EP3227698A1

EP3227698A1 - Control module for an electric motor

Info

Publication number: EP3227698A1
Application number: EP15813551.7A
Authority: EP
Inventors: Goncalo MANUEL; Kamel BOUALLAGA
Original assignee: Valeo Air Management UK Ltd
Current assignee: Valeo Air Management UK Ltd
Priority date: 2014-12-22
Filing date: 2015-12-17
Publication date: 2017-10-11
Also published as: WO2016102934A1

Abstract

An assembly comprising: an electric motor (3), such as a switched reluctance motor, that is repeatedly operable between an idle configuration and an activated configuration; an electrical power source (8) for energising stator coils (9) of the motor; and a control module (1) for controlling the motor. The control module (1) comprises a current sensor (23) arranged to measure the current drawn from the power source (8) when it is energising the coils (9) in the activated configuration, and a processor (25) configured to calculate, as a function of the measured current, the energy that is stored in the motor (3) as a result of the coils (9) having been energised,thereby enabling the heat build-up in the motor (3) to be predicted.

Description

CONTROL MODULE FOR AN ELECTRIC MOTOR

Technical Field The present invention relates to electric motors

(particularly for use in electric superchargers), control modules for such motors, and methods of controlling an

electric motor. Background of the Invention

Some electric motors, such as switched reluctance motors and permanent magnets motors (also known as permanent magnet synchronous motors), can sometimes be susceptible to

overheating. For example if the stator coils in the motor are energised for prolonged periods of time and/or draw a large current, the temperature in the motor can build-up to

potentially damaging levels.

Electric motors, such as switched reluctance motors

(SRMs), are sometimes used to drive a compressor wheel in an electric supercharger. In such an arrangement the motor tends to idle for prolonged periods of time, but must then be able to switch to an activated configuration, for a short period of time, in which the motor rapidly accelerates to a high speed (example from 5000 to 70,000 rpm) to provide a boost. An electric supercharger comprising an electric motor tends to be rated to a particular duty cycle. The duty cycle is the ratio of the time spent in the activated configuration (i.e.

boosting) to the time spent between consecutive operations in the activated configuration (i.e. between boosts) . The rated duty cycle, is the maximum duty cycle at which the heat in the motor can still be effectively dissipated, between

activations. In other words, it is the maximum frequency at which the motor can be repeatedly activated, without causing a net temperature increase in the supercharger.

In some driving scenarios, the rated duty cycle may, however, not be respected. For example, when repeatedly accelerating the car, or when driving up a long incline, the supercharger may be activated more frequently than the rated duty cycle. This can cause heat to build up in the

supercharger, and in extreme cases (for example if the duty cycle is not respected for a prolonged period) this can damage the supercharger (for example by overheating the rotor and/or the control electronics) .

To mitigate this problem, a vehicle designed for extreme supercharger use (for example a sports car), may have a supercharger with a rated duty cycle that is relatively high, whereas for a vehicle designed for more mainstream use, it may be possible to use a supercharger with a lower rated duty cycle. Whilst this may mitigate the risk of the rated duty cycle being exceeded, there still remains a possibility that it could be exceeded in certain driving scenarios.

Furthermore, tailoring a supercharger to achieve a particular duty cycle can be onerous to achieve, and may therefore be undesirable .

Summary of the Invention

The present invention seeks to solve or reduce at least one of the above-mentioned problems.

According to a first aspect of the invention, there is provided an assembly comprising: an electric motor that is repeatedly operable between an idle configuration and an activated configuration; an electrical power source for energising stator coils of the electric motor; and a control module for controlling the electric motor; characterised in that the control module comprises a current sensor arranged to measure the current drawn from the power source when it is energising the coils in the activated configuration, and a processor configured to calculate, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised, thereby enabling the heat build-up in the electric motor to be

predicted .

By providing a current sensor and a processor configured to calculate the energy stored in the electric motor, the heat build-up in the electric motor may be predicted. The present invention recognises that by predicting the temperature build^¬ up in this manner, corrective action can be taken before the temperature reaches excessive (for example, potentially damaging) levels. Thus, the present invention may enable damage to the electric motor to be avoided, when a rated duty cycle is exceeded.

