Methods and control systems for regenerative braking

Abstract

A control system for an electric vehicle configured to perform regenerative braking, the control system comprising one or more processors collectively configured to carry out the following steps: 4020 - determine if a state of charge (SoC) of the vehicle battery meets a SoC charge criterion; and 4040 - upon determining that the SoC meets the SoC criterion, increase a current supplied to an induction motor of the vehicle, compared to a baseline current in the induction motor, to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation. When the SoC meets the SoC criterion, the control system may: increase a stator phase current to be higher than a current for achieving maximum torque per Ampere control; and it may reduce a rotor current frequency, compared to a rotor current frequency for achieving maximum torque per Ampere control. The control system may set the current in the induction motor to the baseline current associated with efficient torque generation in the motor when the SoC of the vehicle battery does not meet the SoC criterion, to perform efficient regenerative braking.

Description

TECHNICAL FIELD The present disclosure relates to methods and control systems for regenerative braking. Aspects of the invention relate to a control system, to a vehicle, to a computer implemented method and to computer readable instructions for controlling an electric vehicle configured to perform regenerative braking. BACKGROUND Electric vehicles typically carry out two types of braking: mechanical braking and regenerative braking. With regenerative braking, kinetic energy from the vehicle’s momentum is converted into electricity to charge the vehicle’s battery. In mechanical braking, brake pads are used to slow the vehicle down and the kinetic energy of the vehicle is dissipated as heat. SUMMARY OF THE INVENTION Although regenerative braking can improve a vehicle’s efficiency, when regenerative braking is applied when the vehicle battery is already at a high state of charge (e.g. at or near full capacity), the additional charging current can damage the battery. Thus, in current systems, when the vehicle battery is in a high state of charge, instead of receiving power generated from braking via a regenerative mode of operative, vehicle braking is carried out with mechanical braking. In mechanical breaking, braking pads of the vehicle wear down more quickly and there is thus increased wear and tear of the vehicle as a result. Furthermore, drivers of the vehicle experience a different driving sensation because regenerative braking feels different to mechanical braking, and this can be jarring to the driver, impacting the perceived smoothness of the driving experience of the vehicle. It is an aim of the disclosure herein to address some of the aforementioned issues amongst others. Aspects and embodiments of the invention provide a control system, a vehicle and a computer implemented method for controlling an electric vehicle as claimed in the appended claims. According to an aspect of the present invention, there is provided a control system for an electric vehicle configured to perform regenerative braking (which may otherwise be thought of as electronic, as opposed to mechanical braking). The control system comprises one or more processors collectively configured to determine if a state of charge of the vehicle battery meets a state of charge criterion. Upon determining that the state of charge meets the state of charge criterion, the one or more processors are collectively configured to increase a current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor. The baseline current is associated with efficient torque generation. This control system can allow regenerative braking to be carried out when the battery is at a high state of charge without supplying a significant amount of further current to the battery, thus protecting the battery from damage. Braking can also be performed electronically even when the battery is full, rather than via mechanical braking, and so a consistent braking sensation can be achieved for the driver. The control system can also result in reduced wear of the brake pads of the vehicle as less mechanical braking is performed. The baseline current is associated with efficient torque generation. For example, the baseline current may be the “default” current e.g. the current used during normal operation of the vehicle (when the battery is not near full capacity). The baseline current may be set, for example, at (or approximately at) the maximum torque per ampere (MTPA) current. Generally, the term “efficient torque generation” may refer to the generation of a desired torque with minimum (or near-minimum) conduction losses. The control system may be configured to increase the current to perform regenerative braking with reduced electrical energy going to the vehicle battery when the state of charge meets the state of charge criterion. The control system may be configured to increase the current supplied to a stator of the induction when the state of charge meets the state of charge criterion. It is easier to cool the stator as it is more thermally accessible and can more easily have a cooling system applied to it compared to the rotor. The control system may be configured to increase a stator phase current to be higher than a current for achieving maximum torque per ampere control. Typically, an induction motor is operated under maximum torque per ampere control to generate a requested torque with the minimum amount of current, to minimise these conduction losses. If a stator phase current is increased to be higher than a current for achieving maximum torque per ampere control, torque is generated in a less efficient manner and the conduction losses increase. In this way, the energy recovered from regenerative braking may be dissipated through conduction losses instead of being stored in the vehicle battery (or with less energy being stored in the battery), and regenerative braking can be carried out when the vehicle battery is at a high state of charge without damaging the battery, or requiring the driver to experience a different driving sensation. The vehicle’s braking pads can also be worn down less and the vehicle can have a greater life span. The control system may be configured to reduce a rotor current frequency compared to a rotor current frequency for achieving maximum torque per ampere control. The induction motor can be configured to create torque in an inefficient manner by increasing the stator-phase current and reducing the rotor current frequency compared to a stator-phase current and rotor current frequency for achieving maximum torque per ampere control. Reducing the rotor current frequency and increasing the stator phase current results in heat dissipation from the stator. It is easier to cool the stator compared to the rotor. The state of charge criterion may comprise a charge threshold. The state of charge may be determined to meet the state of charge criterion if the state of charge is higher than or equal to the state of charge threshold. The state of charge criterion may alternatively comprise a charge range. The state of charge may be determined to meet the state of charge criterion if the state of charge is within the charge range. Beyond the threshold, the amount of charge supplied to the stator may increase e.g. linearly with state of charge of the battery. In this way, as the battery charge increases, more