Field of the invention

The present invention is directed at a control system for controlling a switched reluctance machine, the switched reluctance machine including: a rotor comprising one or more rotor poles; a stator comprising one or more sets of stator poles, each set including one or more stator poles comprising phase windings, such that each phase winding is associated with a respective one of the sets of stator poles; wherein the rotor is moveable relative to the stator by sequential powering of the sets of stator poles by powering the phase windings; wherein the control system is configured for controlling the powering of the sets of stator poles. The invention is further directed at a method of controlling a switched reluctance machine as described above, the method comprising controlling, by a controller, the powering of the sets of stator poles. The invention is further directed at a switched reluctance machine, and an appliance, such as a power generator or a vehicle.

Background

This disclosed invention relates to the control of switched reluctance (SR) motors, and more particularly to the control of a switched reluctance machine operating in continuous conduction mode. In a switched reluctance machine, at low speeds, the torque can be regulated mainly by controlling the magnitude of the phase current by switching the power electronics. When turning off the switching elements, the phase current rapidly drops to zero. The maximum torque depends on the phase current limit. At medium speed, the peak current can be regulated by controlling the relative rotor position at which the phase is turned on. Because of the increased back EMF, the current will decrease even if the switching elements stay active. There is a small current trail after turning off the phase, which can cause a small generative torque. When the speed increases, the switch on angle has to be advanced to reach the same peak current. The current trail after switching off the phase also becomes larger.

In the above, reference is made to‘low speed’ and‘medium speed’, which terms of course do not identify exact speed ranges wherein the switched reluctance machine operates according to the above described characteristics. However, the skilled person will appreciate that the applicable speed ranges are much dependent on the exact motor design and other parameters that vary from switched reluctance machine to switched reluctance machine. For that reason, it is not possible to link the terms‘low speed’ and‘medium speed’ to directly identifiable speed range. In general, the low and medium speed ranges at least have in common that the phase current drops and becomes zero after powering off of the phase and before the next phase starts. The situation where the current always reaches zero before starting the next commutation is called‘discontinuous conduction mode’.

However, as from a certain speed, the current trail will reach up to the angle where the next commutation will start. Reducing the dwell angle (angle between turning on and off the phase) can avoid this situation but also reduces power with increasing speed. Another possibility is to control the motor in‘continuous conduction mode’, also referred to as‘continuous current mode’. In this mode, the phase current does not become zero between commutations. This makes it a lot more difficult to predict the resulting phase current waveform resulting from certain firing angles, because it depends on the current at which the previous commutation ends. Various properties, such as— but not limited to— phase resistance and power electronics voltage drop, therefore have a significant influence on resulting phase current waveform. With fixed firing angles and no other way of control, the current waveform will seem to stabilize at first in continuous conduction mode, but it will continuously change because of changing environmental conditions. For example on a test bench, it can be observed the current waveform changes continuously when the motor coil temperature is changing, while firing angles remain unchanged. Compared to discontinuous conduction mode, very small changes in firing angles have a big influence on the resulting waveform. This makes the transition between discontinuous and continuous conduction mode difficult, especially if the accuracy of the motor model is not extremely good or environmental condition or not exactly known. This results in underperformance of the motor, for example due to undesired generative torque or production of insufficient torque.

To create a stable and predictable current waveform in continuous conduction mode, some kind of feedback control is necessary. Various proposals for improvement have been made in the field. For example, some solutions are based on control of a peak current to enable control of the continuous conduction mode. However, this requires a rather complex algorithm to find the peak current during commutation.

Summary of the invention

It is an object of the present invention to provide a control system that allows to overcome the problems of the prior art, and enables control of a switched reluctance machine in continuous conduction mode in an effective and relative straightforward manner.

To this end, there is provided herewith a Control system for controlling a switched reluctance machine, the switched reluctance machine including: a rotor comprising one or more rotor poles; a stator comprising one or more sets of stator poles, each set including one or more stator poles comprising phase windings, such that each phase winding is associated with a respective one of the sets of stator poles; wherein the rotor is moveable relative to the stator by sequential powering of the sets of stator poles; wherein the control system is configured for controlling the powering of the sets of stator poles, the control system comprising or being operatively connected to a phase current sensor configured for providing a phase current signal for one or more of the sets of stator poles, the phase current signal being indicative of an amount of phase current present in a respective one of the sets of stator poles; wherein the control system comprises a processor configured for, upon powering of one or more of the sets of stator poles or at a predetermined position of the rotor relative to the stator, obtaining the phase current signal of said respective set of stator poles, and for determining based on the obtained phase current signal a timing for controlling the powering of the respective set of stator poles.

