GB2594735A

GB2594735A - Resonant electrical machine

Info

Publication number: GB2594735A
Application number: GB2006694.0A
Authority: GB
Inventors: Abduallah Ahmed
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2020-05-06
Filing date: 2020-05-06
Publication date: 2021-11-10
Anticipated expiration: 2040-05-06
Also published as: GB202006694D0; GB2594735B

Abstract

Driving circuitry for a stator 330 for an electrical machine 300 comprises a plurality of windings 331 and a plurality of groups of capacitors 332. Each capacitor in a group is associated with a different one of the plurality of windings and the capacitance of the capacitors in each group is such that the combination of each group of capacitors and the respective windings has a different electromagnetic resonant frequency. Each capacitor of each group may be arranged in series with its associated winding, and each capacitor may be associated with a different winding. The capacitors may be arranged in a plurality of capacitor banks. The driving circuitry may further comprise one or more switching devices configured to allow switching between the plurality of capacitors in the capacitor bank by selectively electrically connecting or disconnecting one or more capacitors in the capacitor bank from the three-phase driving signal (see fig. 4). The electrical machine may be an electrical motor for a vehicle.

Description

Resonant Electrical Machine

Field of the Invention

The invention relates generally to resonance electrical machines and methods of operating resonance electrical machines.

Background

Many modern electrical motors are three-phase electrical motors. In a three-phase electrical motor, a driving voltage of three phases is provided to windings of a stator. The provision of a three-phase driving voltage in this manner creates a rotating magnetic field in which a rotor of the electrical motor is made to rotate, thus generating mechanical power.

More recently, so-called resonant electrical motors have been developed, which are a type of three-phase electrical motor. In a resonant electrical motor, a capacitor is arranged in series with each of the stator windings. The stator windings and the capacitors arranged in series form part of an electromagnetically resonant circuit having a resonant frequency determined by the inductance values of the windings and the capacitance values of the capacitors. Resonant electrical motors can generate comparatively large power outputs, however the maximum output power of the electrical motor can only be obtained at a very narrow three-phase driving voltage frequency, close to the resonant frequency of the driving circuit of the stator.

The present inventors have identified an improved approach for increasing the maximum power output for a resonant electrical motor, as well as increasing the range of driving frequencies at which the resonant electrical motor can be usefully operated.

Summary of the Invention

Aspects of the invention are set out in the accompanying claims.

According to a first aspect there is provided driving circuitry for a stator for an electrical machine. The driving circuitry comprises: a plurality of windings, and a plurality of groups of capacitors, each group comprising three capacitors, and wherein each capacitor in a group is associated with a different one of the plurality of windings. Each group of capacitors and their associated windings are arranged to produce a rotating magnetic field in the presence of a three-phase driving signal; and the capacitance of the capacitors in each group are such that the combination of each group of capacitors and the respective windings with which the capacitors are associated have a different electromagnetic resonant frequency.

Accordingly, the first aspect provides driving circuitry for a stator of a resonant electrical machine which includes multiple different electromagnetic resonant frequencies. Therefore, as resonant electrical machines output their maximum power close to the resonant frequency, the provision of multiple resonant frequencies increases the power output of the resonant electrical machine over a larger range of driving voltage frequencies.

In some aspects, each capacitor of each group is arranged in series with its associated winding, and each capacitor is associated with a different winding. For example, if the driving circuitry includes two groups of capacitors (each with three capacitors), the driving circuitry includes six windings, each arranged in series with a different capacitor. Therefore, different sets of windings are used to produce the rotating magnetic fields for different resonant frequencies.

As such, a stator with multiple resonant frequencies can be provided in which it is easy and fast to switch between the sets of windings to which the driving voltages are provided. Furthermore, two or more resonant circuits may be run simultaneously, for example during the process of switching between different resonant circuits in order to ensure a seamless transition.

Alternatively in some aspects, the capacitors are arranged in a plurality of capacitor banks, wherein each capacitor bank comprises: a plurality of capacitors arranged in parallel, wherein each capacitor in the capacitor bank belongs to a different group, the plurality of capacitors in the capacitor bank are associated with the same winding, and each of the plurality of capacitors in the capacitor bank are electrically connected to the winding with which they are both associated, wherein each capacitor bank is arranged in series with the winding with which its capacitors are associated. The term 'capacitor bank refers to a collection of capacitors belonging to different groups.

