WO2016092262A1

WO2016092262A1 - An electric supercharger and method of protecting an electric supercharger from high temperatures

Info

Publication number: WO2016092262A1
Application number: PCT/GB2015/053510
Authority: WO
Inventors: Matthew Webster
Original assignee: Valeo Air Management UK Ltd
Current assignee: Valeo Air Management UK Ltd
Priority date: 2014-12-12
Filing date: 2015-11-18
Publication date: 2016-06-16
Anticipated expiration: 2017-06-12
Also published as: EP3231082A1

Abstract

A method of operating a motor in an electric supercharger, the method comprising operating the motor in a first, low-Noise Vibration and Harshness (NVH), mode in which the timings of the current supply of each phase are obtained from a first source in which the timings are selected for low-NVH; monitoring a temperature of the motor; and in response to the monitored temperature exceeding a predetermined threshold, switching operation of the motor into a second, lower- temperature, mode in which the timings of the current supply of each phase are selected for generating lower temperatures in a controller of the switched-reluctance motor.

Description

AN ELECTRIC SUPERCHARGER AND METHOD OF PROTECTING AN ELECTRIC

SUPERCHARGER FROM HIGH TEMPERATURES

The present invention relates to electric superchargers, in particular electric superchargers including motors.

BACKGROUND

A switched reluctance motor (SRM) has a plurality of stator poles and a plurality of rotor poles. One set of poles

(usually the stator poles) are coils that are energized using a plurality of electrical phases. The other set of poles (usually the rotor poles) are typically ferromagnetic

material, such as iron. Successive pairs of the coils are energized in turn, causing the rotor to rotate. In an electric supercharger (eSC) switched reluctance motor, energizing of the coils is typically controlled by a controller. As the rotor rotates, a rotor position transducer (RPT) provides a magnetic waveform from which the speed and position of each of the rotor poles are calculated. The energizing of each pair of coils is carried out as the rotor reaches particular angles in its rotation. For each pair of coils, at an ON angle/time, current is supplied to the coils. At a Freewheel (FW) angle/time, supply of current is stopped, but the current already in the coils is allowed to circulate through the coil via a diode. At an OFF angle/time, the circulating current is switched to ground, turning off the coil. Sets of ON, FW and OFF switching angles/times are provided for different motor speeds and different motor torques in an ON, FW and OFF look-up table, respectively.

Where there is modulation of the current pulse width, a fourth look-up table provides pulse width modulation level (PWM) for different motor speeds and different motor torques. A problem with known eSC using switched-reluctance motors is that they suffer from effects, including noise vibration and harshness (NVH) , which are undesirable for consumers.

It has been found that by starting the supply of current to the plurality of coils earlier (i.e. an earlier timing/smaller angle (relative to the unaligned position) ) the NVH is

reduced. However, using an early ON timing in this manner, has been found to result in a higher peak current and a relatively choppy current waveform is experienced in the electronics controlling the ON/FW/OFF steps. This tends to create higher temperatures in the control electronics,

typically in capacitors (or at the capacitor junctions) when they are exposed to the waveform. The life of the control electronics tends to be a function of their absolute

temperature and the superimposed thermal stress cycle.

Excessive and/or frequent temperature variation can shorten the life of the control electronics and can lead to premature failure of the eSC switched-reluctance motor (specifically the control electronics of the motor) . Although the examples above relate to SRMs, at least some of the same problems are thought to also occur in permanent magnet (PM) motors.

The present invention seeks to mitigate at least some of the above-mentioned problems.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method of operating a motor in an electric

supercharger, the method comprising controlling a supply of current in a plurality of phases to respective pluralities of coils on a stator or rotor of the motor; wherein, for each phase, the control comprises repeatedly (i) starting supply of the current to the plurality of coils supplied by the current of that phase, (ii) stopping the supply of current to those coils but allowing current already supplied to circulate in those coils and (iii) eliminating the circulating current from those coils; wherein the method comprises operating the motor in a first mode in which the timings of the starting, stopping and eliminating steps of each phase are obtained from a first source, characterised in that the method comprises the step of monitoring a temperature of the motor, and in response to the monitored temperature exceeding a predetermined threshold, switching operation of the motor into a second mode in which the timings of the starting, stopping and eliminating steps of each phase are selected for

generating lower temperatures in a controller of the motor.

