Synchronous electric motors, rotors, and methods of operation

Abstract

A rotor for a synchronous electric motor comprises a rotor circuit having a first winding field pole NW1, NW2; a second winding field pole SW1, SW2; and an energy storage device C1 configured to be electrically coupled to the first winding field pole and the second winding field pole. A first current flow control device 3 is arranged between the first winding field pole and the energy storage device, and is configured to allow induced AC half period current components of a single direction to pass from the first winding field pole to the energy storage device. A second current flow control device 4 is arranged between the second winding field pole and the energy storage device, the second current flow control device configured to allow induced AC half period current components of a single direction to pass from the second winding field pole to the energy storage device. The first and second winding field poles may each comprise a plurality of winding field poles, where the plurality of winding field poles may be configured to be in series or in parallel, and the plurality of second winding field poles may be configured to be in parallel.

Description

Field of the invention The present disclosure relates to rotors for use in synchronous electric motors, and methods of operation thereof, in particular rotors configured to minimise torque ripple in synchronous electric motors. Background Synchronous electric motors are used for a variety of applications, including applications requiring precise control of the motor’s rotational frequency. Synchronous motors driven with multi-phase AC have found specific uses in traction applications as the rise of the electric vehicle has accelerated in recent years. They can offer high power density. Many of these types of motors usually use permanent magnets to function, which presents several issues. Firstly, the production of permanent magnets requires scarce resources, such as nickel, cobalt or rare-earth metals. Secondly, the strength of the magnetic field of a permanent magnet wanes overtime, and hence synchronous motors that use them tend to become less efficient with prolonged use. Furthermore, use of permanent magnets means that there always exist magnetic fields within the motor which may interact with other elements of the motor in an adverse manner and cause negative issues during production, functional safety during operation and during recycling. There is therefore a need for an improved synchronous electric motor that does not require the use of permanent magnets. The field-wound type of synchronous motors exert excellent practical utility in speed-variable type rotary electric machines used, for example, for generating power for automotive performances, in that there is no need to place expensive permanent magnets in a rotor core, there is no need to consider resistance to centrifugal force of the magnets, and torque and generated (induced) voltage can be freely controlled by field flux control. However, a great disadvantage of known field-wound synchronous motors is the need to provide the transfer of electrical energy externally to the rotating rotor on which the field windings are situated. Known solutions to this disadvantage include the use of slip-rings, but these will wear over the life of the motor. Alternative known solutions use a rotating transformer or a rotating exciter. Both of these later solutions remove the issue of wear, however, all of these solutions require an external source of electrical energy which increases the cost and size of the overall system. Existing solutions to the disadvantage of the requirement for an external source of energy have so far focused on the use of additional windings or coils situated on the rotor, but this has the disadvantage of requiring additional space and weight. These methods also often result in a motor mechanical torque that contains a great deal of ac ripples which has the disadvantage of increased noise and vibration. There is therefore a need to provide improved structures and methods to supply electric power to rotor windings to reduce current ripples at the rotor, and associated torque ripple produced by the motor. Summary of the invention Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects. An aspect of the invention relates to a rotor for a synchronous electric motor, the rotor comprising: a first plurality of winding field poles; a second plurality of winding field poles; wherein the first plurality of winding field poles are configured to be in series, and the second plurality of winding field poles are configured to be in parallel. This may be advantageous to facilitate a maximum induced voltage peak and current in the first plurality of winding field poles when connected in series, and a minimum induced voltage peak and current when the second plurality of winding field poles are connected in parallel. This configuration may also be advantageous to facilitate a maximum voltage peak without adding additional turns to a winding field pole, merely achieving the higher induced voltage by connecting the first plurality of winding field poles in series. The first plurality of winding field poles and the second plurality of winding field poles may be configured to be winding field poles of the same polarity. For example, wherein the first plurality of winding field poles and the second plurality of winding field poles are configured to be north winding field poles, or wherein the first plurality of winding field poles and the second plurality of winding field poles are configured to be south winding field poles. Alternatively, the first plurality of winding field poles may be configured to be winding field poles of opposite polarity to the second plurality of winding field poles. For example, wherein the first plurality of winding field poles are configured to be north winding field poles, and the second plurality of winding field poles are configured to be south winding field poles, or vice versa. The first plurality of winding field poles and the second plurality of winding field poles may be configured to transition between a first configuration and a second configuration. For example, in the first configuration, the first plurality of winding field poles are configured to be in series, and the second plurality of winding field poles are configured to be in parallel; and in the second configuration, the first plurality of winding field poles are configured to be in parallel, and the second plurality of winding field poles are configured to be in series. This may be advantageous to facilitate a maximum induced voltage peak and current when similar winding field poles are connected in series, and a minimum induced voltage peak and current when similar winding field poles are connected in parallel. In some examples, the first plurality of winding field poles and the second plurality of winding field poles may be configured to be in the first configuration for a first half period of induced winding voltage; and the first plurality of winding field poles and the second plurality of winding field poles are configured to be in the second configuration fora second half period of induced winding voltage; wherein the first half period and the second half period of induced winding voltage have opposing polarity. Because winding field poles spend a first period in the first configuration and a second period in the second configuration, this may allow winding field poles to accommodate a greater current without requiring thicker wires or coils. Thus, the present invention can accommodate wires up to half the cross-sectional area compared to conventional configurations in which all north and south windings carry current at the same time. It is noted, however, that twice the number of turns may be needed at each field pole to get same magnetic field strength. For example, in some examples, the first plurality of winding field poles and the second plurality of winding field poles may be configured to be in the first configuration for a first half period of the induced winding voltage, wherein the first half period corresponds to the half period of positive induced voltage at the first plurality of winding field poles, and the half period of negative induced voltage at the second