Converter design for novel motors

Abstract

A power electronic arrangement, suitable for driving a switched reluctance machine (SRM), comprises a plurality of phase arms, each comprising a phase winding 1, 2, 3, connected in series to form a ri

Description

FIELD OF THE INVENTION

[0001] The present disclosure relates to a power converter and a method of control for said power converter. In particular, the disclosure relates to a power converter for a switched reluctance machine (SRM), the switched reluctance machine acting as either a motor or a generator. BACKGROUND

[0002] A switched reluctance machine (SRM) may be driven using an asymmetric bridge power converter. An asymmetric bridge power converter uses two active switches and two diodes per phase. Each asymmetric bridge provides excitation of one electrical phase of the SRM with a unidirectional current by application of three possible voltage states. The asymmetric bridge has the advantage of requiring a low number of semiconductor devices in order to function, however commercially available power modules are typically available only in multiples of a fully populated half-bridge format, where each device position comprises an active switch and an anti-parallel diode.

[0003] A power electronic converter typically used to drive a three-phase machine uses six switches and six diodes to deliver three balanced currents which are typically sinusoidal with an offset of 120° between phases. Such a power electronic converter supports a maximum total load VA of 3VdcIsw. For an arrangement using fully populated half-bridge devices to implement an asymmetric bridge power converter using commercially available modules, the total supported maximum load VA is also 3VdcIsw. However, this means that the increased number of power semiconductors are being used poorly compared to a conventional three-phase inverter and consequently, the total installed VA rating of an asymmetric bridge power converter implemented using fully populated half bridges will be twice that of a conventional three-phase inverter.

[0004] It is in this context that the teachings of the present disclosure have been devised. SUMMARY OF THE INVENTION

[0005] There is provided a power electronic arrangement for driving a switched reluctance machine, comprising: a plurality of phase arms, each phase arm comprising a phase winding, wherein the plurality of phase arms are connected in series to form a ring; and a main inverter having a plurality of inverter bridge legs, wherein a quantity of the plurality of bridge legs is equal to a quantity of the plurality of phase arms, wherein each of the plurality of bridge legs is connected at a switched node to two of the plurality of phase arms, and each of the plurality phase arms is connected to two of the plurality of bridge legs, and wherein at least one of the phase arms comprises at least one voltage source connected in series with a respective phase winding.

[0006] There is further provided a method of controlling the power electronic arrangement as described in embodiments and examples herein, comprising: controlling the at least one voltage source to provide a circulating bias current in the plurality of phase arms; and controlling the plurality of inverter bridge legs to provide a plurality of currents at a respective switched node.

[0007] There is further provided a switched reluctance machine comprising: a stator; a rotor; a housing; and the power electronic arrangement as described in embodiments and examples herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0009] FIG. 1 shows an asymmetric bridge power converter for driving an SRM, in accordance with various examples and embodiments of the present disclosure; FIG. 2 shows an example of an asymmetric bridge implemented using commercially available power modules, whereby two fully populated half-bridge legs are required per phase, in accordance with various examples and embodiments of the present disclosure; FIG. 3 shows an example of a power electronic converter typically used to drive a three-phase machine, in accordance with various examples and embodiments of the present disclosure; FIG. 4 shows an alternative arrangement comprising asymmetric bridge legs at each end of a series string of phase windings, in accordance with various examples and embodiments of the present disclosure; FIG. 5 shows an arrangement using a controllable voltage source in series with phase windings to form a ring, in accordance with various examples and embodiments of the present disclosure; FIG. 6 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing two voltage sources, in accordance with various examples and embodiments of the present disclosure; FIG. 7 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter implemented by a combination of an IGBT and an antiparallel diode, and an auxiliary converter implemented using MOSFETs, in accordance with various examples and embodiments of the present disclosure; FIG. 8 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter implemented by a combination of an IGBT and an antiparallel diode, and an auxiliary converter implemented using an asymmetric configuration, in accordance with various examples and embodiments of the present disclosure; FIG. 9 shows a generic arrangement for a power converter for driving a three-phase SRM, having two series voltage sources, in accordance with various examples and embodiments of the present disclosure; FIG. 10 shows an alternative example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing a single voltage source, in accordance with various examples and embodiments of the present disclosure; FIG. 11 shows an alternative example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing a single voltage source using an asymmetric configuration, in accordance with various examples and embodiments of the present disclosure; FIG. 12 shows a generic arrangement for a power converter driving a three-phase SRM, having a single series voltage source, in accordance with various examples and embodiments of the present disclosure; FIG. 13 shows an example of a generic arrangement for a power converter for driving a three-phase SRM, comprising a main inverter, and a voltage source in series with each of the phase windings, in accordance with various examples and embodiments of the present disclosure; FIG. 14 shows an example of an arrangement for a power converter implementing the generic arrangement shown in FIG. 13, comprising a main inverter, a first auxiliary converter implementing two voltage sources, and a second auxiliary converter implementing a single voltage source, in accordance with various examples and embodiments of the present disclosure; FIG. 15 shows an example of a generic arrangement for a power converter for driving a three-phase SRM, comprising a main inverter, and two voltages in series with each of the phase windings, in accordance with various examples and embodiments of the present disclosure; FIG. 16 shows an example of an arrangement for a power converter implementing the generic arrangement shown in FIG. 15, comprising a main inverter and three auxiliary converters, each of the three auxiliary converters implementing two voltage sources, in accordance with various examples and embodiments of the present disclosure; FIG. 17 shows an example of a generic arrangement for a power converter for driving a four-phase SRM, comprising a main inverter, and two voltages in series with each of the phase windings, in accordance with various examples and embodiments of the present disclosure. DETAILED DESCRIPTION

[0010] In the following description, for purposes of explanation, numerous specific details of certain examples are set forth. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that example, but not necessarily in other examples.

[0011] FIG. 1 shows an asymmetric bridge power converter for driving an SRM. The asymmetric bridge of FIG. 1 is arranged to drive an SRM having three phases. A first phase comprises a first phase winding 1 electrically coupled to a first bridge leg A and a second bridge leg B. First bridge leg A comprises a first active switch Q1 and a first diode D1, whilst second bridge leg B comprises a second active switch Q2 and a second diode D2. A second phase comprises a second phase winding 2 electrically coupled to a first bridge leg C and a second bridge leg D. First bridge leg C comprises a first active switch Q3 and a first diode D3, whilst second bridge leg D comprises a second active switch Q4 and a second diode D4. A third phase comprises a third phase winding 3 electrically coupled to a first bridge leg E and a second bridge leg F. First bridge leg E comprises a first active switch Q5 and a first diode D5, whilst second bridge leg F comprises a second active switch Q6 and a second diode D6.

[0012] The active switches Q1 to Q6 in FIG. 1 are shown as insulated-gate bipolar transistors (IGBTs), but those skilled in the art will appreciate that alternative functionally equivalent semiconductor devices can be utilized in place of the IGBTs or diodes. For example, any type of power field effect transistor (FET), bipolar junction transistor (BJT) or gate turn-off (GTO) thyristor could be used. The semiconductor device used may be fabricated from a range of semiconductor materials.

