WO2016207617A1

WO2016207617A1 - Control of a switched reluctance motor in clockwise-rotating and anti-clockwise-rotating electric superchargers

Info

Publication number: WO2016207617A1
Application number: PCT/GB2016/051849
Authority: WO
Inventors: Goncalo MANUEL; Phil SPANTON; Kamel BOUALLAGA; Don Lee
Original assignee: Valeo Air Management Uk Limited
Priority date: 2015-06-24
Filing date: 2016-06-21
Publication date: 2016-12-29
Also published as: GB201511118D0; GB2539689A

Abstract

A control unit for use in controlling a switched reluctance motor (SRM) for clockwise rotation in a clockwise-rotating electric supercharger and anti-clockwise rotation in an anti- clockwise-rotating electric supercharger. Each SRM has a rotor (101) and a multiplicity of pairs of stator coils (103). The control unit is configurable in a first mode of operation, for controlling the SRM in the clockwise-rotating supercharger, and the control unit is also configurable in a second mode of operation, for controlling the SRM in the anti-clockwise-rotating supercharger. In that second mode of operation the control unit is configured to energise, at the same time the first and second pairs (105A/105C) of the stator coils, turn off the second pair (105C) of coils, while leaving the first pair (105A) energised; and after the rotor (101) is caused to rotate in the anti-clockwise direction, subsequently accelerating the rotor.

Description

CONTROL OF A SWITCHED RELUCTANCE MOTOR IN CLOCKWISE-ROTATING

AND ANTI-CLOCKWISE-ROTATING ELECTRIC SUPERCHARGERS

FIELD OF THE INVENTION The present invention relates to switched reluctance motors, and in particular to the control of switched

reluctance motors in clockwise-rotating and anti-clockwise- rotating electric superchargers.

BACKGROUND

The use of a switched reluctance motor (SRM) in an electric supercharger is known.

When an SRM is switched off, the rotor tends to have been left in an arbitrary stationary position. A known method of starting a rotor 1 of an SRM is described with reference to

Figures 1A to ID. In this example, the SRM comprises a four- pole rotor 1 and a six-pole stator 3. The six-pole stator 3 comprises three pairs of coils A, B and C. Each pair of coils corresponds to a phase and the terms ^xpairs of coils' and ^xphase' can be used interchangeably.

Referring to Figures 1A and 1C an initial desired

stationary position for the rotor 1 is established by

energising just one pair of stator coils (i.e. a first pair A) which is then held on for some time. These coils attract the nearest pair of poles of the rotor 1 (as shown by the

alignment of the magnetic field 5 of phase A in Figures 1A/C) . The rotor 1, and whatever it is the rotor is attached to (for example the compressor wheel of a supercharger) , gain angular momentum and the poles of the rotor 1 tend to overshoot and oscillate about the position of the pair of coils A. They eventually settle to rest with the poles aligned on the diameter of the first pair of coils A.

To start the SRM, the first pair of coils A are turned off and another pair of coils B is turned on, depending on the desired direction of rotation. For example, the pair of coils B to one side of the first pair A would be switched on to effect rotation in one direction (counter-clockwise in Figure IB) , whereas the pair C to the other side of the first pair A would be switched on to effect rotation in the other direction (clockwise in Figure ID) .

The known starting method described above has, however, the disadvantage that the duration of the first step (or aligning the rotors with the first pair of coils) can be unacceptably long. It can also provide a large current demand for the coils.

To address this problem, an alternative starting method has been suggested. That method is described in

PCT/GB2014/053709 and PCT/GB2014/053711 in the name of Valeo Air Management UK Limited (unpublished at the time of filing this application) . In that method, two adjacent pairs of stator coils (first and second pairs of coils) are initially energised at the same time. Once the rotor has settled to a position between the two pairs of stator coils, the second pair is turned off, while leaving the first pair energised. Thus, the rotor is caused to rotate by the first pair of coils. The rotor is then accelerated by repeatedly turning on and off (only) that first pair of coils (leaving the other coils switched off) .

In such an arrangement it has been found to be beneficial to use the first pair of coils as a reference for the time and/or angle of the rotor. For example, the SRM may comprise a single position sensor located equidistant between the first pair of coils. The single position sensor may be arranged to detect sectors of different magnetisation on a magnetic ring indicator as they rotate past the sensor. From these

measurements (detecting alternating North/South (N/S) sectors of magnetisation) , the angle, timing and speed of the rotor may be determined.