plurality of winding field poles. The first plurality of winding field poles and the second plurality of winding field poles may also be configured to be in the second configuration for a second half period of the induced winding voltage, wherein the second half period corresponds to the half period of negative induced voltage at the first plurality of winding field poles, and the half period of positive induced voltage at the second plurality of winding field poles. The rotor may further comprise a controller, wherein the controller is configured to transition the first plurality of winding field poles and the second plurality of winding field poles between the first configuration and the second configuration. Optionally, the processor / controller may be configured to transition the first plurality of winding field poles and the second plurality of winding field poles between the first configuration and the second configuration based on at least one of (i) an indication of induced winding current, and (ii) an indication of induced winding voltage. In some examples, the controller may be configured to communicate with a control device external to the rotor, wherein the rotor controller is configured to transition the first plurality of winding field poles and the second plurality of winding field poles between the first configuration and the second configuration based on receiving a control signal from the external control device. However, the skilled person will understand that this is not limiting, and in other examples the rotor controller may be configured to transition the first and second plurality of winding field poles based on a program or logic stored at the rotor. The rotor may further comprise at least one energy storage device. The energy storage device may be configured to be charged by (i) the first plurality of winding field poles whilst in series, and / or (ii) the second plurality of winding field poles whilst in series. Charging the energy storage device whilst a plurality of winding field poles are in series may be particularly advantageous to charge the energy storage for the greatest amount of induced voltage (maximum induced voltage peak). Alternatively, or in addition, the energy storage device may be configured to be discharged whilst (i) the first plurality of winding field poles are in parallel, and / or (ii) the second plurality of winding field poles are in parallel. It may be particularly advantageous to discharge the energy storage device whilst a plurality of winding field poles are in parallel in order to discharge the energy storage device for the longest time, having the greatest difference between the stored energy and the field induced voltage (minimum induced voltage peak). Preferably, the energy storage device may be a capacitor, however the skilled person will understand that this is not limiting and that other known energy storage devices may be used. Optionally, the rotor may further comprise a current flow control device electrically coupled to the capacitor, wherein the current flow control device is configured to allow induced AC current components of a single direction pass to the capacitor. For example, the current flow control device may comprise at least one passive element, such as a diode, or at least one active element, such as a switch device. In some examples, the first plurality of winding field poles may be coupled to a circuit, and the second plurality of winding field poles may be coupled to the same circuit. This may be advantageous to provide a single combined circuit that facilitates multiple windings sharing the same magnetic circuits, for example wherein north and south windings share the same magnetic circuit. Alternatively, the first plurality of winding field poles may be coupled to a first circuit, and the second plurality of winding field poles may be coupled to a second circuit, the second circuit being electrically separate to the first circuit. In another aspect of the invention, there is provided a method of operating a rotor for a synchronous electric motor, wherein the rotor comprises a first plurality of winding field poles and a second plurality of winding field poles. The method comprises transitioning the first plurality of winding field poles and the second plurality of winding field poles into a first configuration, wherein the first plurality of winding field poles are configured to be in series, and the second plurality of winding field poles are configured to be in parallel; and transitioning the first plurality of winding field poles and the second plurality of winding field poles into a second configuration, wherein the first plurality of winding field poles are configured to be in parallel, and the second plurality of winding field poles are configured to be in series. Transitioning the first plurality of winding field poles and the second plurality of winding field poles between the first configuration and the second configuration may be based on an indication of induced winding current, and / or an indication of induced winding voltage. In some examples, the method may further comprise transitioning the first plurality of winding field poles and the second plurality of winding field poles into the first configuration for a first period of induced winding voltage; and transitioning the first plurality of winding field poles and the second plurality of winding field poles into the second configuration for a second period of induced winding voltage. Preferably, the first period and the second period may have opposite polarity of induced winding voltage. For example, the first plurality of winding field poles and the second plurality of winding field poles may be transitioned between the first and second configurations when the AC induced current / voltage across the winding field poles is equal to zero. For example, the first plurality of winding field poles may comprise a plurality of north field winding poles, and the second plurality of winding field poles may comprise a plurality of south field winding poles, wherein the plurality of north winding field poles and the plurality of south winding field poles may be configured to be in the first configuration for a first half period of the induced winding voltage, wherein the first half period corresponds to the half period of positive induced voltage at the plurality of north winding field poles, and the half period of negative induced voltage at the plurality of south winding field poles; and wherein the plurality of north winding field poles and the plurality of south winding field poles are configured to be in the second configuration for a second half period of the induced winding voltage, wherein the second half period corresponds to the half period of negative induced voltage at the plurality of north winding field poles, and the half period of positive induced voltage at the plurality of south winding field poles. In another aspect of the invention, there is provided a rotor for a synchronous electric motor. The rotor comprises a winding field pole coupled to a circuit, the circuit comprising (i) a load, (ii) a current flow control device, and (iii) a first current regulator. The current flow control device is coupled between the winding field pole and the load, wherein the current flow control device is configured to allow induced AC current components of a single direction pass across the load. The first current regulator is configured to control the current through the load. This may be advantageous as the current flow control device and current regulator may provide rectification of the AC signal. The load provides an energy sink, hence current flows. In particular, the first current regulator may be configured to divert current through the load when the voltage in the circuit is low. The current regulator may comprise at least one passive device, such as a variable resistor (e.g. temperature-dependent resistor), discrete electronics, such as a comparator and semiconducting switch, or an intelligent semiconductor device, such as a logic or microprocessor-controller semiconductor device. In some examples, the load comprises an energy storage device. The energy storage may advantageously provide field current levelling, thus reducing current ripple, and therefore reducing fluctuations in the field magnetic field