The first mode is preferably a low-Noise Vibration and Harshness (NVH) mode. In the first source the timings are preferably selected for low-NVH. The second mode is

preferably a lower-temperature mode. In the second source the timings are preferably selected for generating lower

temperatures in a controller of the motor, in comparison to those temperatures generated when operating in the first mode. Monitoring the temperature and switching to the second mode when the temperature exceeds a predetermined level, avoids excessive temperatures building up in the supercharger whilst still providing a functioning supercharger. The present invention recognises that it is advantageous to temporarily operate in an alternative mode in which the temperature can be better controlled, even if that mode creates sub-optimal NVH; in effect, the life span of the eSC is recognised as being more important that continuously having low NVH.

The timings in the second mode may be such that greater NVH is generated in comparison to the NVH generated when operating in the first mode.

The first source may comprise one or more look-up tables of timings for the starting, stopping and/or eliminating steps. The timings are preferably provided for a multiplicity of motor speeds and torques.

In the second mode, the timings of the starting, stopping and eliminating steps of each phase are selected for

generating lower temperatures in a controller of the motor. The timings in the second mode may, in principle, be selected in a variety of ways. In some embodiments of the invention, the timings may be selected by applying a factor and/or offset to the timings in the first mode (the factor/offset being such that the temperature is reduced) . In some embodiments of the invention, the magnitude of the timing may remain the same in the first and second modes, but the timings in each mode may be relative to a different datum such that in the second mode the temperature is reduced (for example in the second mode the datum against which the timing is measured (e.g. max

inductance configuration) may be adjusted/offset. The

adjustment/offset is preferably pre-determined to ensure the temperature is reduced.

In preferred embodiments of the invention, in the second mode, the timings are be obtained from a second source. The second source may comprise one or more look-up tables of timings for the starting, stopping and/or eliminating steps. The timings may be provided for a multiplicity of motor speeds and torques.

The timings for the starting step in the first mode (and preferably in the first source) , are preferably earlier than the corresponding timings for the starting step in the second mode (and preferably in the second source) . Such an

arrangement tends to provide lower NVH. The higher

temperatures that may be generated as a result may be

mitigated by virtue of the motor being switchable into the second mode.

The timings for the stopping and/or eliminating steps in the first mode (and preferably in the first source) , may be earlier than the corresponding timings for the respective stopping and/or eliminating steps in the second mode (and preferably in the second source) .

The timings in the second mode may be selected to keep the monitored temperature below a predetermined threshold. For example the timings may be such that the temperature does not exceed a critical temperature that would damage the electronic component of the controller.

It will be appreciated that the first and second sources, for example look-up tables, may contain measurements of time, rotor angles, or any other data that can be used by the controller to implement the timings. The word "timings" is used herein to mean an angle, a time or any of those other alternatives from which the timing of the respective steps is derived .

The skilled person will also understand that look-up tables can be provided as separate look-up tables or as separate portions of one larger look-up table, and references herein to a "look-up table" should be construed to cover both of those possibilities. The first source may, for example comprise three look up tables (one for each of the ON, FW and OFF timings) . Alternatively those timings may all be within a single larger look-up table.

The timings in the first and second modes are preferably such that, for a given motor speed, the torque generated by the motor is substantially the same when in the first or the second mode. Such an arrangement is beneficial because it avoids a noticeable drop/change in performance (boost) when the motor switches between the modes.

The controller may be for controlling (and preferably arranged to control) the starting, stopping and eliminating steps. The temperature being monitored is preferably

representative of a temperature of the controller. The temperature being monitored may be representative of the temperature of one or more electronic components in the controller. The temperature being monitored may be

representative of the temperature of a switch (for example a MOSFET) in the controller. The temperature being monitored may be representative of the temperature of a capacitor in the controller. It will be appreciated that the temperature being monitored need not necessarily be the actual temperature of the controller, or a component in the controller, but is preferably representative of that temperature. For example, the temperature being monitored may be the temperature of coolant in the supercharger, which may be indicative of the temperature of the controller/component.