The magnetic ring indicator needs to be offset from the orientation of the rotor poles by a predetermined amount.

Thus, the angle at which a rotor pole passes a stator pole of a pair of coils is offset from the angle at which the N-S boundary respective to that rotor pole passes the sensor.

This enables the energisation of the coils to be correctly timed (i.e. in advance of the rotor pole centre becoming aligned with them) .

In the field of electric superchargers, one design of supercharger may require the SRM to run in a first direction (e.g. clockwise), but another design of supercharger may require the SRM to run in the opposite direction (e.g. anti^¬ clockwise) . Whilst the above-mentioned control methodology and arrangement may be beneficial in some respects, it has been found to be relatively inflexible in terms of its ability to be used on these different SRMs . In particular, if the direction of rotation were to be changed by changing the energising sequence of the coils, it would also become

necessary to use a different pair of coils as a reference and to make associated changes to the sensor position.

The present invention seeks to reduce or mitigate at least some of the above-mentioned disadvantages.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a control unit for use in controlling a switched reluctance motor for clockwise rotation in a clockwise-rotating electric supercharger and for use in controlling a switched reluctance motor for anti-clockwise rotation in an anti-clockwise- rotating electric supercharger. Each switched reluctance motor has a rotor and a multiplicity of pairs of stator coils, each pair comprising coils that are on opposite sides of the rotational axis of the motor, and wherein the pairs of stator coils comprise: a first pair of coils for forming a first phase; a second pair of coils to one side of the first pair, for forming a second phase; and a third pair of coils to the other side of the first pair for forming a third phase. The control unit is configurable in a first mode of operation, for controlling the switched reluctance motor in the clockwise- rotating supercharger. In that first mode of operation the control unit is configured to perform the following steps: (i) energising, at the same time the first and third pairs of the stator coils, waiting for a period for rotary movement of the rotor to reduce, and then turning off the third pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the clockwise direction by the first pair of coils; and (ii) after the rotor is caused to rotate in the clockwise direction, subsequently accelerating the rotor in the clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off. The control unit is also

configurable in a second mode of operation, for controlling the switched reluctance motor in the anti-clockwise-rotating supercharger, and in that second mode of operation the control unit is configured to perform the following steps: (a)

energising, at the same time the first and second pairs of the stator coils, waiting for a period for rotary movement of the rotor to reduce, and then turning off the second pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the anti-clockwise direction by the first pair of coils; and (b) after the rotor is caused to rotate in the anti-clockwise direction, subsequently

accelerating the rotor in the anti-clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off.

Providing a control unit that is configured to perform these different steps depending on whether it is arranged in the clockwise-rotating or the anti-clockwise-rotating

supercharger, facilitates a relatively flexible arrangement. In particular, it enables the same control unit to be used on two different superchargers without necessarily requiring associated changes to the supercharger (for example in some embodiments it may avoid the need to re-position a sensor in the supercharger) . The first pair of stator coils may be repeatedly turned on and off at a constant frequency (while keeping the other stator coil turned off) . The timing at which the pair of stator coils is turned on and off is preferably independent of the desired torque and/or speed (torque/speed set-point) . For example the timings are preferably not obtained from a look-up table. The timing may be constant. The timings may be in response to an output from a position sensor. For example the pair of stator coils is preferably turned on when the rotor is in a first position (preferably measured by a position

sensor) . The stator coils are preferably turned off when the rotor is in a second position (preferably measured by a position sensor) . Such an arrangement is beneficial because it provides a simple way of accelerating the motor, without requiring a calculation, or other form of selection, of the timings.

The control unit is preferably configured to receive an input indicative of whether it is to control the clockwise- rotating or the anti-clockwise-rotating supercharger. The input indicative may be indicative of whether the control unit is installed in the clockwise-rotating or the anti-clockwise- rotating supercharger. The control unit is preferably

arranged to determine whether to perform steps (i) and (ii), or steps (a) and (b) , in dependence on said input. In

dependence on the input, the control unit may be operable into the first mode of operation in which it is configured to perform steps (i) and (ii) . In dependence on the input, the control unit may be operable into the second mode of operation in which it is configured to perform steps (a) and (b) . In some embodiments of the invention, the control unit may receive the input (indicative of whether it is to control the clockwise-rotating or the anti-clockwise-rotating

supercharger) before installation in that supercharger. For example, the control unit may receive the input before, or during, the assembly of the supercharger. In some embodiments of the invention, the control unit may receive the input during calibration of the supercharger. In some embodiments of the invention, the control unit may read a hardware setting (e.g. a flag) indicative of whether the control unit is installed in a clockwise-rotating or the anti-clockwise- rotating supercharger.

