GB2510186A

GB2510186A - Electric vehicle having regenerative braking using braking resistor and super capacitor

Info

Publication number: GB2510186A
Application number: GB1301545.8A
Authority: GB
Inventors: Patrick Robert Marius Brooking
Original assignee: Protean Electric Ltd
Current assignee: Protean Electric Ltd
Priority date: 2013-01-29
Filing date: 2013-01-29
Publication date: 2014-07-30
Anticipated expiration: 2033-01-29
Also published as: WO2014118678A2; WO2014118678A3; GB201301545D0; GB2510186B

Abstract

An energy system for an electric vehicle system comprises an electric machine arranged to provide a drive torque when placed in a drive mode of operation and arranged to generate current when placed in a braking mode of operation. A braking resistor 801 is placed in series with a switch 802 and a capacitor 812, wherein the switch is arranged to control the flow of current generated by the electric machine through the resistor to allow energy to be stored on the capacitor and for energy to be dissipated in the resistor. A controller 807 may be provided to modulate the switch to control the amount of energy stored on the capacitor and energy dissipated in the resistor. The capacitor may be a super capacitor, electric double-layer capacitor or a high energy density capacitor. The braking resistor may be air cooled or water cooled. An electrical bleed device 803 and/or a low power dc/dc converter may be coupled in parallel with the super capacitor.

Description

AN ENERGY SYSTEM FOR AN ELECTRIC VEHICLE

The present invention relates to an energy system for an electric vehicle.

With increased interest being placed in environmentally friendly vehicles, There has been a corresponding increase in interest in the use of electric vehicles, in which electric motor systems are used to provide a drive torque. L0

Electric motor systems typically include an electric motor and a control unit arranged to control the torque/speed of the electric motor. Examples of known types of electric motor include the DC motor, induction motor, synchronous brushless permanent magnet motor, and switched reluctance motor.

However, as is well known to a person skilled in the art, for a drive torque no be generated by an electric motor it is necessary for the electric motor to be provided with electric current, where batteries are a common energy source for electric motors.

A battery typically stores energy in electrochemical form, where different types of traction batteries used within electric vehicles include lead-acid, nickel-cadmium, nickel-metal-hydride, and lithium batteries.

However, the battery type now most commonly used within electric vehicles is lithium based batteries, which at ambient temperatures have a higher energy density in terms of rcatts hours per Kilogramme (Wh/kg) compared to many other types of batteries.

During operation of an electric vehicle, one or more batteries are typically used to provide energy to an electric motor for providing drive torque while also storing energy generated by an electric motor during regenerative braking.

By way of illustration, Figure 1 illustrates an example of an electrical traction system for an electric vehicle, where an electric traction motor 101 is coupled to an electronic controller 102 for controlling the operation of the electric traction motor 101. The electronic controller 102 is coupled to a DC voltage bus 106, which is connected to a battery 104. The electronic controller uses the energy from the battery to control che torque and/or speed of the electric traction motor.

However, due to the eleotroohemical nature of batteries, their performance an low temperatures, as encountered in the terrestrial automotive operating environment, can be adversely affected. For example, at lower temperatures batteries can exhibit a higher internal resistance, lower capacity, lower charge acceptance, and lower peak charge and discharge rates. Further, higher internal resistances resulting from lower temperatures can cause excessive voltage excursions at high peak battery currents, which can compromise the operation of power electronic motor controllers and auxiliary equipment connected to the battery. Additionally, lithium-ion batteries are specified by some manufacturers as not being able to accept charge below a certain temperature threshold, for example 0°C.

This problem is furTher compounded when transient speed changes of an electric vehicle during acceleration and regenerative braking can cause correspcndinaly high transient current surges into or out of the vehicle battery.

To address this problem super capacitors, for example electric double-layer capacitors, have been used in electric vehicles for storing and supplying the high transient electrical energy and power surges asscciated with rapid speed changes in electric vehicles, with traction batteries being used for providing the lower transient and continuous electrical energy needs of a vehicle's electric motor, for example supplying the energy requirements of an electric motor when maintaining a constant velocity.

As with other capacitors, super capacitors are a static electrical energy snorage element whose stored energy is proportional to the square of the voltage across it.

As such, in order to utilize a significant part of the energy storage capacity of a super capacitor, the terminal voltage of the super capacitor will typically be allowed to fluctuate between a relatively low voltage value to near the maximum rated voltage value. However, such voltage fluctuations will significantly deviate from the DC voltage supply of the electric vehicle's battery.

Accordingly, to be able to use a super capacitor to supply and receive energy no and from a DC battery bus, which is the energy conduit between the electric motor system and the vehicle's traction battery, and capitalizing on the advantages of using a super capacitor, a DC/DC converter is connected between the super capacitor and the battery busbar.

The primary function of the Dc/Dc converter is to convert the voltage level of the DC current from the super capacitor to a level compatible with the voltage levels on the relatively stable DC traction battery bus, as defined by the battery voltage, thereby allowing the super capacitor to supply the high peak power demands associated with the vehicle speed transients.

