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

Electric superchargers are becoming increasingly

attractive for use with internal combustion (IC) engines in the automotive industry. Firstly, they enable lower fuel consumption in the IC compared to conventional (direct engine- driven) superchargers and thus reduce carbon dioxide

emissions. They also tend to be able to be more responsive and may be able to attain higher speeds than direct engine- driven superchargers. Using a switched-reluctance motor in an electric supercharger (to drive the compressor element) has been found to be particularly beneficial.

A problem with some known electric superchargers is that the motor can consume significant battery power. This can be a particular problem when other systems/devices are also run off the battery, because use of the supercharger can leave insufficient battery power to run these systems/devices.

Some arrangements have been suggested for limiting the current consumption by the supercharger. For example

PCT/GB2014/053708 describes a control system for controlling a motor in an electric supercharger in which a full-load curve is selected from a plurality of different full-load curves, each being designed to achieve a different power consumption by the motor. UK application no. GB1420632.0 describes a control system for controlling a motor in an electric

supercharger in which a processor is configured to adjust the target speed in such a way that the peak current is reduced.

It is desirable to provide an alternative, and preferably simpler, arrangement for adjusting the current consumption.

Summary of the Invention

According to a first aspect of the invention, there is provided an assembly comprising: an electric motor; an

electrical power source for energising the stator coils of the 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, and a comparator configured to compare the measured current with a threshold current, and wherein the control module is arranged to control the current drawn from the power source to energise the coils, in response to the comparison between the measured current and the threshold current.

Providing a current sensor and a comparator configured to compare the measured current with a threshold current,

provides a simple way of checking whether the current is too high. Furthermore, by controlling the current to energise the coils, the control module can take corrective action in the event that the current is too high.

The assembly may comprise a speed controller for

controlling the speed of the electric motor. The speed controller may control the speed of the electric motor in dependence on a torque limit. The control module is

preferably arranged to adjust the torque limit in dependence on the comparison between the measured current and the

threshold current, thereby adjusting the current drawn from the power source. The current tends to be dependent on the torque generated by the electric motor; using the torque limit to adjust the current drawn from the power source, has been found to be a particularly effective, yet simple, way (in terms of the required hardware and/or software) to control the current .

The control module may be arranged to iteratively adjust the torque limit until the measured current is less than or equal to the threshold current. Thus, the control module may comprise a control loop in which the torque cap is iteratively adj usted .

The power source may be a DC power source. The voltage of this supply 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.

The control module may comprise controllable switches for connection to the stator coils of the electric motor, to control whether the coils are energised. It will be

appreciated that 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 (that the control module controls) .

When initialising the control of the current, it may be necessary for the control module to provide a starting point for the magnitude of the torque limit. The control module preferably comprises a torque limit estimator configured to estimate an initial torque limit value for a given current threshold. The torque limit estimator may be arranged to receive a motor speed signal representative of the electric motor speed. The control module may be configured to

determine the initial torque limit value in dependence on the speed signal and the current threshold value. For example, in some embodiments of the invention the initial torque limit value may be extracted from a look-up table. The look-up table may comprise values of initial torque limit for given values of speed and current.

The magnitude of the current threshold is preferably able to be varied. The current threshold may be held in a memory module. The memory module may be remote from the control module. The control module may be arranged to receive the current threshold from the memory module. Such an arrangement may be beneficial in that it may enable the performance (for example the speed response) of the electric motor to be varied in dependence on a variation in the current threshold. For example, the control module may be arranged to control the electric motor in a first mode when the current threshold is a first value. The control module may be arranged to control the electric motor in a second mode when the current threshold is a second value. For example, the first mode may correspond to a sports mode, whereas the second mode may correspond to an efficiency mode. Thus, such embodiments of the invention may not only provide a means by which the current can be

controlled to save battery life, but they may (alternatively or additionally) provide a means by which the performance of the electric motor can be tailored based on a user selection. The memory module may be part of an external control system, such as an engine control unit (ECU) . The driver may be arranged to select the current threshold (indirectly through selection of a driving ^λ πιοάθ' ) .

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 an automobile (for which the supercharger may be fitted) often has numerous other systems/devices that are dependent on power from the same power source. 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.

In some embodiments, the control module is contained within the electric supercharger. In some other embodiments, the control module may be distributed between the electric

supercharger and an external control system, such as an engine control unit (i.e. the module need not be a single circuit, but may be distributed between several sources) .

According to a second aspect of the invention, there is provided a method of controlling an electric motor, 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 controlling the speed of the electric motor with a speed controller,

characterised in that the method further comprises the steps of: measuring the current drawn from the power source when it is energising the coils, comparing the measured current with a threshold current, and, in response to the comparison between the measured current and the threshold current, controlling the electric motor in such a way as to adjust the current drawn from the power source.