For example, if the driving circuitry includes two groups of capacitors (each with three capacitors), the driving circuitry includes three windings, where each winding is arranged in series with the capacitor bank, where each capacitor bank includes two capacitors (one from each group) arranged in parallel with one another. Therefore, the same set of windings is used to produce the rotating

magnetic field for each resonant frequency.

As such, a stator with multiple resonant frequencies can be provided without increasing the number of windings. Accordingly, the performance of the electrical machine at a particular resonant frequency can be maintained while increasing the number of possible resonant frequencies for which the electrical machine can operate.

Advantageously in this aspect, the driving circuitry further comprises one or more switching devices configured to allow switching between the plurality of capacitors in the capacitor bank by selectively electrically connecting or disconnecting one or more capacitors in the capacitor bank from its associated winding. Accordingly, the driving circuitry can switch between the capacitor banks to which the three-phase driving voltage is supplied during a single operation such that the electrical machine can be operated across a larger three-phase driving voltage frequency range.

Advantageously, the one or more switching devices include one or more insulated-gate bipolar transistors "IGBTs" and/or one or more metal-oxide-semiconductor field-effect transistors "MOSFETs". Accordingly, the driving circuitry can rapidly and reliably switch between different resonant driving circuits while the electrical machine is operating.

According to a second aspect, there is provided a stator comprising the driving circuitry according to the above-described aspects. That is, some or all of the driving circuitry according to the above aspects may be mounted on the stator of the electrical machine, while other sections of the driving circuitry may be electrically connected to the stator, but not physically mounted on the stator.

According to a third aspect there is provided an electrical machine comprising the stator according to the second aspect and a rotor comprising one or more rotor windings, wherein the rotor is configured to rotate in response to a rotating magnetic field produced by the stator.The rotor may contain a capacitor arranged in series with the rotor windings in order to place the rotor in an electromagnetically resonant state, However the rotor may also be provided without capacitors as the rotor is magnetically coupled to the stator. As such, the arrangement of the components in the rotor can be simplified, without significantly compromising the performance of the machine.

In some aspects, the rotor comprises a rotor core formed of a non-magnetic material. Accordingly, the weight of the rotor can be significantly reduced. Moreover, while the removal of the magnetic core generally reduces the magnetic coupling between the rotor and the stator, the driving circuitry described above allows the electrical machine to output comparable torque values at a wider range of driving frequencies. However, the rotor may instead include a core formed of a magnetic material, such as iron, depending on the particular application. As such, the magnetic coupling between the stator and the rotor may be increased.

In some aspects, the electrical machine is an electrical motor for a vehicle. In particular, as the output torque for the electrical machine can be increased across a wider range of driving frequencies, the electrical machine can be made smaller and lighter, such that it can be utilised in motor vehicles and aeroplanes, as well as consumer devices.

According to a fourth aspect there is provided a method of operating an electrical machine. The method comprises: providing a three-phase driving signal to a first group of capacitors of the plurality of groups of capacitors and their associated windings at a first driving frequency; increasing the driving frequency provided to the first group of capacitors and their associated windings; and upon the driving frequency reaching a threshold value: ending/reducing the provision of the three-phase driving signal to the first group of capacitors of the plurality of groups of capacitors and their associated windings, and providing/increasing a three-phase driving signal to a second group of capacitors of the plurality of groups of capacitors and their associated windings at a second driving frequency.

As such, an electrical machine including multiple groups of capacitors, each group included in an electromagnetically resonant driving circuit, may be operated in a manner which allows multiple resonant driving frequencies to be utilised in a single operation. That is, the optimal resonant frequency circuit may be selected based on the desired driving frequency at a particular point in time.

Advantageously in some aspects, upon the driving frequency reaching the threshold value, one or more switching devices are actuated to electrically disconnect the first group of capacitors from the windings and to electrically connect the second group of capacitors to the windings. As such, a three-phase driving signal can be provided to two different groups of capacitors (with different resonant frequencies) while providing the three-phase driving signal to the same stator windings.