The method may comprise switching to one or more further modes in which the timings of the starting, stopping and eliminating steps of each phase are selected to generate lower temperatures. For example, the timings may be obtained from one or more corresponding further sources. The motor may switch to the one or more further modes in response to the temperature exceeding one or more corresponding further thresholds. Such an arrangement may allow a relatively gradual change from a mode in which low-NVH is optimised, through one or more modes which progressively reduce/restrict the temperature. According to another aspect of the invention, there is provided an electric supercharger including a motor, the motor comprising:

a stator and a rotor;

- a supply providing current in a plurality of phases to respective pluralities of coils on the stator or rotor of the motor;

a controller for controlling supply of current from the supply by repeatedly (i) starting supply of the current to the plurality of coils supplied by the current of that phase, (ii) stopping the supply of current to those coils but

allowing current already supplied to circulate in those coils and (iii) eliminating the circulating current from those coils; and

- a memory module containing a first source of timings of the starting, stopping and eliminating steps of each phase. In the first source the timings are preferably selected for low-NVH. The motor is configured to operate in a first mode in which the timings of the starting, stopping and eliminating steps of each phase are obtained from the first source. The first mode is preferably a low-NVH mode. The supercharger further comprises a temperature measuring device. The motor is configured such that, in response to the monitored

temperature exceeding a predetermined threshold, the motor switches operation into a second mode in which the timings of the starting, stopping and eliminating steps of each phase are selected for generating lower temperatures in the motor in comparison to those temperatures generated when operating in the first mode.

In the second mode, the timings may be obtained from a second source of timings of the starting, stopping and

eliminating steps. The memory module may contain said second source of timings. The second mode is preferably a lower- temperature mode. By configuring the motor to switch between the two modes depending on the temperature, the lifespan of the supercharger may be maintained. Furthermore, the two sources may enable the motor to operate in a low-NVH mode (which is attractive to the user) , but then change to a lower temperature mode when required .

The first source may comprise one or more look-up tables of timings for the starting, stopping and eliminating steps. The timings may be provided for a multiplicity of motor speeds and torques. The second source may comprise one or more look-up tables of timings for the starting, stopping and eliminating steps. The timings may be provided for a multiplicity of motor speeds and torques.

The timings for the starting step in the first mode (and preferably in the first source) , may be earlier than the corresponding timings for the starting step in the second mode (and preferably in the second source) . The stopping and/or eliminating steps in the first mode (and preferably in the first source) may be earlier than the corresponding timings for the corresponding stopping and eliminating steps in the second mode (and preferably in the second source) .

The temperature measuring device may be arranged to measure a temperature representative of a temperature of the

controller. The temperature measuring device may be arranged to measure a temperature representative of a temperature of an electronic component in the controller. The predetermined threshold may be an absolute temperature. The predetermined threshold may be a temperature difference (for example between a coolant in the supercharger, and a temperature of the

controller) .

The motor is an electric motor. In some embodiments, the motor may be a permanent magnet (PM) motor. In preferred embodiments the motor is a switched-reluctance motor (SRM) . The present invention is applicable to switched reluctance motors (SRMs) of 2 phases and to SRMs of more than 2 phases (for example SRMs having three, or more than three phases) . Thus the skilled person will readily understand that in some embodiments of the invention, the method may include,

supplying current in only two phases, whereas in other

embodiments of the invention, the method may include supplying current in 3, or more than 3, phases. The stator may have a plurality of poles, for example 6.

The rotor has a plurality of poles, for example 4. It may be for example that the number of poles on the stator is two more than the number of poles on the rotor.

It will of course be appreciated that features described in relation to embodiments of one aspect of the present invention may be incorporated into other aspects of the present

invention. For example, the method of the invention may incorporate any of the features described with reference to the electric supercharger of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to the accompanying drawings, of which: Figures 1 to 4 show consecutive stages of rotation of the rotor, or phases, in the operation of a known switched

reluctance motor at operational speeds;

Figure 5 is a block circuit diagram of a control circuit for the motor of Figures 1-4; Figure 6 is a block circuit diagram of a control circuit for a motor in an electric supercharger according to a first embodiment of the invention; Figure 7 is a graph showing the behaviour of the current in one of the phases of the switched reluctance motor of the first embodiment, when operating in a first mode and a second mode ; Figure 8a is a graph showing temperatures in the controller of the supercharger when the motor is operating in the first mode; and

Figure 8b is a graph showing the temperatures in the controller of the super charger when the motor is operating in the second mode.