braking energy may be diverted into heat (as opposed to charge to the battery). The control system may be configured to set the current in the induction motor to the baseline current associated with efficient torque generation in the induction motor when the state of charge of the vehicle battery does not meet the state of charge criterion, to perform efficient regenerative braking. The vehicle battery may therefore be charged with energy generated from braking when the state of charge of the vehicle battery is at a level such that charging will not damage the battery. The one or more processors of the control system may further be configured to control a cooling system to cool the induction motor. Cooling the induction motor may for example involve bringing the temperature of the induction motor (e.g., rotor and / or stator) down to its normal operating temperature, such as the temperature of the induction motor under maximum torque per ampere control. This may help the induction motor not to overheat. According to another aspect of the invention, there is provided a vehicle comprising the control system, the induction motor, and the vehicle battery. The vehicle may further comprise a cooling system configured to cool the induction motor. In this way, the excess heat caused can be dissipated. The induction motor may be a traction motor. The induction motor may therefore be used to propel the vehicle. According to a further aspect of the invention, there is provided a computer implemented method for controlling an electric vehicle configured to perform regenerative braking. The method comprises determining if a state of charge of the vehicle battery meets a state of charge criterion. Upon determining that the state of charge of the vehicle battery meets the state of charge criterion, the method may also comprise increasing current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation. Upon determining that the state of charge meets the state of charge criterion, the step of increasing current supplied to the induction motor may comprise increasing the current to perform regenerative braking with reduced electrical energy going to the vehicle battery. Increasing the current supplied to an induction motor of the vehicle may comprise increasing current supplied to a stator of the induction motor to be higher than a current for achieving maximum torque per ampere control. According to yet a further aspect of the invention, there is provided computer readable instructions which, when executed by a computer, are arranged to perform the computer implemented method for controlling an electric vehicle configured to perform regenerative braking. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a graph showing torque isocurves of an induction motor as a function of stator phase current and rotor current frequency; Figure 2a shows a block diagram of a controller according to an embodiment of the present invention; Figure 2b shows a block diagram of a vehicle according to an embodiment of the present invention; Figure 3 shows a vehicle according to an embodiment of the present invention; Figure 4 shows a flowchart of a method according to an embodiment of the present invention; Figure 5 shows a flowchart of a method according to an embodiment of the present invention; Figure 6 shows an induction motor circuit diagram according to an embodiment of the present invention; Figure 7 shows a vector diagram for a generator regime of an induction motor. DETAILED DESCRIPTION Induction motors are widely used in electric vehicles. They comprise a stator, which is stationary, and a rotor, which rotates. Induction motors can also be called induction machines, as they can operate as a motor and as a generator. The stator comprises windings which are connected to an alternating current (AC) power source which causes a rotating magnetic field to be created when AC current is supplied to it. The rotating magnetic field induces a current in the rotor, causing it to rotate. This rotation generates torque to propel the vehicle forwards. Induction motors can generate a particular torque (Ti, Tz, Ta) under different combinations of values of the stator phase current and rotor current frequency. For any given rotor speed, torque isocurves of an induction motor can be drawn as a function of stator phase current and rotor current frequency. Examples of such isocurves are shown in the graph of stator phase current (I) vs rotor current frequency (F) 100 in Figure 1. When the 3 induction motor is operating under Maximum Torque Per Ampere (MTPA) control, the combination of the stator current and the rotor current frequency is selected such that a requested torque (Ti, T2, T3) is generated with the minimum stator phase current (MTPA points (M) • in Figure 1). Conduction losses generated by current flowing through the stator and rotor contribute to most of the energy losses of the induction motor. Typically, an induction motor is operated at MTPA to generate a requested torque with the minimum amount of current, to minimise these conduction losses. MTPA control and the MTPA point is therefore the most efficient point of operation of an induction motor to generate a particular torque value. MTPA is generally applied both when the vehicle is in motion (to generate the most efficient driving conditions) and also when the vehicle is braking (to maximise the energy recovery for a given braking torque). As discussed above, when regenerative braking is applied when the vehicle battery is at a high state of charge (e.g. at or near full capacity), the surplus charge can damage the battery. Moreover, if a braking mode switches from regenerative braking to mechanical braking, a driver will likely notice a different sensation when mechanical braking is carried out, compared to regenerative braking, which reduces driver satisfaction. In mechanical braking, braking pads of the vehicle can also wear down more easily, leading to increased wear and tear of the vehicle. To address these issues, in embodiments herein, when the vehicle battery is determined to be at or close to full capacity, the current supplied to the induction motor is increased (compared to the current required to achieve a requested torque with minimal current). This has the effect of increasing the magnetic field in the induction motor which generates heat. The heat is dissipated, and the induction motor can be cooled at an increased rate, for example to bring the temperature of the induction motor down to its normal operating temperature, such as the temperature of the induction motor under MTPA control. Due to the energy generated from braking being (largely or fully) dissipated in the induction motor rather than being supplied to the vehicle battery, a form of (inefficient) regenerative braking (which might otherwise be thought of as electric braking) can be carried out when the battery is at a high state of charge without supplying a significant amount of further current to the battery, thus protecting the battery from damage. As described below, in this regime, regenerative braking with reduced efficiency (RBRE) is performed. Furthermore, in embodiments herein, braking is still performed electronically even when the battery is full, (rather than via mechanical braking) and thus, in embodiments herein, a consistent braking sensation can be achieved for the driver. Furthermore, embodiments herein allow for reduced wear of the brake pads of the vehicle. A control system for an electric vehicle configured