WO2021165652A1

WO2021165652A1 - A system

Info

Publication number: WO2021165652A1
Application number: PCT/GB2021/050322
Authority: WO
Inventors: Stephen Greetham
Original assignee: Dyson Technology Limited
Priority date: 2020-02-21
Filing date: 2021-02-11
Publication date: 2021-08-26
Also published as: CN115151442A; GB2592246A; GB202002440D0

Abstract

A system comprising an electric motor, a first battery sub-pack (11), a second battery sub-pack (12), and a plurality of switches (SW31- SW34). The switches are configurable into a first configuration in which the first battery sub-pack is connected to the electric motor, a second configuration in which the second sub-pack is connected to the electric motor, and a third configuration in which both the first sub-pack and the second sub-pack are connected in series to the electric motor.

Description

A SYSTEM

Field of the Invention

The present invention relates to a system, and in a particular exemplification, to an electric vehicle having an electric motor supplied with a multilevel voltage.

Background of the Invention

Anxiety about electrical system efficiencies, such as the driving range of an electric vehicle, continues to be a key concern for many consumers. Accordingly, any improvements in the efficiency of electrical systems reliant on energy stored in batteries are clearly desirable.

Summary of the Invention

The present invention provides a system comprising an electric motor, a first battery sub pack, a second battery sub-pack, and a plurality of switches, wherein the switches are configurable into a first configuration in which the first battery sub-pack is connected to the electric motor, a second configuration in which the second sub-pack is connected to the electric motor, and a third configuration in which both the first sub-pack and the second sub-pack are connected in series to the electric motor.

The switches may therefore be configured in order to supply the electric motor with three different voltages. In particular, the electric motor may be supplied with the voltage of the first sub-pack, the voltage of the second sub-pack, or with the sum of the voltages of the first and second sub-packs. As a result, the efficiency and thus the run-time of the system may be increased.

For example, the voltage supplied to the electric motor may be selected according to a desired or commanded speed and/or torque value. For a given voltage, there is typically a single point in the torque-speed characteristic of the electric motor where the efficiency is greatest. Consequently, by selecting a different voltage according to a desired speed and/or torque, the efficiency of the system, and thus the discharge time, may be improved. The system may comprises a controller for configuring the switches. The controller then configures the switches into the third configuration in order to supply a first voltage to the electric motor. Additionally, the controller repeatedly switches or reconfigures the switches between the first configuration and the second configuration in order to supply a second voltage to the electric motor. This arrangement is of particular use when the first and second sub-packs have an identical arrangement of cells, i.e. when the voltages of the two sub-packs are the same. By repeatedly switching or reconfiguring the switches between the first configuration and the second configuration, the two sub-packs discharge at a more even rate.

The controller may reconfigure the switches at a prescribed frequency or when a difference in the voltages of the battery sub-packs exceeds a threshold. As a result, a relatively good balance in the state-of-charge of the two sub-packs may be maintained.

The prescribed frequency or the threshold may be defined in accordance with a voltage of one or both of the battery sub-packs. This then has the advantage that a lower switching frequency may be used whilst also ensuring that both sub-packs reach full discharge at the same time. In particular, a lower switching frequency or a higher threshold may be used when the voltages of the sub-packs are relatively high, and a higher switching frequency or a lower threshold may be used when the voltages of the sub-packs are relatively low.

The controller may configures the switches in order to supply one of the first voltage and the second voltage according to a desired speed and/or torque value. As noted above, by selecting a different voltage according to a desired speed and/or torque, the efficiency of the system, and thus the run-time, may be improved.

The system may comprise an inverter module that comprises a first set of switches and a second set of switches. The first set of switches are then coupled to one or more windings of the electric motor, and the second set of switches are configurable to connect one or both of the battery sub-packs to the first set of switches. More particularly, the second pair of switches may be configurable into a first configuration in which the first battery sub-pack is connected to the first set of switches, a second configuration in which the second battery sub-pack is connected to the first set of switches, and a third configuration in which both the first sub-pack and the second sub-pack are connected to the first set of switches. Existing multilevel inverters typically comprise a large number of switches that convert a single input voltage into a multilevel voltage. By contrast, the system comprises two battery sub-packs that can be connected to the inverter module individually or in series combination. As a result, a more cost-effective multilevel inverter may be achieved.