[0013] Operation of the circuit of FIG. 1 will be described by reference to the first phase. During a magnetisation mode, when the first phase winding 1 needs to establish a magnetic field, active switches Q1 and Q2 are switched on. Current flows through switch Q1, through phase winding 1 from leg A to leg B, and through switch Q2. The voltage across phase winding 1 will be equal to +Vdc. In a freewheeling mode, only one of Q1 or Q2 is switched on. For example, if Q1 is switched on and Q2 is switched off, then current will flow through switch Q1, through phase winding 1 from leg A to leg B, and through diode D2. The voltage IA across phase winding 1 will drop to zero. During a demagnetisation mode, when the first phase winding 1 no longer needs to establish a magnetic field, both of Q1 and Q2 are switched off. Current A flows through diode D1, through phase winding 1 from leg A to leg B, and through diode D2. The voltage IA across phase winding 1 will be equal to -Vdc.

[0014] Each asymmetric bridge provides excitation of one electrical phase with a unidirectional current by application of three possible voltage states: +Vdc, 0, -Vdc. The asymmetric bridge has the advantage of requiring a low number of semiconductor devices in order to function. Using the asymmetric bridge power converter, each phase can be controlled independently. However, commercially available power modules are typically available only in multiples of a fully populated half-bridge format, where each device position comprises an active switch and an anti-parallel diode.

[0015] FIG. 2 shows an example of an asymmetric bridge implemented using commercially available power modules, whereby two fully populated half-bridge legs are required per phase. A first phase comprises a first phase winding 1 electrically coupled to a first bridge leg A and a second bridge leg B. Bridge leg A comprises first and second active switches Q1, Q2 as well as first and second diodes D1, D2. Bridge leg B comprises third and fourth active switches Q3, Q4 as well as third and fourth diodes D3, D4. A second phase comprises four active switches Q5 to Q8, and four diodes D5 to D8. A third phase comprises four active switches Q9 to Q12, and four diodes D9 to D12.

[0016] FIG. 3 shows an example of a power electronic converter typically used to drive a three-phase machine. A total of six switches, Q1 to Q6, and six diodes, D1 to D6, are used to deliver three balanced currents which are typically sinusoidal with an offset of 120° between phases. If the phase voltages are also sinusoidal, then the maximum peak volage across each phase is Vdc ■ V3 / 2, where Vdc is the dc-link voltage. The peak bridge leg currents, given by the difference between the phase currents, are V3 / 2 times the individual peak phase currents. Accordingly, the total load VA supported by the three-phase inverter for a given dc-link voltage Vdc and a switch current rating Isw is equal to 3Vdcisw.

[0017] For the arrangement of FIG. 2, which uses double the number of power semiconductors compared to the arrangement of FIG. 3, the maximum peak voltage available across each phase is equal to the dc-link voltage, and the maximum peak phase current is equal to the switch current, and therefore the total load VA supported by the arrangement of FIG. 2 is also equal to 3VdcIsw. Therefore, the power semiconductors are utilised poorly in the arrangement of FIG. 2 compared to that of the arrangement of FIG. 3.

[0018] Although the current and voltage waveforms applied to an SRM are not typically sinusoidal, the total installed VA rating of the power semiconductors in the arrangement of FIG. 2 can be expected to be much higher than that of the arrangement of FIG. 3, and therefore the use of commercially available power-switching modules to implement the arrangement of FIG. 2 is unlikely to be cost-effective.

[0019] FIG. 4 shows an alternative arrangement comprising asymmetric bridge legs at each end of a series string of phase windings. A plurality of phase windings 1, 2 and 3, are connected in series, with a fully populated bridge leg at each intermediate node. Specifically, bridge leg B is connected at the node common to phase winding 1 and phase winding 2, and bridge leg C is connected at the node common to phase winding 2 and phase winding 3. At the ends of the string, asymmetric bridge legs A and D are connected to phase winding 1 and phase winding 3 respectively. The asymmetric bridge legs A and D at either end of the series string of phase windings carry the full phase current (i.e. the dc-component plus the ac-component), whilst the bridge legs B and C at the intermediate nodes carry only the ac-component of the current. If implemented using fully-populated half-bridge legs in each of the positions (i.e. four half-bridge legs for the example of FIG. 4), then the arrangement of FIG. 4 offers a more cost-effective alternative to the arrangement of FIG. 2, but still falls short of the potential cost-savings possible with the arrangement of FIG. 3.

[0020] FIG. 5 shows an arrangement using a controllable voltage source in series with phase windings to form a phase ring PR. A controllable voltage source Vx is connected in series with a first phase winding 1, a second phase winding 2, and a third phase winding 3. Voltage source Vx delivers current lx through the series of phase windings 1, 2 and 3. The dc-component of current Ix is controlled to provide a substantially constant circulating current. The ac-components iql, iq2 and iq3 are controlled by a three-phase inverter formed of six switches Q1 to Q6. The dc-component of current Ix circulates around the phase ring PR, but does not flow through the switches, Q1 to Q6, of the bridge arrangement, thus reducing the required current rating of the switches Q1 to Q6 and / or reducing losses in the switches.

[0021] The sum of the voltages around the ring PR must be zero. Therefore:

[0022] If Vx is defined as a stiff de voltage source, then the sum of voltages 1^ + 1^+ V3 must also be a fixed voltage, thus removing one degree of freedom from the currents iql, iq2 and iq3. Considering just the ac-component of the voltages V1 + V2 + V3, the sum of these ac-components must be zero, in the same manner as for a conventional three-phase bridge with a delta-connected load. If the three voltages and currents form a balanced, but not necessarily sinusoidal, three-phase set, then the resulting ac voltages cannot contain any triplen harmonics of the fundamental, thus limiting the range of voltage waveforms that may be applied.

[0023] Full flexibility of the arrangement of FIG. 5 can be guaranteed only if Vx also has a controlled ac-component with a voltage bandwidth equivalent to that of the bridge legs. The amplitude of the ac-component, consisting only of the triplen harmonics, can be expected to be smaller than that of the bridge legs. Additionally, Ix must be equal to i3, and therefore the voltage source Vx must be capable of absorbing the ac-component of the phase current i3. As such, for the arrangement of FIG. 5 to control the three phase currents, i2 and i3, with full flexibility of control, voltage source Vx would be required to be a wide-band voltage source with an ac-capability equal to that of the bridge legs, and a dc-capability equal to the average phase current required by the SRM. Voltage source Vx would also need to be electrically isolated from the dc-link, thus increasing the complexity of the arrangement with respect to powering the voltage source.

[0024] The arrangement of FIG. 5 makes use of a series voltage source Vx added to a three-phase inverter having three bridge legs. By adding said series voltage source Vx, a de or slowly varying current may be superimposed on the ac current waveform delivered by the three-phase inverter. The arrangement of FIG. 5 therefore makes better use of commercially available ‘off-the-shelf’ inverter hardware. If the voltage source Vx is considered to be capable of synthesising an arbitrary voltage waveform having wide bandwidth, then the resulting converter arrangement has four independently controllable voltage nodes, and can therefore control three independent currents. The arrangement of FIG. 5 is therefore equivalent in operation to the asymmetric series arrangement of FIG. 4, but with the added benefit of interphase current cancellation in all three bridge legs.