DETAILED DESCRIPTION

A typical switched reluctance motor is shown in Figures 1A, IB and 2 to 4. This example has a combination (which is

frequent) of six, preferably evenly, spaced poles 2 on the stator 1 and four, preferably evenly, spaced poles 3 on the rotor 4. In this example, the poles of the stator project inwardly from a stator ring 5, the ring providing a path of low reluctance material between the stator poles.

The rotor is formed of a stack of cross-shaped laminations, also of low reluctance material. Therefore each rotor pole is connected to the diametrically opposite rotor pole by a low reluctance path, for reasons which will become apparent. So, as marked, pole U is connected by a low reluctance path to pole U' and pole V to pole V .

Each pole of the stator is wound with a coil 6 and the coils are arranged in pairs, each pair comprising the coils at opposite ends of a respective diameter through the rotational axis of the motor. In this case therefore the pairs are coils AA' , BB' and CC , as marked. The coils of a pair are energised at the same time, with current from a motor control circuit 10 (Figure 5), and in a sense such that one provides a magnetic field towards the rotational axis and one away from the axis. In the Figures the arrows on the coils represent the direction of the current in the coil above the plane of the paper and the dashed arrows represent the magnetic flux. Together the magnetic flux lines produced by the energised coils and their respective poles are arranged generally along the diameter between them and then follow the stator ring (in both

circumferential directions) to the other energised coil of the pair . The rotor modifies the distribution of magnetic field lines in the space between the energised pair of stator poles.

Positions of the rotor in which a pair of diametrically opposite poles of the rotor are aligned along the diameter between the energised pair of stator poles are positions of the rotor that have minimum reluctance for the magnetic circuit that comprises the rotor between the aligned rotor poles, the energised stator poles and the stator ring. The example of rotor poles U and U' being aligned between stator poles A and A' is shown in Figure IB. Such a position is therefore a position of minimum magnetic energy. In a non- aligned position, e.g. as in Figure 1A the magnetic flux still flows along the low reluctance path between the poles of rotor and so the flux is diverted from the diameter between the energised poles of the stator, with the result that it has to cross larger air gaps between the poles of the rotor and stator, increasing the reluctance of the magnetic circuit and the magnetic energy. So if the rotor is not aligned there is a torque on it drawing it towards the aligned position.

At operational rotation speeds the motor is driven by energising pairs of stator coils in turn to draw the poles of the rotor forward in the direction of rotation. So when, for example, the rotor is in the position of Figure 1A and the rotor is rotating clockwise, so that rotor poles U and U' are approaching stator poles A and A' , the coils of A and A' are energised so that U and U' are drawn towards A and A' . When the position of Figure IB is reached in which U and U' are aligned with coils A and A', A and A' are turned off (Figure 2) so that the rotor can continue to rotate without being slowed or drawn back to A and A' . At this point also rotor poles V and V are approaching stator poles of coils B and B' so B and B' are energised (Figure 2) to draw stator poles V and V onwards in the clockwise direction towards B and B' .

When the position of Figure 3 is reached in which V and V are aligned with coils B and B' , B and B' are turned off so that the rotor can continue to rotate without being slowed or drawn back to B and B' . At this point rotor poles U' and U are approaching the stator poles of coils C and C so coils C and C are energised to draw rotor poles U' and U onwards in the clockwise direction towards C and C .

When the position of Figure 4 is reached in which U' and U are aligned with C and C the coils C and C are turned off so that the rotor can continue to rotate without being slowed or drawn back to C and C . At this point rotor poles V and V are approaching the stator poles of A and A' so the coils A and A' are energised to draw stator poles V and V onwards in the clockwise direction towards A and A' .