The first battery sub-pack and the second battery sub-pack may be connected in series across a voltage bus. The second set of switches may then comprise a first pair of switches and a second pair of switches connected in series across the voltage bus. Additionally, the first set of switches may be connected between a positive rail and a negative rail. The positive rail is then connected to a first node located between the first pair of switches, and the negative rail is connected to a second node located between the second pair of switches. As a result, a multilevel inverter may be achieved using just four switches. Moreover, controlling the switches in order to achieve a multilevel voltage is relatively straightforward. By contrast, existing multilevel inverters typically comprise a much larger number of switches and their control is relatively complex.

A node located between the first pair of switches and the second pair of switches may be connected to a node located between the first battery sub-pack and the second battery sub pack.

In order that the invention may be more readily understood, reference will now be made by way of example only to the accompanying drawings in which: Figure 1 is a schematic diagram of an exemplification of a system, namely an electric vehicle;

Figure 2 shows a traction drive unit of the electric vehicle;

Figure 3 is a circuit diagram of a battery pack of the electric vehicle;

Figure 4 is a circuit diagram of an embodiment of the electric vehicle comprising a multilevel traction drive unit;

Figure 5 is a circuit diagram of an alternative embodiment of the electric vehicle comprising an alternative multilevel traction drive unit; and

Figure 6 is a circuit diagram of a further embodiment of the electric vehicle comprising a combination of the embodiments of Figures 3 and 5.

The system (electric vehicle) 1 of Figure 1 comprise a battery pack 10 connected to a pair of traction drive units 20. Each traction drive unit 20 is coupled to a pair of wheels 50 via a pair of drive shafts 60. In use, the battery pack 10 supplies electrical power to the traction drive units 20, which in turn drive the wheels 50 of the vehicle 1.

As shown in Figure 2, each traction drive unit 20 comprises an electric motor 21, an inverter module 22, and a gearbox 23. The electric motor 21 is a three-phase motor and comprises three phase windings 25 (see Figures 4 to 6). The inverter module 22 is coupled to the windings 25 of the motor 21 and comprises a plurality of switches SW21- SW26 and a controller 26 for controlling the switches. In use, the inverter module 22 converts the DC voltage supplied by the battery pack 1 into an AC voltage which is applied to windings 25. The gearbox 23 is coupled to the motor 21 and to the drive shafts 60. Torque generated by the motor 21 is then transferred to the wheels 50 via the gearbox 23 and drive shafts 60. Referring now to Figure 3, the battery pack 10 comprises a first sub-pack 11, a second sub-pack 12, a primary bus 14 having a pair of terminals 15,16, an auxiliary bus 17 having a pair of terminals 18,19, a first set of switches SW1-SW6, a second set of switches SW1 1-SW15, and a battery controller 13 for controlling the switches.

Each sub-pack 11,12 comprises a plurality of cells arranged into strings connected in series, with each string comprising a number of cells connected in parallel. The two sub packs have an identical arrangement of cells. In this particular embodiment, each sub pack has a maximum voltage of 462 V (4.2V/cell) and a minimum voltage of 330 V (3.0V/cell).

The primary bus 14 is connected to and is used to supply electrical power to each of the traction drive units 20. More specifically, the primary bus 14 is connected to the inverter module 22 of each traction drive unit 20. The auxiliary bus 17 is connected to and is used to supply electrical power to auxiliary systems (not shown) of the vehicle 1. For example, the auxiliary bus 17 may be connected to a 12 V power supply unit that steps down the voltage of the auxiliary bus 17 and outputs a 12 V supply.