which give rise to torque ripple in the motor. The energy storage device may be configured to discharge to provide energy when the AC voltage is the opposite polarity as that allowed by the current flow control device. Preferably, the energy storage device may comprise a capacitor. The current regulator may be configured to be self-powering, powered by the energy storage device, or powered from a supply derived from a separately rectified feed from the windings. The circuit may comprise (i) a first branch, comprising the first current regulator and the load; and (ii) a second branch, comprising a second current regulator and an energy storage device, wherein the first branch and the second branch are connected in parallel. The second current regulator may be configured to control the current through the energy storage device. The parallel first and second branches may be advantageous to provide electronics that harvest AC energy from the stator to provide constant rotor pole magnetic fields. In particular, the energy storage device can be used for field current levelling, thus reducing current ripple, and therefore reducing fluctuations in the field magnetic field which give rise to torque ripple in the motor. The first current regulator may be operable to have a lower current limit than the second current regulator for a first time period, such that current is configured to flow through the second branch during the first time period and charge the energy storage device during the first time period. The first current regulator may also be operable to have a current limit at least equal to the second current regulator for a second time period, such that current is configured to flow through at least the first branch during the second time period. The first time period and the second time period may be both during a half cycle of induced winding voltage of a first polarity at the winding field pole. For example, wherein the first time period and the second time period are both during a half cycle of positive induced winding voltage at the winding field pole. The energy storage device may be configured to discharge during a third time period, wherein the third time period is during a half cycle of induced winding voltage of a second polarity at the winding field pole, wherein the second polarity is different to the first polarity. The rotor may further comprise a controller, wherein the controller is configured to vary the current limit of at least the first current regulator based on an indication of induced winding current, and / or an indication of induced winding voltage at the winding field pole, and / or an indication of frequency of the induced winding voltage at the winding field pole. Optionally, the controller may also be configured to vary the current limit of the second current regulator. In some examples, the controller may be configured to communicate with a control device external to the rotor, wherein the rotor controller is configured to vary the current limit of at least the first current regulator based on receiving a control signal from the external control device. However, the skilled person will understand that this is not limiting, and in other examples the rotor controller may be configured to vary the current limit of at least the first current regulator based on a program or logic stored at the rotor, for example accounting for an indication of induced winding current, an indication of induced winding voltage at the winding field pole, and / or an indication of frequency of the induced winding voltage at the winding field pole. In some examples, the winding field pole coupled to the circuit comprises at least one first winding field pole; and the rotor further comprising at least one second winding field pole coupled to a second circuit. The second circuit comprises a third branch, comprising a third current regulator and a second capacitor, wherein the third current regulator is configured to control the current through the second energy storage device; and a fourth branch, comprising a fourth current regulator and a second load, wherein the fourth current regulator is configured to control the current through the second load. The third branch and the fourth branch are preferably connected in parallel. Alternatively, the winding field pole coupled to the circuit may comprise at least one first winding field pole coupled to the circuit and at least one second winding field pole coupled to the circuit; such that both the at least one first winding field pole and the at least one second winding field pole are electrically coupled to the first branch and the second branch of the circuit. This may be advantageous to facilitate shared energy storage. The skilled person will understand that the number of windings may be variable, however the number of north winding field poles and the number of south winding field poles are preferably configured to be equal. In another aspect of the invention, there is provided a method of operating a rotor for a synchronous electric motor, the rotor comprising a winding field pole electrically coupled to an energy storage device. The method comprises discharging the energy storage device during a time period, the time period comprising a period wherein voltage across the energy storage device is greater than induced voltage across the winding field pole, and wherein discharging the energy storage device is configured to increase the current across the winding field pole. Discharging the energy storage device during the time period may be configured to minimise current fluctuations, thus reducing current ripple, and therefore reducing fluctuations in the field magnetic field which give rise to torque ripple in the motor. For example, the energy storage device may be discharged during a second time period, wherein the method further comprises charging the energy storage device during a first time period. The first time period may be at least a portion of a half period of induced winding voltage of a first polarity at the winding field pole, and the second time period may be at least a portion of a half period of induced winding voltage of a second polarity at the winding field pole, the first polarity being different to the second polarity. A method of operating the rotor, the rotor comprising a winding field pole coupled to a circuit, the circuit comprising a first branch comprising a first current regulator and a load, and a second branch comprising a second current regulator and an energy storage device, wherein the second current regulator is configured to control the current through the energy storage device, wherein the first branch and the second branch are connected in parallel. The method comprises controlling the current limit of the first current regulator for a first time period to have a current limit lower than the current limit of the second current regulator, such that current is configured to flow through the second branch during the first time period and charge the energy storage device during the first time period. Controlling the current limit of the first current regulator may comprise controlling the resistance of the first current regulator. The method may further comprise controlling the current limit of the first current regulator for a second time period to have a current limit at least equal to the current limit of the second current regulator, such that current is configured to flow through at least the first branch during the second time period. For example, the first time period may comprise a time period wherein voltage across the energy storage device is less than induced voltage across the winding field pole. The method may further comprise discharging the energy storage device during a third time period, wherein the third time period comprises a time period wherein voltage across the energy storage device is greater than induced voltage across the winding field pole. Discharging the energy storage device may be configured to increase the current across the winding field pole. Optionally, the first time period may be at least a portion of a half period of induced winding voltage of a first polarity at the winding field pole, and the third time period may be at least a portion of a half period of induced winding voltage of a second polarity at the winding field pole, the