The present invention is based on the insight that the moment of powering on of the respective sets of stator poles may easily be applied to obtain a phase current value at a fixed moment or predetermined position during each commutation. In fact, the powering on may be applied as trigger to obtain the phase current value from the phase current signal received from the phase current sensor. This value may then be used to control the timing of controlling the powering of respective the set of stator poles. In that manner, in case a build-up in phase current is detected, the control system may control this for example setting the timing of powering-off differently. Alternatively or additionally, the controller may adjust the timing of switching the set of stator poles in a different powering mode, for example a freewheeling mode as explained further below in this document. The timing of various powering modes may thus be adjusted dependent on a desired correction.

Advantageously, since powering on is to be triggered anyway by the control system, such trigger may easily be used as well to perform a reading of the phase current value. Therefore, in some embodiments, each set of stator poles of the switched reluctance machine comprises one or more phase switches for enabling said set of stator poles to be activated and de- activated by operating the phase switches; wherein the processor is configured for providing control signals to at least one of the phase switches of a respective one of the sets of stator poles such as to activate said set of stator poles for enabling said sequential powering. In some of these embodiments, the control signals comprising activation signals and deactivation signals for switching-on and switching-off one or more of the phase switches of the respective set of stator poles, wherein the processor is configured for obtaining the phase current signal of said respective set of stator poles simultaneously with the providing of an activation signal for switching-on the phase switches such as to power the set of stator poles, and for using the phase current signal for determining a timing for providing a deactivation signal to one or more of the at least one of the phase switches for the respective set of stator poles. The activation signal of the switches may conveniently be used to trigger the acquisition of the phase current value from the phase current signal. As may be appreciated, other trigger signals may likewise be used for this purpose, or alternatively a dedicated trigger may be generated. However, the use of the activation signals obviates the need for generation of a dedicated trigger which thereby reduces the overall complexity of the system.

As an alternative to obtaining the phase current signal of the respective set of stator poles upon powering of the set of stator poles, it is also possible to measure the phase current at a predetermined rotor position. This position may for example be a fixed position, a position predetermined for each setpoint, or a position at which the phase switches are scheduled. To determine the position of the rotor relative to the stator, various implementations are possible. For example, the controller may cooperate with a sensor to establish an angular position of the rotor, or obtain such information in a different manner.

In accordance with some embodiments, the processor is configured for comparing the obtained phase current signal with a reference phase current value for said determining of the timing. A comparison with a reference value may be applied to detect a difference and actively control the timing of the powering off of the respective set of stator poles dependent thereon. Various different implementations of such active control are possible, dependent on the control strategy to be implemented. In some embodiments, the control system is configured for obtaining the reference phase current value from at least one of: a memory, a data repository, a wireless data network, a wireline data network, or an application specific network such as a vehicle integrated data network. Typically, the reference values may be obtained ones during initialization or during testing of a switched reluctance machine, and may then be stored in a lookup table for use during operation of the switched reluctance machine.

In accordance with some embodiments, for said determining of the timing, the processor is configured for adjusting the timing dependent on said comparison of the phase current signal with the reference phase current value. For example, in accordance with a preferred embodiment, for performing the adjusting, the processor is configured for at least one of: shorten a duration wherein the respective set of stator poles is powered and/or extend a duration wherein the respective set of stator poles is not powered when the phase current signal indicates a phase current value larger than the phase current reference value, such as by advancing the timing of a switching-off of the respective set of stator poles; and extend the duration wherein the respective set of stator poles is powered and/or shorten the duration wherein the respective set of stator poles is not powered when the phase current signal indicates a phase current value smaller than the phase current reference value, such as by delaying the timing of a

switching-off of the respective set of stator poles. By advancing the timing, the phase current present in the set of stator poles upon commencement of the next commutation will become lower. Likewise, by delaying the timing of the powering off, the set of stator poles will remain activated longer, thereby resulting in the remaining phase current to be higher at the start of the next commutation.