The first set of switches SW1-SW6 are used to connect the two sub-packs 11,12 to the primary bus 14. Moreover, the first set of switches SW1-SW6 can be configured into a first configuration in which the two sub-packs 11,12 are connected in series across the primary bus 14 (i.e. by closing SW1, SW3 and SW5), and a second configuration in which the two sub-packs 11,12 are connected in parallel across the primary bus 14 (i.e. by closing SW1, SW2, SW4 and SW5). Switch SW6 forms part of a primary pre-charge circuit along with resistor R1.

When discharging the battery pack 11, the first set of switches SW1-SW6 are configured into the first configuration. As a consequence, the voltage of the primary bus 14 is between 924 V (fully charged sub-packs) and 660 V (fully discharged sub-packs). In order to charge the battery pack 10, a charge voltage is applied to the terminals 15,16 of the primary bus 14. The battery pack 2 may be charged at a high voltage (e.g. 1000 V) or at a low voltage (e.g. 500 V). When charged at a high voltage, the first set of switches SW1-SW6 are again configured into the first configuration. Conversely, when charged at a low voltage, the first set of switches SW1-SW6 are configured into the second configuration.

The second set of switches comprises a first pair of switches SW11,SW12 and a second pair of switches SW13,S14 connected in series across the primary bus 14. A node 40 located between the two pairs of switches is connected to a node 43 located between the two sub-packs 11,12 when connected in series. The positive terminal 18 of the auxiliary bus 17 is then connected via a positive rail to a node 41 located between the first pair of switches SW11,SW12, and the negative terminal 19 is connected via a negative rail to a node 42 located between the second pair of switches SW13,SW14. The second set of switches further comprises switch SW15, which forms part of an auxiliary pre-charge circuit along with resistor R11.

The second set of switches SW 11-SW15 are used to connect one or both of the two sub packs 11,12 to the auxiliary bus 17. When the sub-packs 11,12 are connected in series across the primary bus 14 (i.e. when the first set of switches SW1-SW6 are in the first configuration), the second set of switches are configurable into a first configuration in which the first sub-pack 11 only is connected across the auxiliary bus 17 (i.e. by closing SW11, SW13 and SW15), and a second configuration in which the second sub-pack 12 only is connected across the auxiliary bus 17 (i.e. by closing SW2, SW4 and SW5). When the sub-packs 11,12 are connected in parallel across the primary bus 14 (i.e. when the first set of switches SW1-SW6 are in the second configuration), the second set of switches are configurable into a third configuration in which the first sub-pack 11 and the second sub-pack 12 are connected in parallel across the auxiliary bus 17 (i.e. by closing SW11, SW14 and SW15). Accordingly, when the two sub-packs 11,12 are connected in series, the first and second configurations are used to connect one only of the sub-packs 11,12 to the auxiliary bus 17. And when the two sub-packs 11,12 are connected in parallel, the third configuration is used to connect both sub-packs 11,12 to the auxiliary bus 17. During discharge of the battery pack 11, the controller 13 configures the second set of switches SW11-SW15 into the first configuration or the second configuration such that only one of the sub-packs 11,12 is connected to the auxiliary bus 17 at any one time. As a result, the voltage of the auxiliary bus 19 is between 462 V (fully charged sub-pack) and 330 V (fully discharged sub-pack). The voltage of the auxiliary bus 19 is therefore half that of the primary bus 14. Moreover, the size or width of the operating voltage range of the auxiliary bus 17, which in this instance is 132 V (i.e. 462 V - 330 V), is also half of that of the primary bus 14. This then has the significant advantages for the auxiliary systems of the vehicle 1. In particular, the auxiliary systems may be rated for a lower maximum voltage and a narrower voltage range. As a result, the complexity, cost, size and/or mass of one or more of the auxiliary systems may be reduced.