first polarity being different to the second polarity. In another aspect of the invention, there is provided a rotor for a synchronous electric motor, the rotor comprising a rotor circuit. The rotor circuit comprises a first winding field pole, a second winding field pole, and an energy storage device configured to be electrically coupled to the first winding field pole and the second winding field pole. The rotor circuit further comprises a first current flow control device arranged between the first winding field pole and the energy storage device, the first current flow control device configured to allow induced AC half period current components of a single direction to pass from the first winding field pole to the energy storage device, and a second current flow device arranged between the second winding field pole and the energy storage device, the second current flow control device configured to allow induced AC half period current components of a single direction to pass from the second winding field pole to the energy storage device. This may be advantageous to provide a single combined circuit that facilitates multiple north and south windings sharing the same magnetic circuits and energy storage. The first winding field pole may comprise a winding field pole of a first polarity, and the second winding field pole may comprise a winding field pole of a second polarity, wherein the second polarity is of opposite polarity to the first polarity. However, alternatively, the first winding field pole may comprise a winding field pole of a first polarity, and the second winding field pole comprises a winding field pole of the first polarity, such that the second winding field pole is of the same polarity to the first winding field pole. The first winding field pole may comprise a plurality of first winding field poles, and the second winding field pole may comprise a plurality of second winding field poles. In a first configuration, the plurality of first winding field poles may be configured to be in series, and the second plurality of winding field poles may be configured to be in parallel. The plurality of first winding field poles and the plurality of second winding field poles may be configured to be winding field poles of the same polarity. For example, wherein the first plurality of winding field poles and the second plurality of winding field poles are both configured to be north winding field poles, or wherein the first plurality of winding field poles and the second plurality of winding field poles are both configured to be south winding field poles. Alternatively, the plurality of first winding field poles may be configured to be winding field poles of opposite polarity to the plurality of second winding field poles. For example, wherein the first plurality of winding field poles are configured to be north winding field poles, and the second plurality of winding field poles are configured to be south winding field poles, or vice versa. The circuit may be configured for the first configuration and a second configuration, wherein in the first configuration, the plurality of first winding field poles are configured to be in series, and the plurality of second winding field poles are configured to be in parallel; and in the second configuration, the plurality of first winding field poles are configured to be in parallel and the plurality of second winding field poles are configured to be in series. The circuit may comprise a first branch comprising a first current regulator and the energy storage device, and a second branch comprising a second current regulator and a load. The first current regulator may be configured to control the current through the energy storage device; and the second current regulator may be configured to control the current through the load. The first branch and the second branch are configured to connected in parallel; and the first branch and the second branch are each connected to the at least one first winding field pole and the at least one second winding field pole. This may be advantageous as coordination of the current regulators can be used to control operation of the energy storage device for field current levelling, thus reducing current ripple, and therefore reducing fluctuations in the field magnetic field which give rise to torque ripple in the motor. A rotor may further comprise a plurality of the rotor circuits, each of the plurality of rotor circuits being electrically independent. Separate rotor circuits may increase robustness and reliability. For example, in the event that one circuit fails, the other independent rotor circuits may still be operational and thus the rotor may continue to operate, albeit with a reduced amount of rotor field strength. In another aspect of the invention, there is provided an electric motor comprising the rotor of any preceding aspect of the invention disclosed herein. The electric motor may further comprise a stator configured to generate a varying magnetic field; wherein the varying magnetic field is configured to induce a voltage at a winding field pole of the rotor. The winding field poles of the rotor are therefore configured to utilise the AC energy from the stator to provide constant rotor pole magnetic fields. Drawings Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 schematically illustrates an embodiment of the present invention comprising two current controlled capacitance charging and current controlled loading circuits, each comprising a single winding. Fig. 2 schematically illustrates current and voltage plots during the operation of a single winding with current controlled capacitance charging and current controlled loading circuits, such as one of the circuits of Fig. 1. Fig. 3 schematically illustrates an alternative embodiment of the present invention comprising two North windings in series with capacitor charging and loading with current control, and two North windings in parallel while discharging the capacitance with current control. Fig. 4 schematically illustrates current and voltage plots during the operation of the circuits of Fig. 3. Fig. 5 schematically illustrates another embodiment of the present invention comprising a single combined circuit that may be used to provide the functions of the circuits of Fig. 3, to provide the operation as shown in the plots of Fig. 4. Figs. 6A to 6B schematically illustrate current and voltage plots during the operation of the circuit of Fig. 5. Fig. 6C schematically illustrates how the fluxes generated by the North and South pole windings combine on the same magnetic flux circuit. Fig. 7 schematically illustrates another embodiment of the present invention comprising a single combined circuit that may be used to provide the functions of multiple North and South windings sharing the same magnetic circuits and energy storage. Fig. 8 schematically illustrates the individual winding currents during the operation of North and South windings, and combined winding currents, with low or zero energy storage. Fig. 9 schematically illustrates the individual winding currents during the operation of North and South windings, and combined winding currents, with higher energy storage compared to Fig. 8. Fig. 10 schematically illustrates another embodiment of the present invention comprising a single combined circuit that may be used to provide the functions of multiple North and South windings sharing the same magnetic circuits and energy storage, with the inclusion of current regulation and loading. Fig. 11 schematically illustrates the individual winding currents during the operation of North and South windings, and the combined winding current, for operation of the circuit of Fig. 10, without current regulation coordination. Fig. 12 schematically illustrates current plots for a North and a South windings, and combined winding currents, without energy storage. Specific description Embodiments of the claims relate to rotors for use in synchronous electric motors, and methods of operation thereof, in particular rotors configured to minimise torque ripple in synchronous electric motors. Synchronous electric motors comprise a stator configured to generate a varying magnetic field, and a rotor configured to rotate about a rotation axis in response to the varying magnetic field. Fig. 1 schematically illustrates two current