to perform regenerative braking in accordance with embodiments herein is described herein with reference to Figures 2a and 2b. The control system comprises a controller 200 comprising one or more processors 202 and a memory 204 with instructions 206 stored therein configured to be operably executed by the one or more processors 202. The control system may be configured (e.g. operative) to perform any methods and functions described herein, such as method 400 described below. The controller 200 comprises processing means and memory means. The processing means may be one or more processors 202 which operably executes computer-readable instructions. The memory means may be one or more memory device 204. The memory means is electrically coupled to the processing means. The memory means is configured to store instructions, and the processing means is configured to access the memory means and execute the instructions stored thereon. The instructions, when executed by the processing means, may cause the one or more processors to perform any of the methods herein, such as the method 400 described below. The one or more processors 202 (e.g. processing circuitry or logic) may be any type of processor 202, such as, for example, a central processing unit (CPU), a Neural Processing Unit (NPU), or any other type of processing unit. The one or more processors may comprise one or more sub-processors, processing units, multi-core processors or modules that are configured to work together in a distributed manner to control the system in the manner described herein. The memory 204 of the controller 200 can be configured to store program code or instructions that can be executed by the processor 202 of the controller 200 to perform the functionality described herein. The memory 204 may be configured to store any data or information referred to herein, such as for example, requests, resources, information, data, signals, or similar that are described herein. The processor 202 may be configured to control the memory 204 to store such information. In brief, in embodiments herein, the one or more processors 202 are configured to determine if a state of charge of the vehicle battery 214 meets a state of charge criterion and, upon determining that the state of charge meets the state of charge criterion, increase a current supplied to an induction motor 216 of the vehicle 300 compared to a baseline current to increase thermal losses in the induction motor 216. The controller 200 shown in Figure 2a is configured to receive vehicle battery charge data from a vehicle battery charge gauge 212 and determine whether the state of charge of a vehicle battery 214 meets a state of charge criterion, such as meeting or exceeding a state of charge threshold. Upon determining that the state of charge of the vehicle battery 214 meets the state of charge criterion, the controller outputs a control signal to control the current supplied to an induction motor 216 of the vehicle. The current supplied to the induction motor 216 is increased compared to a baseline current in the induction motor 216 associated with efficient torque generation, to increase thermal losses in the induction motor 216. It will be appreciated that the controller 200 may comprise other components in addition to those illustrated in Figures 2a and 2b. For example, the controller 200 may comprise an input means 208 and an output means 210. The input means 208 may comprise an electrical input of the controller. The input means may comprise a wired or wireless connection to receive messages from one or more other modules in the vehicle (e.g., vehicle battery, charge gauge, induction motor, regenerative braking system, and / or cooling system) and / or one or more modules external to the vehicle. For example, the input means 208 can be arranged to receive a battery charge signal from a charge gauge 212. The battery charge signal is an electrical signal which is indicative of a state of charge of a vehicle battery 214. The output means 208 may comprise an electrical output of the controller 200. The output means may comprise a wired or wireless connection to send messages to other modules in the vehicle. The output means 210 is arranged to output a current control signal (e.g., an instruction), indicative of the current to be supplied to an induction motor 216. As shown in Figure 2b, the control system 200 can interface with other modules in a vehicle. For example, the control system 100 may send and / or receive electronic messages (e.g. via the input means 208 and output means 210 respectively). Such modules may comprise, for example, a regenerative braking system 218, an induction motor 216, vehicle battery 214, charge gauge 212 and / or cooling system 220, as will be described in more detail below. As shown in Fig, 3, the control system and the modules above can be embedded in a vehicle 300. Although the vehicle 300 depicted in Figure 3 is a passenger vehicle, it will be appreciated that the processes described herein apply to a great many different types of vehicles configured to perform regenerative braking. Generally, the principles herein can be applied to electric or hybrid-electric vehicles. Examples of vehicles include but are not limited to cars, lorries, vans, scooters, bikes, trains, and buggies. The vehicle 300 may comprise an induction motor 216, vehicle battery 214, battery charge gauge 212 and regenerative braking system 218. The vehicle 300 has a battery 214 which can be any rechargeable battery suitable for use in powering an electric vehicle or hybrid electric vehicle. The battery charge gauge 212 may be configured to measure the charge status of the vehicle battery 214 (e.g., how much charge is in the vehicle battery) and send data relating to the charge status of the vehicle battery 214 to the processor 200. The regenerative braking system 218 may be configured to carry out regenerative braking, that is slow down a vehicle 300 while generating energy from braking, and convert this kinetic energy into electrical energy so it can be supplied to the vehicle battery 214 to charge it. In embodiments herein, the regenerative braking system 218 may also be configured to transform energy generated from braking in the induction motor 216, to be dissipated as heat, as discussed in mere detail below. Thermal losses may refer to conduction losses from current flowing through the induction motor, or the dissipation of thermal energy through other means. The induction machine may receive the braking power through a shaft of the induction motor, and this power may be transformed into heat in the machine though the magnetic field and electrical currents. The energy may leave the induction machine through a cooling system. The skilled person will be familiar with the operation of an induction motor 216, which is discussed in more detail below. The induction motor 216 may comprise a stator with electrical windings and a rotor with electrical conductors. The induction motor may be operated by supplying an alternating current (AC) supply to the stator windings. The AC supply to the stator windings generates a rotating magnetic field (a stator field, or stator flux) which induces current flow in the conductors of the rotor. Interaction between the rotating magnetic field and induced current flow in the rotor conductors causes movement of the rotor. In embodiments herein, the induction motor 216 may be configured to receive a current, and particularly an increased current relative to a baseline