The control module may comprise a comparator configured to compare the stored energy, with a threshold level. In

dependence on the stored energy exceeding the threshold level, the control module may lower the energy input during a

subsequent energising of the coils. Such an arrangement provides a simple manner in which to assess whether/when to take corrective action to prevent further heat build-up.

Furthermore, by providing a threshold level (against which the stored energy is compared) , the control module facilitates a relatively flexible arrangement because the threshold can be tailored to the particular heat tolerance of a given electric motor/supercharger. The module preferably lowers the energy input to prevent the electric motor overheating.

The comparator may be configured to compare the stored energy with a plurality of threshold levels. For example the threshold levels may progressively increase. In dependence on the stored energy progressively exceeding each threshold level, the control module may progressively lower the energy input during a subsequent energising of the coils. Such an arrangement may provide a smoother (more gradual) response to managing any heat build-up in the electric motor.

In principle, there are several ways in which the energy input into the coils may be lowered:

In some embodiments of the invention, the control module may adjust the maximum power available from the power source (for example by limiting the maximum current available) . The control module may adjust the duration of the period in which the electric motor is in the activated configuration. The control module may adjust the duration of the period in which the electric motor is in the idle configuration between activated configurations. The electric motor may be rated to perform at a maximum duty cycle, defined by the ratio of the time spent in the activated configuration to the time spent between consecutive operations in the activated configuration. The control module may adjust the duration of the period in which the electric motor is in the activated configuration and/or the idle configuration, such that the electric motor does not exceed the maximum duty cycle.

The control module may comprise a speed controller arranged to adjust the speed of the electric motor. In response to the stored energy exceeding the threshold level, the speed

controller may reduce the maximum allowable speed of the electric motor. In embodiments in which the current required to drive the electric motor is dependent on the speed of the electric motor, it may be possible to adjust the energy input by lowering the speed. Such an arrangement is especially beneficial in embodiments in which the electric motor is in an electric supercharger because the user (for example the driver of a car) is less likely to notice a reduction in boost than, for example, the electric motor simply remaining idle instead.

The control module may comprise controllable switches for connection to stator coils of the electric motor, to control how, and/or whether, the coils are energised. The control module may be arranged such that in response to the stored energy exceeding the threshold level, the control module adjusts the timings of the controllable switches such that electric motor operates more efficiently. For example, the timings on the controllable switches may be designed to minimise noise vibration and harshness (NVH) during normal operation, but may be switched to a more efficient operation if the energy exceeds the threshold level.

The control module may be arranged such that in response to the stored energy exceeding the threshold level, the control module requests cooling of the electric motor by a cooling device. For example, the electric motor may be cooled by a coolant, preferably a liquid coolant. The control module may request cooling of the electric motor by increasing the flow- rate of the coolant through the cooling device. The control module may request cooling of the electric motor by decreasing the temperature of the coolant passing through the cooling device .

It will be appreciated that the control module may adjust the energy input through any combination of the above- mentioned approaches.

The processor is configured to calculate the energy that is stored in the electric motor as a result of the coils having been energised. The energy stored is a function of the measured current. For example, the energy stored may be calculated by integrating the power consumed (current

multiplied by voltage (IV) over time. Energy is also

dissipated from the electric motor over time (for example through heat loss to the surrounding environment) . The energy stored may be calculated by subtracting the energy dissipated over time, from the total energy input over time. The energy is preferably the thermal energy.

It will be appreciated that the processor need not

necessarily calculate the exact amount of energy actually stored in the electric motor. The calculation may be an estimation of the energy stored in the electric motor. In some embodiments, the rate of energy loss from the electric motor may be an estimation, for example based on a previously- determined rate (for example a rate that has been empirically determined) .