controlled, capacitance charging, and current controlled loading circuits (100, 102), each circuit 100, 102, comprising a single field winding. In this example, a first circuit 100 comprises a north field winding NW1, and a second circuit 102 comprises a south field winding, SW1. Both circuits 100 and 102 may be provided on a rotor for use in a synchronous motor. The circuit 100 the North winding NW1 and a Current flow control device 1. The circuit 100 further comprises (i) a first branch comprising Current Regulator 1 and a capacitor C1, and (ii) a second branch comprising a Current Regulator 2 and Load 1, wherein the first branch and the second branch are connected in parallel. The Current Regulator 1 is configured to control the current through capacitor C1, and Current Regulator 2 is configured to control the current through Load 1. The rotor further comprises second circuit 102, wherein circuit 102 comprises the South winding SW1 and Current flow control device 2. The circuit 102 further comprises (i) a first branch comprising Current Regulator 3 and a capacitor C2, and (ii) a second branch comprising a Current Regulator 4 and Load 2, wherein the first branch and the second branch are connected in parallel. The current flow control elements (Current flow control 1, Current flow control 2) may be passive elements such as diodes or active elements such as switch devices. The loading elements (Load 1, Load 2) may be passive elements such as resistors or temperature dependent resistors, where the impedance of the load could be fixed or varying and have values including zero ohms. The current regulator elements (Current regulator 1, Current regulator 2, Current regulator 3, Current regulator 4) may include passives such as temperature dependent resistors, discrete electronics such as comparators and semiconducting switches, or intelligent such as logic / microprocessor-controlled semiconductor devices. The skilled person will understand that the current regulators may be self-powering, or drawn on a supply derived from the energy storage of capacitors, C1 or C2, or from a supply derived from a separately rectified feed from the windings, NW1 and SW1. Fig. 2 schematically illustrates current and voltage plots during the operation of a single winding with current controlled capacitance charging and current controlled loading circuits. For the purposes of illustration, Fig. 2 will be described with reference to the circuit 100 of Fig. 1. 1001 denotes the voltage or current axis, and 1002 denotes the time axis. 1003 denotes the voltage induced on the winding, such as NW1, and 1006 denotes the voltage across the capacitor, such as C1, in the same circuit, such as 100. 1004 denotes the current through the capacitor, C1, during charging when current regulator 1 has no limit and when current regulator 2 has a limit of zero amps. In this case, the circuit 100A is equivalent to a simple diode (provided by current flow control 1), and capacitor C1 in series with the NW1 winding. This current is unidirectional and will thus produce a unidirectional magnetic field in the NW1 winding pole. However, the current is not held constant and is not at a controlled level. The magnetic flux will thus also not be constant and at the required level. Those skilled in the art will understand that a simple, low-cost version of the invention can be produced with simple diodes as the current flow control, and no current regulators or loads. 1007 denotes the current through the NW1 winding when current regulator 1 has a limit of zero, and current regulator 2 has a current limit equal to the plateau in plot 1005. In this case, the field current is constant for part of the induced voltage wave. However, the energy is lost in the load. The desired winding current level is the plateau in plot 1005. An embodiment of the present invention uses two current regulators (current regulator 1 and current regulator 2) to provide the winding current wave 1005, and also to charge the capacitor C1 as per plot 1006. During section 1007 (bounded by plot 1005 and plot 1004) the current flows through current regulator 2. Both current regulators have a limit set at the desired winding current level. The winding current, 1005, cannot immediately reach the desired winding current level as the induced winding voltage is too small. As the induced winding voltage rises, so does the winding current until it reaches the desired winding current level where it is held by current controller 2. During this stage in the induced winding voltage, there is not enough voltage to charge the capacitor C1 at the required winding current level. At the boundary of section 1007 and 1008 (which is part of the plot 1004) current regulator 2 reduces its limit so that current can flow through current regulator 1 to charge the capacitor, C1. Those skilled in the art will understand how the control of the current regulator’s limits can be based on the induced winding voltage level, by logic / microprocessor sequencing, or otherwise. During section 1008, the current flow has changed from fully through the load, Load 1, to fully charging the capacitor, C1, where it is limited by current regulator 1. At the boundary of sections 1008 and 1009, the balance of current flow is reversed by increasing the current limit of current regulator 2 so that current flows through the load, Load 1, once more. This method is repeated during the next positive half cycles of the induced winding voltage until the voltage across the capacitor C1, denoted by 1006, results in no current flowing through current regulator 1. Figs. 4 and 6A to 6C will explain how the invention utilises the stored energy in capacitor, C1, which results in the capacitor charging during the positive half cycle of the induced winding voltage wave. The previous description relating to Fig. 2 has been disclosed in relation to the NW1 winding which is a North pole winding of circuit 100 of Fig. 1. However, those skilled in the art will appreciate the mirrored symmetry of currents which will be provided by the circuit 102 in Fig. 1 which contains SW1 winding which is a South pole winding. In this case, it is the negative half cycle of the induced winding voltage that is used to charge the capacitor, C2 through current regulator 3 and is loaded by Load 2 through current regulator 4. Fig. 3 schematically illustrates two North windings, NW1 and NW2, in series with capacitor charging and loading in a first circuit 300 with current control, and two North windings, NW1 and NW2, in parallel while discharging the capacitance in a second circuit 302 with current control. Both circuits 300 and 302 may be provided on a rotor for use in a synchronous motor. Although only North windings are shown in Fig. 3, those skilled in the art will appreciate mirrored symmetry may be provided by equivalent circuits of Fig. 3, or otherwise, which include South pole windings, such as SW1 and SW2 in the place of North windings N1 and N2. The structure of circuit 300 is similar to that of circuit 100 of Fig. 1 as described above, however instead of providing only one North winding pole NW1, the circuit 300 comprises two North winding poles NW1 and NW2 in series. Circuit 302 comprises a capacitor C1, Current regulator 5, and Current flow control device 11 which are electrically coupled to a first branch of the circuit and a second branch of the circuit, the first and the second branches being arranged in parallel. Each of the first and second parallel branches of circuit 302 comprise a North winding pole, NW1 and NW2, respectively. Turning to circuit 300 of Fig. 3, the two North windings, each on its own field pole, NW1 and NW2, are connected in series to give the maximum induced voltage peak which is then current flow controlled by Current flow control 1. Current flow control 1 could be a simple diode or a switch, etc. Current Regulator 1 controls the current charging the capacitor, C1. Current regulator 2 controls the current through the load, Load 1. Turning to circuit 302 of Fig. 3, the two North windings, NW1 and NW2, are connected in parallel resulting in the minimum induced voltage peak which maximises the time for which the voltage on the C1 is greater so current can flow through Current flow control 11 under the control of Current Regulator 5. Fig. 4 schematically illustrates current and voltage plots during