GB2456351A

GB2456351A - Hub motor with individually controlled stator coils provides safe braking

Info

Publication number: GB2456351A
Application number: GB0800588A
Authority: GB
Inventors: Martin Boughtwood
Original assignee: PML FLIGHTLINK Ltd; QED Group Ltd
Current assignee: PML FLIGHTLINK Ltd; QED Group Ltd
Priority date: 2008-01-14
Filing date: 2008-01-14
Publication date: 2009-07-15
Anticipated expiration: 2028-01-14
Also published as: GB0800588D0; GB2456351B

Abstract

An electric vehicle having a hub motor in each wheel is disclosed. The motor of each wheel includes coils sets (44, 46, 48, fig.7) arranged to produce a magnetic field of the motor. The motor also includes a plurality of magnets (242, fig.3; fig.9). Each set of coils (44, 46, 48, fig.7) comprises a plurality of individually energised coils. Each individually energised coil may comprise more than one winding (74A, 74B, 74C, fig.8; fig.9). The motor also includes a plurality of control devices (80, fig.10), each device housed within the casing of the motor (80, fig.3) and connected to a respective individually energised coil. A common power connection links the control devices and a power source. Each wheel motor can provide regenerative braking. In braking mode, the control devices (80, fig.11) are arranged to switch their respective coils using pulse width modulation so that the current profile delivered by each control device (80) to the common power connection is offset in relation to that from other control devices. Full braking torque may thus be provided whilst avoiding current peaks on the common power connection. The redundancy in the design of the motor allows it to continue to provide both drive and braking torque even if some of the switching circuits fail. A dump resistor may also be incorporated and may be distributed around the wheel. DC plug braking may be used at very low speeds. A separate mechanical braking arrangement may not be needed.

Description

VEHICLE WITH IN-WHEEL MOTOR BRAKE

FIELD OF THE INVENTION

The invention relates to electric vehicles and electric motors, and in particular electric vehicles of the type using in-wheel electric motors.

BACKGROUND OF THE INVENTION

Vehicles using in-wheel electric motors are known. The challenge in recent years has been to develop electric motor technology to provide greater power and efficiency to extend the speed, acceleration and range of electric vehicles. Various designs of in-wheel electric motor are known, including multi-phase designs. A known three phase design will first be briefly described by way

of background.

Figure 1 shows a schematic representation of a typical three phase motor.

In this example, the motor includes three coil sets. Each coil set produces a magnetic field associated with one of the three phases of the motor. In a more general example, N coil sets can be used to produce an N-phase electric motor.

Each coil set can include one or more sub-sets of coils which are positioned around a periphery of the motor. In the present example, each coil set includes four such sub-sets -the coil sub-sets of each coil set are labelled 14, 16 and 18, respectively in Figure 1. As shown in Figure 1, the coil sub-sets 14, 16, 18 are evenly distributed around the motor 10 to co-operate in producing a rotating magnetic field within which a central rotor 12, which typically incorporates one or more permanent magnets, can rotate as shown by the arrow labelled C. The coil sub-sets of each coil set are connected together in series as shown by the connections 24, 26 and 28 in Figure 1. This allows the currents in the coils of each coil set to be balanced for producing a substantially common phase. The wires of each coil set are terminated as shown at 34, 36 and 38 in Figure 1.

Typically, one end of the wire for each coil set is connected to a common reference terminal, while the other wire is connected to a switching system for controlling the current within all of the coils of that coil set. Typically then, current control for each coil set involves controlling a common current passing through a large number of coils.

As shown in Figure 2, each coil sub-set can include one or more coils. In particular, Figure 2 shows the coils 24A, 248 in one of the coil sub-sets 14. In this example, there are two coils per coil sub-set. For a three phase electric motor, the switching system is almost invariably a three phase bridge circuit including a number of switches. Typical power electronic switches including the Metal Oxide Silicon Field Effect Transistor (MOSFET) and the Insulated Gate Bipolar Transistor (IGBT) exhibit two principal losses: switching losses and conduction losses.

While switching losses decrease with switching speed, a faster switching speed also leads to increased electromagnetic interference (EMI) noise. This problematic trade off between switching speed and EMI noise is compounded at higher power ratings (e.g. for a larger motor), since larger switches are required.

The inductance associated with a power switch and its connection system increases with the physical size of the switch. This inductance impacts the switching speed of the power device and the switching speed of a power device is typically therefore limited by its physical size. Accordingly, for high power ratings larger switches must be used, but larger switches involve slower switching speeds and therefore larger switching losses. Moreover, the cost of a power device increases roughly with the square of the size of the device. Conduction losses also increase with increased power.

Including switching losses and conduction losses, the total losses are approximately proportional to the square of the power. This imposes senous thermal management problems for the motor since, for example, a doubling of the power leads to a four fold increase in thermal losses. Extracting this heat without elevating the temperature of the device above its safe operating level becomes the limiting factor in what power the device can handle. Indeed, today larger power devices having intrinsic current handling capabilities of, for example, 500A are restricted to 200A due to thermal constraints.