current associated with efficient torque generation and normal operation. The increased current being supplied to the induction motor 216 generates a greater magnetic field and greater thermal losses than when a current associated with efficient torque generation is supplied to the induction motor. In embodiments herein, the vehicle 300 may also comprise a cooling system 220 configured to cool the induction motor 216. The cooling system 220 may be configured to cool the rotor and / or the stator of the induction motor 216. In embodiments herein, when the battery state of charge meets a state of charge criterion, instead of applying efficient regenerative braking, the induction motor is caused to perform inefficient regenerative braking, to dissipate the braking energy as heat (rather than further / over charge the battery). As such, a cooling system 220 may be fitted to the vehicle to help dissipate the generated heat. The skilled person will be familiar with cooling systems suitable for the purposes described herein, for example a cooling system may pump cold water through a sleeve surrounding the stator to cool down the winding. Figure 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method for controlling an electric vehicle configured to perform regenerative braking, such as the vehicle illustrated in Figure 2. The method 400 may be performed by the control system 200 illustrated in Figure 2a. In particular, the memory may comprise computer-readable instructions which, when executed by the processor, perform the method 400 according to an embodiment of the invention. The method 400 is for use in an electric vehicle configured to perform regenerative braking. The skilled person will be familiar with regenerative braking, but in brief, as described above, regenerative braking involves converting the kinetic energy of the vehicle into electricity, which is stored in the vehicle battery 214 so it can be used at a later date to power the vehicle. In brief, the method 400 comprises determining, at step 402 if a state of charge of the vehicle battery meets a state of charge criterion and, at step 404, upon determining that the state of charge of the vehicle battery meets the state of charge criterion, increasing current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation. In more detail, in step 402, the processor receives a state of charge reading for the vehicle battery. As will be familiar to the skilled person, a state of charge can be measured, for example, using a charge gauge. The state of charge may indicate how full the battery is. State of charge may be expressed, for example, as a percentage (or fraction) of the total possible, or theoretical maximum amount of charge that the battery can store. As another example, the state of charge may be expressed in terms of the absolute amount of charge in the battery (at the point of time that the measurement is made). The state of charge criterion may comprise (e.g. be expressed in terms of), for example, a charge threshold or charge range. If the state of charge criterion is expressed as a threshold, then step 404 may be triggered if the state of charge of the vehicle is above the level of the state of charge threshold. It will be appreciated that the threshold may be set at different levels according to preference, however, as an example, the threshold may be set at about 80 percent full, about 85 percent full, about 90 percent full, or about 95 percent full. In some embodiments, the threshold may be set at the same value that is used in current (e.g. prior art) systems to trigger mechanical braking. In such embodiments, instead of triggering mechanical braking, (inefficient) regenerative braking is triggered instead, in a manner that causes heat production. 6 Inefficient regenerative braking may be referred to as Regenerative Braking with Reduced Efficiency (RBRE) or electronic braking. The efficiency of regenerative braking may vary from the MTPA regime which is highly efficient braking, to zero efficiency (where all the energy generated from braking is converted into heat in the induction machine), depending on the control mode determined by the battery charge state. RBRE may include braking which is less efficient than when the system is operating under MTPA control. It will be appreciated that the state of charge criterion may be expressed in other ways, for example, it may be expressed as a range of state of charge values over which the amount of charge supplied to the stator is changed, as described in more detail below. In step 404, if the state of charge meets the state of charge criterion, then the processor 202 will send a message to cause a current supplied to the induction motor of the vehicle to be increased compared to a baseline current in the induction motor. As noted above, this is to increase thermal losses in the induction motor. The baseline current is associated with efficient torque generation. For example, the baseline current may be the “default” current e.g. the current used during normal operation of the vehicle (when the battery is not near full capacity). The baseline current may be set, for example, at (or approximately at) the MTPA current, as described above with respect to Figure 1. Generally, the term “efficient torque generation” may refer to the generation of a desired torque with minimum (or near-minimum) losses. Losses may for example include conduction losses, core losses, A.C. winding losses, windage, and / or friction losses. In step 404, when the state of charge criterion is met, the current (e.g. the stator phase current) sent to the induction motor is increased to cause energy to be lost as heat. With respect to Figure 1, it will be appreciated that the induction motor can be caused to create torque (for braking) in an inefficient manner that leads to heat-loss, either by increasing the stator-phase current and reducing the rotor current frequency, or by increasing both the rotor current frequency and the stator phase current. In the first example, reducing the rotor current frequency and increasing the stator phase current moves the system from the MTPA point, to the left and into box 102 and results in heat dissipation from the stator. In the second example, increasing both the rotor current frequency and the stator phase current moves the system from the MTPA point, to the right and into box 104, and causes heat loss in the rotor. Although the advantages herein can be achieved in both regimes (e.g. by causing heating of the stator or rotor), there are additional advantages associated with reducing the rotor current frequency and increasing the stator phase current, as this causes the stator to heat up and it is easier to cool the stator, compared to the rotor. Direct cooling implemented in and around the stator may provide a conductive path for heat dissipation. As noted above, increasing the current (e.g. the stator phase current) supplied to the induction motor relative to a current associated with efficient torque generation leads to inefficient torque generation, whereby a desired torque is generated with more than minimal conduction losses and thus, the energy recovered from regenerative braking is dissipated though conduction losses instead of being stored in the vehicle battery. In this way, regenerative braking (with reduced efficiency - RBRE) can be carried out (rather than mechanical braking) even when the vehicle battery is near or at maximum capacity. This enables regenerative braking (RBRE) to be