[0025] With the arrangement of FIG. 4, the number of bridge legs can be reduced to one more than the number of phases. For example, an arrangement for a three-phase SRM can be implemented using four bridge legs. However, the outermost bridge legs in the series string of phase windings of this arrangement do not benefit from the inter-phase current cancellation, and thus the losses for this series arrangement are greater than for an unbroken ring arrangement, such as shown in FIG. 5.

[0026] The series voltage source of the arrangement of FIG. 5 requires an external source of power. Providing an external source of power is inconvenient in applications where the only readily available source of significant power is that associated with the main inverter, which is typically a high voltage ac or de source. For example, a typical electric vehicle de bus will be in the range of 200 V to 1000 V, whereas the voltage source required to deliver the de current will typically be less than 10% of this voltage. When the requirement for isolation is also taken into account, a power electronic solution will typically involve an isolated dc-dc converter with a power rating of 10% to 20% of the rated power of the motor (dependent on peak-dc losses, which may be up to 20 kW on a 100 kW peak rated traction motor). In a typical three-phase application the additional cost of a galvanically-isolated series voltage source may easily exceed the cost of the asymmetric series arrangement utilising an additional bridge leg.

[0027] Compared to a conventional asymmetric bridge arrangement implemented using fully populated commercially-available half-bridge modules, a ring or series topology offers a more cost-effective converter solution for driving SRMs. However, the ring and series arrangements of FIG. 4 and FIG. 5 have drawbacks as discussed above.

[0028] It is in this context that the described embodiments of FIG. 6 to FIG. 17 have been devised. In particular, there is provided herein a power electronic arrangement comprising a main inverter and one or more voltage sources, said one or more voltage sources implemented by one or more auxiliary converters. The one or more voltage sources are connected in series with the phases to form a ring. Arrangements are also described herein that make use of more than one voltage source in a phase arm. Accordingly, the total number of series voltage sources can be between one voltage source and twice the number of voltage sources as there are bridge legs in the main inverter.

[0029] The auxiliary converters are operable to drive a circulating bias current through the SRM phase windings, with said bias current being combined with currents supplied by the main inverter in order to regulate the net current supplied to each phase winding. In the case of steady-state excitation, the bias current comprises a dc-component along with selected harmonics of the SRM excitation current. In a more general case of varying excitation due to changes in desired SRM phase winding currents, an average value of the bias current will follow that of the desired SRM phase winding currents.

[0030] The various embodiments provided herein have an advantage in that the power to drive the circulating bias current in the one or more series voltage sources is derived directly from the ac-components of the phase currents synthesised by the main inverter. These synthesised currents may be the SRM excitation currents themselves, or may be a current injected into the phase winding having a different frequency spectrum to that of the SRM excitation currents. The one or more series voltage sources that support the circulating bias current are synthesised using the same auxiliary converter as is used to provide the power to drive the circulating bias current. Therefore, no additional power source is required, and thus there is no requirement for the voltage source(s) to be galvanically isolated from the main inverter.

[0031] Additionally, the series voltage source(s) may be integrated into or onto the SRM without having to provide a separate or additional power connection. The series voltage source(s) may be co-packaged with the main inverter. The series voltage sources may be packaged separately from the main inverter and / or the SRM.

[0032] A power electronic arrangement as described herein may be used to drive a switched reluctance machine (SRM). The power electronic arrangement may comprise a plurality of phase arms. The term “phase arm” as used herein describes a phase winding, optionally in series with one or more voltage sources, wherein a phase arm is connected between two bridge legs. The plurality of phase arms may be connected in series so as to form a ring, wherein a flow of current around the phase arms is enabled.

[0033] The power electronic arrangement may further comprise a main inverter. The main inverter may comprise a plurality of inverter bridge legs. The quantity of the plurality of inverter bridge legs may be equal to a quantity of phase arms. For example, for a power electronic arrangement configured to drive a two-phase SRM the main inverter may have two bridge legs. For a power electronic arrangement configured to drive a three-phase SRM, the main inverter may have three bridge legs. Each of the bridge legs may be connected at a switched node to two of the plurality of phase arms, in such a way that each of the phase arms is connected to two of the plurality of bridge legs. This is to say, each of the phase arms is connected to exactly two bridge legs, and each bridge leg is connected to exactly two phase arms.

[0034] At least one of the phase arms may comprise at least one voltage source connected in series with the respective phase winding. In some examples, a phase arm may comprise a single voltage source in series with the respective phase winding. In some examples, a phase arm may comprise two voltage sources in series with the respective phase winding. In some examples only one of the phase arms may comprise one or two voltage sources in series with the respective phase winding. In some examples a plurality of the phase arms may comprise one or two voltage sources in series with the respective phase winding.

[0035] In some examples the at least one voltage source connected in series with a respective phase winding may be operable to provide a voltage which supports a circulating bias current in the plurality of phase arms. The circulating bias current may be a bias current with a de component. In some examples the circulating bias current may comprise harmonics of the SRM excitation current. The circulating bias current may be combined with currents supplied by the main inverter to regulate a net current supplied to each phase winding. In some examples the circulating bias may be controlled independently of currents provided by the main inverter.

[0036] In some examples the at least one voltage source may be operable to provide a voltage having components at a fundamental electrical frequency of the SRM. In some examples the at least one voltage source may be operable to provide a voltage having components at one or more harmonics of the fundamental electrical frequency of the SRM. In some examples the at least one voltage source may be operable to provide a voltage having components at a frequency different from the fundamental electrical frequency of the SRM. In some examples the at least one voltage source may be operable to provide a voltage having components at a frequency different from the harmonics of the fundamental electrical frequency of the SRM.

[0037] In some examples the at least one voltage source may be implemented by means of an auxiliary converter. In some examples two voltage sources in series, connected to a common bridge leg, may be implemented by means of an auxiliary converter having three half-bridge legs, the three half-bridge legs having a common dc-link. In some examples a single voltage source may be implemented by means of an auxiliary converter having two halfbridge legs, the two half-bridge legs having a common dc-link. In some examples a plurality of voltage sources may be implemented by means of a plurality of auxiliary converters. In some examples each of the plurality of auxiliary converters may have a separate local dc-link. In examples, the dc-link may be provided by means of one or more capacitors.

[0038] In some examples a first terminal of a first voltage source and a first terminal of a second voltage source may be electrically coupled to a common switched node of a bridge leg of the main inverter. A second terminal of the first voltage source may be connected to a first phase winding. A second terminal of the second voltage source may be connected to a second phase winding. In some examples a first terminal of a voltage source may be electrically coupled to a switched node of a bridge leg of the main inverter. A second terminal of the voltage source may be connected to a first phase winding.

[0039] In some examples a voltage rating of a plurality of switches which comprise the halfbridge legs of the auxiliary converter(s) may be less than a voltage rating of a plurality of switches which comprise the main inverter bridge legs. In some examples a current rating of a plurality of switches which comprise the main inverter bridge legs is less than a current rating of a plurality of switches which comprise the half-bridge legs of the auxiliary converter(s).

