine, or a synchronous reluctance machine. The stator windings of the electric machine can be in a high-speed configuration, for example, such that configuration switches Cs1, Cs2, Cs3, Cs4, and Cs5 are conductive and configuration switches Cs6, Cs7, and Cs8 are non-conductive. In this high-speed configuration, the stator windings have two star-connected winding systems U1-V1-W1 and U2-V2-W2 connected in parallel with the output terminals of the converter 101. The stator windings can be in a low-speed configuration, for example, with configuration switches Cs1-Cs5 in a non-conductive state and configuration switches Cs6-Cs8 in a conductive state. In this low-speed configuration, winding portions U1 and U2 are connected in series. Correspondingly, winding portions V1 and V2 are connected in series, and winding portions W1 and W2 are connected in series. Thus, the number of series-connected turns of the stator winding in the high-speed configuration is less than that in the low-speed configuration.

[0022] Converter 101 includes converter stage 102 configured to provide a stator voltage to the stator windings. Converter 101 includes a control system 103 configured to control converter stage 102 to control the stator windings to be in a low-speed configuration or a high-speed configuration and to provide the stator voltage. Control system 103 is configured to provide control signals to controllable switches S1, S2, S3, S4, S5, and S6 of converter stage 102. Controllable switches S1-S6 can be, for example, bipolar transistors, field-effect junction transistors, metal-oxide-semiconductor field-effect transistors (MOSFETs), gate-turn-off thyristors (GTOs), insulated-gate bipolar transistors (IGBTs), or some other suitable controllable switches. Control system 103 is configured to provide control signals to configuration switches Cs1-Cs8. Each of the constituent switches Cs1-Cs8 may include, for example, an anti-parallel connected bipolar transistor, an anti-parallel connected field effect junction transistor, an anti-parallel connected metal oxide semiconductor field effect transistor (MOSFET), an anti-parallel connected gate turn-off thyristor (GTO), or an anti-parallel connected insulated gate bipolar transistor (IGBT). The constituent switches Cs1-Cs8 may also be, for example, electromechanical relays or contactors.

[0023] The control system 103 is configured to limit the torque of the electric machine 105 so that the torque limit is higher when the stator windings are in a low-speed configuration than when the stator windings are in a high-speed configuration. For example, the torque can be limited by limiting the stator current of the electric machine 105. The control system 103 is configured to deactivate the converter stage 102, i.e., set all controllable switches S1-S6 to a non-conducting state, during the change between the low-speed and high-speed configurations. An advantage of the stator windings and control system 103 described above is that they can be configured to give the electric machine 105 different characteristics and, advantageously, the configuration can be changed relatively quickly while the electric machine 105 is operating, thereby resembling a gear change in a mechanical gearbox.

[0024] 1b illustrates torque limits implemented in a converter according to an exemplary non-limiting embodiment. In the exemplary case shown in FIG. 1b, the control system 103 is configured to ramp the torque limits between a higher torque limit T1 corresponding to a low-speed configuration and a lower torque limit T2 corresponding to a high-speed configuration when the rotational speed is in the transition region between rotational speed n1 and rotational speed n2. In the converter according to an exemplary non-limiting embodiment, the control system 103 may ramp the torque limits between a higher torque limit T1 corresponding to a low-speed configuration and a lower torque limit T2 corresponding to a high-speed configuration when the rotational speed is in the transition region between rotational speed n1 and rotational speed n2. i) setting the controllable switches S1 to S6 of the converter stage 102 to a non-conducting state; ii) a procedure for monitoring the stator current of the electric machine 105; iii) after the stator current drops below a threshold, setting all of the switches Cs1 to Cs8 constituting the stator winding to a non-conducting state, and waiting for a first predetermined time; iv) setting the stator winding configuration switches Cs1 to Cs8 to positions corresponding to the new stator winding configuration and waiting a second predetermined time period; v) operating the converter stage 102 to apply a stator voltage to the stator windings; It is configured to use

[0025] Figure 1c shows exemplary measurement results associated with accelerating the motor at full torque and changing from the low-speed configuration of the stator windings to the high-speed configuration of the stator windings. In this exemplary case, changing the winding configuration from the low-speed configuration to the high-speed configuration takes approximately 100 milliseconds.