If the second set of switches SW11-SW15 were to remain indefinitely in, say, the first configuration, the first sub-pack 11 would discharge quicker than the second sub-pack 12. As a result, the first sub-pack 11 would reach a state of full discharge before the second sub-pack 12, which is undesirable. Accordingly, during discharge of the battery pack 10, the controller 13 repeatedly switches or reconfigures the second set of switches SW11- SW15 between the first configuration and the second configuration. As a result, the two sub-packs 11,12 discharge at a more even rate.

The controller 13 may switch between the first and second configuration at a prescribed frequency. The prescribed frequency may be set such that, when the power demanded by the auxiliary systems is at a maximum, a relatively good balance in the state-of-charge of the two sub-packs 11,12 is nevertheless maintained. If the prescribed frequency is fixed at this value, a potential disadvantage arises when the power demanded by the auxiliary systems is relatively low. In this instance, switching between the two configurations would occur at a higher frequency then that required to maintain a good balance in the states of charge. Accordingly, the prescribed frequency may be defined according to the power drawn by the auxiliary systems. In particular, a higher prescribed frequency may be used in response to a higher power draw. Rather than switching at a prescribed frequency, the controller 13 may instead switch between the two configurations whenever a difference in the voltages of the two sub-packs 11,12 exceeds a threshold. This then has the advantage that, irrespective of the power drawn by the auxiliary systems, switching between the two configurations occurs at the lowest possible frequency that ensures a good balance in the states of charge.

It is desirable to minimise the switching frequency whilst also ensuring that both sub packs 11,12 reach a state of full discharge at the same time. A lower switching frequency may therefore be used when the voltages of the sub-packs 11,12 are relatively high, and a higher switching frequency may be used when the voltages of the sub-packs 11,12 are relatively low. Accordingly, the controller 13 may define the prescribed frequency or the difference threshold according to the voltage of one or both of the sub-packs 11,12. More particularly, as the voltage(s) decreases, the controller 13 would use a higher prescribed frequency or a lower difference threshold.

When charging the battery pack 10 at a high voltage (i.e. when the sub-packs 11,12 are connected in series), the controller 13 again configures the second set of switches SW11- SW15 in the manner described above. That is to say that the controller 13 repeatedly switches or reconfigures the second set of switches SW11-SW15 between the first configuration and the second configuration. As a consequence, the voltage of the auxiliary bus 17 continues to be held at a voltage of between 426 V (fully charged sub packs) and 330 V (fully discharged sub-packs). The controller 13 may use a prescribed frequency or a difference threshold, as described above, in order to control the switching. Should the controller 13 use a prescribed frequency or a difference threshold that depends on the voltage of one or both of the sub-packs 11,12, the controller 13 would in this instance use a higher prescribed frequency or a lower difference threshold as the voltage(s) increases. This is in contrast to the situation described above for discharging, in which the controller 13 uses a higher prescribed frequency or a lower difference threshold as the voltage(s) decreases. When charging the battery pack 10 at a low voltage (i.e. when the sub-packs 11,12 are connected in parallel), the controller 13 configures the second set of switches SW11- SW15 into the third configuration. As a consequence, the auxiliary bus 17 is held at the same voltage as that of the primary bus 14. The auxiliary bus 17 is therefore held at the charge voltage of 500 V.

The battery pack 10 comprises two voltage buses: a primary bus 14 for delivering high power to the primary systems of the vehicle 1, including the traction drive units 20; and an auxiliary bus 17 for delivering low power to the auxiliary systems of the vehicle 1. The primary bus 14 is held at a relatively high voltage (e.g. between 660 V and 924 V). As a result, a given power may be delivered to the primary systems at a lower current. This then has the advantage that losses may be reduced, and electrical components of the primary systems, such as the switches SW21-SW26 of the inverter module 22 of the traction drive units 20, may be rated for lower current. The auxiliary bus 17 by contrast is held at a relatively low voltage. Moreover, the width of the operating voltage range is narrower than that of the primary voltage bus. As a result, the auxiliary systems, which are typically required to operate over the full voltage range of the battery pack, may be simpler, cheaper, smaller and/or lighter. Importantly, the battery pack 10 is able to provide two different voltages on the buses 14,17 simultaneously without any step-down power conversion.