[0040] A power electronic arrangement as disclosed herein may be controlled by a method that includes steps of controlling at least one voltage source to provide a circulating bias current in the plurality of phase arms, and controlling the plurality of inverter bridge legs to provide a plurality of currents at a respective switched node of the bridge legs. Controlling at least one voltage source may comprise switching one or more active switch devices of one or more auxiliary converters. In some examples switching one or more active switch devices of one or more auxiliary converters may comprise using a pulse-width-modulation (PWM) technique to generate a desired voltage at the voltage source, or to provide a desired circulating bias current. Controlling the plurality of inverter bridge legs may comprise switching one or more active switch devices of the main inverter. In some examples switching one or more active switch devices of the main inverter may comprise using a pulse-width-modulation (PWM) technique to provide one or more desired currents.

[0041] In examples controlling the circulating bias current and / or controlling the plurality of currents provided by the main inverter may be operable to regulate a current flowing in one or more phase arms of a plurality of phase arms. In some examples the electrical power used by a voltage source of the at least one voltage source(s) to provide the circulating bias current may be derived from a current flowing in said voltage source. In some examples said electrical power may be derived from one or more ac-components of the plurality of currents provided by the main inverter. In examples there may be no additional or external power source connected to the auxiliary converter to generate the one or more voltage sources. In some examples the one or more voltage sources may not be galvanically isolated from the main inverter.

[0042] The plurality of currents provided by the main inverter may comprise components at a fundamental electrical frequency of the SRM. The plurality of currents provided by the main inverter may comprise components at harmonics of the fundamental electrical frequency of the SRM. The plurality of currents provided by the main inverter may comprise components at frequencies different from a harmonic spectrum of the excitation currents of the SRM.

[0043] Controlling the power electronic arrangement may comprise measuring an average phase current for one or more of the phase arms. In some examples the at least one voltage source may be controlled to maintain a measured average phase current at a value equal to a specified circulating bias current. Controlling the power electronic arrangement may comprise controlling the at least one voltage source to maintain a dc-link voltage of the auxiliary converter at a value substantially the same as a specified value, or within a threshold of a specified value.

[0044] The power electronic arrangement of various embodiments herein may be used as part of a switched reluctance machine comprising a stator, a rotor, and a housing. In some examples one or both of the at least one voltage source and the main inverter may be integrated within said housing. In some examples one or both of the at least one voltage source and the main inverter may be housed separately from the housing of the SRM. In some examples the at least one voltage source and the main inverter may be integrated in a single package.

[0045] FIG. 6 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing two voltage sources.

[0046] The main inverter comprises bridge legs A, B and C, said bridge legs formed from switches Q1 to Q6. The auxiliary converter comprises bridge legs S, T and U, said bridge legs formed from switches Q7 to Q12. The auxiliary converter comprises a local dc-link capacitor, Caux. A first phase winding 1 is electrically coupled between bridge leg A and bridge leg B of the main inverter. A second phase winding 2 is electrically coupled between bridge leg A of the main inverter and bridge leg S of the auxiliary converter. A third phase winding 3 is electrically coupled between bridge leg U of the auxiliary converter and bridge leg A of the main inverter. Bridge leg C of the main inverter is electrically coupled to bridge leg T of the auxiliary converter, such that voltage vc = vt.

[0047] The combination of a phase winding and synthesised voltage source in series may be referred to as a phase arm. A first phase arm may comprise the first phase winding 1. The first phase arm is connected between legs A and B of the main inverter. A second phase arm may comprise the second phase winding 2 in series with the voltage source vy. The second phase arm is connected between legs B and C of the main inverter. A third phase arm may comprise the third phase winding 3 in series with the voltage source vx. The third phase arm is connected between legs C and A of the main inverter. The voltages, vphl, vph2 and vph3, in the first, second and third phase arms, respectively, are therefore: Vphl =va-vb=v1 Vph2 =Vb-Vc = V2-Vy Vph3 =Vc~Va = V3-Vx

[0048] Each inverter phase arm therefore supports one phase winding voltage, vlt v2 or v3, and in addition the second and third phase arms, respectively, support voltages -vy and -vx.

[0049] The auxiliary converter is operable to synthesise a voltage to support a circulating bias current through the phase windings. The auxiliary converter comprises a local dc-link capacitor, Caux. The local dc-link capacitor Caux extracts power from the ac currents synthesised by the main inverter and converts it into power that supports the circulating bias current through the phase windings.

[0050] Additionally, the auxiliary converter is operable to synthesise an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltages vx and vy are generated by switching of bridge legs S, T and U of the auxiliary converter. Voltages vx and vv are defined as: a y Vx = Vu~ vt and, Vy = Vt - Vs

[0051] Each of vx and vy can take on three possible voltage states: +Vaux, 0, -Vailx. The voltage of each of vx and vy will depend upon the state of the switches in adjacent bridge legs. The voltages of vx and vy according to various switch states are as follows: Q7 Q8 Q9 Q10 Q11 Q12 vx vy off on off on on off ' ¥aux 0 off on on off on off 0 +y 1 yaux off on off on off on 0 0 off on on off off on ^aux TKlUX on off off on on off TKiux _y yaux on off on off on off 0 0 on off off on off on 0 Vaux on off on off off on KlUX 0

[0052] A time averaged intermediate voltage can be generated using the auxiliary converter by techniques such as pulse width modulation (PWM) of the switches at a suitable rate. In some examples, PWM may be performed in the range of 1 kHz to 100 kHz.

[0053] Since the voltages vx and vy are primarily associated with the development of the average or dc-bias component of the circulating current, their magnitudes will be of the order of the bias current multiplied by the sum of the phase resistances. Accordingly, vx and vy will be relatively small compared to the dc-link voltage, Vdc, of the main inverter. The value of the auxiliary inverter dc-link, Vaux, can therefore be chosen to be much smaller than Vdc. In some examples, the value of the Vaux is between 10% to 20% of Vdc. Accordingly, the use of low-voltage switching devices in the auxiliary converter is permitted for switches Q7 to Q12. Examples of suitable low-voltage switching devices may comprise silicon MOSFETs, GaN FETs, or similar.

[0054] In operation of the arrangement of FIG. 6, the currents carried by the switches of the main inverter consist solely of the difference components of the phase current. This is to say: hegA llegB = l2 ~ ll HegC = 'l2

[0055] If we break the phase currents down into the circulating bias current and the ac excitation components, where / 0 is the average circulating bias current, and i± is the ac excitation component consisting of the fundamental electrical frequency and the harmonics thereof, then can be written as: h = / 0 + ii thus showing that the difference currents carried by the main inverter phase legs do not include the circulating current. On the other hand, the currents carried by sources vx and vy do include the circulating currents, and are equal to the total respective phase currents j3 and i2.