The electric motor may be a switched reluctance motor (SRM) . The electric motor may be a permanent magnets motor (also known as a permanent magnets synchronous motor (PMSM) ) . The present invention is especially beneficial for controlling these two types of electric motor in an electric supercharger because in such an arrangement the electric motor tends to be subjected to a wide variety of different usage patterns (e.g. for different drivers and/or different terrain/usage

profiles) . Furthermore, a supercharger tends to either be at idle, or be fully activated. The electric motor therefore tends to experience repeated high acceleration, which can lead to temperature build-up if performed in quick succession. The electric motor may be in an electric supercharger. The electric supercharger preferably comprises a compressor element arranged to be driven by the electric motor, to provide a compressed charge to an engine. The engine is preferably for use in an automobile. The engine is preferably a relatively small capacity engine. The engine is preferably 4 litres or less, more preferably 3 litres or less, and yet more preferably 2 litres or less) . The engine may be in an automobile. The automobile may be less than 3.5 tonnes, and more preferably less than 2 tonnes.

The power source for energising the coils may be a DC power source. The voltage of this power source depends on the application and might be 12V, 24V, 48V or 300V, for example. The power source may be a car battery.

The power source preferably provides a DC bus current. The stator coils may be provided with respective phase currents. It will be appreciated that the phase current is the current that has been controlled to determine how and/or whether the coils are energised. For example, in embodiments in which the control module comprises controllable switches for connection to the stator coils of the electric motor, the phase current is the current that has been controlled by these switches.

The current sensor is preferably arranged to measure the DC bus current. Such an arrangement provides an effective yet simple way of measuring the current.

In some embodiments, the control module is contained within the electric supercharger. For example, in embodiments comprising a threshold level, the threshold may be held in a memory module within the supercharger (for example in an electronics module) . In some other embodiments, the control module may be distributed between the electric supercharger and an external control unit, such as the control unit for the engine (i.e. the module need not be a single circuit, but may be distributed between several sources) . Such an arrangement may be beneficial because it enables the control unit for the engine to influence the control of the supercharger

(particularly how to manage the lowering of the energy input during a subsequent energising of the coils, to prevent the electric motor overheating) . The part of the control module in the control unit of the engine may be arranged to provide the threshold value. The part of the control module in the control unit of the engine may manage the lowering of the energy input during a subsequent energising of the coils, to prevent the electric motor overheating. The processor may be configured to output a signal representative of the predicted heat build-up in the electric motor, to the control unit of the engine. The control unit of the engine may manage the lowering of the energy in response to this signal. The control module may be arranged to output a warning to the control unit, when the estimated heat build-up exceeds a predetermined level.

According to another aspect of the invention there is provided an automobile comprising the assembly described herein .

According to another aspect of the invention, there is provided a method of controlling an electric motor that is repeatedly operable between an idle configuration and an activated configuration, the method comprising the steps of: energising the stator coils of the electric motor by an electrical power source, measuring the current drawn from the power source when it is energising the coils in the activated configuration, and calculating, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised, thereby enabling the heat build-up in the electric motor over that cycle, to be predicted.

The method may comprise the step of predicting the heat build-up in the electric motor. The method may comprise the steps of comparing the stored energy, with a threshold level, and in dependence on the stored energy exceeding the threshold level, lowering the energy used during a subsequent energising of the coils, to prevent the electric motor overheating.

According to another aspect of the invention, there is provided a processor for use as the processor described herein. The processor may be configured to calculate, as a function of a measured current, the energy that is stored in the electric motor as a result of the coils having been energised .

According to another aspect of the invention, there is provided a control module for use as the control module described herein. The control module comprises a current sensor arranged to measure the current drawn from the power source when it is energising the coils in the activated configuration, and a processor configured to calculate, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised .

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

applicable to any other aspect of the invention, and vice versa. For example any features described with reference to the control module may be equally applicable to the method of the invention and vice versa.