Consider a conventional three phase motor with a given power rating. If a larger power rating is desired, this can be achieved by producing a motor with a larger diameter. For a larger motor diameter, the peripheral speed of the rotor increases for a given angular velocity. For a given supply voltage this requires that the motor coils to have a reduced number of turns. This is because the induced voltage is a function of the peripheral speed of the rotor and the number of turns in the coils. The induced voltage must always be at or below the supply voltage. However, the reduced number of turns in the coils leads to a reduced inductance for the motor, since the inductance of the motor is proportional to the square of the number of turns.

Almost all electronic control units for electric motors today operate by some form of pulse width modulation (PWM) voltage control. PWM control works by using the motor inductance to average out an applied pulse voltage to drive the required current into the motor coils. Using PWM control an applied voltage is switched across the motor windings for a minimum period dictated by the power device switching characteristic. During this on period, the current rises in the motor winding at a rate dictated by its inductance and the applied voltage. The PWM control is then required to switch off before the current has changed too much so that precise control of the current is achieved.

As discussed above, the use of larger power devices leads to a slower switching speed, while a larger motor also has a lower inductance. For higher power motors, these two factors inhibit the effectiveness of PWM as a control system because the current in the motor coils rises more rapidly (due to the low inductance of the motor due to the fewer number of turns in the coils) but the PWM control is more coarse (due to the slow switching speed achievable using high power switching devices).

A known solution to this problem is to introduce additional inductance in the motor in the form of current limiting chokes in series with the motor windings.

This added inductance increases the rise time of the current in the motor coils.

However, the chokes are typically as large or larger than the motor itself and as they carry the full current they dissipate a large additional heat loss as well as being a substantial extra volume, weight and cost.

In all such known motor arrangements, development effort has been concentrated on providing higher power for acceleration and low losses for greater vehicle range from a given battery source.

SUMMARY OF THE INVENTION

We have appreciated that in-wheel electric motors may also be used to provide a braking torque and, furthermore, that an in-wheel electric motor can provide the full braking torque needed for a vehicle without the need for additional mechanical braking. The invention resides first in the appreciation that an in-wheel electric motor can provide the full braking torque needed by a vehicle.

Also, the invention resides in the appreciation that, to provide full braking torque, an in-wheel electric motor must be able to convert high transient power loads into electricity and be able to deliver this back to a source or load. The invention further resides in an arrangement which allows high power to be converted into electrical current by an in-wheel motor without producing high transient currents.

The invention is defined in the accompanying independent claims, with preferred features set out in the dependent claims.

An embodiment of the invention uses a combination of techniques by which the kinetic energy of a vehicle may be converted to electrically power in an in-wheel motor without producing currents beyond the capabilities of the electrically system including coils, switched and power connections. The invention is applicable to electric vehicles such as cars with 4 in-wheel electric motors (one per wheel), capable of high speed travel at speeds in excess of 60Mph. The embodiment of the invention is capable of producing a braking force that delivers in excess of 200kW of power over a few seconds, thereby decelerating a vehicle of mass on the order 1.5 tons from 60Mph in around 5 seconds. A motor embodying the invention is capable of such power conversion and yet only has a mass of the order 30Kg.

The motor includes one or more separate coil sets arranged to produce a magnetic field of the motor. Each coil set includes a plurality of coil sub-sets.

Each coil sub-set includes one or more coils. The magnetic field produced by the coils in each coil set have a substantially common phase. The motor also includes a plurality of control devices each coupled to a respective coil sub-set for controlling a current in the coils of that respective coil sub-set.

The electric motor embodying the invention uses a new technique of coil switching for the purpose of avoiding high transient currents. The control devices can include one or more switches for applying a pulsed voltage to the one or more coils of a coil sub-set. Pulse width modulation (PWM) control of the currents in the motor coils can be enhanced due to the increased number of turns which can be included in the coils. Because smaller switching device can be used, significant savings in cost, weight and heat dissipation can be made. The new technique of switching involves staggering the switching of the switches so that switching pulses of a coil are staggered in relation to switching pulses of other coils. The staggering of the pulses is such that the currents received from each of the coils has peaks and troughs of waveform at different times, thereby summing to an approximately DC current with a ripple rather than high peaks and troughs.

Control of the currents in the coils of the motor is further enhanced because the current in each coil sub-set can be controlled independently of the current in another coil sub-set. Because all of the coils of each coil set are not connected in series, the coil or coils of each coil sub-set can have a larger number of turns. The increased number of turns in each coil increases the overall inductance of the motor. This means that lower currents can be used in the coils of each coil sub-set, which leads to fewer heat dissipation problems, and which allows smaller switching devices to be used. The use of smaller switching devices in turn allows for faster switching speeds and lower switching losses.

Some of the control devices can include means for monitoring a back EMF within the coils of that coil sub-set. The control device can adjust a pulse of the pulsed voltage (e.g. a width of the pulse) in response to the monitored back EMF for high speed power control.