Additionally, when The vehicle is above a certain operational speed the energy in the super capacitor can also be discharged into The DC voltage bus at lower power and over a longer timescale than during vehicle acceleration or deceleration. This allows the super capacitor to be reset1 to a lower voltage level, thereby allowing the super capacitor to accept energy from the next regenerative braking cycle.

Bidirectional DC/Dc converters are used to allow both the charging and discharging of a super capacitor to occur, where charging occurs when the vehicle is undergoing regenerative braking and discharging occurs when the vehicle is accelerating. Additionally, charging or discharging of a super capacitor can also occur at any time to set the voltage levels on the super capacitor, which is effectively decoupled from the DC voltage bus by the DC/DC converter.

While charging and discharging of the super capacitor occurs, to maximize the efficiency of the super capacitor, the DC/DC converter allows the voltage across the super capacitor to both fall below and exceed the DC traction bus voltage. As such, the DC/DC converter will typically have a buck-boost voltage conversion capability as well as having bi-directional power flow.

By way of illustration, Figure 2 illustrates an example of an electrical traction system for an electric vehicle, where the electrical tracrion system includes an energy system having a super capacitor 202 coupled to a DC voltage bus via a Dc/Dc converter 201. As with the electrical traction system illustrated in Figure 1, the electrical traction system also includes an electric traction motor 101 coupled to an electronic controller 102 for controlling the operation of the electric traction motor 101. The electronic controller 102 is coupled to a DC voltage bus 106, which in addition to being connected to the DC/DC converter 201 is also connected to a battery 104. If the vehicle needs to accelerate, the super capacitor 202 supplies transient power into the DC bus via the DC/DC converter.

Accordingly, by using the super capacitor for transient power needs, this places less stress on the battery, which consequently does not have to supply energy for accelerating the vehicle.

However, the peak power levels required for a super capacitor to meet the power demands of the electric motor during vehicle speed changes require a correspondingly high power rating for the DC/DC converter. Indeed, the peak power levels can be in the regions of hundreds of kilowatts for the super capacitor to fully supply the transient power demands of the vehicle traction system.

As a result, DC/DC converters within an electric vehicle are typically sophisticated, complex and costly pieces of electronic equipmenr that typically have a low duty cycle of operation, being used for only a small percentage of the overall vehicle journey time. Further, because of the size of electrical inducrors used in the power conversion stage of a Dc/Dc converter, the size and weight of a DC/DC converter within an electric vehicle can be quite large and heavy.

To avoid the complexity and cost associated with a Dc/DC converter, an alternative solution for dealing with surplus energy generated by an electric motor during regenerative braking has been the use of braking resistors, which dissipate surplus energy as heat. Such resistors have been used for many years in railway locomotives, but have to be of substantial consuruction to dissipate the peak powers present in a braking cycle. The energy released by the locomotive during braking is wasted as heat and is not recoverable.

It is desirable to improve this situation.

In accordance with an aspect of the present invention there is provided an energy system and method according to the accompanying claims.

The invention as claimed allows current to flow into a super capacitor, from a DC voltage bus, where the current flow is controlled by means of a braking resistor and a controllable switch. Some of the energy flow is dissipated as heat in the braking resistor, with the remaining energy stored on the super capacitor. As the braking resistor does not have to dissipate all of the energy taken from the DC bus, the invention as claimed provides the advantage of reducing the power rating of the braking resistor, while also allowing energy to be stored on a super capacitor.

If a unidirectional switch is used to control the current flow to the super capacitor, the energy in the super capacitor can be dissipated resistively at a low rate using a separate energy dissipation oirouit, or used to supply a low power Dc/Do converter for powering vehicle accessories.

If a bi-directional switch is used, some of the energy in the super capacitor can be fed back on to the DO bus in order to assist powering the electric motors for generating drive torgue. The braking resistor inherently limits the current into and our of the super capacitor without the need for a high power DO/DO converter.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a prior art energy system for an electric vehicle; Figure 2 illustrates a prior art energy storage system having a super capacitor and a Dc/DO converter; Figure 3 illustrates a vehicle according to an embodiment of the present invention; Figure 4 illustrates an exploded view of an electric motor as used in an embodiment of the present invention; Figure 5 illustrates an exploded view of the electric motor shown in figure 4 from an alternative angle; Figure 6 illustrates an example arrangement of coil sets for an electric motor according to an embodiment of the present invention; Figure 7 illustrates an energy system according to a first embodiment of the present invention; Figure 8 iliustrates a switch inccrporated in an energy system according to an embodiment of the present invention; Figure 9 illustrates an energy system according to a second embodiment of the present invention; Figure 10 illustrates voltage variations associated with an energy system according to an embodiment of the present invention; Figure 11 illustrates a controller according to a first embodiment of the present invention; Figure 12 illustrates a controller according to a first embodiment of the present invention.