The steps performed by the control unit are preferably arranged to start the switched reluctance motor. For example, they may be (or form part of) a starting procedure.

Each supercharger may comprise a magnetic ring indicator having a set of boundaries between sectors of different magnetisation, the magnetic ring indicator being arranged to rotate with the rotor. Each supercharger may comprise a sensor arranged to detect the sectors of different

magnetisation of the ring indicator as they rotate past the sensor, such that the angle of the rotor may be determined. The control unit has been found to be especially beneficial for use in superchargers having such a magnetic ring indicator and a sensor because such arrangements would otherwise tend to need significant structural modifications to be run in the opposite direction (for example the sensor may otherwise need to be re-positioned) .

In the clockwise-rotating supercharger, the ring

indicator may be offset from the poles of the rotor by a first offset. In the anti-clockwise-rotating supercharger the ring indicator may be offset from the poles of the rotor by a second offset. The second offset is preferably in the

opposite direction to the first offset, but equal in

magnitude. It has been found that the control unit can be used on the two different superchargers with such oppositely offset indicator rings, but without tending to require other structural modifications to the supercharger and/or control system. In both the clockwise-rotating and the anticlockwise-rotating superchargers, the ring indicator may be arranged such that the sensor detects the same order of transition between sectors of magnetisation (e.g. from North- to-South rather than South-to-North) as the rotor rotates. Thus the ring indicator may, in effect, be rotated not only a first amount to provide the above-mentioned opposing offset, but also a second amount (e.g. 45 degrees for an 8-pole ring indicator) to provide the same order of transition between sectors of magnetisation.

The first pair of coils may be repeatedly turned on and off dependent on the output on the position sensor. Thus the first pair of coils may be used as a reference phase for time and/or angle of rotation of the rotor. In both the clockwise- rotating and the anti-clockwise-rotating superchargers, the sensor may be a single sensor arranged to detect the sectors of different magnetisation of the ring indicator as they rotate past the sensor. The single sensor may be positioned equidistant between the pair of coils in the first pair of coils .

For each pair of stator coils, the coils of the pair are preferably diametrically opposite on the stator. In principle the second and third pairs of coils need only be to one side of the first pair (but could be spaced apart therefrom) .

However, in preferred embodiments of the invention, the second pair of coils is the directly neighbouring pair of coils to one side of the first pair. The third pair of coils is preferably the directly neighbouring pair of coils to the other side of the first pair.

A pair of coils may be referred to as a phase, and reference herein to a ^xpair of coils' and ^xphase' may be interchangeable.

According to a second aspect of the invention, there is provided an electric supercharger comprising: a switched reluctance motor, a control unit, as described herein with reference to the first aspect, arranged to control the

switched reluctance motor, and a compressor wheel coupled to the rotor of the motor to be driven thereby. The electric supercharger may be a clockwise-rotating electric

supercharger; the control unit may be configured in the first mode. The electric super charger may be an anti-clockwise- rotating electric supercharger; the control unit may be configured in the second mode.

According to a third aspect of the invention, there is provided a method of controlling a switched reluctance motor to rotate in either a clockwise direction in a clockwise- rotating electric supercharger or an anti-clockwise direction in an anti-clockwise-rotating electric supercharger. Each switched reluctance motor has a rotor and a multiplicity of pairs of stator coils, each pair comprising coils that are on opposite sides of the rotational axis of the motor, and wherein the pairs of stator coils comprise: a first pair of coils for forming a first phase, a second pair of coils to one side of the first pair, for forming a second phase; and a third pair of coils to the other side of the first pair for forming a third phase. The method comprises the steps of: determining whether the switched reluctance motor being controlled is in the clockwise-rotating supercharger or the anti-clockwise-rotating supercharger; and if controlling the switched reluctance motor in the clockwise-rotating

supercharger, operating in a first mode to conduct a method comprising the steps of: (i) energising, at the same time the first and third pairs of the stator coils, waiting for a period for rotary movement of the rotor to reduce, and then turning off the third pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the clockwise direction by the first pair of coils; and (ii) after the rotor is caused to rotate in the clockwise direction, subsequently accelerating the rotor in the clockwise

direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off. Whereas, if controlling the switched reluctance motor in the anti-clockwise-rotating supercharger, operating in a second mode to conduct a method comprising the steps of: (a) energising, at the same time the first and second pairs of the stator coils, waiting for a period for rotary movement of the rotor to reduce, and then turning off the second pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the anti-clockwise direction by the first pair of coils; and (b) after the rotor is caused to rotate in the anti-clockwise direction, subsequently

According to yet another aspect of the invention, there is provided a computer program product arranged, when executed on a computing device, to perform the method of the above- mentioned aspect of the invention.