Description of the Drawings

Various embodiments of the invention will now be

described, by way of example only, with reference to the accompanying schematic drawings of which:

Figure 1 is a graph showing the variation in electrical power consumption, and in stored thermal energy, over time, in the SRM of a known electric supercharger;

Figure 2a is a close up view of zone a in Figure 1 ;

Figure 2b is a close up view of zone b in Figure 1; Figure 3 is a schematic of an assembly comprising a control module controlling a switched reluctance motor in a first embodiment of the invention;

Figure 4 is a graph showing the variation in electrical power consumption, and in stored thermal energy, over time, in the SRM in Figure 3;

Figure 5 is a schematic of an assembly comprising a control module controlling a switched reluctance motor in a second embodiment of the invention;

Figure 6 is a graph showing the variation in electrical power consumption, and in stored thermal energy, over time, in the SRM in Figure 5; and

Figure 7 is a schematic of an assembly comprising a control module controlling a permanent magnets motor in a third embodiment of the invention.

Detailed Description

Figure 1 is a graph showing the variation in electrical power consumption, and in stored thermal energy, over time, in the switch reluctance motor (SRM) of a known electric

supercharger (not shown) . The SRM drives a compressor wheel to provide a compressed charge to a car engine (not shown) .

In accordance with known SRMs, the motor comprises a plurality of stator coils, surrounding a rotor. The stator coils are selectively energised in order to rotate the rotor. DC electrical current is supplied to the stator coils, from a power source in the form of a car battery.

The SRM is repeatedly operable between an idle

configuration in which an idling current is supplied to the coils (to rotate the rotor at an idle speed of 5, OOOrpm) , and an activated configuration in which a higher current is supplied in order to accelerate the motor to 70, OOOrpm. In this example, the SRM is rated to a duty cycle of 33%. This rated duty cycle is the maximum allowable ratio of the time in which the SRM is in the activated configuration, to the total time between activations.

Figure 1 shows the SRM (already at operating temperature) operating in a first zone (zone a) in which SRM is operating at its rated duty cycle, and then a second zone (zone b) in which the SRM consistently exceeds its rated duty cycle.

Zones a and b are shown in more detail in Figures 2a and 2b to which reference is now made:

Figure 2a is a close up view of zone a in Figure 1. As shown by the lowermost line in Figure 2a, the SRM repeatedly switches between idle (negligible power) to activated (100% power) . The total time between subsequent switches t_T is 6 seconds, and for each activation the time the SRM is in the activated configuration t_a is 2 seconds. The SRM is thus operating at the rated duty cycle of 33% (2s/6s) .

The uppermost line if Figure 2a shows the stored thermal energy in the motor. When the motor is in the activated configuration and the coils are being energised to accelerate the motor, the thermal energy in the motor increases (the motor has inherent inefficiencies which result in some of the energy input being converted into heat) . However, when the motor returns to idle, the heat dissipates into the

surrounding environment (through a combination of forced cooling from the liquid coolant in the supercharger and natural cooling to the surrounding environment) . The rated duty cycle is such that the net stored thermal energy after each activation cycle is zero, i.e. the thermal energy input during activation is dissipated during the idle time.

Figure 2b is a close up view of zone b in Figure 1. In zone b, the motor is repeatedly activated t_a for 4 seconds, but the total time between subsequent switches t_t remains 6 seconds. The motor is therefore operating well above the rated duty cycle. As a result, the thermal energy in the motor steadily increases over time. Although some of the thermal energy dissipated during the idle period (see the saw- tooth profile) that dissipation is insufficient to cool the motor back down. As shown in Figure 1, after only 300 seconds in zone b, the stored thermal energy is approaching 90% of the maximum allowable without causing permanent damage to the motor. If the motor were to continue running in zone b for longer, the stored thermal energy would reach 100% and the motor would fail as a result of overheating.

Figure 3 is a schematic of an assembly in a first

embodiment of the invention. The assembly comprises a control module 1 controlling a switched reluctance motor (SRM) 3. The SRM 3 comprises a six-pole stator 5 and a four-pole rotor 7. The stator 5 comprises a plurality of stator coils 9 that are selectively energised by a 12V DC car battery 8DC 12V car battery 8, via a series of controllable switches 11. The SRM 3 is in a supercharger (not shown) and drives a compressor wheel such that the supercharger provides an engine with a compressed charge.