In the following preferred embodiment, an energy system is described in which high transient electrical energy and power surges from an electric motor, typically associated with rapid speed changes in an electric vehicle, can be dissipated without che need for a high power Dc/Dc converter and without the need to use the vehicles traction battery.

For the purposes of the present embodiment, reference to electric motor is also intended to include reference to an electrical machine chat is able to operate in both a motoring mode and generating mode.

Figure 3 illustrates an electric vehicle 100 having a plurality of in-wheel electric motors 101 arranged to provide drive torgue to a respective wheel of the vehicle, an energy storage device 104 such as a battery and an energy system 105. The plurality of in-wheel electric motors are coupled to the energy storage device 104 and the energy system 105 via a DC voltage bus 106.

When placed in a mororing mode the in-wheel electric motors 101 are arranged to provide torque for driving the vehicle 100, as is well known to a person skilled in the art.

Typically an in-wheel eleotric motor 101 will be incorporated within at least two wheels of the vehicle 100, where each in-wheel eleotrio motor 101 is arranged to provide & drive torque to the respective wheel for moving the vehicle. For example, in a oar having four wheels, in-wheel electric motors may be inoorporated within all four of the wheels or within two of the wheels that are preferably located on the same axis.

When placed in a generating mode the in-wheel electric motors 101 are arranged to provide a braking torque, as is well known to a person skilled in the art, with regenerative braking current being generated into the DC voltage bus 106 by the in-wheel electric motors 101.

The energy storage device 104 and the energy system 105 are arranged to provide energy to the plurality of in-wheel electric motors when the plurality of in-wheel electric motors are placed in the motoring mode and arranged to receive energy from the plurality of in-wheel electric motors when the plurality of in-wheel electric motors are placed in the generating mode.

For the purposes of the present embodiment the electric traction motor or motors is of the type of a DC motor, or an induction motor, or a permanent magnet motor, or a switched reluctance motor, or a hybrid combination of these.

Typically the DC moror is controlled using a control device such as a power electronic system known as a voltage chopping circuit, and the other types of motor by an electronic variable frequency inverter that uses pulse width modulation (PWM) to vary the speed and torque of the motor.

Although the control device, which is arranged to control current flow in the electric motor, may be located anywhere within the vehicle, for the purposes of the present embodiment the control device is mounted on the electric motor.

Although the presenu embodiment describes an electric vehicle 100 having in-wheel electric motors 101, as would be appreciated by a person skilled in the art, an electric vehicle according to an embodiment of the present invention may use any form of electric motor arranged to generate drive torque, for example a single electric motor connected to a drive system that is arranged to transfer the drive torque generated by the electric motor to two or more of the wheels of the vehicle.

However, for the purposes of the present embodiment, as illustrated in Figure 4, the in-wheel electric motor includes a stator 252 comprising a heat sink 253, multiple coils 254 and an electronics module 255 mounted in a rear portion of the stator for driving the coils. The coils 254 are formed on stator tooth laminations to form coil windings. A stator cover 256 is mounted on the rear portion of the stator 252, enclosing the electronics module 255 to form the stator 252, which may then be fixed to a vehicle and does not rotate relative to the vehicle during use.

Tn a preferred embodiment, the electronics module 255 includes two control devices 400, where each control device 400 includes two inverters and control logic, which in the -10 -present embodiment includes a processor, for controlling the operation of the inverters.

As illustrated in Figures 4 and 5, a rotor 240 oomprises a front portion 220 and a oylindrioal portion 221 forming a cover, which substantially surrounds the stator 252. The rotor includes a plurality of permanent magnets 242 arranged around the inside of the cylindrical portion 221. For the purposes of the present embodiment 32 magnet pairs are mounted on the inside of the cylindrical portion 221.

However, any number of magnet pairs may be used.

The magnets are in close proximity to the coils windings 254 on the stator 252 so that magnetic fields generated by the coils interaot with the magnets 242 arranged around the inside of the oylindrioal portion 221 of the rotor 240 to cause the rotor 240 to rotate. As the permanent magnets 242 are utilized to generate a drive torque for driving the electric motor, the permanent magnets are typically called drive magnets.

The rotor 240 is atuaohed to the stator 252 by a bearing block 223. The bearing block 223 can be a standard bearing block as would be used in a vehicle to which this motor assembly is to be fitted. The bearing block comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to a central portion of the wall of the stator 252 and also to a central portion 225 of the front portion 220 of the rotor 240. The rotor 240 is thus rotationally fixed to the vehicle with which it is to be used via the bearing block 223 at the central portion 225 of the rotor 240. This has an advantage in that a wheel rim and tyre can then be fixed to the rotor 240 at the central portion 225 using the normal wheel bolts -11 -to fix the wheel rim to the central portion 225 of the rotor 240 and consequently firmly onto the rotatable side of the bearing block 223. The wheel bolts may be fitted through the central portion 225 of the rotor through the bearing block itself. With both the rotor 240 and the wheel being mounted to the bearing block 223 there is a one to one correspondence between the angle of rotation of the rotor and the wheel.