[0026] In a converter according to an exemplary, non-limiting embodiment, the control system 103 is configured to maintain a computational model of the electric machine 105. The computational model can be, for example, a two-axis model including the direct “d” and quadrature “q” inductances of the stator and rotor 106 of the electric machine 105 and the winding resistance of the electric machine 105. The converter 101 is configurable to perform an identification operation to find parameters of the computational model. Advantageously, the identification operation is performed when the stator windings are in a low-speed mode and when the stator windings are in a high-speed mode. The control system 103 is configured to perform vector control of the electric machine 105 to control the speed and / or torque of the electric machine 105 based on the stator current, the stator voltage, and the computational model of the electric machine. The control system 103 is configured to change the parameters of the computational model of the electric machine 105 during a change between a low-speed configuration and a high-speed configuration.

[0027] In a converter according to an exemplary non-limiting embodiment, the control system 103 is configured to estimate an electromotive force E induced in a low speed configuration of the stator winding as a result of the rotational speed of the rotor 106 of the electric machine 105 and the magnetic flux carried by the rotor of the electric machine. The control system 103 determines whether the estimated electromotive force E in the low speed configuration is below a threshold (e.g., V DC / √2 or less (V DC is the DC voltage of the converter stage 102). In the exemplary case where the electric machine 105 is a permanent magnet machine, the electromotive force E is substantially equal to C PM n act Ψ PM (However, n act is the rotor speed, and Ψ PM is the magnetic flux due to the rotor permanent magnets on the stator windings, and C PM In the exemplary case where the electric machine 105 is an induction machine, the electromotive force E is substantially equal to C IM n act Ψ R0 e -t / τ (However, n act is the rotor speed, and ΨR0 is the rotor flux when the stator current becomes zero, and C IM is a constant, and τ is L R / R R (However, L R is the rotor inductance when the stator current is zero, and R R is the rotor resistance. In an induction machine, the rotor magnetic flux is maintained by the rotor cage winding, and the rotor magnetic flux decays according to the time constant τ described above.

[0028] In a converter according to an exemplary non-limiting embodiment, the control system 103 is configured to be able to change from a high speed configuration to a low speed configuration if all of the following conditions are met: 1) A high-speed configuration is currently being used 2) Enough time has passed since the last change between the slow and fast configurations 3) The estimated electromotive force, i.e., back electromotive force (emf), in the slow configuration is less than a threshold (e.g., V DC / √2)

[0029] In a converter according to an exemplary non-limiting embodiment, the control system 103 is configured to be able to change from a slow configuration to a fast configuration if both of the following conditions are met: 1) A slow configuration is currently being used 2) Enough time has passed since the last change between the slow and fast configurations

[0030] In the converter according to an exemplary non-limiting embodiment, the control system 103 is configured to automatically change from a high speed configuration to a low speed configuration upon fulfillment of the following requirements and permitting conditions: 1) Requirements: Actual rotation speed n act is the downshift speed n down is less than the actual torque T act is the specified torque limit T low speed limit Exceeds. 2) Requirements: The electric machine operates for a specified time t low speed limit , downshift speed ndown It is operating at less than 3) Permission condition: Enough time has passed since the last change between the slow and fast configurations 4) Enabling condition: The estimated electromotive force E, i.e., the back electromotive force (emf) in the slow configuration, is less than a threshold (e.g., V DC / √2)

[0031] The rationale for the above requirements 1) and 2) is that the rotational speed n act is the downshift speed n down The main advantage of this configuration is that it is not necessary to immediately change to the low-speed configuration if the current flow rate is less than 100 kJ / s. The change should only be performed if very high torque is required that cannot be produced in the high-speed configuration. Changing to the low-speed configuration reduces converter current and converter current losses, and therefore it is desirable to change to the low-speed configuration if it appears that the electric machine will operate at a speed corresponding to the low-speed configuration for more than a predetermined length of time.