[0056] The average phase current of a typical SRM may be of a greater magnitude compared to the fundamental frequency component. For example, taking an offset sinewave current with a minimum of 100 A and a maximum of 700 A, the average value of said current is 400 A, whereas the alternating current amplitude is 300 A. Using the example arrangements of FIG. 1 or FIG. 2, each of the inverter bridge legs must be capable of carrying the combined current, which has an RMS value of 453 A. However, assuming a balanced three-phase excitation in the main inverter of the arrangement of FIG. 6, the RMS current in each of the main inverter bridge legs would be that of the alternating difference current component alone, which has an RMS value of 367 A. Accordingly, using the arrangement of FIG. 6, the average RMS current carried by any of the main inverter bridge legs will be lower than for arrangements such as those of FIG. 1 or FIG. 2, where the inverter bridge legs must carry the total phase current.

[0057] Accordingly, for implementations where the inverters employ identical switching devices, the losses in the main inverter of FIG. 6 can be expected to be lower than those of the inverters of FIG. 1, FIG. 2 or FIG. 4. This is due to (i) a smaller number of bridge legs, and (ii) lower losses in some or all of said bridge legs.

[0058] Although FIG. 6 shows the switching devices a MOSFETs, the semiconductor devices used to implement the main inverter and the auxiliary converter may be chosen in different ways according to desired operating parameters of the system. FIG. 7 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter implemented by a combination of an IGBT and an antiparallel diode, and an auxiliary converter implemented using MOSFETs. The operation of the arrangement of FIG. 7 is substantially the same as that of FIG. 6.

[0059] FIG. 8 shows an example of a power converter for driving a three-phase SRM, comprising a main inverter implemented by a combination of an IGBT and an antiparallel diode, and an auxiliary converter implemented using an asymmetric configuration. If individual phase winding currents are unidirectional, then some of the bridge legs of the auxiliary voltage source may be implemented using an asymmetric configuration, with bridge legs having one active switching device and one diode in each leg. According to the example shown in FIG. 8, bridge legs S and U of the auxiliary converter are implemented with an asymmetric configuration, whilst bridge leg T of the auxiliary converter is implemented as a fully populated bridge leg.

[0060] Provided the currents i2 and i3 are greater than zero, each of vx and vy can take on three possible voltage states: +Vaux, 0, -Vaux. The voltage of each of vx and vy will depend upon the state of the switches in adjacent bridge legs. The voltages of vx and vy according to various switch states are as follows: D7 Q8 Q9 Q10 Q11 D12 Vx vy off on off on on off TKrux 0 off on on off on off 0 TKlUX off on off on off on 0 0 off on on off off on ^aux TKlUX on off off on on off ' vaux -y *aux on off on off on off 0 0 on off off on off on 0 -V yaux on off on off off on ^aux 0

[0061] The general operation of the arrangement of FIG. 8 is substantially the same as that of FIG. 6 and FIG. 7, however supporting unidirectional phase winding currents only.

[0062] FIG. 9 shows a generic arrangement for a power converter for driving a three-phase SRM, having two series voltage sources. This generic arrangement represents a genericised version of the arrangements shown in FIG. 6, FIG. 7 and FIG. 8. In the example of FIG. 9, a first bridge leg A comprises first switching device Q1 and second switching device Q2. A second bridge leg B comprises first switching device Q3 and second switching device Q4. A third bridge leg C comprises first switching device Q5 and second switching device Q6. First phase winding 1 is electrically coupled between bridge leg A and bridge leg B. Second phase winding 2 is electrically coupled in series with voltage source vy, between bridge leg B and bridge leg C. Third phase winding 3 is electrically coupled in series with voltage source vx, between bridge leg C and bridge leg A. Voltage source vy and voltage source vx share a common connection at bridge leg C.

[0063] For each of the arrangements of FIG. 6, FIG. 7, FIG. 8 and FIG. 9, the two series voltage sources vx and vy are not galvanically isolated from one another, having a point of common connection. The resulting auxiliary converter has a common dc-link, which allows for any ripple current components which arise to be compensated, either partially or fully, between the two series voltage sources vx and vy. Consequently, the amount of local energy storage and size of dc-link capacitor Caux associated with Vaux may be minimised.

[0064] FIG. 10 shows an alternative example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing a single voltage source. The main inverter comprises bridge legs A, B and C, said bridge legs formed from switches Q1 to Q6. The auxiliary converter comprises bridge legs T and U, said bridge legs formed from switches Q7 to Q10. The auxiliary converter may be used to synthesise a single arbitrary voltage. The voltage of vx according to various switch states is as follows: Q7 Q8 Q9 Q10 Vx off on on off TKzux on off on off 0 off on off on 0 on off off on Vaux

[0065] FIG. 11 shows an alternative example of a power converter for driving a three-phase SRM, comprising a main inverter and an auxiliary converter implementing a single voltage source using an asymmetric configuration. The main inverter comprises bridge legs A, B and C, said bridge legs formed from switches Q1 to Q6. The auxiliary converter comprises bridge legs T and U, and may be used to synthesise a voltage source vx for an implementation where the individual phase winding currents are unidirectional. Bridge leg T comprises a diode D7 and an active switch Q8. Bridge leg U comprises an active switch Q9 and a diode D10. Provided the current i3 is greater than zero, the voltage of vx according to various switch states is as follows: D7 Q8 Q9 D10 off on on off +y 1 yaux on off on off 0 off on off on 0 on off off on —V vaux

[0066] FIG. 12 shows a generic arrangement for a power converter driving a three-phase SRM, having a single series voltage source. This generic arrangement represents a genericised version of the arrangements shown in FIG. Wand FIG. 11. In the example of FIG. 12, a first bridge leg A comprises first switching device Q1 and second switching device Q2 A second bridge leg B comprises first switching device Q3 and second switching device Q4. A third bridge leg C comprises first switching device Q5 and second switching device Q6. First phase winding 1 is electrically coupled between bridge leg A and bridge leg B. Second phase winding 2 is electrically coupled between bridge leg B and bridge leg C. Third phase winding 3 is electrically coupled in series with voltage source vx, between bridge leg C and bridge leg A. Voltage source vx is coupled between bridge leg C and the third phase winding 3.

[0067] FIG. 13 shows an example of a generic arrangement for a power converter for driving a three-phase SRM, comprising a main inverter, and a voltage source in series with each of the phase windings. In the example of FIG. 13, a first bridge leg A comprises first switching device Q1 and second switching device Q2. A second bridge leg B comprises first switching device Q3 and second switching device Q4. A third bridge leg C comprises first switching device Q5 and second switching device Q6. First phase winding 1 is electrically coupled in series with voltage source vz, between bridge leg A and bridge leg B. Second phase winding 2 is electrically coupled in series with voltage source vy, between bridge leg B and bridge leg C. Third phase winding 3 is electrically coupled in series with voltage source vx, between bridge leg C and bridge leg A. Voltage source vy and voltage source vx share a common connection at bridge leg C. Voltage source vz is coupled between bridge leg A and the first phase winding 1.

[0068] As a result of having one voltage source per phase, the de bias voltage may be divided equally between the phases and therefore the individual voltage sources can each have a lower rated voltage compared to embodiments utilising fewer voltage sources. Additionally, the ac-components of and additional voltages and / or currents synthesised by the main inverter may be balanced across the phases of the SRM, thus simplifying control and providing a straightforward route to minimising any additional alternating energy exchange with the dc-link of the main inverter.