performed when the vehicle battery is at a high state of charge without damaging the vehicle battery. It further means the driver does not experience a different braking sensation when the vehicle battery is near or at maximum capacity (which they wouId experience if the braking mode were to switch from regenerative braking to mechanical braking). It also means that the vehicle’s braking pads are worn down less and that the vehicle has a greater life span. It will be appreciated that the method 400 may comprise further steps to those illustrated in Figure 4. For example, steps 402 and 404 may be performed in an iterative manner, to ensure that the current is only increased to the induction motor when the battery state of charge actually meets the state of charge criterion. Furthermore, if in step 402 it is determined that the state of charge does not meet the state of charge criterion, then the method 400 may comprise performing regenerative braking so as to charge the battery e.g. in the normal, or most efficient, manner. E.g. using the baseline current. In such embodiments, the charge to the stator and the rotor current frequency may be set (or maintained) at the MTPA values for the required torque. Furthermore, it will be appreciated that the state of charge criterion is not necessarily merely a threshold. For example, beyond the threshold, the amount of recovered braking energy supplied to the stator may increase e.g. linearly with state of charge of the battery. In this way, as the battery charge increases, more braking energy may be diverted into heat (as opposed to charge to the battery). Figure 5 illustrates a method 500 according to an embodiment of the present invention comprising method steps and a feedback loop. At step 502 the state of charge of the vehicle battery is measured, for example using a charge gauge. Step 402 is then performed, as described above, and the one or more processors determine whether the state of charge of the vehicle battery meets a state of charge criterion. There is a feedback loop such that if the state of charge of the vehicle battery does meet the state of charge criterion, the method proceeds to step 404 (as described above) and the current supplied to an induction motor is increased compared to a baseline current associated with efficient torque generation. However, if the state of charge of the vehicle battery does not meet the state of charge criterion, the method returns to step 502 and the state of charge of the vehicle battery is measured again. In this way, continuous monitoring and protection of the battery is provided to prevent damage from over-charging due to regenerative braking. A summary of various principles of induction motors and how they can be applied to the described control system is provided below. Definitions of the symbols used can be found in the appendix. In the expressions provided, the phasors of base harmonics of currents, voltages and flux linkages are used. The phasors are complex numbers, which, beside the magnitude (intensity), expresses the phase position of quantities. The rotor quantities are denoted with “ ’ ”, this means these are transformed on the stator side. This transformation is carried out because the effective number of turns of the stator and rotor winding systems are different. An advantage of the transformation to the stator side is that the stator quantities (current and voltage) can be measured. The operation of induction motors is based on the speed difference between the rotating magnetic field generated by the stator, and the rotation speed of the rotor. This speed difference results in a rotating magnetic field which rotates relative to the rotor. The magnetic flux lines of rotating magnetic field intersect the conductors of the rotor (these conductors are bars in slots of a rotor of a squirrel cage induction motor 216) and in this way induced voltages are generated in the rotor bars which will result in currents in the rotor conductors. The interaction between the currents in the rotor bars and rotating magnetic field results in torque on the rotor shaft. Two key speed values can be distinguished in the induction motor. They are the speed of the rotating magnetic field (w1 = 2 ■ rr ■ f, where “f is the frequency of the base harmonic of the currents), and the rotating speed of the rotor (wrotor = • rr, where “RPM” is the number of rotations of the rotor per minute). Practically it is a “mechanical speed”, until wi is the “electrical speed”. The relationship between these two speeds when the rotor rotates together with the rotating magnetic field (in synchronism) is described with formula (1). When the rotor rotates together with the rotating magnetic field (in synchronism), the relationship between the angular speed of the rotating magnetic field and rotor can be determined on the base of following expression: = P^rotor (1-) When Wj >pwro£or the machine operates in the motor regime, until in the case a)1 <pMrotOr when the machine operates in the generator regime. -The multiplication with “p (number of pole pairs) scales the rotor speed in mechanical domain to electrical domain. For the cases when the rotor does not rotate together with the rotating magnetic field, the relationship between these key speed quantities can be determined by the following expression: ^1(1 S) P^rotor (2-) Where “s” is the slip. The slip is negative for generator regime and positive for motor regime. The machine capability is frequently measured with its power, which is proportional with the machine torque. The ratio of power and torque is equal to the angular speed of rotor shaft. The relationship between the power and torque at the rotating electrical machine is expressed with the following formula: Pm — ^rotor ' Tshaft (3-) At the induction motor 216, the shaft torque can be calculated as the vectorial product of the space vectors of the base harmonics of the currents ( / ) and flux linkages (r / i): Tshaft =k-Is-ips- sin(yrs) = k ■ Ir' ■ tyr' ■ sin^ = k ■ Ir’ ■ $r’ (4.) As seen from expression (4.), the torque depends on the electromagnetic quantities (currents and flux linkages). The <ps angle between the stator current Is and flux linkage i / is changes with the operation point of the machine, until the angle <pr between the rotor current Ir' and flux linkage is constant, and its value is 90°. The k is a machine coefficient (its value depends on the design). The torque expression with the rotor quantities expresses the simple relationship between the torque, current and flux linkage. On the basis of the equivalent circuit (Figure 6) of the induction motor, the relationship between the rotor current and flux linkage can be expressed by the following expression: 4'' = ~j ' "i' ' P ' ^ / (1 - «) ' (5 ) The equation (5.) is based on reference directions in the equivalent circuit of the induction motor for base harmonics (Figure 6.). In the breaking regime, “s” has a negative sign (generator regime), which is reflected in the direction of -l_r’*R_27s in the vector diagram for the generator regime of the induction motor (Figure 7.). The formula for the calculation of “s” can be determined on the base of equation (2.). As seen from expression (2.), the “s” slip can be adjusted with the speed difference between the rotating magnetic field and the rotor. This “adjustment” of “s” slip is carried out by the change of frequency of base harmonic of voltage of an inverter (at the constant rotor speed the speed of the rotating magnetic field is changed by