[0032] In the converter according to an exemplary non-limiting embodiment, the control system 103 is configured to automatically change from a slow configuration to a fast configuration upon fulfillment of the following requirements and permitting conditions: 1) Requirements: Actual rotation speed n act is the upshift speed n up Exceeds. 2) Requirements: The electric machine operates for a specified time t up speed limit , upshift speed n up is operating beyond 3) Permission condition: Enough time has passed since the last change between the slow and fast configurations 4) Permission condition: Actual torque T act is the specified torque limit T high speed limit is less than

[0033] Permission Condition 4:T above act <T high speed limitWhen using the high-speed configuration, the current torque is a necessary condition for the electric machine to be able to generate actual torque. The low-speed configuration can generate higher torque than the high-speed configuration, which can lead to a situation where the configuration cannot be changed because the high-speed configuration cannot generate enough torque. As the speed increases, the electric machine enters the field-weakening region, and eventually, converter current limiting reduces the torque so that the high-speed configuration can be switched on. However, going into field-weakening offers little benefit compared to changing the configuration before field-weakening. On the other hand, field-weakening should be avoided because the phase current should be zero, or at least near zero, to operate the winding configuration switches. When converter modulation is stopped in field-weakening, the freewheeling diodes of the converter stage 102 rectify power into the DC voltage link, which appears as a braking torque on the electric machine shaft.

[0034] To avoid field weakening in the low-speed configuration and to match torque when changing configurations, the torque limits shown in FIG. 1b can be used. In the low-speed configuration when the rotational speed is lower than limit n1, the torque is limited according to what can be generated in the low-speed configuration. If the rotational speed is increasing, the torque limit decreases, for example linearly, so that at rotational speed limit n2, the torque limit is equal to the maximum torque that can be generated in the high-speed configuration. Advantageously, at upshift speed n up is equal to or greater than n2, that is, n up ≧n2. As a result, the rotation speed n act is the upshift speed n up , the torque is low enough to allow a change of configuration. Furthermore, only when necessary, i.e., when the actual torque T act is so high that it is limited by the torque limit, torque limiting is performed.

[0035] 1a can be based on one or more analog circuits, one or more digital processing circuits, or a combination of analog and digital processing circuits. Each digital processing circuit can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor (e.g., an application specific integrated circuit "ASIC"), or a configurable hardware processor (e.g., a field programmable gate array "FPGA"). Furthermore, control system 103 may include one or more memory circuits, each of which can be, for example, a random access memory "RAM" circuit.

[0036] 2 illustrates a flowchart of a method according to an exemplary non-limiting embodiment for driving an electric machine in which the stator windings are modifiable to a low speed configuration or a high speed configuration with fewer series-connected turns than the low speed configuration. an operation 201 of controlling the stator windings to be in a low speed configuration or a high speed configuration; an operation 202 of controlling a converter stage to supply a stator voltage to the stator winding; - operation 203 of limiting the torque of the electric machine so that the torque limit is higher when the stator windings are in a low speed configuration than when the stator windings are in a high speed configuration; an operation 204 of deactivating a converter stage during a change between a slow configuration and a fast configuration and performing the change; Includes.

[0037] A method according to an exemplary, non-limiting embodiment includes ramping (e.g., linearly ramping) the torque limit as a function of rotational speed, ramping the torque limit between a higher torque limit corresponding to a low speed configuration and a lower torque limit corresponding to a high speed configuration.

[0038] A method according to an exemplary non-limiting embodiment includes, during a change from a current configuration of stator windings to a new configuration of stator windings, i) setting the controllable switches of the converter stages to a non-conducting state; ii) a procedure for monitoring the stator current of the electric machine; iii) setting all of the switches in the stator winding to a non-conducting state after the stator current drops below a threshold value and waiting a first predetermined time period; iv) setting the stator winding configuration switch to a position corresponding to the new stator winding configuration and waiting a second predetermined time period; v) operating the converter stage to apply a stator voltage to the stator winding; The method includes using the following steps:

[0039] A method according to an exemplary non-limiting embodiment includes maintaining a computational model of the electric machine, controlling a rotational speed and / or torque of the electric machine based on a stator current, a stator voltage, and the computational model of the electric machine, and changing parameters of the computational model of the electric machine during a change between a low-speed configuration and a high-speed configuration.

[0040] A method according to an exemplary non-limiting embodiment includes estimating an emf induced in a low speed configuration of a stator winding as a result of the rotational speed of a rotor of an electric machine and the magnetic flux carried by the rotor of the electric machine, and allowing a change from the high speed configuration to the low speed configuration only if the estimated emf is below a threshold value.