the operation of the circuits of Fig. 3. Similar reference numerals to Fig. 2 denote features as before. The half period of the positive induced voltage, 1003, when the two windings, NW1 and NW2, are in series (circuit 300) is denoted by 1203 during which the current, 1005, is controlled by Current Regulator 1 while charging the capacitor, C1 and Current regulator 2 while current passes through the load, Load 1. The half period of the negative induced voltage when the two windings, NW1 and NW2, are in parallel (circuit 302) is denoted by 1204. During this time the capacitor, C1, is discharged, when its voltage is greater than the induced voltage on the windings, NW1 and NW2. This occurs in two places where the current is denoted 1201 and 1202. During these times, the current is controlled by Current flow control 11 and Current Regulator 5. Fig. 5 schematically illustrates a single combined circuit 500 for a rotor that may be used to provide the functions of Fig. 3, giving results as in the plots of Fig. 4. Combined circuit 500 comprises a first branch comprising Current Regulator 6 and a capacitor C1, and a second branch comprising a Current Regulator? and Load 1, wherein the first branch and the second branch are connected in parallel. Current regulator 6 is configured to provide the charging control of Current Regulator 1, and the discharge current control of Current Regulator 5. Current regulator 7 provides the same function as Current Regulator 2. Combined circuit 500 further comprises current flow control 3 coupled between (i) the North windings and (ii) the first and second branches. Current flow control 3 provides the functions of Current flow control 1 (when current flows out of the series connected windings NW1 and NW2 to charge the capacitor C1), and Current flow control 11 (when current flows out of the capacitor C1 and into the parallel connected windings NW1 and NW2). The transition between the parallel and series connection of the two North winding poles, NW1 and NW2, is made by a parallel and series controller 1 arranged therebetween. The parallel and series controller 1 is also coupled to a circuit branch parallel to the two North winding poles, NW1 and NW2. The parallel and series controller 1 is therefore configured to transition the two North winding poles, NW1 and NW2, between a series connection wherein the current flow is shown by path 504, and a parallel configuration wherein the current flow path is shown by path 506. The combined winding current polarity is controlled by the H bridge made up of a plurality of switches, S1, S2, S3 and S4. Alternatively, the skilled person will understand that some or all of the switches, S1, S2, S3, S4, can be exchanged for diodes. In addition, those skilled in the art will understand that alternative electrical connection methods may also be used to provide similar functionality. During the positive half cycle of the induced winding voltage, the current flow is shown by path 504. The windings NW1 and NW2 are connected in series by parallel and series controller 1, and only switches S2 and S3 are conducting in a closed position. The current path 504 is either / or charging C1 through Current Regulators, or / and through Load 1 and Current Regulator 7. During the negative half cycle of the induced winding voltage, the current flow is shown by path 506. The windings NW1 and NW2 are connected in parallel by parallel and series controller 1, and only switches S1 and S4 are conducting in a closed position. The current path 506 is discharging capacitor C1 through Current Regulator 6. Whilst only North windings are shown in Fig. 5, those skilled in the art will appreciate mirrored symmetry may be provided by an equivalent circuit of Fig. 5, or otherwise, which includes South pole windings, such as SW1 and SW2 in the place of North windings N1 and N2. Fig. 6A schematically illustrates a current and voltage plot during the operation of a circuit, including North windings. In this example, the North windings, such as NW1 and NW2, are employed as per the circuit and function of Fig. 5. Fig. 6B schematically illustrates a current and voltage plot during the operation of a mirror circuit, including South windings. In this example, the South windings (SW1, SW2. not shown) are provided in a similar combined mirror circuit configuration as the North windings, NW1 and NW2, of Fig. 5. Fig. 6C schematically illustrates the fluxes generated by the North and South pole windings of Figs. 6A and 6B combined. Each unique North winding, NW1 or NW2, shares the same magnetic circuit as a unique South winding, SW1 or SW2. Alternatively, the skilled person will understand that all windings can be on the same magnetic circuit, or in groups so long as there is the same number of Norths and South windings per group. For each of Figs. 6A to 6C, the time denoted by 1002 is indicated as three time periods, 1401, 1402 and 1403. During time section 1401, the induced winding voltages 1003 of all windings (NW1, NW2, SW1, SW2) are positive. As shown in Fig. 6A, during 1401, the North windings, NW1 and NW2, charge their capacitor, C1 (as shown by plot 1006), and / or are loaded, by Load 1, as described above. During time section 1402, the induced winding voltages 1003 of all windings (NW1, NW2, SW1, SW2) are negative. As shown in Fig. 6A, the North winding circuit 500 discharges the capacitor C1 through the North windings, NW1 and NW2 at time portions 1201 and 1202 when its voltage is greater than the induced voltage on the windings. By contrast, as shown in Fig. 6B, the South winding circuit charges its capacitor, C2, during time period 1402. During time section 1403, the induced winding voltages 1003 of all windings (NW1, NW2, SW1, SW2) are positive once more. As shown in Fig. 6A, the North winding circuit 500 charges the capacitor C1 through the North windings, NW1 and NW2; whilst the South winding circuit (not shown) discharges its capacitance via capacitor C2 through its windings, SW1 and SW2, as shown in Fig. 6B. Time sections 1402 and 1403 then cyclically repeat. Fig. 6C shows the combined currents through a North and a South winding. The control of the capacitance charging, loading, and discharging may be configured to provide a constant magnetic flux, 1404, around a shared magnetic circuit. This may be advantageous to reduce and / or prevent torque ripples during operation of a synchronous electromagnetic motor using a rotor described herein. It is noted that each North and South winding passes current only half the time. As such, each winding may require twice the number of the turns to provide the same magnetic field level as would be provided if both the North and South windings carried current at the same time. Despite this, by virtue of passing current only half the time, the wire used for each winding may be half the cross-sectional area compared to that needed for a winding which carries current constantly, so ultimately each winding will fit in approximately the same volume as a conventional winding configuration. Those skilled in the art will understand how the control of the regulator’s limits can be based on the induced winding voltage level in either the windings that are currently in series (this voltage will be affected by the loading), or more preferably, the windings currently in parallel (this voltage is not affected by loading), or by logic / microprocessor sequencing. Fig. 7 schematically illustrates a single combined circuit that may be used to provide the functions of multiple North and South windings, NW1, NW2, SW1, SW2, sharing the same magnetic circuits and energy storage, C1. Fig. 7 illustrates how a shared energy storage, capacitor C1, can be used by combining instances of the circuits described earlier. Current flow control 4 controls the current from the South windings and switches S5, S6, S7 and S8 can control the flow polarity. It is noted that there is no current regulation or loading shown in this circuit. The circuit 700 comprises a first H bridge made up of a plurality of switches, S1, S2, S3 and S4. However, the skilled person will understand that some or all of the switches, S1, S2, S3, S4, can be exchanged for diodes. The load of the H bridge comprises two North windings NW1 and NW2 with a parallel and series controller 1 therebetween. As described with reference to Fig.5, the parallel and