[0069] FIG. 14 shows an example of an arrangement for a power converter implementing the generic arrangement shown in FIG. 13, comprising a main inverter, a first auxiliary converter implementing two voltage sources, and a second auxiliary converter implementing a single voltage source. The main inverter comprises bridge legs A, B and C, said bridge legs formed from switches Q1 to Q6. The first auxiliary converter comprises bridge legs S, T and II, said bridge legs formed from switches Q11 to Q16. The first auxiliary converter comprises a first local dc-link capacitor Cauxl. The second auxiliary converter comprises bridge legs Q and R, said bridge legs formed from switches Q7 and Q10. The second auxiliary converter comprises a second local dc-link capacitor Caux2.

[0070] A first phase winding 1 is electrically coupled between bridge leg R of the second auxiliary converter and bridge leg B of the main inverter. A second phase winding 2 is electrically coupled between bridge leg B of the main inverter and bridge leg S of the first auxiliary converter. A third phase winding 3 is electrically coupled between bridge leg U of the first auxiliary converter and bridge leg A of the main inverter. Bridge leg A of the main inverter is electrically coupled to bridge leg Q of the second auxiliary converter, such that voltage va = vq. Bridge leg C of the main inverter is electrically coupled to bridge leg T of the first auxiliary converter, such that voltage vc = vt.

[0071] A first phase arm may comprise the first phase winding 1 in series with the voltage source vz. The first phase arm is connected between legs A and B of the main inverter. A second phase arm may comprise the second phase winding 2 in series with the voltage source vy. The second phase arm is connected between legs B and C of the main inverter. A third phase arm may comprise the third phase winding 3 in series with the voltage source vx. The third phase arm is connected between legs C and A of the main inverter. The voltages, vphl, vph2 and vph3, in the first, second and third phase arms, respectively, are therefore: Vphl =va-vb = v1-vz Vph2 =Vb-Vc = V2-Vy Vph3 =Vc~Va = V3-Vx

[0072] Each inverter phase arm therefore supports one phase winding voltage, v2 or v3, and in addition each of the phase arms, supports voltages -vz, —vy and -vx, respectively.

[0073] The first auxiliary converter is operable to synthesise an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltages vx and vy are generated by switching legs S, T and U of the first auxiliary converter. Voltages vx and vy are defined as: vx = vu - vt and, Vy = Vt - Vs

[0074] The second auxiliary converter is operable to synthesise an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltage vz is generated by switching legs Q and R of the second auxiliary converter. Voltages vz is defined as: Vz = Vr - vq

[0075] Each of vx, vy and vz can take on three possible voltage states: +Vaux, 0, -Vaux. The voltage of each of vx, vy and vz will depend upon the state of the switches in adjacent bridge legs in the respective auxiliary converter.

[0076] FIG. 15 shows an example of a generic arrangement for a power converter for driving a three-phase SRM, comprising a main inverter, and two voltages in series with each of the phase windings. In the example of FIG. 15, a first bridge leg A comprises first switching device Q1 and second switching device Q2. A second bridge leg B comprises first switching device Q3 and second switching device Q4. A third bridge leg C comprises first switching device Q5 and second switching device Q6. First phase winding 1 is electrically coupled in series with voltage sources vza and vzb, between bridge leg A and bridge leg B. Second phase winding 2 is electrically coupled in series with voltage sources vya and vyb, between bridge leg B and bridge leg C. Third phase winding 3 is electrically coupled in series with voltage sources vxa and vxb, between bridge leg C and bridge leg A. Voltage source vxb and voltage source vza share a common connection at bridge leg A. Voltage source vzb and voltage source vya share a common connection at bridge leg B. Voltage source vyb and voltage source vxa share a common connection at bridge leg C.

[0077] The arrangement of FIG. 15 combines the advantages of the embodiments of FIG. 6 and FIG. 13. Specifically, each phase is supported by two voltage sources, with one voltage source on each side of the respective phase winding. Each bridge leg is therefore associated with a separate auxiliary converter, which can be implemented in the same manner as for the embodiment of FIG. 6.

[0078] FIG. 16 shows an example of an arrangement for a power converter implementing the generic arrangement shown in FIG. 15, comprising a main inverter and three auxiliary converters, each of the three auxiliary converters implementing two voltage sources. The main inverter comprises bridge legs A, B and C, said bridge legs formed from switches Q1 to Q6.

[0079] A first auxiliary converter comprises bridge legs S, T and U, said bridge legs formed from switches Q19 to Q24. The first auxiliary converter comprises a first local dc-link capacitor Cauxl. A second auxiliary converter comprises bridge legs P, Q and R, said bridge legs formed from switches Q13 to Q18. The second auxiliary converter comprises a second local dc-link capacitor Caux2. A third auxiliary converter comprises bridge legs L, M and N, said bridge legs formed from switches Q7 to Q12. The third auxiliary converter comprises a third local dc-link capacitor Caux3.

[0080] A first phase winding 1 is electrically coupled between bridge leg N of the third auxiliary converter and bridge leg P of the second auxiliary converter. A second phase winding 2 is electrically coupled between bridge leg R of the second auxiliary converter and bridge leg S of the first auxiliary converter. A third phase winding 3 is electrically coupled between bridge leg U of the first auxiliary converter and bridge leg L of the third auxiliary converter.

[0081] Bridge leg A of the main inverter is electrically coupled to bridge leg M of the third auxiliary converter, such that voltage va = vm. Bridge leg B of the main inverter is electrically coupled to bridge leg Q of the second auxiliary converter, such that voltage vb = vq. Bridge leg C of the main inverter is electrically coupled to bridge leg T of the first auxiliary converter, such that voltage vc = vt.

[0082] A first phase arm may comprise the first phase winding 1 in series with voltage sources vza and vzb. The first phase arm is connected between legs A and B of the main inverter. A second phase arm may comprise the second phase winding 2 in series with voltage sources vya and vyb. The second phase arm is connected between legs B and C of the main inverter. A third phase arm may comprise the third phase winding 3 in series with voltage sources vxa and vxb. The third phase arm is connected between legs C and A of the main inverter. The voltages, vphl, vph2 and vph3, in the first, second and third phase arms, respectively, are therefore: Vphl = va-vb = v1- vza - vzb Vph3 Vc Va Vxa ^xb

[0083] The first auxiliary converter is operable to synthesize an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltages vxa and vyb are defined as: Vxa = vu — Vt and, Vyb =Vt~Vs

[0084] The second auxiliary converter is operable to synthesize an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltages vya and vzb are defined as: Vya Vr Vq and, Vzb Vq Vp

[0085] The third auxiliary converter is operable to synthesize an alternating voltage that interacts with the ac-components of the current through the phase windings. Voltages vza and vxb are defined as: Vza Vn Vm and, Vxb = Vm-Vt

[0086] Each of vxa, vxb, vya, vyb, vza and vzb can take on three possible voltage states: +Vaux, 0, -Vaux. The voltage of each of vxa, vxb, vya, vyb, vza and vzb will depend upon the state of the switches in adjacent bridge legs in the respective auxiliary converter.