[0041] A method according to an exemplary non-limiting embodiment includes preventing a change from a slow configuration to a fast configuration if less than a predetermined time has elapsed since the last change from a fast configuration to a slow configuration, and preventing a change from a fast configuration to a slow configuration if less than a predetermined time has elapsed since the last change from the slow configuration to the fast configuration.

[0042] A computer program according to an exemplary, non-limiting embodiment includes computer-executable instructions for controlling a programmable processing system to perform operations related to the method described in any of the above exemplary, non-limiting embodiments.

[0043] A computer program according to an exemplary, non-limiting embodiment includes a software module for driving an electric machine whose stator windings are variable to a low speed configuration or a high speed configuration with fewer turns connected in series than the low speed configuration. controlling the stator windings to be in a low speed configuration or a high speed configuration; controlling a converter stage to supply a stator voltage to the stator winding; limiting the torque of the electric machine such that the torque limit is higher when the stator windings are in a low speed configuration than when the stator windings are in a high speed configuration; - deactivating a converter stage during a change between a slow and fast configuration and performing the change; The programmable processing system includes computer executable instructions for controlling the programmable processing system to:

[0044] A software module can be, for example, a subroutine or function implemented in a programming tool suitable for a programmable processing system.

[0045] A computer program product according to an exemplary, non-limiting embodiment includes a computer readable medium (eg, a compact disc "CD") having encoded thereon a computer program according to an exemplary, non-limiting embodiment.

[0046] The signal, according to the exemplary, non-limiting embodiment, is encoded to carry information that defines a computer program, according to the exemplary, non-limiting embodiment of the present invention.

[0047] The specific examples provided in the above description should not be construed as limiting the applicability and / or interpretation of the appended claims. It should be noted that the lists and groupings of examples provided in this document are non-exhaustive lists and groupings unless expressly specified otherwise.

Claims

1. 1. A converter (101) for driving an electric machine whose stator windings are changeable to be in a low speed configuration or in a high speed configuration having fewer series-connected turns than the low speed configuration, a converter stage (102) adapted to supply a stator voltage to said stator windings; a control system (103) configured to control the stator windings to be in the low-speed configuration or the high-speed configuration and to control the converter stages to provide the stator voltage; Including, the control system is configured to limit the torque of the electric machine such that a torque limit is higher when the stator windings are in the low speed configuration (T1) than when the stator windings are in the high speed configuration (T2), and the control system is configured to deactivate the converter stages during a change between the low speed configuration and the high speed configuration to effect the change; When changing from the low-speed configuration to the high-speed configuration and the rotational speed is increasing, the control system reduces the torque limit when the rotational speed is in a predetermined transition region, and limits the torque of the electric machine so that at the rotational speed at the upper limit of the transition region, the torque limit is equal to the maximum torque that can be generated in the high-speed configuration. A converter (101).

2. 2. The converter of claim 1, wherein the control system is configured to ramp the torque limit as a function of rotational speed, ramping the torque limit between a higher torque limit (T1) corresponding to the low speed configuration and a lower torque limit (T2) corresponding to the high speed configuration.

3. The control system, during a change from a current configuration of the stator windings to a new configuration of the stator windings, i) setting the controllable switches (S1 to S6) of said converter stage (102) in a non-conducting state; ii) a procedure for monitoring the stator current of said electric machine; iii) setting all of the stator winding switches (Cs1 to Cs8) to a non-conducting state after the stator current drops below a threshold, and waiting for a first predetermined time; iv) setting the configuration switches (Cs1-Cs8) of the stator windings to positions corresponding to the new configuration of the stator windings and waiting a second predetermined time period; v) operating said converter stage (102) to supply a stator voltage to said stator winding; 3. A converter according to claim 1 or 2, configured to use:

4. 4. The converter of claim 1, wherein the control system is configured to maintain a computational model of the electric machine and to control the rotational speed and / or torque of the electric machine based on a stator current, a stator voltage and the computational model of the electric machine, and wherein the control system is configured to change parameters of the computational model of the electric machine during the change between the low speed configuration and the high speed configuration.