In accordance with some embodiments, the control system further comprises or is operatively connected to a position sensor configured for providing the processor with a position signal indicative of an angular position of the rotor relative to the stator. This enables the control system to control the powering off position, i.e. the angular position wherein the respective set of stator poles is de-energized or switched off. For example, for determining said timing, the processor is configured for determining based on the phase current signal a reference angular position of the rotor, the control system being configured for powering off of the respective set of stator poles upon the rotor reaching the reference position.

In accordance with a second aspect, there is provided a switched reluctance machine comprising a control system according to one or more of the preceding claims. Yet in accordance with a third aspect, there is provided an appliance comprising a switched reluctance machine according to claim 10, the appliance being at least one of a power generator, a vehicle, or a motor driven apparatus.

The present invention in accordance with a fourth aspect thereof relates to a method of controlling a switched reluctance machine, the switched reluctance machine including: a rotor comprising one or more rotor poles; a stator comprising one or more sets of stator poles, each set including one or more stator poles comprising phase windings, such that each phase winding is associated with a respective one of the sets of stator poles;

wherein the rotor is moveable relative to the stator by sequential powering of the sets of stator poles; wherein the method comprises: controlling, by a controller, the powering of the sets of stator poles; and obtaining, from a phase current sensor, a phase current signal for one or more of the sets of stator poles, the phase current signal being indicative of an amount of phase current present in a respective one of the sets of stator poles; the method further comprising: upon powering of one or more of the sets of stator poles, obtaining the phase current signal of said respective set of stator poles; and determining based on the obtained phase current signal a timing for powering off of the respective set of stator poles.

Brief description of the drawings

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figure 1 schematically illustrates, in cross-section, a rotor and stator of a 4-phase 16/12 switched reluctance motor;

Figure 2 illustrates a schematic circuit topology of a typical inverter for a four-phase switched reluctance machine;

Figure 3A provide operational characteristics of a multiphase switched reluctance machine at low rotor speeds;

Figure 3B provide operational characteristics of a multiphase switched reluctance machine at medium rotor speeds;

Figure 3C provide operational characteristics of a multiphase switched reluctance machine at high rotor speeds;

Figure 4 provides a typical performance characteristic torque vs. rotor speed for a conventionally controlled multiphase switched reluctance machine when only using discontinuous conduction mode;

Figure 5 schematically illustrates a phase current characteristic for a multiphase switched reluctance machine operated using a control system and/or method of the invention; Figure 6 provides a typical performance characteristic torque vs. rotor speed for a conventionally controlled multiphase switched reluctance machine when also using continuous conduction mode;

Figure 7 schematically illustrates a method in accordance with an embodiment of the invention.

Detailed description

Figure 1 schematically illustrates a multiphase switched reluctance machine (SRM or SR-machine), in particular a multiphase switched reluctance machine motor 1. The motor 1 comprises a stator 2 including a plurality of coils 6 and stator poles 7. In figure 1, the motor 1 and coils 6 of the stator 2 are schematically illustrated in cross-section around the cores 8. Thus, in figure 1, the windings of each coil 6 are visible on either side of the core 8. The stator poles 7 form the cores 8 of the coils 6.

The motor 1 further comprises a rotor 3 including a plurality of counter poles 10 for interacting with the stator poles 7. The rotor 3 is rotatable relative to the stator 2, for example by means of an axis 4. The coils 6 of the stator 2 are associated with phase stages 12, 13, 14 and 15 of the motor 1, such that each coil 6 of the plurality of coils of the stator 2 is associated with one of the phase stages 12-15 respectively. In the figure, the phase stages 12-15 are also designated by the phase stage numbers O (phase stage 12), Q (phase stage 13), Q (phase stage 14) and O (phase stage 15).

In figure 1, phase stage O (12) is powered and the counter poles 10 of the rotor 3 are aligned with phase stage O (12). By subsequently powering phase stage Q (13), the rotor 3 will rotate clockwise to align counter poles 10 with phase stage Q. Alternatively, by powering phase stage O (13), the rotor 3 will rotate counter-clockwise to align counter poles 10 with phase stage O. Hence, the rotor 3 can be rotated in either direction dependent on the powering sequence of the phase stages O, q, Q and © (12- 15).