Figure 5 shows an exploded view of the same assembly as Figure 4 from the opposite side showing the stator 252 and rotor. The rotor 240 comprises the outer rotor wall, which includes the front portion 220, and circumferential portion 221 within which magnets 242 are circumferentially arranged.

As previously described, the stator 252 is connected to the rotor 240 via the bearing block at the central portions of the rotor and stator walls.

A V shaped seal is provided between the circumferential wall 221 of the rotor and the outer edge of the stator 252.

The rotor also includes a set of magnets 227 for position sensing, otherwise known as commutation magnets, which in conjunction with sensors mounted on the stator allows for a rotor flux angle to be estimated. The rotor flux angle defines the positional relationship of the drive magnets to the coil windings. Alternatively, in place of a set of separate magnets the rotor may include a ring of magnetic material that has multiple poles that act as a set of separate magnets.

To allow the commutation magnets to be used to calculate a rotor flux angle, preferably each drive magnet has an associated commutation magnet, where the rotor flux angle is -12 -derived from the flux angle associated with the set of commutation magnets by calibrating the measured commutation magnet flux angle. To simplify the correlation between the commutation magnet flux angle and the rotor flux angle, preferably the set of commutation magnets has the same number of magnet or magnet pole pairs as the set of drive magnet pairs, where the commutation magnets and associated drive magnets are approximately radially aligned with each other. Accordingly, fcr the purposes of the present embodiment the set of commutation magnets has 32 magnet pairs, where each magnet pair is approximately radially aligned with a respective drive magnet pair.

A sensor, which in chis embodiment is a Hall sensor, is mounted on the stator. The sensor is pcsitioned so that as the rotor rotates each of the commutation magnets that form the commutation magnet ring respectively rotates past the sensor.

As the rotor rotates relative to the stator the commutation magnets correspondingly rotate past the sensor with the Hall sensor outputting an AC voltage signal, where the sensor outputs a complete voltage cycle of 360 electrical degrees for each magnet pair that pass the sensor.

To aid in the determination of the direction of the rotor, the sensor may also have an associated second sensor placed electrical degrees apart.

As illustrated in Figure 6, the motor in this embodiment includes eight coil sets 60 with each coil set 60 having three coil sub-sets 61, 62, 63 that are coupled in a wye configuration to form a three phase sub-motor, resulting in the motor having eight three phase sub-motors.

-13 -Pairs of the coil sets 60 are connected in parallel with each pair of parallel coupled coil sets 60 being connected to a respective three phase inverter included on a control device 400. As is well known to a perscn skilled in the art, a three phase invercer contains six switches, where a three phase alternating voltage may be generated by the controlled operation of the six switches.

The star points of the parallel coupled coil sets 60 are not electrically coupled. Tn an alternative embodiment each of the coil sets 60 may be connected to a separate inverter.

As stated above, the electronics module 255 includes two control devices 400, with each control device 400 having two inverters that are coupled to a respective pair of parallel coupled coil sets 60. Additionally, each control device 400 includes an interface arrangement, where in a first embodiment the interface arrangement on each control device 400 is arranged to allow communication between the respective control devices 400 housed in the electronics module 255 with one control device 400 being arranged to communicate with a vehicle controller mounted external to the electric motor, where the processor on each control device 400 is arranged to handle communication over the interface arrangement.

The processors on the respective oontrcl devices 400 are arranged to control the inverters included on the respective control device 400 to allow each of the electric motor coil sets 60 to be supplied with a three phase voltage supply, thereby allowing the respective coil sub-sets 61, 62, 63 to generate a rotating magnetic field. Although the present embodiment describes each coil set 60 as having three coil -14 -sub-sets 61, 62, 63, the present invention is not limited by this and it would be appreciated that each coil set 60 could have two or more coil sub-sets. Equally, although the present embodiment describes an electric motor having eight coil sets 60 (i.e. eight sub motors) the motor could have one or more coil seus with an associated control device.

Under the control of the respective prccessors, each three phase bridge inverter is arranged to provide PWI'4 voltage control across the respective coil sub-sets 61, 62, 63, thereby generating a current flow in the respective coil sub-sets to provide a required torque for the respective sub-motors.

For a given coil sec 60 the three phase bridge inverter switches are arranged to apply a single voltage phase across each of the coil sub-sets 61, 62, 63.

The inverter switches can include semiconductor devices such as MOSFETs or IGBTs. In the present example, the switches comprise IGETs. However, any suitable known switching circuit can be employed for controlling the current. One well known example of such a switching circuit is the three phase bridge circuic having six switches configured to drive a three phase electric motor. The six switches are configured as three parallel sets of two switches, where each pair of switches is placed in series and from a leg of the three phase bridge circuit.

The plurality of switches are arranged to apply an alternating voltage across the respective coil sub-sets.