Alternatively, upon the driving frequency reaching the threshold value, one or more switching devices are actuated to electrically disconnect the first group of capacitors and their associated windings from the three-phase driving signal and to electrically connect the second group of capacitors and their associated windings to the three-phase driving signal. As such, a stator with multiple resonant frequencies can be operated in a manner in which it is easy and fast to switch between the sets of windings to which the driving voltages are provided. Furthermore, the electrical connection of the second group of capacitors and their associated windings to the three-phase driving signal may occur before the electrical disconnection the first group of capacitors and their associated windings from the three-phase driving signal, for example in order to ensure a seamless transition between the two sets of windings.

In some aspects, the method further comprises: increasing the driving frequency provided to the second group of capacitors and their associated windings; upon the driving frequency reaching a second threshold value: ending/reducing the provision of the three-phase driving signal to the second group of capacitors of the plurality of groups of capacitors and their associated windings, and providing/increasing the three-phase driving signal to a third group of capacitors of the plurality of groups of capacitors and their associated windings at a third driving frequency. As such, the electrical machine can be operated close to three different resonant driving frequencies in a single operation, allowing higher power outputs to be obtained for a greater range of driving frequencies.

In some aspects, the method further comprises: decreasing the driving frequency provided to the second group of capacitors and their associated windings; and upon the driving frequency falling to the threshold value: ending/decreasing the provision of the three-phase driving signal to the second group of capacitors of the plurality of groups of capacitors and their associated windings, and providing/increasing the three-phase driving signal to the first group of capacitors of the plurality of groups of capacitors and their associated windings at the first driving frequency.

As such, not only can the method of this aspect switch between capacitor groups in response to an increasing driving frequency, but the method can additionally switch between capacitor groups in response to a decreasing driving frequency. In some examples, the driving circuitry may be arranged to switch between different operational modes based on a detected resistive load on the output of the electrical machine. For example, for an electrical machine placed in a motor vehicle, the electrical machine may be arranged to detect that the motor vehicle is travelling up an incline. In response, the electrical machine may switch to a different operating mode to provide an immediate increase in output torque.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the following figures.

In accordance with one (or more) embodiments of the present invention the Figures show the following: Figure 1 depicts an example of a traditional resonant electrical machine.

Figure 2 depicts a graph shown the output torque of a traditional resonant electrical machine for a range of driving frequencies.

Figure 3 depicts an electrical machine according to a first example teaching of the disclosure.

Figure 4 depicts an electrical machine according to a second example teaching of the disclosure.

Figure 5 depicts a graph shown the output torque for an electrical machine according to the first and second examples shown in Figures 3 and 4.

Figure 6 depicts a graph showing the output torque for an electrical machine including six resonant frequencies.

Figure 7 depicts a flowchart of a method for operating a resonant electrical machine having multiple resonant frequencies.

Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words "comprises", "comprising", and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean "including, but not limited to". The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein.

The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.

Detailed Description

Figure 1 shows an example traditional resonant electrical machine 100 and a representation of the circuitry for the electrical machine 100. The electrical machine 100 includes a rotor 110, a stator 130 and an air gap 120 between the rotor 110 and the stator 130.

The stator 130 includes stator windings 131 comprising three winding phases 131a, 131b, 131c each arranged to receive a different phase of a three-phase driving signal generated by an inverter, or by any other suitable known means. In response to a three-phase driving signal, the stator windings 131 produce a rotating magnetic field. The rotor includes rotor windings 111 configured to magnetically couple to the rotating magnetic field produced by the stator windings 131 to cause the rotor to rotate.

The stator windings 131 in Figure 1 are shown spaced apart around the circumference of the stator 130, however in reality the stator windings 131 will be grouped close together in sequence. In other words, stator winding 131a will be located proximal to stator windings 131b, which will in turn be located proximal to stator winding 131c. Stator winding 131c will then be located proximal to another stator winding configured to receive the same phase of the three-phase driving voltage as stator winding 131a. Accordingly, each stator winding is arranged next to two stator windings which each receive a different phase of the three-phase driving voltage and which receive a phase of the three-phase driving voltage different to that particular stator winding. The rotor windings 111a-111c are arranged in a corresponding manner to the stator windings 131a-131c.

The electrical machine 100 of Figure 1 also includes a plurality of capacitors 132 arranged in series with the stator windings 131. Capacitor 132a is arranged in series with stator winding 131a, capacitor 132b is arranged in series with stator winding 131b, and capacitor 132c is arranged in series with stator winding 131c. The inductances of the stator windings 131a-131c are equal (or approximately equal) to one another, and the capacitances of the capacitors 132a-132c are equal (or approximately equal) to one another.