The motor illustrated in figure 1 is a 4-phase 16/12 switched reluctance motor, consisting of 4 switchable phase stages wherein each phase stage comprises 4 stator poles 7 distributed across a full revolution, and 12 rotor poles. Application of the calibration method of the present invention is not limited to this type of motor, but may be applied to other types of switched reluctance motors, e.g. 2-phase 4/2, 4-phase 8/6, 3-phase 6/4, 3-phase 12/8, 5-phase 10/8, 6-phase 12/10, 7-phase 14/12, 8-phase 16/14 or any other configuration. Moreover, even though many of the

embodiments described herein explain the invention as applied to a radial flux motor, the present teachings are not limited thereto and may likewise be applied to an axial flux motor. The most commonly used topology of an inverter 18 for controlling SR-machines is shown in figure 2. Inverters for SR-machines with a different number of phases can be similar, although the number of phase associated switching stages may be different. In figure 2, a switching stage for phase A of an SR-machine has been generally designated as I. Each phase has two switching elements and four diodes, of which two to clamp at -UDC voltage level when de-energizing the phase. This has been illustrated for phase A, element 24 schematically illustrates a coil of a stator pole of phase A. The semiconductor type switching elements 22 and 23 enable switching of the phase, such as to power-on and power-off the coil 24 and to switch the phase into a freewheeling state, as explained below. The clamping diodes 27 and 28 enable to clamp the coil at -UDC voltage level, upon powering-off phase A.

In the circuit illustrated in figure 2, when both switching elements 22 and 23 are closed, the phase voltage is +U _DC . The phase A is energised (ON’), and the conductive path is from switching element 22 via coil (or coils) 24 to switching element 23. When only one switching element is closed, which may be either switching element 22 or switching element 23, the phase voltage is near 0V. The phase is‘freewheeling’ (‘FW’), and current is allowed to flow freely through the phase (free-wheeling). When both switching elements 22 and 23 are open, the applied phase voltage is - UDC (if there is any current flowing through the phase). The phase is de energising (OFF’). The conductive path is from diode 27 via coil (or coils) 24 to diode 28.

Operational characteristics of an SR-machine are illustrated in figures 3A to 3C for low rotor speeds, medium rotor speeds and high rotor speeds. Considering figure 3A for low rotor speeds, curve 29 illustrates the phase current i dependent on the angular position of the rotor for one of the phase stages of an SR-machine. The generated torque T is illustrated as curve 35, whereas curve 36 illustrates the flux linkage y. In the flux linkage vs. phase current diagram 37, the surface 40 spanned by curve 38 is indicative of the amount of work delivered by a single commutation of the phase stage. Similarly, figures 3B and 3C illustrate these characteristics for medium rotor speeds and high rotor speeds. In figure 3B for medium rotor speeds, curve 53 illustrates the phase current i dependent on the angular position of the rotor for the respective phase stage. The generated torque T is illustrated as curve 55, whereas curve 56 illustrates the flux linkage y. In the flux linkage vs. phase current diagram 57, the surface 58 spanned by curve 59 is indicative of the amount of work delivered by a single

commutation of the phase stage. In figure 3C for high rotor speeds, curve 63 illustrates the phase current i dependent on the angular position of the rotor for the respective phase stage. The generated torque T is illustrated as curve 65, whereas curve 66 illustrates the flux linkage y. In the flux linkage vs. phase current diagram 67, the surface 68 spanned by curve 69 is indicative of the amount of work delivered by a single commutation of the phase stage.

Referring to figure 3A, at low speed of the rotor 3, the torque T 35 can be regulated mainly by controlling the magnitude of the phase current I 29 by switching the power electronics. The phase stage is powered at switch- on angle 30, the phase current i 29 thereafter rapidly building up in the phase stage. When switching-off the switching elements 22 and 23 at angular position 31, the phase current i rapidly drops to zero. The maximum torque generated during the commutation depends on the phase current limit.

Referring to figure 3B, at medium speed the peak current 53 can be regulated by controlling the relative rotor position 51 at which the phase is turned on. Because of the increased counter electromotive force (the back EMF), the current 53 will decrease even if the switching elements stay active prior to the turn-off angle 52. There is a small current trail after turning off the phase at 52, which can cause a small generative torque T as can be seen in 55. When the speed increases, the switch on angle 51 has to be advanced (moved to the left in the diagram) to reach the same peak current 53. The current trail after switching off the phase at 52 also becomes larger (the end-point thereof moving further to the right in curve 53).