As described above, the plurality of switches are configured to form an n-phase bridge circuit. Acccrdingly, as is well -15 -known to a person sUlled in the art, the number of switches will depend upon the number of voltage phases to be applied to the respective sub motors. Although the current design shows each sub motor having a three phase construction, the sub motors can be constructed to have any number of phases.

The wires (e.g. copper wires) of the ccil sub-sets 61, 62, 63 may be connected directly to the switching devices that form part of the inverters, as appropriate. Lo

Tn the present embodiment, one of the control devices 400 included in the electronics module 255 is arranged to receive torque demand requests over a communication line from an external vehicle controller, where the torque demand requests received ac the control device 400 is transmitted by the control device 400 to the other control device 400 housed in the electronics module 255 over a different communication line. Optionally, status information may also be provided from the control devices 400 to the vehicle controller.

As stated above, the energy requirements for the plurality of electric motors is provided by the energy storage device 104, for example a battery, and the energy system 105, where the energy storage device 104 and energy system 105 are coupled to the respective in-wheel electric motors via a DC voltage bus 106.

Upon the occurrence of a predetermined condition during regenerative braking, the energy system 105 is arranged to receive the energy generated by the plurality of in-wheel electric motors, as described below. Examples of the predetermined condirion include the vehicles traction battery 104 being unable to accept charge, the battery 104 -16 -having a high internal resistance, the battery 104 is fully charged, or the batnery 104 is unable to accept significant current because of The ambient temperature of the battery.

As illustrated in Figure 7, the energy system includes a resistor 801, a swioch 802 and a capacitor 812 that are coupled in series. The capacitor is preferably a super capacitor or an electric double-layer capacitor or a high energy density capacitor, however for the purposes of the present invention reference will be made to a super capacitor.

One terminal of the resistor 801 is coupled to the positive DC voltage busbar with the other terminal of the resistor 801 being coupled to a terminal of the switch 802. The other terminal of the swicch 802 is coupled to a terminal of the super capacitor 812 with the other terminal of the super capacitor 812 being coupled to the negative DC voltage busbar.

Preferably, an energy bleed device 803 and/or a low power DO/DC converter 820 are coupled in parallel with the super capacitor 812, as described below.

Preferably, the operation of the switch 802 is controlled via a controller 807, as described below, where activation of the energy system 105 is controlled via operation of the switch 802.

To aid the cooling of the resistor 801, otherwise known as a braking resistor, the resistor 801 can be of watercooied or aircooled construction, with a large heat capacity to absorb the energy pulses dissipating into it when current flows through the resistor. The braking resistor 801 is arranged -17 -to absorb some of the regenerative braking energy when used with a super capacinor, where the braking resistor 801 is configured to absorb a high peak power for a short time or low duty cycle.

A typical range of resistance for the braking resistor 801 is of the order of hundreds of milliohms to ohms. A typical range of capacitance for the super capacitor 812 is of the order of several tenths of Farads to Farads of capacitance.

The actual values of resistance and capacitance will depend upon the weight of rhe vehicle. For a battery voltage of several hundred volcs, the peak current rating of the components is likely to be at least several hundred amps and the peak power rating of the braking resistor in the tens or hundreds of kilowatcs.

The energy system is activated by closing the switch 802 to allow current to flow through the resistor 801.

Upon activation of The energy system, that is to say upon activation of the switch 802, the energy delivered to the DC voltage bus 106 from the electric motors during regenerative braking is partially dissipated by the braking resistor 801 with the remaining energy being stored on the super capacitor 812. The use of the resistor 801 ensures that the super capacitor 812 does not need to absorb all of the energy generated by the in-wheel electric motors, thereby allowing the capacicy of the super capacitor to be reduced.

Similarly, the energy absorbed by the super capacitor 812 during regenerative braking, for example during vehicle deceleration, reduces the reguired power rating of the braking resistor 801, which would otherwise have to absorb -18 -the kinetic energy cf the vehicle if the battery 104 were unable to do so.

Additionally the braking resistor 801 limits the current flow into the super capacitor 812 by virtue of its resistance and the voltage drop across the resistor, which increases linearly with current. Accordingly, this avoids the need to have a heavy inductor and high freguenoy modulated current control of the switch 802 for limiting the instantaneous current into the super capacitor 812.

For improved effioienoy, during regenerative braking the voltage variation across the super capacitor 812 is preferably kept within plus or minus 20 percent of the nominal or open circuit battery voltage, which would allow approximately 90 percent of the energy generated by the electric motors and placed on the DC vcltage bus by the control devices to be stored on the super capacitor 812, with approximately 10 peroent of the energy being dissipated in the braking resistor.

This efficiency level is similar to that of a high power Dc/Dc converter.

Other ranges of voloages may be placed across the super capacitor, however at low capacitor voltages there will less energy instantaneously being stored on the super capacitor and more energy being dissipated in the braking resistor 801, resulting in lower overall efficiency.