Rotor 110 also includes a plurality of capacitors 112 arranged in series with the rotor windings 111. Capacitor 112a is arranged in series with rotor winding 111a, capacitor 112b is arranged in series with rotor winding 111b, and capacitor 112c is arranged in series with rotor winding 111c. The inductances of the rotor windings 111a-111c are equal (or approximately equal) to one another, and the capacitances of the capacitors 112a-112c are equal (or approximately equal) to one another.

The combination of the stator windings 131 and the capacitors 132 has an electromagnetic resonant frequency, determined by the inductance of the stator windings 131 and the capacitors 132, referred to as the resonant frequency of the stator. Similarly, the combination of the rotor windings 111 and the capacitors 112 has an electromagnetic resonant frequency, determined by the inductance of the rotor windings 111 and the capacitors 112, referred to as the resonant frequency of the rotor. Generally, the resonant frequency of the stator and the resonant frequency of the rotor are set to be approximately equal to one another.

In use, a three-phase driving signal is provided to the stator windings 131 with a particular frequency called the driving frequency. When the driving frequency is close to the resonant frequencies of the stator 130 and rotor 110, the magnetic coupling between the stator 130 and rotor 110 significantly increases, thereby increasing the output torque of the electrical machine 100. However, when the driving frequency is not close to the resonant frequencies, the output torque of the electrical machine 100 is significantly lower. As such, the overall range of driving frequencies at which the resonant electrical machine 100 can be operated with adequate output torque is small.

Figure 2 shows a graph illustrating the output torque for a traditional resonant electrical machine such as that shown in Figure 1. As can be seen from Figure 2, the output torque of the resonant electrical machine 100 is generally relatively low (particularly compared to a conventional electrical machine without capacitors 112, 132), except for the region close to the resonant frequency where the output torque significantly increases. The graph shown in Figure 2 merely depicts the theoretical maximum output of the electrical machine 100 and in reality the output torque may be limited close to the resonant frequency by the maximum current that may pass through the electrical machine 100.

Figure 3 shows an electrical machine 300 according to a first example teaching of the disclosure. The electrical machine 300 includes a rotor 310, a stator 330, and an air gap 320 between the rotor 310 and the stator 330.

The stator 330 includes first stator windings 331 comprising three winding phases 331a, 331b, 331c each arranged to receive a different phase of a three-phase driving signal generated by an inverter, or by any other suitable known means, and are similar to the windings 131 discussed above in relation to Figure 1. Each phase of the first stator windings 331 is arranged in series with a capacitor 332a-332c of a first group of capacitors 332 in a similar manner to the capacitors 132a-132c of the electrical machine 100 of Figure 1. As such, the combination of the winding 331 and the capacitors 332 has a first electromagnetic resonance frequency, as described in relation to Figure 1.

The rotor 310 includes rotor windings 311 similar to rotor windings 111 described above in relation to Figure 1. However, the example rotor 310 of Figure 3 does not include any capacitors arranged in series with the windings 311a-311c. As such, the rotor is not in an electromagnetically resonant state. However, the present inventors have identified that the rotor 310 may magnetically couple to the stator 330 despite the rotor 310 not being in an electromagnetically resonant state. Accordingly, the design of the rotor 310 can be significantly simplified compared to the rotor 110 of Figure 1.

The stator 330 further includes second stator windings 333a comprising three winding phases 333a, 333b, 333c each arranged to receive a different phase of a three-phase driving signal generated by an inverter, or by any other suitable known means. Stator winding 333a is configured to receive the same driving voltage phase as stator winding 331a, stator winding 333b is configured to receive the same driving voltage phase as stator winding 331b, and stator winding 333c is configured to receive the same driving voltage phase as stator winding 331c. Each phase of the second stator windings 333 is arranged in series with a capacitor 334a-334c of a second group of capacitors 334.

The capacitance of the capacitors in the second group of capacitors 334 is set such that the combination of the second group of capacitors 334 and the second stator windings 333 has a second electromagnetic resonance frequency different to the first resonance frequency (of the combination of the first group of capacitors 332 and the first stator windings 331). For example, if the inductances of the first and second stator windings 331, 333 are identical, the capacitance of the first and second groups of capacitors 332, 334 will be different. However, if the inductances of the first and second stator windings 331, 333 are different, the capacitance of the first and second groups of capacitors 332, 334 may be the same or different in order to attain different resonance frequencies.