[0087] FIG. 17 shows an example of a generic arrangement for a power converter for driving a four-phase SRM, comprising a main inverter, and two voltages in series with each of the phase windings.

[0088] In the example of FIG. 17, a first bridge leg A comprises first switching device Q1 and second switching device Q2. A second bridge leg B comprises first switching device Q3 and second switching device Q4. A third bridge leg C comprises first switching device Q5 and second switching device Q6. A fourth bridge leg D comprises first switching device Q7 and second switching device Q8. First phase winding 1 is electrically coupled in series with voltage sources vza and vzb, between bridge leg A and bridge leg B. Second phase winding 2 is electrically coupled in series with voltage sources vya and vyb, between bridge leg B and bridge leg C. Third phase winding 3 is electrically coupled in series with voltage sources vxa and vxb, between bridge leg C and bridge leg D. Fourth phase winding 4 is electrically coupled in series with voltage sources vwa and vwb, between bridge leg D and bridge leg A. Voltage source vwb and voltage source vza share a common connection at bridge leg A. Voltage source vzb and voltage source vya share a common connection at bridge leg B. Voltage source vyb and voltage source vxa, share a common connection at bridge leg C. Voltage source vxb and voltage source vwa share a common connection at bridge leg D.

[0089] In an implementation of the arrangement of FIG. 17, voltage sources vxb and vwa may be implemented by a first auxiliary converter comprising three bridge legs and a first local delink capacitor. Voltage sources vyb and vxa may be implemented by a second auxiliary converter comprising three bridge legs and a second local dc-link capacitor. Voltage sources vzb and vya may be implemented by a third auxiliary converter comprising three bridge legs and a third local dc-link capacitor. Voltage sources vwb and vza may be implemented by a fourth auxiliary converter comprising three bridge legs and a fourth local dc-link capacitor.

[0090] Advantageously, for arrangements according to the present disclosure, the switches of the main inverter can be sized to carry only the difference current, as opposed to the full phase currents. Under conditions of symmetrical excitation, each of the main inverter legs is subject to the same loading. Therefore, due to the absence of any de current, both switch positions in each bridge leg will be subject to the same average loading, thus leading to equidistribution of losses across the main inverter switches. Accordingly, the function of the main inverter switches can be implemented using commercially available power switching modules comprising one or more fully populated half bridges having equal current rating. This contrasts with the inverter arrangements of FIG. 1, FIG. 2 or FIG. 4, where the presence of a de component in one or more of the bridge legs will lead to an asymmetric distribution of losses across the inverter switch positions.

[0091] The lower voltage devices in the auxiliary converter can be expected to be of a lower cost and generate lower losses when compared to the higher voltage devices of the main inverter. The overall cost of the main inverter and the auxiliary converter can therefore be expected to be lower than that of the inverter according to the arrangement of FIG. 4, where the number of bridge legs must be one greater than the number of phases (e.g. four bridge legs for a three-phase implementation), and where at least two of the bridge legs must be rated for the full phase current. Additionally, the overall efficiency of the combined main inverter and auxiliary converter can be expected to be higher than that of the arrangement of FIG. 4.

[0092] Those skilled in the art will recognise that there are many possible alternative arrangements which may use different numbers of SRM phases and / or different numbers of voltage sources. These alternative arrangements have the same advantages discussed in relation to the various specific embodiments described herein.

[0093] Embodiments and examples of the present disclosure provide a power electronic arrangement for driving a switched reluctance machine, comprising: a plurality of phase arms, each phase arm comprising a phase winding, wherein the plurality of phase arms are connected in series to form a ring; and a main inverter having a plurality of inverter bridge legs, wherein a quantity of the plurality of bridge legs is equal to a quantity of the plurality of phase arms, wherein each of the plurality of bridge legs is connected at a switched node to two of the plurality of phase arms, and each of the plurality phase arms is connected to two of the plurality of bridge legs, and wherein at least one of the phase arms comprises at least one voltage source connected in series with a respective phase winding.

[0094] In some examples or embodiments disclosed herein the at least one voltage source is operable provide a voltage to support a circulating bias current in the plurality of phase arms.

[0095] In some examples or embodiments disclosed herein the circulating bias current is controlled independently of the main inverter.

[0096] In some examples or embodiments disclosed herein, the at least one voltage source is operable to provide a voltage having components at one or more frequencies, wherein the one or more frequencies include one or more of: a fundamental electrical frequency of the switched reluctance machine; a harmonic of the fundamental electrical frequency of the switched reluctance machine; and a frequency different from the fundamental electrical frequency of the switched reluctance machine or the harmonics of the fundamental electrical frequency of the switched reluctance machine.

[0097] In some examples or embodiments disclosed herein the at least one voltage source is provided by an auxiliary converter.

[0098] In some examples or embodiments disclosed herein the at least one voltage source comprises two voltage sources in series, and wherein the auxiliary converter comprises three half-bridge legs, the three half-bridge legs having a common direct current (DC) link.

[0099] In some examples or embodiments disclosed herein the two voltage sources comprise a first voltage source having a first terminal and a second terminal, and a second voltage source having a first terminal and a second terminal, wherein a first terminal of the first voltage source and a first terminal of the second voltage source are electrically coupled with the switched node of an inverter bridge leg of the main inverter, and wherein the second terminal of the first voltage source is electrically coupled to a first phase winding and the second terminal of the second voltage source is electrically coupled to a second phase winding.

[0100] In some examples or embodiments disclosed herein the at least one voltage source comprises a single voltage source, and wherein the auxiliary converter comprises two halfbridge legs, the two half-bridge legs having a common direct current (DC) link.

[0101] In some examples or embodiments disclosed herein the single voltage source comprises a first terminal and a second terminal, the first terminal electrically coupled to the switched node of an inverter bridge leg of the main inverter, and the second terminal electrically coupled to the respective phase winding.

[0102] In some examples or embodiments disclosed herein a voltage rating of a plurality of switches which comprise the half-bridge legs of the auxiliary converter is less than a voltage rating of a plurality of switches which comprise the plurality of main inverter bridge legs.

[0103] In some examples or embodiments disclosed herein a current rating of a plurality of switches which comprise the plurality of main inverter bridge legs is less than a current rating of a plurality of switches which comprise the half-bridge legs of the auxiliary converter.

[0104] Further embodiments and examples of the present disclosure provide a method of controlling the power electronic arrangement as described in embodiments and examples herein, comprising: controlling the at least one voltage source to provide a circulating bias current in the plurality of phase arms; and controlling the plurality of inverter bridge legs to provide a plurality of currents at a respective switched node.

[0105] In some examples or embodiments disclosed herein controlling the circulating bias current and controlling the plurality of currents provided by the main inverter is operable to regulate a current flowing in each phase arm of the plurality of phase arms.

[0106] In some examples or embodiments disclosed herein an electrical power used by a voltage source of the at least one voltage source to provide the circulating bias current is derived from a current flowing in said voltage source.

[0107] In some examples or embodiments disclosed herein an electrical power for the at least one voltage source is derived from one or more alternating current (AC) components of the plurality of currents provided by the main inverter.