In the embodiment described above and illustrated in Figure 3, the battery pack 10 comprises an auxiliary pre-charge circuit SW15, Rl l. Conceivably, if the controller 13 were to switch or reconfigure the second set of switches SW11-SW14 between the first configuration and the second configuration at a sufficiently high frequency or if the controller 13 used a different threshold that was sufficiently small, the pre-charge circuit may be omitted. Pre-charge would then be required only on initial power up of the electric vehicle 1. However, this could be handled by momentarily operating the switches SW 11- SW14 in linear mode. Each of the first set of switches SW1-SW6 is a contactor. By contrast, each of the second set of switches SW11-SW15 is a power device (i.e. a semiconductor switch). Contactors have the advantage that they are simple, robust, cheap and easy to control. However, contactors have a relatively limited number of open-and-close cycles, and have a relatively slow response time. As such, contactors are unsuitable for the second set of switches SW11-SW15. Contactors also suffer from the problem that they may open inadvertently in response to vibration or a mechanical impulse, such as that which may arise when the vehicle 1 hits a pothole. Accordingly, whilst the first set of switches SW1- SW6 are contactors, one or more of the switches may alternatively be a power device. Although more expensive and their control is more complex, power devices are not susceptible to opening in response to vibration or a mechanical impulse, and they are generally more compact and lighter than contactors.

In the embodiment described above, the auxiliary bus 17 and the second set of switches SW11-SW15 form part of the battery pack 10. However, it will be appreciated, particularly when viewing Figure 3, that the auxiliary bus 17 and the second set of switches SW11-SW15 may be located outside of the battery pack 10 and may form a separate stage of the power system of the vehicle 1. The battery pack 10 would then comprise a midpoint terminal 42, such as that shown in Figure 5, in order to provide a connection between the midpoint node 40 of the second set of switches SW 11 -SW 15 and the midpoint node of the two sub-packs 11,12. Accordingly, in a more general sense, the vehicle 1 may be said to comprise a power system having a first sub-pack 11, a second sub-pack 12, a primary bus 14, an auxiliary bus 15, and a plurality of switches SW11- SW14. The first sub-pack 11 and the second sub-pack 12 are then connectable in series to the primary bus 14 to provide a first voltage at the primary bus 14. And the switches SW 11-SW14 are configurable to selectively connect one only of the first sub-pack 11 and the second sub-pack 12 to the auxiliary bus 17 to provide a second voltage at the auxiliary bus 17.

In addition to supplying power to the auxiliary systems of the vehicle 1, the auxiliary bus 17 may be used to supply power to one or more of the primary systems. In particular, the auxiliary bus 17 may be used to supply power at a lower voltage to the traction drive units 20

For a given voltage applied to the windings 25 of the motor 21, there is typically a single point in the torque-speed characteristic of the motor 21 where the efficiency of the motor 21 is greatest. Consequently, by selectively applying different voltages to the windings 25 according to a desired speed and/or torque of the motor 21, the efficiency of the motor

21 may be improved. As a result, the driving range of the vehicle 1 may be increased. By way of example only, the higher voltage of the primary bus 14 (e.g. 924 V) may be applied to the motor 21 when operating at high torque conditions (e.g. when accelerating), and the lower voltage of the auxiliary bus 17 (e.g. 462 V) may be applied to the motor 21 when operating at low torque conditions (e.g. when coasting).