series controller 1 is also coupled to a circuit branch parallel to the two North winding poles, NW1 and NW2. The parallel and series controller 1 is therefore configured to transition the two North winding poles, NW1 and NW2, between a series connection wherein the current flow is shown by path 504, and a parallel configuration wherein the current flow path is shown by path 506. The circuit 700 also comprises second mirrored H bridge, the second mirrored H bridge comprises two South winding poles, SW1 and SW2, instead of North winding poles. The circuit 700 further comprises a capacitor C1 coupled between the first and second H bridges, wherein current flow control 3 is arranged in between the fist H bridge and the capacitor C1, and current flow control 4 is arranged in between the second H bridge and the capacitor C1. The value of the energy storage, in this example capacitor C1, can be optimised to provide a unidirectional current offset versus the time required to initially charge the capacitor, C1. Fig. 8 schematically illustrates the North winding current 802, South winding current 804, and a combined current 806 provided by the North and South windings, with low or no energy storage. As the energy storage is low or zero, the combined current 806 from one North winding 802 and one South winding 804 follows a fully rectified shape of AC with a low average. It is noted that in the example shown in Fig. 8, the current passes through two windings of the same polarity in series, into two windings of the same polarity in parallel. This results in the parallel winding having half as much current flowing through it that the series connected winding, thus providing the asymmetric current profile for each of the North windings and South windings respectively, 802 and 804. Winding utilisation is good as current flows through all windings at the same time. Fig. 9 schematically illustrates North winding current 902, South winding current 904, and a combined current 906 with higher energy storage compared to the Fig. 8. Providing greater energy storage means that the combined current 906 from a North winding 902 and a South winding 904 has a greater average and does not drop to zero, unlike the combined current 806 of Fig. 8. The shape of the individual winding currents is similar to those in Fig. 2. Fig. 10 schematically illustrates a single combined circuit 1000 that may be used to provide the functions of multiple North and South windings sharing the same magnetic circuits and energy storage with the inclusion of current regulation and loading. Circuit 1000 comprises first and second H bridges as described above with reference to Fig. 7. Combined circuit 1000 further comprises a first branch comprising Current Regulator 6 and a capacitor C1, and a second branch comprising a Current Regulator? and Load 1, wherein the first branch and the second branch are connected in parallel, and wherein the first and second branches are coupled between the first and second H bridges. Combined circuit 1000 further comprises current flow control 3 coupled between (i) the first H-bridge comprising the North windings and (ii) the first and second branches. Current flow control 3 provides the functions of Current flow control 1 (when current flows out of the series connected windings NW1 and NW2 to charge the capacitor C1), and Current flow control 11 (when current flows out of the capacitor C1 and into the parallel connected windings NW1 and NW2). Combined circuit 1000 also comprises current flow control 4 coupled between (i) the second H-bridge comprising the South windings and (ii) the first and second branches. The circuit requires only one load (Load 1) and one load current regulator (Current Regulator 7) and current regulators 6 and 7 may be controlled in a coordinated manner. Fig. 11 schematically illustrates the individual winding currents for a North winding current 1102 and a South winding current 1104, for operation of the circuit of Fig. 10 without current regulation coordination. Fig. 11 also shows a combined winding current 1106 of both the North and South winding currents 1102, 11O4.The combined winding currents 1106 can be made constant by tuning the coordinated current regulators. Fig. 12 schematically illustrates current plots for North windings 2001, South windings 2003, and the combined current 2001 of the North and South windings when the is no energy storage. The time axis 1002 is separated into time periods 2004 through to 2009. During sections 2004 and 2006, the current is provided by the controlled loading of the North windings. During section 2005, the current is shared between the North and South windings. The North windings are in series at this stage so pass the full current and the South windings are in parallel so pass half that current. Hence the plot shows that the combined current is one third through the South windings, and two thirds through the North windings. The skilled person will understand that this ratio is variable depending on the number of windings that are combined in a single circuit. In this example, the minimum set of windings would be 4, which comprises two North windings and two South windings. However, the skilled person will understand that additional pairs of winding poles (the pair comprising a North winding and a South winding) may be added to the same circuit. This may be advantageous to increase the voltage during series connection. The roles of the South and North windings are reversed in sections 2007, 2003 and 2009. Generally, the case with low or zero energy storage will provide less winding current and hence less magnetic field for the same induced voltage conditions. Those skilled in the art will understand that one or more combined groups of windings, each with a separate circuit, can be used on the same rotor. For example, a plurality of sets of windings, for example each set being a group of four windings (two North and two South), may be provided on separate circuits. A primary advantage of providing separate circuits is to improve robustness and reliability. For example, if one group of windings on a shared circuit fails, providing multiple separate circuits will enable other groups(s) of windings on said separate circuits to continue working, and so the rotor will continue to function, having only a reduced amount of rotor field strength. The skilled person will also understand from the description above that it is possible to alter the current regulation level, and thus control the rotor field strength during operation. This may be particularly beneficial at higher speed levels, as it extends the speed range for the same stator voltage. This is a region often called “field weakening” or the “constant power region.” For example, in a simple case, this may take the form of a frequency detector which measures the frequency of the induced rotor winding voltage. Alternatively, in embodiments comprising multiple groups of four windings (two North and two South) using separate circuits, it may be possible to selectively to disable one or more of these circuits to reduce the total rotor field strength. Additionally or instead, an “over speed” protection method could employ the techniques above and result in removing the rotor field winding current entirely if the rotational speed of the rotor exceeds a pre-determined threshold. The figures described herein indicate simple configurations of rotor windings around their associated rotor poles. However, the skilled person will understand that the magnetic circuits may also include the metal, such as iron or magnetic steel, which joins the rotor poles. This is often referred to as the “back iron”. The skilled person will also understand that an alternative approach may be employed by placing the windings on the metal between the poles, instead of around the poles. Embodiments of the present invention operate with both positions of the rotor windings. Those skilled in the art will also understand that the preceding methods and circuits can be combined and also implemented in different forms. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed, or replaced as described herein and as set out in the claims. 5 In the context of the present disclosure other examples and variations of the apparatus and methods described herein will be apparent to a person of skill in the art.