the control). As seen from expression (5.), at the constant rotor speed and flux linkage, through “s” slip the rotor current can be controlled. This feature shows the flexibility of the control of the rotor current through the frequency of the stator voltages. On the basis of the equivalent circuit (Figure 6.), a vector diagram can be constructed which shows the space vectors of voltages, currents and flux linkages in generator regime (a similar vector diagram can be constructed for the motor regime). The vectors are determined in a coordinate system “dq”, where the vectors behave as complex numbers (practically the complex numbers behave as two dimensional (2D) vectors with the extended function, because the multiplication with “j” practically rotates the initial vector with 90°). The Real axis is denoted with “d” and it is aligned with the rotor flux linkage ipr. The l_r’ rotor current is perpendicular to the ip_r’ rotor flux linkage in the vector diagram, as assumed at the torque expression (4.). This is also reflected in the expression (5.). This means that the rotor flux linkage is linearly independent from the rotor current, because the length of projection of the rotor current to the vector of rotor flux linkage has zero value. This relationship between the rotor current and flux linkage is evidence that the magnetizing current which results in the rotor flux linkage is the part of “d” component of the l_s stator current (projection of l_s to the “d” axis). The stator current beside the projections to the “d” and “q” axes, can be split up to the components parallel and perpendicular to the voltage vector of machine (V_m). The component perpendicular to the voltage vector of machine (V_m) is responsible to developing of magnetic flux in the magnetic circuit of the induction motor. The component parallel with the voltage vector (V_m) is the active component of the stator current vector. The product of the magnitudes of voltage vector (V_m) and reactive component of stator current is proportional with the reactive power provided by power source (inverter). The product of the magnitudes of voltage vector (V_m) and active component of stator current is proportional with the active power provided by the power source (inverter). The losses in induction motors can be divided into two groups. The iron losses are in the first group, which changes with the saturation of magnetic circuit, and with the machine speed. At speed values lower than 30-40% of the peak speed, the iron losses are significantly lower compared to the copper losses, which are in the second group. This ratio of losses (iron and copper) determines a method which can be used to determine the most efficient way to increase the losses in the motor. This practically means, that the losses can be increased efficiently by increasing the current in the motor and rotor conductors. These currents result in conduction losses, which, beside the power returned to the battery, also contribute to increase the breaking torque of the induction machine. The conduction losses in the rotor and the stator at the three phase induction motors can be calculated on the base of the following expression: 3 , p — _Ji2 , p 1 rconducting-loss-rot 2 ' p . ..     ,       . . = 2 / 2 . D                                                                                          / 6 1 rconducting-loss-stator 2Is The decomposition of stator current into “d” and “q” components, allows a simplified approach of the analysis of the magnetic circuit and Magnetomotive Forces in induction motors. The magnetic circuit of the induction motors usually has a nonlinear characteristic (similar to electrical motors), which means it saturates as the flux density increase. This means a small percentage (for example 5%) increase of the qj_r’ rotor flux linkage, could result in significantly higher “d” component of stator current (40-50%). This increase of “d” component of stator current also has a significant impact on the resultant stator current, which increases by a similar percentage. The “q” component of stator current has a different role. It is responsible for maintaining the balance of Magneto Motive Forces (MMF) in the induction motor. This means, that the MMF-s developed by rotor current, and the “q” component of the stator current have identical amplitude, but they point in the opposite directions. This means they cancel each other. Figure 7 shows a vector diagram of an induction motor 216 in the generator regime. As already discussed, the stator current can be divided into active and reactive components. The active component is parallel with the machine voltage vector (V_m), and the reactive is perpendicular to the vector of machine voltage. These current components result in the power components, which the induction motor receives from the invertor. The first component is the active power, which is negative in the generator regime. The negative sign shows the direction of the power flow (positive from the inverter to the motor, negative from the motor to the inverter). Practically, the active power component (denoted with P) is responsible for the current flow between the inverter and battery, which charges the battery in the generator regime. This means if the active component is zero, the induction machine will not charge the battery despite that the induction machine current is not zero. At the vector diagram level, the active power has zero value when the l_s stator vector is perpendicular to the vector of machine voltage Vm. The second power component is the reactive power (denoted with Q). This component is responsible for the magnetisation of magnetic circuit. Practically, it has a constant value and expresses the circulation of the magnetic field energy between the phases. The inverter controls this component with the three-phase voltage source (the voltages are shifted with 120 electrical degrees). The inverter controller may use the “d” component of stator current (Id) and the “q” component of stator current (lq) (or any other parameters) and indirectly control the stator current and the rotor current frequency. In the case when generator breaking is applied at a high state of charge of the battery 214, the charging current can damage the battery 214. To avoid this and still exploit the breaking capability of the induction motor 216, a high-power loss breaking regime can be applied. This high-power loss breaking regime can be achieved when the induction machine operates in the generator regime at the non-optimal angle shift between the stator current and voltage vectors. The power returned to the battery 214 can be controlled depending on the state of charge of the battery 214, and the necessary breaking torque. In the case when the returned power to the battery 214 is zero, losses in the induction motor 216 may be equivalent with the breaking power. A sophisticated management of the breaking losses can be implemented if the returned power is regulated according to the demand of auxiliary applications in the car (for example heating or cooling of the passenger cabin). The breaking efficiency may vary depending on the demand. As mentioned earlier, the active power in the regime of generator breaking can be zero at the non-zero stator current. With the control of the induction motor, the current returned to the battery can be adjusted together with the losses in the machine. This makes it possible to change the efficiency of the generator braking regime from its highest level to zero, when