Figure 4 shows an alternative embodiment of the electric vehicle 1. The inverter module

22 continues to comprise a pair of terminals 27,28 connected to the primary bus 14, a link capacitor C926 connected across the primary bus 14, a set of switches SW21-SW26 coupled to the windings 25 of the motor 21, and an inverter controller 26 for controlling the switches SW21-SW26. In this alternative embodiment, the inverter module 22 additionally comprises a second set of terminals 30,31 connected to the auxiliary bus 17, a second link capacitor C462 connected across the auxiliary bus 17, and a second set of switches SW31-SW34 for selectively coupling the first set of switches SW21-SW26 to one of the primary bus 14 and the auxiliary bus 17. In particular, the second set of switches SW31-SW34 are configurable into a first configuration (i.e. SW31,SW34 are closed and SW32,SW33 are open) in which the first set of switches SW21-SW6 are connected to the primary bus 14, and a second configuration (i.e. SW32,SW33 are closed and SW31,SW34 are open), in which the first set of switches SW21-SW26 are connected to the auxiliary bus 17. The controller 26 then configures the second set of switches SW3 1-SW34 into the first configuration or the second configuration as required. By way of example only, the controller 26 may execute a vector control algorithm to generate a voltage vector, which is then used to determine the configuration of the second set of switches SW31-SW34. For example, if the amplitude of the voltage vector is greater than a threshold, the controller 26 may configure the second set of switches SW31-SW34 into the first configuration such that the voltage of the primary bus 14 is applied to the windings 25. Conversely, if the amplitude of the voltage vector is less than the threshold, the controller 26 may configure the second set of switches SW31-SW34 into the second configuration such that the voltage of the auxiliary bus 17 is applied to the windings 25.

The provision of a traction drive unit 20 having a dual bus may be achieved without the need for the auxiliary bus 17, as will now be described with reference to Figure 5.

Figure 5 illustrates a further embodiment of the electric vehicle 1. In this further embodiment, the battery pack 10 no longer comprises an auxiliary bus 17 or the second set of switches SW11-SW15. Instead, the battery pack 10 comprises an additional terminal 44 and a further switch SW7. The additional terminal 42 is connected via switch SW7 to the node 41 located between the two sub-packs 11,12 when connected in series. For the purposes of the present discussion, this additional terminal 42 will be referred to as the midpoint terminal, since it is connected to the midpoint between the two sub-packs 11,12. Switch SW7 forms part of the first set of switches SW1-SW6 and is provided only to ensure that the midpoint terminal 42 can by isolated from the first sub-pack 11 when the battery pack 10 is powered off.

The inverter module 22 again comprise a pair of terminals 27,28 connected to the primary bus 14, a first set of switches SW21-SW26 coupled to the windings 25 of the motor 21, and an inverter controller 26 for controlling the switches SW21-SW26. In this further embodiment, the inverter module 22 additionally comprises a single mid-point terminal 32, a second set of switches SW31-SW34, and a pair of link capacitors C462-1, C462-2.

The second set of switches comprises a first pair of switches SW31,SW32 and a second pair of switches SW33,S34 connected in series across the primary bus 14. A node 33 located between the two pairs of switches is connected to the midpoint terminal 32 via a midpoint rail. The first set of switches SW21-SW26 may be said to be connected between a positive or upper rail and a negative or lower rail. The positive rail is then connected to a node 34 located between the first pair of switches SW31,SW32, and the negative rail is connected to a node 35 located between the second pair of switches SW33,SW34.

The two link capacitors C462-1, C462-2 are connected in series across the primary bus 14. Moreover, a first link capacitor C462-1 is connected in parallel with the first pair of switches SW31,SW32 between the positive rail of the primary bus 14 and the midpoint rail, and the second link capacitor C462-2 is connected in parallel with the second pair of switches SW33,SW34 between the midpoint rail and the negative rail of the primary bus 14.

The second set of switches SW31-SW34 of the inverter module 22 are similar to the second set of switches SW11-SW14 of the battery pack 10 of Figures 3 and 4, both in terms of their arrangement and their operation, as will now be described.