The energy extracted from the DC voltage bus 106 into the super capacitor can be oontrolled by modulating the on time of the switch 802. In turn this allows the peak voltage of the DC voltage bus 106 during regenerative braking to be -19 -controlled and limbed to a maximum value, in addition to providing control of the amount of energy going into the battery 104 and into the super capacitor 812.

Tn a first embodiment, the operation of the energy system is controlled via operation of a unidirectional switch, where upon activation of the unidirectional switch, the unidirectional switch allows current to flow in a single direction through the brake resistor into the super capacitor.

In this configuration, a diode 810 is preferably placed in parallel with the braking resistor 801 to provide a freewheel path for any inductive energy in the braking resistor 801.

The current flow moo the super capacitor 812, and consequently the amount of energy dissipated by the braking resistor 801, is decermined based on the current generated by the electric motors into the DC voltage bus 106 and upon the operational duty cycle of the switch. In other words, the duty cycle at a fixed freguenoy at which the switch is modulated on and off is selected based upon the required average current flow into the super capacitor 812. A preferred fixed freguenoy for modulating the switch would be in the region of 5 co 10 kHz, although higher freguencies may be used so operation produces no audible noise.

As stated above, the controller 807 is arranged to control the operation of the switch. To allow the controller 807 to determine whether to activate the energy system 105 the controller 807 is arranged to measure the DC bus voltage, the super capacitor voltage, and, using a current transformer 806 the braking resistor current. To allow the -20 - controller 807 to determine the operational state of the in-wheel electric motors and/or the battery so that the energy system 105 can be operated to provide optimum charge to the super capacitor, preferably the controller 807 is arranged to communicate with a vehicle controller over a communication bus, where the vehicle controller is arranged to received the operational state of the respective electric motors 101 and the battery 104.

Figure 8 illustrates a preferred embodiment of the unidirectional switch, where a power diode 901 is placed in series with a conventional IGRI having an integral inverse diode 902. However, other switch devices may be used, for example MOSFETs. The series diode 901 may be omitted if the flow of energy from the super capacitor 812 back into the DC bus 106 is allowable, however this will not be controllable but will occur whenever the DC bus voltage is less than the super capacitor voluage, for example under acceleration of the vehicle.

As stated above, preferably a bleed device 803 is connected in parallel across uhe super capacitor 812. The bleed device 803 is arranged to slowly thermally dissipate the energy stored in the super capacitor. The dissipation rate would correspond to a discharge rate of the super capacitor 812 to discharge the super capacitor 812 to a specified voltage level, for example 20 percent below the nominal battery voltage, in the expected interval between regenerative braking cycles of the vehicle.

The bleed device 803 includes a lower rated power bleed resistor 805, and an auxiliary bleed switch 804 placed in series. Preferably, the bleed resistor 805 resistance will -21 -be 5 to 10 times the value than the resistance value of the braking resistor 801, or greater.

Upon activation of The auxiliary bleed switch 804, energy stored on the super capacitor 812 is dissipated through the bleed resistor 805, which causes the vcltage across the super capacitor 812 to drop, thereby enabling the super capacitor 812 to store the next regenerative braking power surge delivered to The DC voltage bus 106 from the respective electric motors 101.

Preferably, a diode 811 is placed in parallel with the bleed resistor 805, thereby providing a freewheel path for any inductive energy in the bleed resistor 805, if required.

Alternatively, or in addition, preferably a low power DC/DC converter 820, for example a DC/DC converter having a power rating in the region of 1kw, is connected in parallel across the super capacitor 812. The low power Dc/DC converter 820 is arranged to feed the energy stored on the super capacitor 812 to the vehicle low voltage DC supply, for example 12y, for use by the vehicle's electric accessories or to charge the 12V vehicle batcery, thereby discharging the super capacitor voltage and enabling the super capacitor 812 to store the next regenerative braking power surge delivered to the DC voltage bus 106 from the respective electric motors 101. Preferably, the operation of the low power DC/DC converter is controlled via an enable line.

Using a 1kw DC/DC converter 820 with a nominal 320V traction battery would typically be expected to provide several minutes of power at the designated power level when used with a super capaciror consisting of several Farads of capacitance and the super capacitor is being discharged from -22 -percent above nominal battery voltage to 20 peroent below nominal battery voliage.

The auxiliary bleed switch 804 may be implemented using any suitable switoh, for example a standard IGET with optional inverse diode or a I'4CSFET.

Preferably, the operation of the auxiliary bleed switch 804 is ccntrolled via the controller 807, as desoribed below.

Tn a second embodiment, the activation of the energy system is oontrolled via the operation of a bidirectional switch, where upon activation of the bidirectional switch, the bidirectional switch allows current to flow through the brake resistor 801 into the super capacitor 812 and for energy to be returned to the DC voltage bus 106 from the super capacitor 812.

Allowing energy to be returned to the DC voltage bus 106 from the super capacitor 812 helps to maintain the DC bus voltage and the batcery voltage when the electric motor's current reguirements are increased, for example when the electric motors 101 are being used to accelerate the vehicle.