When V and V reach A and A' the rotor has turned 90°, so, since the rotor has four-fold rotational symmetry it is in effect in the same position as Figure 2 (with poles U' and U re-labelled as V and V, and vice versa) and so the cycle of energising coils B and B' then C and C and then A and A' is repeated to advance the rotor the next 90°, and so on. As is known in the art the coils are switched off and on at particular angles of rotation of the rotor, for example in response to sensing signals generated by the coils as they are both driven by the currents and their inductance changes as the rotor poles pass by them. A first motor control circuit 10 is shown in Figure 5. This comprises the stator coil pairs connected in parallel across a DC power supply 20. Coils A and A' , connected in parallel with each other, are energised by closing switches 21 and 22, and similarly coils B and B' by switches 23 and 24 and coils C and C by switches 25 and 26. These switches are operated by the control circuit 10, which closes the switches when the coils are to be energised. Having the coils A and A' operated by a common pair of switches

(similarly each coil pair B and B' , and C and C , having its own pair of common switches) is sufficient to provide the patterns of coil energisation described above. The switches 21 to 26 are provided, for example, as FET or IGBT transistors. A measure of the current is used by the motor control circuit 10 to determine the position of the rotor and in turn to

determine the timings of the operation of the switches 21 to

26. The coil pairs AA' , BB' and CC , and associated switches, are in parallel with respective capacitors 43. In more detail, the control circuit 10 of Figure 5 senses signals generated by the coils as they are both driven by their currents and their inductance changes as the rotor poles pass by them. This inductance comprises the stator coil pairs connected in parallel across DC power supply 20. The voltage of this supply depends on the application and might be 12V, 24V, 48V or 300V, for example. Coils A and A', connected in parallel with each other, are energised by closing switches 21 and 22, and similarly coils B and B' by switches 23 and 24, and coils C and C by switches 25 and 26. These switches are operated by a controller 44, comprising a switch control unit

27, which closes the switches when the coils are to be

energised. The current in each coil pair is sensed by a resistor 28 connected in series with it to provide a resulting voltage signal that is proportional to the current, which is used to determine the rotor positions, which are used in turn to determine the timings of the operation of the switches 21 and 22, 23 and 24, and 25 and 26. The motor control circuit 10 processes the signals from the coils in a number of stages, forming a control loop. A

position estimator 30 receives the signals indicative of the coil currents and continuously calculates from them the position of the rotor and outputs a rotor position signal 31. The calculation is performed by a microcontroller. A speed estimator 32 differentiates this signal with respect to time, to provide a rotor speed signal 33. The control loop is designed to control the speed of the motor to be as set by an input signal, speed command signal 35, and the difference between the speed command signal and the rotor speed signal is formed by a subtractor 36 to form a speed error signal 37. A loop controller 38, for example in this case a proportional- integral controller, uses this signal to adjust a torque command 39 for the motor. The relationship between the torque applied by a motor to its steady state speed is generally monotonically increasing. So the controller 38 increases the torque commanded if the speed error indicates that the motor is running slower than required and reduces torque commanded if the motor is running faster than commanded. The controller 38 also filters the signals circulating round the control loop in order to smooth the response of the loop.

The motor 1 is of course not controlled directly by a torque command and the torque command 39 is converted to control angles 42 for the switches of the motor. These angles are the angles of the rotor at which the switches of the motor operated, in particular the angles at which a coil pair is turned ON, the angle at which it is allowed to "freewheel", and the angle at which it is turned OFF. It will be appreciated that the look-up table, may contain measurements of time, rotor angles, or any other data that can be used by the controller 44 to implement the timings. Although angles are referred to in the example described with reference to Figures 1 to 5, these could equally be times, for example.