According to yet another aspect of the invention there is provided a control unit for an electric supercharger,

comprising the computer program product of the above-mentioned aspect. The control unit may comprise a microprocessor for executing the computer program product.

It will be appreciated that features described with reference to one aspect of the invention are equally

applicable to another aspect, or vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to the accompanying drawings, of which:

FIGURES 1A to ID show a known starting sequence for a switched reluctance motor;

FIGURES 2A and 2B show a starting sequence in a first SRM controlled by a control unit of a first embodiment of the invention;

FIGURE 3 shows the output from the sensor in Figures 2A and 2B, and the energisation of Phase A;

FIGURES 4A and 4B show a starting sequence in a second SRM controlled by the control unit of a first embodiment of the invention; and

FIGURE 5 is a flow chart showing steps taken by the control unit of the first embodiment of the invention. DETAILED DESCRIPTION

FIGURES 2A and 2B, and 4A and 4B show a schematic of a first, anti-clockwise-rotating, switched reluctance motor (SRM) and a second, clockwise-rotating, SRM respectively.

Features in Figures 4A-B that correspond to similar features in Figures 2A-B, are shown with the same reference numerals as in the first embodiment, but with the prefix ^λ2' (or ^λ20' where appropriate) instead of ^λ1' or ^λ10'. In these

embodiments, the directions of rotation are defined by viewing the SRM from the rear end.

Each SRM comprises a six-pole stator 103/203 with three pairs of stator coils A, B, and C. Each pair of coils A, B ,C comprises two diametrically opposite coils arranged to provide a magnetic field when energised (each pair A, B, C may

subsequently be referred to herein as a Phase) . Each SRM also comprises a four-pole rotor 101/201 arranged to rotate within the stator 103/203. In Figures 2A-B, and 4A-B, a reference point 107/207 is shown on the rotor to assist in understanding the invention and visualising the rotation of the rotor

101/201.

The shaft (not shown) to which the rotor is attached also supports a magnetic ring indicator 109/209 having four pairs of north poles 111/211 and south poles 113/213 (i.e. eight alternating magnetic sectors) . In the first SRM of Figures 2A and 2B, the ring indicator 109 is co-axial with the rotor 101, but offset by +4 degrees from the rotor poles. In the second SRM of Figures 4A and 4B, the ring indicator 209 is co-axial with the rotor 201, but offset by -4 degrees from the rotor poles (although those offsets are too small to be visible in the Figures) . The differing offset is explained in more detail below with reference to the direction in which the SRMs are run. Each SRM comprises a single position sensor 115/215. The sensor 115/215 is positioned equidistant between the pair of coils A, and is arranged to detect the sectors of different magnetisation of the ring indicator 109/209 as they rotate past the sensor 115/215.

The first SRM is located in an anti-clockwise-rotating electric supercharger (not shown) that is designed to run in the anti-clockwise direction as viewed in Figures 2A and 2B. That anti-clockwise-rotating supercharger comprises a control unit (not shown) according to a first embodiment of the invention. The control unit is arranged to start the rotation of the rotor 101 of the first SRM in the correct (anti^¬ clockwise) direction as is explained below.

Referring first to Figure 2A the SRM is started by initially energising two directly neighbouring pairs of stator coils (phases A and C in this case) . The magnetic fields 105a, 105c of pairs A and C are shown in Figure 2A, and they act on the rotor 101 so as to hold it in the intermediate position, between the two phases A/C shown in Figure 2A. The next step is to release (i.e. no longer energise) phase C, but maintain the energisation of phase A. This causes the rotor 101 to rotate in an anti-clockwise direction as shown by the arrow in Figure 2B.