Therefore, the stator 330 of this first example has two possible electromagnetic resonance frequencies, depending on which of the first and second windings 331, 333 and the first and second groups of capacitors 332, 334 is provided with the three-phase driving voltage. Accordingly, a wider range of operation of the electrical machine 300 can be achieved by supplying the three-phase driving voltage to the winding and capacitor combination having a resonant frequency closest to the driving frequency.

Furthermore, it is possible to switch between windings and capacitor combinations during operation of the electrical machine 300. For example, the three-phase driving voltage may be initially supplied to the first stator windings 331 and first group of capacitors 332 at a driving frequency below the first resonant frequency. The driving frequency can then be increased up to and beyond the first resonant frequency. Then, upon reaching a threshold driving frequency, the three-phase driving voltage is supplied to the second stator windings 333 and the second group of capacitors 334, having the second resonant frequency, and the three-phase driving voltage is no longer supplied to the first stator windings 331 and first group of capacitors 332. The threshold driving frequency may, for example, be the driving frequency at which the maximum output torque of the electrical machine operating with the first resonant frequency is equal (or approximately equal) to the maximum output torque of the electrical machine operating with the second resonant frequency.

The switching between different resonant frequency modes at the threshold driving frequency may occur in a number of different ways. For example, the electrical machine 300 may stop supplying the three-phase driving voltage in the first resonant frequency mode before the electrical machine 300 begins supplying the three-phase driving voltage in the second resonant frequency mode, or may stop supplying the three-phase driving voltage in the first resonant frequency mode and begin supplying the three-phase driving voltage in the second resonant frequency mode simultaneously.

Alternatively, the electrical machine 300 may begin supplying the three-phase driving voltage in the second resonant frequency mode as the driving frequency approaches the threshold driving frequency. As such, the three-phase driving voltage may be supplied to the first and second windings 331, 333 simultaneously before ending the supply of the three-phase driving voltage to the first stator windings 331. Accordingly, it is possible to seamlessly switch between different resonant frequency modes in the example of Figure 3. In some examples, the three-phase voltage may be supplied to the first and second windings 331, 333 simultaneously (i.e. at the same time) in general usage of the electrical machine 300. Accordingly, the output of both sets of windings may be combined to increase the overall output torque of the electrical machine.

The driving circuitry may additionally comprise switching devices, such as insulated-gate bipolar transistors "IGBTs" and/or one or more metal-oxide-semiconductor field-effect transistors "MOSFETs" in order to facilitate the switching between different stator windings 331, 333. Alternatively, any other suitable means for selectively supplying a three-phase driving voltage to different sets of windings may be used, such as using different inverters for different winding sets.

The driving circuitry may additionally comprise one or more processors and/or storage devices to control the driving circuitry and determine which set of windings should be supplied with the three-phase driving voltage at any given time.

The rotor 310 of the electrical machine may have a rotor core formed of a magnetic material or formed of a non-magnetic material. For example, the rotor 310 may have a magnetic core such as iron, in a similar manner to traditional electrical machines. As such, the magnetic coupling between the rotor 310 and the stator 330 may be increased, particularly when the second group of windings are connected to the three-phase driving signal. Alternatively, the rotor core may be formed of a non-magnetic material such as a plastic material. Such a rotor may have a reduced weight and will also prevent magnetic saturation of the magnetic core.

Figure 4 shows an electrical machine 400 according to a second example teaching of the disclosure. The electrical machine 400 includes a rotor 410, a stator 430, and an air gap 420 between the rotor 410 and the stator 430. The rotor 410 of electrical machine 400 is identical to the rotor 310 shown in Figure 3.

The stator 430 of electrical machine 400 includes stator windings 431 each arranged to receive a different phase of the three-phase driving voltage, but does not include second windings corresponding to the second windings 333 shown in Figure 3. Instead, in order to provide the stator 430 with multiple resonant frequencies, the electrical machine 400 includes three capacitor banks 450 arranged in series with the stator windings 431. Capacitor bank 450a is arranged in series with the stator winding 431a, capacitor bank 450b is arranged in series with stator winding 431b, and capacitor bank 450c is arranged in series with stator winding 431c.