Thus it will be appreciated that ^xangle' and ^>time' can be used interchangeably. The word "timings" is used herein to mean an angle, a time or any of those other alternatives from which the timing of the respective steps is derived. To turn the pair of coils on both its associated switches are turned on (for coils AA' switches 21 and 22) . In the freewheel mode the switch (e.g. 21) connecting the coils to the positive supply is opened but the current continues to circulate through a diode and at the off angle both switches are opened and the current in the coil passes through the other marked diode to ground, dissipating over a short period after the switches are opened. (Alternatively, for the

freewheel mode the switch connecting the coils to the negative supply may be opened instead, with the current continuing to flow through the coils of the pair and the other marked diode. Which of the two switches is open in the freewheel mode can be alternated in order share balance the power dissipated by the switches between them. )

The conversion of the torque command signal to these angles is performed by a lookup table 41 (although only one look up tables is shown, there is effectively a (sub-) table for each of the ON, FW and OFF timings) . The angles needed to provide the torque desired are dependent on the speed of the rotor, so the rotor speed signal 33 is also provided to the lookup table 41, to provide the angles for that torque and speed. These angles are determined empirically while driving the motor while connected to its desired load. The angles 42 produced by the lookup table 41 are passed to the controller 44, and more specifically to the switch control unit 27, which operates the switches at the angles 42

accordingly when those angles match the rotor position signal 31. In more detail, the angles 42 supplied are the same for each coil pair and are relative to the angular position of the coil pair. The switch control unit 27 keeps track of which coil pair is to be operated next and uses the rotor position value 31 modulo 30° for the comparison with the angles 42. Circuit blocks 30, 32, 36, 38, 41 and 27 are preferably implemented by the microcontroller.

Other forms of control circuit are known. One similar circuit example uses Hall Effect sensors rather than the coil currents to note when the rotor passes various positions. As is known in the art, other combinations of stator and rotor pole numbers are possible for the motor. These have different cycles of energisation of the coils in order to keep the torque on the rotor in the forward direction. A common relationship between the numbers of poles is to have two more stator poles than rotor poles and to have both even in number. The choice of the number of poles usually takes into account the operating speed of the motor, the operating power, the acceptable level of torque ripple (variation in torque

supplied by the motor with angle of the rotor) , and the circuitry required.

It is usually generally preferred in such motors for reasons of balance of torque to energise coils in pairs that are diametrically opposite to each other.

A known problem with electric superchargers comprising switched reluctance motors is that they can generate

undesirable levels of noise vibration and harshness (NVH) . In the example described above with reference to Figures 1 to 5 the timings in look-up table 41 have been selected to minimise NVH. Specifically, it has been recognised that starting the supply of current to the coils relatively early (i.e. at a relatively low angle (from the unaligned position)) results in relatively low NVH. Whilst this arrangements addresses the problem of NVH, it tends to create relatively high

temperatures in the controller 44. In particular, the early energisation of the coils results in a choppy current

waveform. This means the capacitors 43 in the controller 44 absorb significant current ripple, and hence energy. This causes significant temperature rises in the capacitors 43. If the core temperature of the capacitors 43 exceeds a certain level the capacitors 43 can be permanently damaged and the lifespan of the motor is reduced. Embodiments of the present invention seek to address this problem. An example switched-reluctance motor, for an

electric supercharger, according to a first embodiment of the invention comprises a stator 1 and a rotor 4 as discussed above in respect of Figs. 1 to 5. Figure 6 is a block circuit diagram of the control circuit for the motor in the electric supercharger according to a first embodiment of the invention. The circuit diagram is similar to that of Figure 5 in that the DC power supply 20, in conjunction with switches 21 and 22, 23 and 24, and 25 and 26, provides current in three phases, A, B and C. The supply of current is controlled by the switch control unit 27 in the controller 44, with the current in phase A supplied to a first pair of coils AA' on the stator 1, the current in phase B supplied to a second pair of coils BB' on the stator 1, and the current in phase C supplied to a third pair of coils CC on the stator 1. Also as described above, for each phase A, B, C, the switch control unit 27 repeatedly (i) starts supply of the current to the pair of coils AA' , BB' or CC supplied by the current of that phase (i.e. the ON step), (ii) stops the supply of current to those coils but allows current already supplied to circulate in those coils (i.e. the

Freewheel step) and (iii) eliminates the circulating current from those coils (i.e. the OFF Step) . The switch control unit 27 delays the starting, stopping and eliminating steps for the phase C relative to the corresponding steps for phase B, and the starting, stopping and eliminating steps for phase B relative to the corresponding steps for phase A. In contrast to the example in Figure 5 however, the motor comprises a temperature monitoring device 101 arranged to measure the temperature of the controller 44. This monitored temperature is representative of the core temperature in the capacitors 43. The output of the temperature monitoring device 101 is received by the microcontroller. In dependence on whether the monitored temperature has exceeded a

predetermined threshold T the microcontroller selects a different source (different lookup tables 41, 141) for the timings of the ON, FW and OFF steps. In other words, when the monitored temperature is below the threshold, the system used the first look-up table 41 to obtain timings, and when the monitored temperature is above the threshold the system uses a second look-up table 141 to obtain timings.