Figure 9 illustrates an energy system 105 having a bidirectional switch 1001. The features in Figure 9 that correspond to the features in Figure 7 have been given the same reference numerals as those given in Figure 7.

Preferably, the bidirectional switch 1001 is implemented using two back-to-back IGBTs 1002, 1004 with inverse diodes 1003, 1005, respectively. However, as would be appreciated -23 -by a person skilled in the art other type of switch configurations may be used.

For the bidirectional switch configuration illustrated in Figure 9, the current flow into the super capacitor 812 is controlled via the operation of IGBT 1002 and inverse diode 1005, whereas the current flow into the DC voltage bus 106 from the super oapaoitor 812 is controlled via the operation of IGBT 1004 and diode 1003.

By way of illustration, the operation of the energy system illustrated in Figure 9, with the associated change in voltage levels, will now be described with reference to Figure 10, where the bleed device 803 and low power DC/DC converter have been turned off to avoid the dissipation of energy stored on the super capacitor 812 through these mechanisms.

For reference purposes, the capacitor voltage VC 1101 corresponds to the voltage across the super capacitor 812, the battery voltage VE 1102 corresponds to the voltage across the battery 104. The nominal battery voltage VBnom 1103 is approximately the open circuit voltage appropriate to the battery's state of charge.

During time period 701, the super capacitor 812 is charged up to the nominal battery voltage VBnom 1103 by activating ICET switch 1002. In the absence of any vehicle regenerative braking this charge would ordinarily be expected to come from the battery.

During time period 702 the capacitor voltage \TC 1101 is maintained at VBnom 1103 by having the bidirectional switch -24 - 1001 turned off, that is to say both ICBI 1002 and IGBT 1004 are switched off.

Period 703 corresponds to the electric motors 101 being placed in generating mode, for example during regenerative braking of the vehicle. During period 703 the regenerative current generated by the electric motors 101 causes the battery voltage to rise. To allow the super capacitor 812 to absorb the excess energy on the DC voltage bus, IGBT switch 1002 is switched on, thereby allowing the voltage across the super capacitor 812 to increase to the peak level. Once the voltage across the super capacitor 812 has reached the peak level, to allow the energy to be stored on the super capacitor 812 during period 704 the IGBT switch 1002 is turned off.

Period 705 corresponds to the electric motors 101 being placed in a motoring mode for accelerating the vehicle.

During period 705, TGBT switch 1004 is turned on upon detection that the battery voltage has dropped, which results from the bartery 104 providing energy to the electric motors 101. Upon IOBT switch 1004 being turned on, the super capacitor 812 discharges energy into the DC voltage bus 106, thereby meeting some of the energy reguirements for accelerating the vehicle.

During period 706 the charging of the capacitor is repeated by turning ICET swirch 1002 on until the super capacitor voltage \C 1101 corresponds to the nominal battery voltage VBnom 1103.

For the purposes of illustration, Figure 11 illustrates a first embodiment of a preferred controller 807 for controlling the operation of the bidirectional switch 1001.

-25 -However, as would be appreciated by a person skilled in the art other controllers may be used, which can be implemented either with analog or digital / microprocessor techniques, or both.

The controller includes a first comparator 1201, a second comparator 1202, exclusive logic 1203, a current control modulator 1204 and IGBT drivers 1205.

The first comparator 1201 is arranged to identify whether a charge cycle of the capacitor should occur by comparing the capacitor voltage to the battery voltage, where the inputs to the first comparator 1201 are provided the capacitor voltage VC 1101 and the battery voltage VE 1102.

The second comparator 1202 is arranged to identify if a capacitor discharge cycle should occur by comparing the nominal battery volcage to the actual battery voltage, where the inputs to the second comparator 1202 are provided the nominal battery volcage VBnom 1103 and the battery voltage VE 1102.

The exclusive logic 1203 is arranged to compare the output from the first comparator 1201 and the second comparator 1202 to prevent a charge or a discharge cycle occurring at the same time.

The current control modulator 1204 determines the modulation depth of the IGBT switches 1002 and 1004 based on the absolute value of voltage error between the battery voltage and its nominal value, and the braking or acceleration demand scaling as set by the vehicle controller. By determining the modulation depth of the IGBT switches 1002 and 1004 it is possible to control the IGBT drivers 1205 for -26 -setting the average current levels thrcugh the braking resistor and super capacitor by selecting an appropriate modulation depth for the IGBT switches 1002 and 1004, thereby allowing the energy stored on the super capacitor 812 to be kept within predetermined values.

It is also possible to turn both IGBT switches 1002 and 1004 on continuously. In this scenario there would be no control of capacitor currenr other than by the limiting effect of the braking resistor 801.

For the purposes of illustration, Figure 12 illustrates a seccnd embodiment of a preferred oontrcller 807 for controlling the operation of the unidirectional switch 802 and the auxiliary bleed switch 804. However, as would be appreciated by a person skilled in the art other controllers may be used, which can be implemented either with analog or digital / microprocessor techniques, or both.