Each capacitor bank 450 includes a plurality of capacitors arranged in parallel. Capacitor bank 450a includes capacitor 432a of a first group of capacitors 432 and capacitor 434a of a second group of capacitors 434b arranged in parallel. Similarly, capacitor hank 450b includes capacitors 4326 and 434b from the first and second groups of capacitors 432, 434 respectively, and capacitor bank 450c includes capacitors 432c and 434c from the first and second groups of capacitors 432, 434 respectively.

The combination of the stator windings 431 and each of the first and second groups of capacitors 432, 434 has an electromagnetic resonant frequency. The capacitance of the capacitors in the first group of capacitors 432 are chosen to be different to the capacitance of the capacitors in the second group of capacitors 434. As such, the combination of the stator windings 431 and each of the first and second groups of capacitors 432, 434 has a different resonant frequency.

The capacitor banks 450 also include switching devices 440 configured to selectively connect and/or disconnect capacitors from the three-phase driving signal. The switching devices 440 may, for example, include one or more insulated-gate bipolar transistors "IGBTs" and/or one or more metaloxide-semiconductor field-effect transistors "MOSFETs", however other switching devices may be used. As such, it is possible to switchably provide the three-phase driving voltage to different capacitors in the capacitor banks. For example, the operation of the electrical machine 400 may begin with the first group of capacitors 432 connected to the three-phase driving signal, giving the stator 430 a first resonant frequency. The driving frequency can then be increased up to and beyond the first resonant frequency. Then, upon reaching a threshold driving frequency, the switching devices 440 actuate to disconnect the first group of capacitors 432 from the three-phase driving voltage and to connect the second group of capacitors 434 to the three-phase driving voltage. In this manner, the electrical machine 400 can switch between different resonant frequency modes during operation.

Figure 5 shows a graph illustrating the maximum output torque at a given driving frequency for electrical machine 300 shown in Figure 3 and electrical machine 400 shown in Figure 4. The maximum output torque is the maximum output torque that can be attained from operating the electrical machine with either of the resonant frequency modes. As can be seen, when compared to the output of a traditional resonant electrical machine shown in Figure 2, the provision of a second resonance peak increases the maximum output torque at frequencies closer to the second resonance peak than to the first resonant peak.

The first and second examples shown in Figures 3 and 4 include two groups of capacitors and hence two different resonance frequencies and two resonant peaks. However, substantially any number of resonant frequencies can be provided by increasing the number of groups of capacitors. For example, in the first example of Figure 3 a third group of capacitors can be provided in series with third stator windings in a similar manner to the first and second stator windings 331, 333 and first and second groups of capacitors 332, 334. Similarly in Figure 4, the capacitor banks 450 may include a third capacitor from a third group of capacitors and a further switching device 440 to allow the third capacitor of the capacitor bank to be switchably connectable to the three-phase driving voltage. The stators of the electrical machines 300 and 400 can therefore be provided with substantially any number of resonant frequencies and hence resonant peaks.

Figure 6 shows a graph illustrating the maximum output torque at a given driving frequency for electrical machine, such as those shown in Figures 3 and 4, including six groups of capacitors and hence six resonant frequencies. Such an electrical machine includes six resonance peaks and as such the maximum output torque of the electrical machine (that is the maximum output torque attainable at a particular driving frequency in any resonant frequency mode) is greatly increased across a large range of frequencies as compared to the traditional resonant electrical machine shown in Figure 1 and its output torque shown in Figure 2.

Figure 7 shows a flowchart of a method 700 for operating a resonant electrical machine having multiple resonant frequencies. The method 700 begins at step 701 by providing a three-phase driving signal to a first group of capacitors of the plurality of groups of capacitors and their associated windings at a first driving frequency. The method then proceeds to step 702 by increasing the driving frequency provided to the first group of capacitors and their associated windings.

Then at step 703, upon the driving frequency reaching a threshold value, the method proceeds by ending the provision of the three-phase driving signal to the first group of capacitors of the plurality of groups of capacitors. The method then concludes at step 704 by providing a three-phase driving signal to a second group of capacitors of the plurality of groups of capacitors and their associated windings at a second driving frequency. While step 703 is shown as being carried out before step 704, step 704 may, in some examples, be carried out before step 703 in order to improve the seamlessness of the transition.