The first look-up table is the same as that in Figure 5 and contains timings of ON, FW and OFF that are selected for low- NVH. The second table, however, contains timings that are selected for lower-temperature (higher efficiency) operation. In the second table, the timings are later (higher angles) than the corresponding timings in the first look-up table.

This is demonstrated in Figure 7 which is a graph overlaying the behaviour of the current in one of the phases of the switched reluctance motor of the first embodiment, when operating with timings from the first table (i.e. in a low-NVH mode) and a with timings from the second look-up table (i.e. in the lower temperature mode) .

Referring to Figure 7, the current waveform 103 when the timings are from the first look-up table 41 is shown in thin line. The current is switched ON at time tl, supply of current is stopped at time t2 (i.e. freewheel) and the

circulating current is eliminated at time t3 (i.e. OFF) . The current waveform 105 when the timings are from the second look-up table 141 is shown in thicker line. The current is switched ON at time t'l, supply of current is stopped at time t'2 (i.e. freewheel) and the circulating current is eliminated at time t'3 (i.e. OFF) . As is evident from Figure 7 tl < t'l, t2 < t'2 and t3 < t'3 and the peak current is significantly lower when using timings from the second look-up table. The current waveform 105 when using timings from the second table results in the capacitors 43 being exposed to a less choppy current. Although it may result in higher-NVH

generated by the supercharger, the temperature in the

capacitors 43 does not rise as high and they are kept below a temperature at which irreparable damage would be caused.

By providing a motor that is configurable between these two modes (in which the different look-up tables 41 and 141 are used) , the electric supercharger can demonstrate low-NVH characteristics during low temperature operation, but the lifespan of the supercharger is preserved at higher

temperature operation by moving to a higher efficiency, albeit with temporarily worse NVH.

Figures 8a and 8b evidence the difference in capacitor core temperature and the difference in MOSTFET junction temperature, experienced when operating in the first, low-NVH, mode (Figure 8a) and the second, lower-temperature, mode

(Figure 8b) . Each graph has been generated when running the supercharger in the same mission profile. The behaviour of the variation in temperature over time is primarily the result of the different demands on the supercharger over the mission profile (for example, after 5000s the supercharger is

relatively unstressed and does not therefore experience high temperatures) .

Referring to Figure 8a, it can be seen that the temperature of the capacitor core (upper graph) rises to be consistently above the pre-determined threshold T for most of the usage period. The capacitor core temperatures being this far above the threshold T would be enough to shorten the lifespan of the controller 44in the supercharger. The supercharger is water cooled. The temperature difference between the capacitor core and the coolant of the water-cooled supercharger, is also indicative of potentially damaging temperatures in the

capacitors .

Referring next to Figure 8b (in which the second look-up table 141 is used) , it can be seen that the temperature of the capacitor core (upper graph) is relatively constant and does not exceed the pre-determined threshold T for any significant time. Although this may result in higher NVH, it is

preferably in terms of preserving the life of the

supercharger .

Whilst the present invention has been described and

illustrated with reference to particular embodiments, 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, in another embodiment of the invention (not shown) the timings of the starting, stopping and eliminating steps of each phase are not obtained from a second look-up table, but are instead selected by applying a predetermined factor and/or offset to the timings in the first mode (the factor/offset being such that the temperature is reduced) . In another embodiment (not shown) the output of the rotor position transducer (RPT) is offset in the second mode, such that the timing is relative to a different maximum inductance (L_max) . The offset is such that the magnitude of the timings remain the same in the first and second modes, but such that (due to the different datum to which they are measured) the temperature in the second mode is lower than the first.