The control module 1 comprises a speed control module 13 for controlling the energising of the coils (including

determining the ON, Freewheel, OFF timings) and thus, the speed of the motor. The speed control module 13 itself, comprises the controllable switches 11, look-up tables 15, speed controller 17, position estimator 19, and speed

estimator 21. The operation of such a speed control module 13 has been suggested previously. For example, see PCT

application PCT/GB2014/053710 the content of which is hereby incorporated by reference. In Figure 3 the control module 1 is shown in dashed lines, whereas the speed control module 13 is shown in dot- dashed lines.

In contrast to known arrangements, the control module 1 comprises a current sensor 23 and a processor 25. The current sensor 23 is arranged to measure the DC bus current drawn from the battery 8 when the coils are energised. The output of the current sensor 23 is fed to the processor 25 which is arranged to calculate the stored thermal energy in the motor.

The processor 25 does this by measuring the power output from the battery over time (shown in Figure 3 as an integral of the voltage (V(t)) multiplied by the current (I(t))) . This is representative of the total energy input into the motor. In some embodiments, the processor factors this energy input by an efficiency factor to calculate the approximate magnitude of the electrical energy being converted into thermal energy. It also substracts an energy loss estimation X(t) (not shown in Figures 3) to take into account thermal energy dissipated from the motor over time. The energy loss estimation X(t) is an estimate based on a previously-obtained empirical

measurement .

It will be appreciated from the above, that the processor 25, thus provides an estimate of the thermal energy that is stored in the motor, at any one time.

The processor outputs an estimation of this stored thermal energy to a comparator 27, which compares the estimate to a threshold value TH1 29. If the stored thermal energy is below the threshold TH1 29, it is deemed safe to continue using the SRM in its current operation. However, if the stored thermal energy is above the threshold TH1 the energy

controller 31 is arranged to take remedial action to prevent an excessive temperature rise in the motor. That remedial action will now be explained with reference to Figure 4. Figure 4 is a graph showing the variation in electrical power consumption, and in stored thermal energy, over time, in the SRM in Figure 3. The SRM is initially operating above the rated duty cycle. Accordingly, the stored thermal energy in the SRM steadily rises (see 300 to 400 seconds) . In this embodiment the threshold value TH1 29 is equal to 80% of the maximum stored thermal energy (i.e. at which permanent damage would occur) . Once the processor 25 calculates that the energy has reached that level, the energy controller 31 instructs the speed controller 17 to reduce the power input into the motor by 20%. Thus, when activated, the power input to the coils is 80% of the previous value. By reducing the power input on each activation, the motor is able to dissipate energy at a greater rate than it builds up in the motor, until it reaches a steady-state in which the thermal energy is approximately 75% of the maximum allowable. This is a safe level at which the motor is able to operate without causing permanent damage.

The control module 1 in the first embodiment of the invention is therefore arranged to lowers the energy input during a subsequent energising of the coils, to prevent the motor overheating. The first embodiment of the invention thus provides a self-protecting function before the heat build-up becomes too great.

In the first embodiment the control module 1 lowers the energy input during a subsequent energising of the coils, by restricting the power available from the battery. In other embodiments, the energy input may be controlled in other ways. For example, in other embodiments (not shown) the control module may adjust the duration of the period in which the motor is in the activated configuration; it may adjust the maximum speed of the motor, and/or it may adjust the timings of the switching of the coils. Figure 5 is a schematic of an assembly according to a second embodiment of the invention. Features in the second embodiment of the invention that correspond to similar

features in the first embodiment of the invention, are shown with the same reference numerals as in the first embodiment, but with the addition of the prefix ^λ1' (or ^λ10' where

appropriate) . The features in the second embodiment are the same as the first with the exception of those described below:

Firstly, in contrast to the first embodiment, the control module 101 is distributed between the supercharger and the engine control unit 130 (often referred to as an ECU) . The current sensor 123 and processor 125 are housed on the

supercharger itself, whereas the comparator 127, threshold 129 and energy controller 131 are part of the control unit 130 of the engine. The output of the processor 125 is thus received by the engine control unit 130. Only when this stored energy value from the processor 125 exceeds the threshold 1 Thl

(stored in a memory of the engine control unit 130) does the energy controller in the engine control unit 130 adjust the energy input to prevent the motor overheating. This may be beneficial because it enables the car/engine manufacturer to decide the most appropriate manner to adjust the energy input, rather than have it fixed by having the relevant parts of the control module on the supercharger.