Therefore, from one perspective there has been described driving circuitry for a stator for an electrical machine, the driving circuitry comprising: a plurality of windings; and a plurality of groups of capacitors, and wherein each capacitor in a group is associated with a different one of the plurality of windings; and wherein the capacitance of the capacitors in each group are such that the combination of each group of capacitors and the respective windings with which the capacitors are associated have a different electromagnetic resonant frequency.

Claims

Claims 1. Driving circuitry for a stator for an electrical machine, the driving circuitry comprising: a plurality of windings; and a plurality of groups of capacitors, each group comprising three capacitors, and wherein each capacitor in a group is associated with a different one of the plurality of windings; wherein each group of capacitors and their associated windings are arranged to produce a rotating magnetic field in the presence of a three-phase driving signal; and wherein the capacitance of the capacitors in each group are such that the combination of each group of capacitors and the respective windings with which the capacitors are associated have a different electromagnetic resonant frequency.
2. The driving circuitry according to claim 1, wherein each capacitor of each group is arranged in series with its associated winding, and each capacitor is associated with a different winding.
3. The driving circuitry according to claim 1, wherein the capacitors are arranged in a plurality of capacitor banks, wherein each capacitor bank comprises: a plurality of capacitors arranged in parallel, wherein each capacitor in the capacitor bank belongs to a different group, the plurality of capacitors in the capacitor bank are associated with the same winding, and each of the plurality of capacitors in the capacitor bank are electrically connected to the winding with which they are both associated, wherein each capacitor bank is arranged in series with the winding with which its capacitors are associated.
4. The driving circuitry according to claim 3, wherein the driving circuitry further comprises: one or more switching devices configured to allow switching between the plurality of capacitors in the capacitor bank by selectively electrically connecting or disconnecting one or more capacitors in the capacitor bank from the three-phase driving signal.
5. The driving circuitry according to claim 4, wherein the one or more switching devices include one or more insulated-gate bipolar transistors "IGBTs" and/or one or more metal-oxidesemiconductor field-effect transistors "MOSFETs".
6. A stator for an electrical machine comprising the driving circuitry according to any of claims 1-5.
7. An electrical machine, the electrical machine comprising: the stator according to claim 6; and a rotor comprising one or more rotor windings, wherein the rotor is configured to rotate in response to a rotating magnetic field magnetic field produced by the stator.
8. The electrical machine according to claim 7, wherein the rotor comprises a rotor core formed of a non-magnetic material.
9. The electrical machine according to claim 7 or claim 8, wherein the one or more rotor windings are provided without a capacitor arranged in series with the one or more rotor windings.
10. The electrical machine according to any of claims 7-9, wherein the electrical machine is an electrical motor for a vehicle.
11. A method of operating the electrical machine according to any of claims 7-10, the method comprising: providing a three-phase driving signal to a first group of capacitors of the plurality of groups of capacitors and their associated windings at a first driving frequency; increasing the driving frequency provided to the first group of capacitors and their associated windings; and upon the driving frequency reaching a threshold value: ending the provision of the three-phase driving signal to the first group of capacitors of the plurality of groups of capacitors, and providing a three-phase driving signal to a second group of capacitors of the plurality of groups of capacitors and their associated windings at a second driving frequency.
12. The method according to claim 11, wherein upon the driving frequency reaching the threshold value, one or more switching devices are actuated to electrically disconnect the first group of capacitors from the windings and to electrically connect the second group of capacitors to the windings.
13. The method according to claim 11, wherein upon the driving frequency reaching the threshold value, one or more switching devices are actuated to electrically disconnect the first group of capacitors and their associated windings from the three-phase driving signal and to electrically connect the second group of capacitors and their associated windings to the three-phase driving signal.
14. The method according to any of claims 11-13, further comprising: increasing the driving frequency provided to the second group of capacitors and their associated windings; upon the driving frequency reaching a second threshold value: ending the provision of the three-phase driving signal to the second group of capacitors of the plurality of groups of capacitors and their associated windings, and providing the three-phase driving signal to a third group of capacitors of the plurality of groups of capacitors and their associated windings at a third driving frequency.
15. The method according to any of claims 11-14, further comprising: decreasing the driving frequency provided to the second group of capacitors and their associated windings; upon the driving frequency falling to the threshold value: ending the provision of the three-phase driving signal to the second group of capacitors of the plurality of groups of capacitors and their associated windings, and providing the three-phase driving signal to the first group of capacitors of the plurality of groups of capacitors and their associated windings at the first driving frequency.