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. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A method of operating a motor in an electric

supercharger, the method comprising controlling a supply of current in a plurality of phases to respective pluralities of coils on a stator or rotor of the motor;

wherein, for each phase, the control comprises repeatedly (i) starting supply of the current to the plurality of coils supplied by the current of that phase, (ii) stopping the supply of current to those coils but allowing current already supplied to circulate in those coils and (iii) eliminating the circulating current from those coils;

wherein the method comprises operating the motor in a first, low-Noise Vibration and Harshness (NVH) , mode in which the timings of the starting, stopping and eliminating steps of each phase are obtained from a first source, in which first source the timings are selected for low-NVH;

characterised in that the method comprises the step of monitoring a temperature of the motor, and

in response to the monitored temperature exceeding a predetermined threshold, switching operation of the motor into a second, lower-temperature, mode in which the timings of the starting, stopping and eliminating steps of each phase are selected for generating lower temperatures in a controller of the motor, in comparison to those temperatures generated when operating in the first mode.

2. A method according to claim 1, wherein the first source comprises one or more look-up tables of timings for the starting, stopping and eliminating steps, the timings being provided for a multiplicity of motor speeds and torques.

3. A method according to claim 1 or claim 2, wherein in the second mode, the timings are obtained from a second source, the second source comprising one or more look-up tables of timings for the starting, stopping and eliminating steps, the timings being provided for a multiplicity of motor speeds and torques .

4. A method according to any preceding claim, wherein the timings for the starting step in the first mode, are earlier than the corresponding timings for the starting step in the second mode.

5. A method according to claim 4, wherein the timings for the stopping and/or eliminating steps in the first mode, are earlier than the corresponding timings for the respective stopping and/or eliminating steps in the second mode.

6. A method according to any preceding claim, wherein the timings in the first and second modes are such that, for a given motor speed, the torque generated by the motor is substantially the same when in the first or the second mode.

7. A method according to any preceding claim, wherein the controller is for controlling the starting, stopping and eliminating steps, and wherein the temperature being monitored is representative of a temperature of the controller.

8. A method according to claim 7, wherein the temperature being monitored is representative of the temperature of one or more capacitors in the controller.

9. A method according to any of claims 1 to 8, wherein the motor is a switched reluctance motor.

10. An electric supercharger including a motor, the motor comprising :

- a stator and a rotor;

- a controller for controlling supply of current from the supply by repeatedly (i) starting supply of the current to the plurality of coils supplied by the current of that phase, (ii) stopping the supply of current to those coils but allowing current already supplied to circulate in those coils and (iii) eliminating the circulating current from those coils; and

- a memory module containing a first source of timings of the starting, stopping and eliminating steps of each phase, in which first source the timings are selected for low-NVH;

wherein the motor is configured to operate in a first, low- NVH, mode in which the timings of the starting, stopping and eliminating steps of each phase are obtained from the first source,

characterised in that

the supercharger further comprises a temperature

measuring device,

and wherein the motor is configured such that, in

response to the monitored temperature exceeding a

predetermined threshold, the motor switches operation into a second, lower-temperature, mode in which the timings of the starting, stopping and eliminating steps of each phase are selected for generating lower temperatures in the controller of the motor in comparison to those temperatures generated when operating in the first mode.

11. An electric supercharger according to claim 10, wherein in the second mode, the timings are obtained from a second source of timings of the starting, stopping and eliminating steps, the memory module containing said second source of timings .

12. An electric supercharger according to claim 11, wherein the first source comprises one or more look-up tables of timings for the starting, stopping and eliminating steps, the timings being provided for a multiplicity of motor speeds and torques, and the second source comprises one or more look-up tables of timings for the starting, stopping and eliminating steps, the timings being provided for a multiplicity of motor speeds and torques.

13. An electric supercharger according to any of claims 10 to

12, wherein the timings for the starting step in the first mode, are earlier than the corresponding timings for the starting step in the second mode.

14. An electric supercharger according to any of claims 10 to

13, wherein the temperature measuring device is arranged to measure a temperature representative of a temperature of the controller .

15. An electric supercharger according to any of claims 10 to

14, wherein the motor is a switched reluctance motor.

16. An electric supercharger and a method of operating a motor in an electric supercharger, as described herein with reference to Figures 6 to 8b.