At a certain speed, the current trail will reach up to the angle where the next commutation will start. This is where the high rotor speed region starts, which is visualized in figure 3C. Reducing the dwell angle (angle between turning on and off the phase between locations 61 and 62) can avoid this situation but also reduces power and generated work W with increasing speed. The situation where the current always reaches zero before starting the next commutation is called‘discontinuous conduction mode’. In the high speed region, the motor 1 may be controlled in

‘continuous conduction mode’, also referred to as‘continuous current mode’.

In continuous conduction mode, the phase current 63 does not become zero between commutations. This makes it a lot more difficult to predict the resulting phase current waveform resulting from certain firing angles 61 and 62, because it depends on the current i at which the previous commutation ends. Properties like phase resistance, power electronics voltage drop therefore have a significant influence on resulting phase current waveform. With fixed firing angles 61 and 62 and no other way of control, the current waveform will seem to stabilize at first in continuous conduction mode, but it will continuously change because of changing environmental conditions. For example on a test bench, it can be observed the current waveform changes continuously when the motor coil

temperature is changing, while firing angles 61 and 62 remain unchanged. Compared to discontinuous conduction mode, very small changes in firing angles have a big influence on the resulting waveform. This makes the transition between discontinuous and continuous conduction mode difficult, especially if the accuracy of the motor model is not extremely good or environmental condition or not exactly known.

Performance characteristics of a conventionally controlled SR- machine are illustrated in figure 4 in a torque vs. speed diagram. At low speeds, the amount of delivered torque T is determined by the current limit though the coils and is therefore a constant value (region 70). At medium speeds, the amount of torque will decrease as the speed of the rotor increases. The decrease will be proportional with l/w, as illustrated in region 71. At high rotor speeds, in region 72, reduction of the dwell angle to control the phase current in the phase stage results in a decrease of the torque T which is proportional with 1/w ² .

Figure 5 illustrates a the behavior of an SR-machine control in accordance with the principles of the present invention. In figure 5, the phase current vs. time is illustrated. Upon switching-on the phase stage at t=0 at reference 73 in the figure, the phase current rapidly builds up in the stator poles of the phase stage as illustrated by curve 75. Simultaneously with the switching-on of the phase stage at 73, the phase current in the phase stage is compared to a reference phase current 78. Based on the difference 74 between the actual phase current at switch-on and the reference phase current 78, the processor of the control system determines how to adjust the ON-state of the phase stage (i.e. the set of stator poles associated with the phase stage) such as to approach the reference phase current level 78 upon the start (i.e. the switch-on angular position) of the next commutation. The ON-state is adjusted by adjusting one or more of the OFF-angle and/or FW-angle. Herein, the OFF-angle is the angular position at which the phase stage is switched to the OFF-state; with reference to figure 2, this is the state wherein both switching elements 22 and 23 are switched off. Moreover, the FW-angle is the angular position at which the phase stage is switched to the freewheeling state; with reference to figure 2, this is the state wherein one of the switching elements 22 or 23 is switched off, while the other one is switched on.

As will be appreciated, the processor only has a certain budget in terms of angular position (and hence time within the commutation) to extend the length of ON-state of the phase stage. This is because suboptimal performance due to, for example, a generation of a counter torque that interferes with operation of the next phase stage is to be prevented.

However, within that budget, the processor calculates the ON-state adjustment Ai (reference 76) required to close the gap between the phase current at switch-on and the reference level 78 at the start of the next commutation.

During the first commutation 75, a maximum extension Ai of the ON-state is added to the first commutation. In accordance with the present invention, such an adjustment of the ON-state may be implemented by postponing the switch-off moment at which the phase stage is powered off, in other words by shifting the angular position (OFF-angle) at which the phase stage is switched off. This will extend the freewheeling state with duration Ai. Alternatively, or additionally, in accordance with embodiments of the invention, adjusting the phase current at switch-on of the next commutation may also be obtained by adjusting the FW-angle. This will affect the phase current level during the ON-state, and thereby also the remaining phase current after switching-off.