The second set of switches SW31-SW34 are used to connect one or both of the two sub packs 11,12 to the first set of switches SW21-SW26. Let us assume that the two sub packs 11,12 are connected in series across the primary bus 14 (i.e. SW1, SW3, SW5 are closed), and that the midpoint terminal 42 is connected to the midpoint between the two sub-packs (i.e. SW7 is closed). The second set of switches SW31-SW34 are then configurable into a first configuration in which the first sub-pack 11 only is connected to the first set of switches SW21-SW26 (i.e. by closing SW31 and SW33), a second configuration in which the second sub-pack 12 only is connected to the first set of switches SW21-SW26 (i.e. by closing SW32 and SW34), and a third configuration in which both sub-packs 11,12 are connected in series to the first set of switches SW21- SW26 (i.e. by closing SW31 and SW34). The first and second configurations are therefore used to connect one only of the sub-packs 11,12 to the first set of switches SW21-SW26, and the third configuration is used to connect both sub-packs 11,12 to the first set of switches SW21-SW26.

The inverter controller 26 configures the second set of switches SW31-SW34 such that either a first voltage or a second voltage is supplied to the first set of switches SW21- SW26. As a result, either the first voltage or the second voltage is applied to the windings 25 of the motor 21 by the first set of switches SW21-SW26. The first voltage corresponds to the combined or series voltage of the two sub-packs 11,12, whereas the second voltage corresponds to the voltage of just one of the sub-packs 11,12. The second voltage is there half that of the first voltage. In this particular embodiment, the first voltage is between 924 V (fully charged sub-packs) and 660 V (fully discharged sub-packs), and the second voltage is between 462 V (fully charged sub-pack) and 330 V (fully discharged sub pack).

If the second set of switches SW31-SW34 were to remain indefinitely in, say, the first configuration, the first sub-pack 11 would discharge quicker than the second sub-pack 12. As a result, the first sub-pack 11 would reach a state of full discharge before the second sub-pack 12, which is undesirable. The inverter controller 26 therefore repeatedly switches or reconfigures the second set of switches SW31-SW34 between the first configuration and the second configuration. As a result, the two sub-packs 11,12 discharge at a more even rate.

As noted above in connection with the second set of switches SW11-SW15 of the battery pack 10, the inverter controller 26 may switch between the first and second configuration at a prescribed frequency. The prescribed frequency may be fixed or it may be defined according to the power drawn by the motor 21, a desired speed/or torque of the motor 21, or by some other parameter. For example, the controller 26 may execute a vector control algorithm to generate a voltage vector. The voltage vector may then be used to determine whether the first voltage or the second voltage should be applied to the windings 25. Additionally, the voltage vector may be used to define the frequency at which the second set of switches SW31-SW34 are switched between the first configuration and the second configuration. Rather than switching at a prescribed frequency, the controller 26 may instead switch between the two configurations whenever a difference in the voltages of the two sub-packs 11,12 exceeds a threshold. Again, as noted above in connection with the battery pack of Figure 3, the controller 26 may define the prescribed frequency or the difference threshold according to the voltage of one or both of the sub-packs 11,12. In particular, as the voltage(s) of the sub-pack(s) decreases, the controller 26 may use a higher prescribed frequency or a lower difference threshold.

With the embodiments shown in Figures 4 and 5, a cost-effective multilevel inverter module 22 is provided. Multilevel inverters are known, but they typically comprises a large number of switches and the required control is relatively complex. By contrast, with the embodiments shown in Figures 4 and 5, a multilevel inverter can be achieved with the addition of just four switches, namely SW31-SW34. Moreover, control of the switches SW31-SW34 is relatively straightforward. By way of example, if all switches SW21-SW26 and SW31-SW34 of the inverter module 22 were opened, current in the windings 25 would still have a return path to the link capacitors (C926 of Figure 4, and C462-1 and C462-2 of Figure 5) via the body diodes of switches SW31 and SW34. By contrast, with existing multilevel inverters, the switches must be carefully controlled and/or additional diodes must be provided in order to provide the necessary paths back to the link capacitors.