In the example embodiment shown in Figure 5, the energy controller 131 is arranged to adjust the energy input by adjusting the speed reference 132, normally supplied by the engine control unit. When necessary, the speed reference 132 is lowered, to reduce the target speed that the speed

controller 117 is seeking to achieve. Thus the speed

controller 117 does not drive the motor as hard as it would otherwise do and the energy consumption is reduced. In the second embodiment the control module 101 is also arranged to have multiple thresholds to progressively lower the energy input when energising the coils. Figure 6 is a simplified graph showing the power consumption and the thermal energy stored in the motor 103 when a car, comprising the supercharger of the second embodiment, is driven up a long incline. The motor 103 is activated for prolonged periods (either at 100% or 50% power consumption) . This is well in excess of the rated duty cycle and the stored thermal energy in the motor gradually rises. During the brief idle periods, the energy level drops, but it does not recover to a

sustainable level. The control module 101 uses two thresholds TH1 60% and TH2 80%. At TH1 the power is reduced by 17% (as shown by the third activation being 17% lower power) .

Although this reduces the rate of energy build-up, it is not sufficient to prevent it increasing and when the stored thermal energy reaches the second threshold TH2, the power is reduced yet further (by a total of 33%) . This is sufficient to lower the stored thermal energy and damage to the motor is prevented. The second embodiment thus, progressively reduces the power consumption, as consecutive thresholds are exceeded. This arrangement tends to provide a more gradual (and less noticeable) response for the user, which can be beneficial.

Finally, in the second embodiment the processor does not calculate an exact value of the stored energy, as it does not adjust the stored energy value by the efficiency of the motor. Instead, the processor adjusts the energy loss estimation X(t) by the efficiency factor and the threshold, to which this value is compared, is unadjusted by an efficiency factor. The efficiency factors thus cancel out during any calculation. The comparison of the stored energy (from the processor calculation) with the threshold is therefore still a valid one, for judging whether the heat build-up in the motor is becoming excessive. It will be appreciated that there are also other ways (not necessarily described herein) in which to calculate whether the stored thermal energy in the motor is becoming excessive.

Figure 7 is a schematic of an assembly according to a third embodiment of the invention. Features in the third embodiment of the invention that correspond to similar

features in the first embodiment of the invention, are shown with the same reference numerals as in the first embodiment, but with the addition of the prefix ^λ2' (or ^λ20' where

appropriate) .

The third embodiment of the invention is the same as the first embodiment, except for the differences described below:

The motor is a permanent magnet motor (also known as a permanent magnets synchronous motor (PMSM) ) . As is known for such a motor, the motor comprises a permanent magnet rotor (i.e. a rotor with magnets inserted in it, or otherwise associated with it) and a stator having a plurality of stator coils connected to an energy source for energising the coils.

In contrast to the first embodiment, the assembly does not comprise a look up table 15 for use in controlling the speed. Instead, the speed controller 217 directly sends command signals (labelled ^λΟηά signals' in Figure 7) to controllable switches 211 to vary the current flow in the coils. The speed controller 217 also receives feedback on these phase currents, for use in controlling the speed of the PMSM.

In common with the first embodiment described herein, the control module 201 comprises a current sensor 223 arranged to measure the DC bus current drawn from the battery 208 when it is energising the stator of the motor. The output of the current sensor 223 is fed to the processor 225 which is arranged to calculate the stored thermal energy in the motor. 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. 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. An assembly comprising:

An electric motor that is repeatedly operable between an idle configuration and an activated configuration;

an electrical power source for energising stator coils of the electric motor; and

a control module for controlling the electric motor; characterised in that the control module comprises

a current sensor arranged to measure the current drawn from the power source when it is energising the coils in the activated configuration, and

a processor configured to calculate, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised,

thereby enabling the heat build-up in the electric motor to be predicted.