The speed of the electric motor may be controlled in dependence on a torque at which the electric motor may

operate. The method may comprise the step of adjusting the current drawn from the power source by adjusting the magnitude of the torque.

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 control module for use as the control module as described herein. The control module may comprise a current sensor arranged to measure the current drawn from the power source when it is energising the coils, and a comparator configured to compare the measured current with a threshold current. The control module is arranged to control the current drawn from the power source to energise the coils, in response to the comparison between the measured current and the threshold current.

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 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 schematic of an assembly according to a first embodiment of the invention; and

Figure 2 is a schematic of an assembly according to a second embodiment of the invention. Detailed Description

Figure 1 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 DC 12V car battery 8, via a series of controllable switches 11. The SRM and control module 1 are part of an electric supercharger in a car (not shown) ; the SRM 3 is arranged to drive a compressor element (not shown) for providing a compressed charge.

The assembly also 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 sensor 18, position estimator 19, and speed estimator 21. The operation of speed control modules per se has been suggested previously. For example, a speed control module is described in 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.

The control module 1 of the first embodiment of the invention, comprises a current sensor 23 arranged to measure the DC bus current drawn from the battery 8 when it is

energising the coils 9, and a comparator 27 configured to compare the measured current with a threshold current 29. The threshold current 29 is one of a plurality of thresholds, held in a memory module 29' in the engine control unit 30

(described in more detail below) . The control module 1 also comprises a controller 31 configured to receive the output from the comparator 27. In dependence on the comparison between the measured current and the threshold current, the controller 27 outputs a torque limit 35. That torque limit 35 is then passed to the

controller 17 in the speed control module 13. The speed controller 17 controls the speed of the motor 3 to ensure that the torque limit 35 is not exceeded (i.e. the torque generated by the motor 3 does not exceed the torque limit 35 output from the controller 31) . Since the current consumed by the motor 3 is dependent on the torque generated by that motor, it has been recognised that the magnitude of the current can be controlled by adjusting the torque limit. This has been found to be an especially effective and simple manner by which to control the current consumed by the motor.

The torque limit 35 is calculated as part of an iterative control process. In each iterative loop of that control process the controller 31 generates a new torque limit by factoring a baseline torque limit 37 by a factor that depends on the magnitude of the difference between the measured current and the threshold current. Thus, if there is a large difference between the measured current and threshold current, then the torque limit 35 is rapidly decreased (i.e. a factor <1 is applied to the baseline torque limit 37) . However, as that difference approaches zero, the torque limit 35 tends to stabilise (i.e. a factor of, or close to, 1 is applied to the baseline torque limit 37) .

The baseline torque limit 37 is estimated using a torque limit estimator 39. The torque limit estimator 39 comprises a look up table 41 containing values of torque limit for

different values of current and motor speed. The estimator 39 is arranged to receive both the current threshold 29 from the memory module 29' , and the output of the speed estimator from the speed control module 13. In dependence on these values, the estimator 39 outputs (based on the data in the look-up table 40) the baseline torque limit 37. This baseline torque limit 37 is effectively an initial estimate of the torque limit required to obtain the current threshold (based on the empirically-obtained data in the look-up table) . It is received by the controller 31, which subsequently refines the torque limit 35 using the above-described control loop.

In the first embodiment of the invention, the memory module 29' is part of the engine control unit (ECU) . The memory module 29' hold a plurality of different current thresholds 29 that correspond to different modes of operation of the supercharger. For example, a high current threshold corresponds to a sports mode, whereas a low threshold

correspond to a fuel-efficient mode. The first embodiment of the invention, thus allows a driver to select a preferred mode, and adjusts the performance of the supercharger

accordingly .

The ECU 30 is also arranged to adjust the current

threshold in the event that the battery 8 is running low. For example, to preserve the operation of other engine systems running off the battery 8, the ECU 30 automatically selects a lower current threshold 29 once the battery power drops too far. The first embodiment of the invention, thus provides a simple and effective way of preserving battery life.

Figure 2 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 ^λ 1' (or ^λ 10' where

appropriate) . The second 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 having magnets inserted 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 117 directly sends command signals (labelled ^λ Οηά signals' in Figure 2) to controllable switches 111 to vary the current flow in the coils. The speed controller 117 also receives feedback on these phase currents, for use in controlling the speed of the PMSM.

The control module 101 still comprises a current sensor 123 arranged to measure the DC bus current drawn from the battery 108 when it is energising the stator coils, and a comparator 127 configured to compare the measured current with a threshold current 129. The current controller 131 receives this comparison.

The torque setpoint received by the current controller is not, however, received from a look up table. Instead, the torque setpoint is set by a torque controller (T(p)) (in dependence on the maximum current received from the ECU and in dependence on the output of the speed estimator) .

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.