In the embodiment of Figure 5, the battery pack 10 comprises sub-packs 11,12 having the same number and configuration of cells. As a result, the two sub-packs 11,12 have the same nominal voltage and capacity. Conceivably, however, the battery pack 10 may comprise sub-packs 11,12 having different nominal voltages. The inverter module 22 would then be capable of supplying the electric motor 21 with three voltage levels rather than two. In particular, when the second set of switches SW31-SW34 are in the first configuration, the inverter module 22 would supply the voltage of the first sub-pack 11. When the second set of switches SW31-SW34 are in the second configuration, the inverter module 22 would supply the voltage of the second sub-pack 12. And when the second set of switches SW31-SW34 are in the third configuration, the inverter module 22 would supply the sum of the voltages of the first and second sub-packs 11,12. It will be appreciated that when operating in this mode, the inverter controller 26 would not repeatedly switch or reconfigure the switches SW31-SW34 between the first configuration and the second configuration. In the embodiment of Figure 5, a multilevel inverter module 22 is achieved without the need for the auxiliary bus 14 of the battery pack 10 of Figure 3. However, the two embodiments of Figures 3 and 5 are not mutually exclusive, and the vehicle 1 may comprise the combination of the two embodiments, as illustrated in Figure 6. In comparing the embodiments of Figures 4 and 6, the embodiment of Figure 6 has at least two advantages over that of Figure 4. First, the total capacitance requirement of the inverter module 22 is smaller, and thus the size and cost of the capacitors are reduced. Second, the second set of switches SW11-SW15 of the battery pack 10 are not required to carry the relatively high currents drawn by the traction drive unit 20. As a result, cheaper switches rated for lower current may be used for the second set of switches

SW 11 -SW 15 of the battery pack 10.

Claims

1. A system comprising an electric motor, a first battery sub-pack, a second battery sub-pack, and a plurality of switches, wherein the switches are configurable into a first configuration in which the first battery sub-pack is connected to the electric motor, a second configuration in which the second sub-pack is connected to the electric motor, and a third configuration in which both the first sub-pack and the second sub-pack are connected in series to the electric motor.

2. A system as claimed in claim 1, wherein the electric vehicle comprises a controller for configuring the switches, the controller configures the switches into the third configuration in order to supply a first voltage to the electric motor, and the controller repeatedly reconfigures the switches between the first configuration and the second configuration in order to supply a second voltage to the electric motor.

3. A system as claimed in claim 2, wherein the controller reconfigures the switches at a prescribed frequency or when a difference in the voltages of the battery sub-packs exceeds a threshold.

4. A system as claimed in claim 3, wherein the prescribed frequency or the threshold is defined in accordance with a voltage of one or both of the battery sub-packs.

5. A system as claimed in any one of claims 2 to 4, wherein the controller configures the switches in order to supply one of the first voltage and the second voltage according to a desired speed and/or torque value.

6. A system as claimed in any one of the preceding claims, wherein the system comprises an inverter module, the inverter module comprises a first set of switches and a second set of switches, the first set of switches are coupled to one or more windings of the electric motor, and the second set of switches are configurable to connect one or both of the battery sub-packs to the first set of switches.

7. A system as claimed in claim 6, wherein the second pair of switches are configurable into a first configuration in which the first battery sub-pack is connected to the first set of switches, a second configuration in which the second battery sub-pack is connected to the first set of switches, and a third configuration in which both the first sub pack and the second sub-pack are connected to the first set of switches.

8. A system as claimed in claim 6 or 7, wherein the first battery sub-pack and the second battery sub-pack are connected in series across a voltage bus, the second set of switches comprise a first pair of switches and a second pair of switches connected in series across the voltage bus, the first set of switches are connected between a positive rail and a negative rail, the positive rail is connected to a first node located between the first pair of switches, and the negative rail is connected to a second node located between the second pair of switches.

9. A system as claimed in claim 8, wherein a node located between the first pair of switches and the second pair of switches is connected to a node located between the first battery sub-pack and the second battery sub-pack.