The controller 807 includes a first comparator 901, a second comparator 910, a first AND gate 902, a second AND gate 903, a third AND gate 911, a first TGBT driver 904, a second LOST driver 912, a brake enable line 905, a brake demand line 906, a modulator 907, an inverter 908 and a scaler 909.

To control the operation of the unidirectional switch 802 the first comparator 901 compares the battery voltage 1102 with the nominal bartery voltage VBnom 1103. If the battery voltage \7S 1102 is greater than the nominal battery voltage VBnom 1103 and the regenerative brake is enabled as indicated by the brake enable line 905 being set high by the vehicle controller, then switch 802 is turned on according to the output of the duty cycle modulator 907. The first AND gate 902 provides a positive output signal when the output -27 -from the first comparator 901 and the brake enable line 905 have been set high. The second AND gate 903 provides output signal when the output from the first AND gate 902 and the duty cycle modulator 907 have been set high. The operation of the unidirectional switch 802 is controlled by the first IGBT driver 904.

To control the operation of the auxiliary bleed switch 804, a fraction of the battery voltage VB 1102 is obtained by the scaler 909 and compared with the super capacitor voltage \TC 1101 using the second comparator 910. Tf the super capacitor voltage VO 1101 is greater than the voltage value output by the scaler 909, the second comparator 910 output is set high and if the enable line 905 is set low to indicate that regenerative braking is not enabled the auxiliary switch 804 is turned on to discharge the super capacitor voltage via resistor 805. The auxiliary switch 804 is turned on by the third AND gate 911 being set high as a result of the inverter 908 inverting the low brake enable line signal and the output from the second comparator 910 being set high, where the operation of the auxiliary switch 804 is controlled by the second IGBT driver 912.

A limited continuous braking mode of operation is also possible, where the energy on the DC voltage bus 106 that results from regenerative braking is continuously dissipated in the braking resistor 801 and the bleed resistor 805. As the bleed resistor 805 will typically have a much larger resistance value than the brake resistor 801, the bleed resistor 805 will typically determine the continuous braking power from the DC voltage bus 106.

-28 -

Claims

C LA I MS1. An energy system for a vehicle, the energy system comprising an electric machine arranged to provide a drive tcrgue when placed in a drive mcde of cperation and arranged to generate current when placed in a braking mode of operation; a first resistor placed in series with a first switch and a capaciror, wherein the first switch is arranged to control the flow of current generated by the electric machine through the first resistor to allow energy to be stored on the capacitor and for energy to be dissipated in the first resistor.
2. An energy system according to claim 1, wherein the capacitor is an electric double-layer capacitor, a super capacitor, or a high energy density capacitor.
3. An energy system according to claim 1 or 2, wherein the first switch is arranged to be modulated to control the amount of energy stored on the capacitcr and energy dissipated in the first resistor.
4. An energy system according to claim 3, further comprising a controller arranged to operate the first switch.
5. An energy system according to claim 4, wherein the controller is arranged to operate the first switch so that the energy stored on the capacitor is kept within predetermined voltage values.
6. An energy system according to claim 4, wherein the controller is arranged to operate the first switch so that -29 -energy is stored on the capacitor when the battery is unable to store additional charge.
7. An energy system aocording to any one of the preceding claims, wherein the first switch is a unidirectional switch for allowing electrical energy generated by the electrical machine to be stored on the capacitor.
8. An energy system according to any one of claims 1 to 6, wherein the switch is a bidirectional switch.
9. An energy system according to claim 8, wherein the bidirectional switch is arranged to allow energy stored on the capacitor to be directed to the electric machine for providing drive torque.
10. An energy system according to any one of the preceding claims, further comprising an electrical bleed device connected in parallel to the capacitor for dissipating energy stored on the capacitor.
11. An energy system according to claim 10, wherein the bleed device includes a second resistor and a second switch, wherein the second switch is arranged to allow energy stored on the capacitor to be dissipated in the second resistor.
12. An energy system according to claim 10, wherein the bleed device is a Dc/Dc converter for dissipating energy stored on the capacitor by providing the stored energy to at least one electrical or electronic device located in the vehicle at a voltage compatible with the at least one electrical or electronic device.

-30 -
13. An energy system according to any one of the preceding claims, wherein when the first switch is configured to allow current generated by the electrical machine to flow through the first resistor, the first resistor is arranged to dissipate less than half the energy generated by the electric machine when placed in a braking mode of operation and more than half rhe energy generated by the electric machine is stored on the capacitor.
14. A method of storing electrical energy generated by an electrical machine when placed in a braking mode of operation, the method comprises operating a first switch placed in series wirh a first resistor and a capacitor to control the flow of current generated by the electric machine through the first resistor to allow energy to be stored on the capacitor and for energy to be dissipated in the first resistor such that the first resistor is arranged to dissipate less than half the energy generated by the electric machine and more than half the energy generated by the electric machine is stored on the capacitor.-31 -