[0108] In some examples or embodiments disclosed herein the plurality of currents provided by the main inverter comprise components at a fundamental electrical frequency of the switched reluctance machine and harmonics of said fundamental electrical frequency .

[0109] In some examples or embodiments disclosed herein the plurality of currents provided by the main inverter comprise components at frequencies different from a harmonic spectrum of the switched reluctance machine excitation currents.

[0110] Some examples or embodiments disclosed herein further comprise measuring an average phase current; and controlling the at least one voltage source to maintain the measured average phase current at a specified circulating bias current.

[0111] Some examples or embodiments disclosed herein further comprise controlling the at least one voltage source to maintain a DC link voltage of the auxiliary converter within a threshold of a specified value.

[0112] Further embodiments and examples of the present disclosure provide a switched reluctance machine comprising: a stator; a rotor; a housing; and the power electronic arrangement as described in embodiments and examples herein.

[0113] In some examples or embodiments disclosed herein the at least one voltage source is integrated within the housing of the switched reluctance machine.

[0114] In some examples or embodiments disclosed herein the at least one voltage source and the main inverter are integrated in a single package.

[0115] In some examples or embodiments disclosed herein the at least one voltage source is housed separately from the switched reluctance machine and the main inverter.

[0116] In some examples or embodiments disclosed herein the main inverter is integrated within the housing of the switched reluctance machine.

[0117] In some examples or embodiments disclosed herein the main inverter is housed separately from the switched reluctance machine.

[0118] All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be combined in any combination, except combinations where some features are mutually exclusive. Each feature disclosed in this specification, including any accompanying claims, abstract, and drawings, may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed in one example of a generic series of equivalent or similar features.

[0119] The present teachings are not restricted to the details of any of the foregoing examples. Any novel combination of the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be envisaged. The claims should not be construed to cover merely the foregoing examples, but also any variants which fall within the scope of the claims.

Claims

1. A power electronic arrangement for driving a switched reluctance machine, comprising:a plurality of phase arms, each phase arm comprising a phase winding, wherein the plurality of phase arms are connected in series to form a ring; anda main inverter having a plurality of inverter bridge legs, wherein a quantity of the plurality of bridge legs is equal to a quantity of the plurality of phase arms,wherein each of the plurality of bridge legs is connected at a switched node to two of the plurality of phase arms, and each of the plurality phase arms is connected to two of the plurality of bridge legs, andwherein at least one of the phase arms comprises at least one voltage source connected in series with a respective phase winding.

2. The power electronic arrangement of claim 1, wherein the at least one voltage source is operable provide a voltage to support a circulating bias current in the plurality of phase arms.

3. The power electronic arrangement of claim 2, wherein the circulating bias current is controlled independently of the main inverter.

4. The power electronic arrangement of claim 1, wherein the at least one voltage source is operable to provide a voltage having components at one or more frequencies, wherein the one or more frequencies include one or more of:a fundamental electrical frequency of the switched reluctance machine;a harmonic of the fundamental electrical frequency of the switched reluctance machine; anda frequency different from the fundamental electrical frequency of the switched reluctance machine or the harmonics of the fundamental electrical frequency of the switched reluctance machine.

5. The power electronic arrangement of claim 1, wherein the at least one voltage source is provided by an auxiliary converter.

6. The power electronic arrangement of claim 5, wherein the at least one voltage source comprises two voltage sources in series, andwherein the auxiliary converter comprises three half-bridge legs, the three half-bridge legs having a common direct current (DC) link.

7. The power electronic arrangement of claim 6, wherein the two voltage sources comprise a first voltage source having a first terminal and a second terminal, and a second voltage source having a first terminal and a second terminal,wherein a first terminal of the first voltage source and a first terminal of the second voltage source are electrically coupled with the switched node of an inverter bridge leg of the main inverter, andwherein the second terminal of the first voltage source is electrically coupled to a first phase winding and the second terminal of the second voltage source is electrically coupled to a second phase winding.

8. The power electronic arrangement of any of claims 5 to 7, wherein the at least one voltage source comprises a single voltage source, andwherein the auxiliary converter comprises two half-bridge legs, the two half-bridge legs having a common direct current (DC) link.

9. The power electronic arrangement of claim 8, wherein the single voltage source comprises a first terminal and a second terminal, the first terminal electrically coupled to the switched node of an inverter bridge leg of the main inverter, and the second terminal electrically coupled to the respective phase winding.

10. The power electronic arrangement of any of claims 1 to 9, wherein a voltage rating of a plurality of switches which comprise the half-bridge legs of the auxiliary converter is less than a voltage rating of a plurality of switches which comprise the plurality of main inverter bridge legs.

11. The power electronic arrangement of any of claims 1 to 10, wherein a current rating of a plurality of switches which comprise the plurality of main inverter bridge legs is less than a current rating of a plurality of switches which comprise the half-bridge legs of the auxiliary converter.

12. A method of controlling the power electronic arrangement of any of claims 1 to 11, comprising:controlling the at least one voltage source to provide a circulating bias current in the plurality of phase arms; andcontrolling the plurality of inverter bridge legs to provide a plurality of currents at a respective switched node.

13. The method of claim 12, wherein controlling the circulating bias current and controlling the plurality of currents provided by the main inverter is operable to regulate a current flowing in each phase arm of the plurality of phase arms.

14. The method of claim 13, wherein an electrical power used by a voltage source of the at least one voltage source to provide the circulating bias current is derived from a current flowing in said voltage source.

15. The method of any of claims 12 to 14, wherein an electrical power for the at least one voltage source is derived from one or more alternating current (AC) components of the plurality of currents provided by the main inverter.

16. The method of any of claims 12 to 15, wherein the plurality of currents provided by the main inverter comprise components at a fundamental electrical frequency of the switched reluctance machine and harmonics of said fundamental electrical frequency.

17. The method of any of claims 12 to 16, wherein the plurality of currents provided by the main inverter comprise components at frequencies different from a harmonic spectrum of the switched reluctance machine excitation currents.

18. The method of any of claims 12 to 17, further comprising measuring an average phase current; andcontrolling the at least one voltage source to maintain the measured average phase current at a specified circulating bias current.

19. The method of any of claims 12 to 18, further comprising controlling the at least one voltage source to maintain a DC link voltage of the auxiliary converter within a threshold of a specified value.

20. A switched reluctance machine comprising:a stator;a rotor;a housing; andthe power electronic arrangement of any of claims 1 to 11.

21. The switched reluctance machine of claim 20, wherein the at least one voltage source is integrated within the housing of the switched reluctance machine.

22. The switched reluctance machine of claim 20 or 21, wherein the at least one voltage source and the main inverter are integrated in a single package.

23. The switched reluctance machine of claim 20, wherein the at least one voltage source is housed separately from the switched reluctance machine and the main inverter.

24. The switched reluctance machine of claim 20 or 21, wherein the main inverter is integrated within the housing of the switched reluctance machine.

25. The switched reluctance machine of any of claims 20 to 23, wherein the main inverter is housed separately from the switched reluctance machine.31

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