Claims

1. A rotor for a synchronous electric motor, the rotor comprising a rotor circuit, the rotor circuit comprising:a first winding field pole;a second winding field pole;an energy storage device, electrically coupled to the first winding field pole and the second winding field pole;a first current flow control device arranged between the first winding field pole and the energy storage device, the first current flow control device configured to allow induced AC half period current components of a single direction to pass from the first winding field pole to the energy storage device;a second current flow control device arranged between the second winding field pole and the energy storage device, the second current flow control device configured to allow induced AC half period current components of a single direction to pass from the second winding field pole to the energy storage device.

2. The rotor of claim 1 wherein the first winding field pole comprises a winding field pole of a first polarity, and the second winding field pole comprises a winding field pole of a second polarity, wherein the second polarity is of opposite polarity to the first polarity.

3. The rotor of claim 1 wherein the first winding field pole comprises a winding field pole of a first polarity, and the second winding field pole comprises a winding field pole of the first polarity, such that the second winding field pole is of the same polarity to the first winding field pole.

4. The rotor of any one of claims 1 to 3, wherein the first winding field pole comprises a plurality of first winding field poles, and the second winding field pole comprises a plurality of second winding field poles.

5. The rotor of claim 4, wherein the plurality of first winding field poles and the plurality of second winding field poles are configured to be winding field poles of the same polarity.

6. The rotor of claim 4, wherein the plurality of first winding field poles are configured to be winding field poles of opposite polarity to the plurality of second winding field poles.

7. The rotor of claim 5 or claim 6, wherein in a first configuration, the plurality of first winding field poles are configured to be in series, and the plurality of second winding field poles are configured to be in parallel.

8. The rotor of any claim 7, wherein in a second configuration, the plurality of first winding field poles are configured to be in parallel and the plurality of second winding field poles are configured to be in series.

9. The rotor of any preceding claim wherein the rotor circuit comprises:a first branch, comprising a first current regulator and the energy storage device, wherein the first current regulator is configured to control the current through the energy storage device; anda second branch, comprising a second current regulator and a load, wherein the second current regulator is configured to control the current through the load.

10. The rotor of claim 9, wherein the first branch and the second branch are connected in parallel; and wherein the first branch and the second branch are each connected to the at least one first winding field pole and the at least one second winding field pole.

11. The rotor of any preceding claim, wherein the first and second current flow control devices comprise one or more of diodes, switch devices or a combination thereof.

12. The rotor of any preceding claim further comprising a plurality of the rotor circuits, each of the plurality of rotor circuits being electrically independent.

13. A method of operating a rotor for a synchronous electric motor, the rotor comprising a first plurality of winding field poles and a second plurality of winding field poles, each electrically coupled to an energy storage device, wherein the method comprises the steps of:operating a first current flow control device arranged between the first plurality of winding field poles and the energy storage device to allow induced AC half period current components of a single direction to pass from the first winding field pole to the energy storage device; andoperating a second current flow control device arranged between the second plurality of winding field poles and the energy storage device to allow induced AC half period current components of a single direction to pass from the second winding field pole to the energy storage device.

14. The method of claim 13, wherein the method further comprises the steps of: controlling, using a first current regulator, the current through the energy storage; and controlling, using a second current regulator, a current through a load.

15. An electric motor comprising the rotor of any preceding claim.

16. The electric motor of claim 15 further comprising:a stator configured to generate a varying magnetic field;wherein the varying magnetic field is configured to induce a voltage at a winding field pole of the rotor.

17. The electric motor of claims 15 or 16 further comprising a control device, wherein the control device is configured to cause a processor on the rotor to perform the method of claims 13 or 14.

18. A computer readable non-transitory storage medium comprising a program for a computer configured to cause a processor to perform the method of claims 13 or 14.

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