2. An assembly according to claim 1, wherein the control module comprises a comparator configured to compare the stored energy, with a threshold level, and

wherein, in dependence on the stored energy exceeding the threshold level, the control module lowers the energy input during a subsequent energising of the coils, to prevent the electric motor overheating.

3. An assembly according to claim 2, wherein the control module adjusts the maximum power available from the power source .

4. An assembly according to claim 2 or claim 3, wherein the control module adjusts the duration of the period in which the electric motor is in the activated configuration.

5. An assembly according to any of claims 2 to 4, wherein the control module adjusts the duration of the period in which the electric motor is in the idle configuration between activated configurations.

6. An assembly according to any of claim 4 or 5, wherein the electric motor is rated to perform at a maximum duty cycle, defined by the ratio of the time spent in the activated configuration to the time spent between consecutive operations in the activated configuration, and wherein the control module adjusts the duration of the period in which the electric motor is in the activated configuration and/or the idle

configuration, such that the electric motor does not exceed the maximum duty cycle.

7. An assembly according to any of claims 2 to 6, wherein the control module comprises a speed controller arranged to adjust the speed of the electric motor, and in response to the energy stored exceeding the threshold level, the speed controller reduces the maximum allowable speed of the electric motor.

8. An assembly according to any of claims 2 to 7, wherein the control module comprises controllable switches for connection to the stator coils of the electric motor, to control how the coils are energised, and the control module is arranged such that in response to the stored energy exceeding the threshold level, the control module adjusts the timings of the

controllable switches such that electric motor operates more efficiently.

9. An assembly according to any of claims 2 to 8, wherein the control module is arranged such that in response to the stored energy exceeding the threshold level, the control module requests cooling of the electric motor by a cooling device.

10. An assembly according to any of claim 1 to 9 wherein the electric motor is in an electric supercharger, the electric supercharger comprising a compressor element arranged to be driven by the electric motor, to provide a compressed charge to an engine .

11. An assembly according to claim 10, wherein the control module is contained within the electric supercharger.

12. An assembly according to claim 10, wherein the control module is distributed between the electric supercharger and a control unit for the engine.

13. An assembly according to claim 12, wherein the processor is configured to output a signal representative of the predicted heat build-up in the electric motor, to the control unit of the engine.

14. An assembly according to claim 13, arranged to output a warning to the control unit, when the estimated heat build-up exceeds a predetermined level.

15. An assembly according to any preceding claim wherein the electric motor is a switched reluctance motor.

16. An assembly according to any of claims 1 to 14, wherein the electric motor is a permanent magnets motor.

17. A method of controlling an electric motor that is repeatedly operable between an idle configuration and an activated configuration, the method comprising the steps of: energising the stator coils of the electric motor by an electrical power source,

measuring the current drawn from the power source when it is energising the coils in the activated configuration, and calculating, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised,

thereby enabling the heat build-up in the electric motor over that cycle, to be predicted.

18. A method according to claim 17, wherein the method

comprises the step of predicting the heat build-up in the electric motor.

19. A method according to claim 17 or 18, comprising the steps of comparing the stored energy, with a threshold level, and in dependence on the stored energy exceeding the threshold level, lowering the energy used during a subsequent energising of the coils, to prevent the electric motor overheating.

20. A processor for use as the processor of any of claims 1 to 17, the processor being configured to calculate, as a function of a measured current, the energy that is stored in the electric motor as a result of the coils having been energised.

21. A control module for use as the control module in any of claims 1 to 17, the control module comprising

a current sensor arranged to measure the current drawn from the power source when it is energising the coils in the activated configuration, and a processor configured to calculate, as a function of the measured current, the energy that is stored in the electric motor as a result of the coils having been energised.