5. 5. A converter according to claim 1, wherein the control system is configured to estimate an emf induced in the low speed configuration of the stator winding as a result of the rotational speed of a rotor of the electric machine and the magnetic flux carried by the rotor of the electric machine, and to change from the high speed configuration to the low speed configuration only if the estimated emf is below a threshold value.

6. 6. The converter of claim 1, wherein the control system is configured to prevent a change from the slow speed configuration to the fast configuration if less than a predetermined time has elapsed since a previous change from the fast configuration to the slow speed configuration, and to prevent a change from the fast configuration to the slow speed configuration if less than the predetermined time has elapsed since a previous change from the slow speed configuration to the fast configuration.

7. 1. A method of driving an electric machine in which a stator winding is changeable to be in a low speed configuration or a high speed configuration having fewer series connected turns than the low speed configuration, comprising: - controlling (201) the stator windings to be in the low speed configuration or the high speed configuration; - controlling (202) a converter stage to supply a stator voltage to said stator winding; Including, limiting (203) the torque of the electric machine so that a torque limit is higher when the stator windings are in the low speed configuration than when the stator windings are in the high speed configuration; and shutting down the converter stages during a change between the low speed configuration and the high speed configuration to effect the change; The torque limiting step (203) involves limiting the torque of the electric machine when the rotational speed is increasing from the low-speed configuration to the high-speed configuration, by decreasing the torque limit when the rotational speed is in a predetermined transition region, and limiting the torque of the electric machine at the rotational speed at the upper limit of the transition region so that the torque limit is equal to the maximum torque that can be generated in the high-speed configuration. A method characterized by:

8. 8. The method of claim 7, including the step of ramping the torque limit as a function of rotational speed, ramping the torque limit between a higher torque limit corresponding to the low speed configuration and a lower torque limit corresponding to the high speed configuration.

9. During a change from a current configuration of the stator windings to a new configuration of the stator windings, i) setting the controllable switches of said converter stages to a non-conducting state; ii) a procedure for monitoring the stator current of said electric machine; iii) setting all of the switches in the stator winding to a non-conducting state after the stator current drops below a threshold value, and waiting a first predetermined time; iv) setting the configuration switches of the stator windings to positions corresponding to the new configuration of the stator windings and waiting a second predetermined time period; v) operating the converter stage to supply a stator voltage to the stator winding; 9. The method of claim 7 or 8, comprising using:

10. 10. A method according to any one of claims 7 to 9, comprising the steps of maintaining a computational model of the electric machine, controlling the rotational speed and / or torque of the electric machine based on a stator current, the stator voltage and the computational model of the electric machine, and changing parameters of the computational model of the electric machine during the change between the low speed configuration and the high speed configuration.

11. 11. A method according to any one of claims 7 to 10, comprising the steps of estimating an emf induced in the low speed configuration of the stator windings as a result of the rotational speed of a rotor of the electric machine and the magnetic flux carried by the rotor of the electric machine, and allowing a change from the high speed configuration to the low speed configuration only if the estimated emf is below a threshold.

12. 12. The method of claim 7, further comprising the steps of: preventing a change from the slow configuration to the fast configuration if less than a predetermined time has elapsed since a previous change from the fast configuration to the slow configuration; and preventing a change from the fast configuration to the slow configuration if less than the predetermined time has elapsed since a previous change from the slow configuration to the fast configuration.

13. 1. A computer program for driving an electric machine whose stator windings are changeable to be in a low speed configuration or a high speed configuration having fewer series connected turns than the low speed configuration, comprising: - controlling the stator windings to be in the low speed configuration or the high speed configuration; - controlling a converter stage to apply a stator voltage to said stator windings computer-executable instructions for controlling a programmable processing system to: limiting the torque of the electric machine such that the torque limit is higher when the stator windings are in the low speed configuration than when the stator windings are in the high speed configuration; - deactivating the converter stage during the change between the slow and fast configurations and performing the change; The torque limiting step includes, when changing from the low speed configuration to the high speed configuration and the rotational speed is increasing, decreasing the torque limit when the rotational speed is in a predetermined transition region; limiting the torque of the electric machine so that at the upper rotational speed of the transition region, the torque limit is equal to the maximum torque that can be generated in the high-speed configuration; a computer program product comprising computer executable instructions for controlling said programmable processing system to:

14. 14. A non-volatile computer readable medium encoded with the computer program of claim 13.

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