all breaking power is dissipated (converted into heat) in the motor. At the constant stator current vector amplitude, the level of breaking efficiency can be regulated with the angle between the stator current and voltage vectors (zero breaking efficiency is achieved when the stator current vector is perpendicular to the stator voltage vector). This concept is illustrated in Figure 1. The breaking regimes with zero breaking efficiency can be implemented in two different ways: • Minimising the magnetising current, this will result in high rotor current • Maximising the magnetising current, this will result in low rotor current Minimising the magnetising current: In this case, at the given stator current l_s, the lowest rotor flux linkage (ip_r ) is determined, where the breaking torque is equivalent to the required value. This results in a higher rotor current frequency, and these operation points are to the right of the MTPA points in Figure 1. A disadvantage of these operation points is that it is difficult to remove the heat generated by the high rotor current in the rotor. The amount of losses generated by stator and rotor currents are in the similar range. Maximising the magnetising current: In this case, at the given stator current Ls, the lowest rotor current (l_r’) is determined, where the breaking torque is equivalent with the required value. This results in a low rotor current frequency, and these operation points are to the left of the MTPA points in Figure 1. An advantage of these operation points is that a low amount of heat is generated in the rotor. The heat losses are mostly generated in the stator conductors, which can be more efficiently removed by a cooling system compared to the heat generated in the rotor. The overall amount of the losses at the same stator current will be lower in this case compared to the previous case (when minimising the magnetising current), but by applying a higher stator current (the magnetising component of the stator current increased) the losses will increase. The applied breaking strategy and the value of the contribution of motor losses to the breaking torque may be limited by boundary conditions. These include: • The current limit of the inverter and motor This could be a limiting factor when maximizing the magnetising current because the saturation of the magnetic circuit allows a higher stator current to be applied than the maximum machine current, but the inverter and induction motor current limits will not allow this. • Voltage limit of the system This could be a limiting factor when maximizing the magnetising current. The high rotor flux linkage goes together with the high stator flux linkage, which at the lower speed values will reach the voltage limit compared to the breaking regimes with optimal-operation point, • The cooling capability of the cooling system. This also can limit the allowed losses in the machine, because the current and voltage limits may make it possible to develop more losses in the machine. This higher loss level could overheat the machine, which can lead to the damage of the motor. Turning now to other embodiments, it will be appreciated that the method 400 may be embodied in one or more computer programs. For example, computer readable instructions can be arranged such that, when executed by a computer, the computer is caused to perform the methods herein, such as the method 400. Computer readable instructions (e.g. a computer program) may take different forms, for example, source code, compiled code, executable code, or any other type of code. It will be appreciated that the source code of computer programs may be written in a wide variety of different programming languages, and may take different architectural designs. For example, the functionality described herein may be split across various different sub-routines. Furthermore, the skilled person will appreciate that many different ways of splitting the functionality between the different sub-routines will be possible. The sub-routines may be stored together in one executable file to form a self-contained program. Furthermore, computer programs may call external and / or standard libraries of computer code for performing certain sub-tasks associated with the functionality described herein. In another embodiment, there is a computer-readable data carrier having stored thereon the computer readable instructions as above. A computer-readable data carrier may comprise non-transitory computer readable media, having stored thereon computer readable instructions as described above. Examples of computer readable media include, but are not limited to: ROM, such as a CD ROM, a semi-conductor ROM or a magnetic recording medium such as a hard disk. Examples of computer readable data carriers include but are not limited to an electronic signal, optical signal, radio signal, computer storage medium, or similar. The carrier of a computer program maybe any entity or device (e.g. hardware) capable of carrying the program. As an example, a carrier may be a computer readable media as described above. In other examples a carrier may be a transmissible carrier such as an electronic or optical signal, which may be conveyed via electrical or optical cable or by radio or other means. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these claims cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. Text in the following table may be used in combination with the figures to aid readability: 4020 Determine if a state of charge of the vehicle battery meets a state of charge criterion 4040 Upon determining that the state of charge of the vehicle battery meets the state of charge criterion, increase current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation 5020 Measure state of charge of vehicle battery 5022 Does state of charge of vehicle battery meets state of charge criterion? 5040 Increase current supplied to an induction motor compared to a baseline current associated with efficient torque generation 5000 No 5001 Yes APPENDIX Pm- mechanical power on the rotor shaft, w,.otor-angular speed of the rotor shaft, TShaft-torque on the rotor shaft, / s-the length of space vector of the base harmonic of stator current, ^>s-the length of space vector of the base harmonic of stator flux linkage, <ps-shift angle between the stator flux linkage and stator current vectors, Ir'-the length of space vector of the base harmonic of rotor current, i / v’-the length of space vector of the base harmonic of rotor flux linkage, g)r-shift angle between the rotor flux linkage and rotor current vectors, s- slip ((w1 - wro£or ■ p) / (^), where “p”-is the number of pole pairs, oij- the frequency of the stator current, j- is the Imaginary operator, it rotates the vectors by 90° in the complex coordinate system, V m-macine voltage space vector (machine phase voltage), Rs- stator resistance, f?r'-rotor resistance, L_scr -leakage inductance of the stator, L_ra’ -leakage inductance of the rotor, L_m - main field inductance, l_m - magnetising current (or the current of main field inductance), E - back emf in the machine, practically it is the voltage on the main field inductance, ^>m-the length of space vector of the base harmonic of main field flux linkage, i / )s<T-the length of space vector of the base harmonic of stator leakage flux linkage, rpr<T'-the length of space vector of the base harmonic of rotor leakage flux linkage, Xso = w_1 -L so -leakage impedance of the stator, X_ra’ = w_1 L_ ra -leakage impedance of the rotor.