After the rotor 101 is caused to rotate in this

direction, it is subsequently accelerated (in that anti^¬ clockwise direction) by repeatedly turning on and off phase A, while keeping the other pairs of stator coils (phases) turned off. The top half of Figures 3 shows the output of the position sensor 115, and the bottom half of Figure 3 shows the energisation of phase A, during a 180 degree rotation of the rotor 101. The position sensor 115 alternates between high and low outputs depending on the magnetisation 111/113 of the sector of the ring indicator 109 as it passes it. When the sensor output 115 is high, the phase A is repeatedly

energised, but is not energised when the output is low. Thus, each time a pole of the rotor 101 approaches the pair of coils A, it is drawn towards them, thereby accelerating the rotor 101.

Embodiments of the present invention provide a control unit that is not only operable on the anti-clockwise-rotating supercharger (i.e. to be run in an anti-clockwise direction), but one that is also operable on a clockwise-rotating

supercharger (i.e. to be run in the opposite direction) . The control of the SRM in the second supercharger is shown in Figures 4A and 4B, to which reference is now made.

Referring first to Figure 4A, the SRM is started by initially energising phase A, but instead of also energising phase C (as per Figure 2A) , phase B (the pair of coils the other side of pair A) is energised. The magnetic fields 205a, 205b of pairs A and B are shown in Figure 4A, and they act on the rotor 201 so as to hold it in the intermediate position, between the two phases A/B shown in Figure 4A.

The next step is to release (i.e. no longer energise) phase B, but maintain the energisation of phase A. This causes the rotor 201 to rotate in a clockwise direction as shown by the arrow in Figure 4B. After the rotor 201 is caused to rotate in this

direction, it is subsequently accelerated (in that clockwise direction) by repeatedly turning on and off phase A, while keeping the other pairs of stator coils turned off. This final step of the starting sequence is the same as that employed on the first SRM of Figures 2A-B. Since the ring indicator 209 is offset by -4 degrees in the second SRM

(rather than + 4 degrees in the first) the energising of phase A accelerates the rotor 201 in the clockwise direction. The ring indicator is also rotated 45 degrees to effectively reverse the north-south poles on the ring indicator (i.e. it is 53 degrees from the position of the ring indicator in the first supercharger) ; this ensures the detector is arranged to detect a transition from North-to-South magnetisation.

It will be appreciated from the description above, that the same control unit is thus arranged to be able to be used on both the anti-clockwise-rotating and the clockwise-rotating superchargers, despite those superchargers being arranged to be run in opposite directions.

A schematic of the control unit 117 and some of the steps processed by it, is shown in Figure 5. The control unit 117 is first arranged to receive an input indicative of whether it is to control an anti-clockwise-rotating or clockwise-rotating supercharger. In this embodiment, the input is received from an output in the respective supercharger, which flags the direction in which the supercharger is designed to run. In other embodiments (not shown) the input may be received in another manner (for example) it may be input during

calibration .

In dependence on this input, the control unit 117 either executes the steps in the left, or right-hand side of Figure 5. The left-hand side describes some of the process for the anti-clockwise-rotating supercharger, whereas the right-hand side describes some of the process for the clockwise-rotating supercharger .

The control unit 117 conducts the initial step of holding the rotor 101/201 using two phases for a timer period T_Hi which is sufficient for the rotor to stabilise. Once the required phase (C or B) is released the control unit checks (for a time period T_H2) for a position sensor 115/215 signal to determine if the rotor is rotating. If it is rotating, the control unit 117 repeatedly energises phase A (not shown in Figure 5) , but if the position sensor signal is not detected with period T_H2 the control unit recognises a failed start has occurred.

Whilst the present invention has been described and illustrated with reference to a particular embodiment, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For example, the stator need not necessarily comprise 6-poles. The stator may comprise more than 6-poles. The rotor need not necessarily comprise 4- poles. The stator may comprise more than 4-poles. The choice of stator/rotor poles typically depends on the topology required. Where in the foregoing description, integers or elements are mentioned which have known, obvious or

foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to

encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the

independent claims.

Claims

1. A control unit for use in controlling a switched reluctance motor for clockwise rotation in a clockwise-rotating electric supercharger and for use in controlling a switched reluctance motor for anti-clockwise rotation in an anti-clockwise- rotating electric supercharger,

wherein each switched reluctance motor has a rotor and a multiplicity of pairs of stator coils, each pair comprising coils that are on opposite sides of the rotational axis of the motor, and wherein the pairs of stator coils comprise:

a first pair of coils for forming a first phase

a second pair of coils to one side of the first pair, for forming a second phase; and

a third pair of coils to the other side of the first pair for forming a third phase wherein the control unit is configurable in a first mode of operation, for controlling the switched reluctance motor in the clockwise-rotating supercharger, and wherein in that first mode of operation the control unit is configured to perform the following steps:

(i) energising, at the same time the first and third pairs of the stator coils,

waiting for a period for rotary movement of the rotor to reduce, and then

turning off the third pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the clockwise direction by the first pair of coils; and

(ii) after the rotor is caused to rotate in the clockwise direction, subsequently accelerating the rotor in the

clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off, characterised in that the control unit is also configurable in a second mode of operation, for controlling the switched reluctance motor in the anti-clockwise-rotating supercharger, and in that second mode of operation the control unit is configured to perform the following steps:

(a) energising, at the same time the first and second pairs of the stator coils,

waiting for a period for rotary movement of the rotor to reduce, and then

turning off the second pair of coils, while leaving the first pair energised so that the rotor is caused to rotate in the anti-clockwise direction by the first pair of coils; and

(b) after the rotor is caused to rotate in the anti^¬ clockwise direction, subsequently accelerating the rotor in the anti-clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off.

2. A control unit according to claim 1, wherein each

supercharger comprises a magnetic ring indicator having a set of boundaries between sectors of different magnetisation, the magnetic ring indicator being arranged to rotate with the rotor

and each supercharger comprises a sensor arranged to detect the sectors of different magnetisation of the ring indicator as they rotate past the sensor, such that the angle of the rotor may be determined.

3. A control unit according to claim 2, wherein in the clockwise-rotating supercharger, the ring indicator is offset from the poles of the rotor by a first offset, and wherein in the anti-clockwise-rotating supercharger the ring indicator is offset from the poles of the rotor by a second offset, the second offset being in the opposite direction to the first offset, but equal in magnitude.

4. A control unit according to claim 2 or claim 3, wherein in both the clockwise-rotating and the anti-clockwise-rotating superchargers, the sensor is a single sensor arranged to detect the sectors of different magnetisation of the ring indicator as they rotate past the sensor.

5. A control unit according to claim 4, wherein the single sensor is positioned equidistant between the pair of coils in the first pair of coils.

6. A control unit according to any preceding claim, wherein for each pair of stator coils, the coils of the pair are diametrically opposite on the stator.

7. A control unit according to any preceding claim, wherein the second pair of coils is the directly neighbouring pair of coils to one side of the first pair, and the third pair of coils is the directly neighbouring pair of coils to the other side of the first pair.

8. An electric supercharger comprising:

a switched reluctance motor,

a control unit as claimed in any of claims 1 to 7

arranged to control the switched reluctance motor, and a compressor wheel coupled to the rotor of the motor to be driven thereby.

9. An electric supercharger according to claim 8, wherein the electric supercharger is a clockwise-rotating electric supercharger, and the control unit is configured in the first mode .

10. An electric supercharger according to claim 8, wherein the electric super charger is an anti-clockwise-rotating electric supercharger, and the control unit is configured in the second mode .

11. A method of controlling a switched reluctance motor to rotate in either a clockwise direction in a clockwise-rotating electric supercharger or an anti-clockwise direction in an anti-clockwise-rotating electric supercharger, wherein each switched reluctance motor has a rotor and a multiplicity of pairs of stator coils, each pair comprising coils that are on opposite sides of the rotational axis of the motor, and wherein the pairs of stator coils comprise:

a first pair of coils for forming a first phase

a third pair of coils to the other side of the first pair for forming a third phase wherein the method comprises the steps of: determining whether the switched reluctance motor being controlled is in the clockwise-rotating supercharger or the anti-clockwise-rotating supercharger; and if controlling the switched reluctance motor in the clockwise-rotating supercharger, operating in a first mode to conduct a method comprising the steps of:

(i) energising, at the same time the first and third pairs of the stator coils,

waiting for a period for rotary movement of the rotor to reduce, and then

clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off, characterised in that if controlling the switched reluctance motor in the anti- clockwise-rotating supercharger, operating in a second mode to conduct a method comprising the steps of:

waiting for a period for rotary movement of the rotor to reduce, and then

(b) after the rotor is caused to rotate in the anti- clockwise direction, subsequently accelerating the rotor in the anti-clockwise direction by repeatedly turning on and off the first pair of stator coils, while keeping the other pairs of stator coils turned off.

12. A computer program product arranged, when executed on a computing device, to perform the method of claim 11.

13. A control unit for an electric supercharger, comprising the computer program product of claim 12.

14. A control unit, supercharger and/or method as described herein with reference to the Figures.