During the first communication, the maximum adjustment is implemented by the processor, and although the gap 80 between the phase current at the switch on time 79 and the reference level 78 is smaller upon the start of the second commutation, there still is a relatively large gap. Upon switching-on the phase stage during the second commutation at 79, the processor again obtains the actual phase current from the phase current sensor in the switched reluctance machine, and compares it to the reference phase current 78. An adjustment of the ON-state is calculated by the processor, and the ON-state is adjusted by postponing the switch off moment by switching off at a later angular position. This again adjusts the ON-state of the phase stage by an amount A ₂ (reference 82) during the freewheeling state. Like Di, A ₂ resembles the maximum amount of

adjustment during that commutation. If the rotor speed has not changed between the first commutation and the second commutation, the adjustment A ₂ 82 is more or less equal to the adjustment Ai 76. After switching-off at 83, the phase current drops, and upon switch-on at 88, a measurement of the actual phase current is obtained by the processor to calculate a difference 89 with the reference phase current 78. During the third commutation, only a short extension of the ON-state is desired, resulting in an adjustment of D3 as illustrated in figure 5.

The reference phase current level at switch on time 78 is set such that, at the given rotor speed and under the given operational conditions (temperature, required torque, etc.) a maximum amount of torque is generated by obtaining a peak phase current 91 that approximates a safety level 90 under the given conditions. Both the reference phase current 78 and the safety level 90 may be determined during test runs or simulations of the switched reluctance machine, for example during factory tests. These values may for example be stored in a look up table, which may be available from a memory in the control system. Optionally, such a look up table may also store the desired adjustments or A’s of the ON-state dependent on the measured difference in phase current upon switching on the phase stage. These adjustments may be stored as individual adjustments to one or more of the OFF-angle or FW-angle, or as absolute or relative OFF-angles and/or FW-angles. As may be appreciated, obtaining these values from a look up table during operation could provide more flexible control possibilities.

The increased performance obtained using a control method of the present invention is illustrated in figure 6. In figure 6, the deliverable torque dependent on the rotor speed is illustrated for a switched reluctance machine controls using the control system or control method in accordance with the present invention. The curve 70’, 71’, 72’ illustrates the torque in the low rotor speed range, the mid rotor speed range and the high rotor speed range. These parts 70’, 71’ and 72’ of the curve are comparable to the corresponding parts of the curve of figure 4 (70, 71 and 72). The advantages of the present control system and control method are obtained in the high speed region, in continuous conduction mode. This is illustrated by the area 92 between curve 72’ of the high rotor speed region, and 72 of the

corresponding curve obtained using a conventional control method. Clearly, the amount of torque that may be delivered in the high speed range 72’ is larger than with the conventional control methods. In fact, the decrease of the torque T which is proportional to l/w in the mid speed range 71’, is continued at rate l/w in the high speed range 72’. The amount of torque gained is thus considerable as compared to conventional control methods.

A control method in accordance with the present invention is schematically illustrated in figure 7. The method of figure 7, in a first step 100, ... processor to obtain control parameters from the look up table stored in the memory. These control parameters are for example based on the amount of requested torque and the rotor speed, as indicated above. For example, from the look up table, the reference phase current level and the safety level may be obtained. As may be appreciated it is not necessary, but merely optional, to obtain the safety level from the look up table. The skilled person will appreciate that the reference phase current values provided in accordance with the look up table and the resulting adjustments of the on- time, will cause the system to operate within the safety level. Therefore, to obtain the safety level of the current phase (reference 90 in figure 5) may purely be advantageous for monitoring purposes, e.g. to detect whether the switch reluctance machine malfunctions.

In step 102, upon start of the commutation, the switching elements of the phase stage are switched-on such as to power the phase stage at the ON-angle. Simultaneously, the phase current is measured by the phase current sensor and the actual phase current value is obtained by the processor. Next, in step 104, the obtained phase current value is compared with the reference phase current value obtained from the memory in step 100. Either from a look up table or from a different algorithm or data obtained from a network or other data repository, the processor determines the required adjustment of the ON-state for approaching the reference phase current value at the start of the next commutation.

At step 106, the processor may shift the freewheeling- and OFF angles in order to extend or reduce the duration of the powered on state of the phase stage. The effect of this is illustrated for example in figure 5, as discussed hereinabove. In step 108, the actual switching to the freewheeling state and the off- state during commutation will be performed by the processor, at the adapted angles.

The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be restrictive on the invention. It is believed that the operation and

construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims.

In the claims, any reference signs shall not be construed as limiting the claim. The term 'comprising' and‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression ‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any claim. Furthermore, the words‘a’ and‘an’ shall not be construed as limited to‘only one’, but instead are used to mean‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope. Expressions such as:

"means for ...” should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.