Claims

1. A control system for an electric vehicle configured to perform regenerative braking, the control system comprising one or more processors collectively configured to:determine if a state of charge of the vehicle battery meets a state of charge criterion; andupon determining that the state of charge meets the state of charge criterion, increase a current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation.

2. The control system of Claim 1, wherein the control system is configured to increase the current supplied to a stator of the induction motor when the state of charge meets the state of charge criterion.

3. The control system of claim 2, wherein the control system is configured to increase a stator phase current to be higher than a current for achieving maximum torque per ampere control.

4. The control system of Claim 3, wherein the control system is configured to:reduce a rotor current frequency compared to a rotor current frequency for achieving maximum torque per ampere control.

5. The control system of any one of the preceding claims, wherein:the state of charge criterion comprises a charge threshold and the state of charge is determined to meet the state of charge criterion if the state of charge is higher than or equal to the state of charge threshold; orthe state of charge criterion comprises a charge range and the state of charge is determined to meet the state of charge criterion if the state of charge is within the charge range.

6. The control system of any one of the preceding claims, wherein the control system is further configured to set the current in the induction motor to the baseline current associated with efficient torque generation in the induction motor when the state of charge of the vehicle battery does not meet the state of charge criterion, to perform efficient regenerative braking.

7. The control system of any preceding claim, wherein the one or more processors are further configured to control a cooling system to cool the induction motor.

8. A vehicle comprising: the control system of any one of claims 1 to 7; the induction motor; and the vehicle battery.

9. The vehicle of Claim 8, further comprising a cooling system configured to cool the induction motor.

10. The vehicle of Claims 8 or 9, wherein the induction motor is a traction motor.

11. A computer implemented method for controlling an electric vehicle configured to perform regenerative braking, the method comprising:determining if a state of charge of the vehicle battery meets a state of charge criterion; andupon determining that the state of charge of the vehicle battery meets the state of charge criterion, increasing current supplied to an induction motor of the vehicle compared to a baseline current in the induction motor to increase thermal losses in the induction motor, wherein the baseline current is associated with efficient torque generation.

12. The computer implemented method of Claim 11, wherein increasing the current supplied to an induction motor of the vehicle comprises increasing current supplied to a stator of the induction motor to be higher than a current for achieving maximum torque per ampere control. 5 13. The computer implemented method of Claim 11 or 12, wherein increasing the current supplied to an induction motor of the vehicle comprises reduce a rotor current frequency compared to a rotor current frequency for achieving maximum torque per ampere control.

14. The computer implemented method of Claim 11,12 or 13, further comprising: setting the current in the induction motor to the baseline current associated with efficient torque generation in the induction motor when the state of charge of the vehicle battery does not meet 10 the state of charge criterion, to perform efficient regenerative braking.

15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claims 11,12,13 or 14.15

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