Since smaller components (e.g. switching devices) can be used, they can be housed within a casing of the motor, in contrast to known systems using large, bulky switching devices. For example, the control devices can be located adjacent their respective coil sub-sets within the motor thereby simplifying termination of the coil windings. The casing of the motor can include one or more apertures dimensioned such that the control devices can be accessed one at a time, depending on the orientation of the rotor/casing and the control devices.

The electric motor is thus operable in a braking mode. In the braking mode control devices coupled to a respective coil sub-set for controlling a current in the one or more coils of the respective coil sub-set are operable by current drawn from the coils. Since the control devices can operate from current drawn from the coils, a fail-safe braking arrangement is provided as the control devices can continue to operate (and thereby control braking) even in the event of failure of the power supply. Preferably, each control device is arranged so that it is operable from current from one sub-set of coils when in a braking mode. This ensures that there is redundancy built into the braking arrangement, as, in the event of failure of a coil, other coils and control devices would still be operable to provide a braking force.

The motor preferably also includes a capacitance coupled between the coils and a connection for a power supply. The capacitance ensures that current can continue to be supplied to the control devices when a transition occurs between a power consuming mode and non-power consuming mode. The motor also includes a resistance selectively coupled to the control devices such that in an emergency braking mode power from the coils may be consumed by the resistance. An emergency braking mode is one in which a power supply is unable to receive power from the coils, for example, because the power supply such as a battery has failed, a battery is full or a connection has failed. The resistance is preferably arranged very close to the control devices and coils thereby reducing the risk of connection failure.

The embodiment of the invention is a motor comprising a cooling arrangement. The motor includes a plurality of coils arranged around a circumference and a cooling channel disposed immediately adjacent the plurality of coils through which a coolant fluid may be pumped. Circulated -gives the option of convective flow. Key point here is the multi faceted cooling plate. It encloses the windings on three sides and provides faces for the attachment of the electronic power devices, the dump power devices and the dump resistor. In fact another key point is that the stator assembly comprising the coils, teeth and back iron is assembled directly onto the cooling plate. The assembly is then potted onto the cooling plate using thermally conductive material -epoxy filled with aluminium oxide or aluminium nitride or carbon for example. This potting process is important due to the mechanical integrity imparted to the whole assembly -all parts are as one and more able to withstand vibration and shock.

The potting further improves the electrical strength of the insulation system in that it prevents any air pockets within the winding system. Because of the high switching speeds dv/dt is high and this induces electrical stress in the insulation medium of the windings. Air pockets would risk ionisation and lead to early failure of the insulation. In electronically controlled motors or generators this insulation breakdown brought on by the repeated electrical stress induced through the switching events is a major reliability issue -potting reduces this risk by a very large degree. Potting is best done under vacuum, but low viscosity potting material can be used in atmospheric pressure. The potting is of critical value in improving the thermal conductivity between the heat generating windings and laminations and the heat sinking cooling plate with it's cooling fluid inside.

The potting is further of great benefit in that it allows the winding system to be fully immersed in water with no risk of electrical failure. This is important due to the need to make the electrical system immune to condensation or other water ingress.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figure 1 schematically shows an example arrangement for a three phase motor; Figure 2 schematically shows the arrangement of coils in one of the coil sub-sets shown in Figure 1; Figure 3 is an exploded view of a motor embodying the invention; Figure 4 is an exploded view of the motor of Figure 3 from an alternative angle; Figure 5 schematically shows an example coil arrangement for a three phase motor according to an embodiment of this invention; Figure 6 schematically shows an example arrangement of coils in one of the coil sub-sets shown in Figure 3 according to an embodiment of the invention; Figure 7 schematically shows schematically shows an example arrangement for a three phase motor according to an embodiment of this invention; Figure 8 schematically shows an example arrangement of coils in one of the coil sub-sets shown in Figure 7 according to an embodiment of the invention; Figure 9 schematically shows the coils of the embodiment in relation to the magnets; Figure 10 schematically shows an example of a control device in accordance with an embodiment of this invention; Figure 11 is a circuit diagram of the switching arrangement; and Figure 12 schematically shows an arrangement in which a common control device is used to coordinate the operation of a plurality of control devices.

DETAILED DESCRIPTION

The embodiment of the invention described is an electric vehicle and an electric motor for use in a wheel of a vehicle. The motor is of the type having a set of coils being part of the stator for attachment to a vehicle, radially surrounded by a rotor carrying a set of magnets for attachment to a wheel. The vehicle is operable in a braking mode in which the electric motors provide the full braking torque.

The physical arrangement of the embodying assembly is best understood with respect to Figures 3 and 4. The assembly can be described as a motor with built in electronics and bearing, or could also be described as a hub motor or hub drive as it is built to accommodate a separate wheel.

Referring first to Figure 3, the assembly comprises a stator 252 comprising a rear portion 230 forming a first part of the housing of the assembly, and a heat sink and drive arrangement 231 comprising multiple coils and electronics to drive the coils as well as a heat sink. The coil drive arrangement 231 is fixed to the rear portion 230 to form the stator 252 which may then be fixed to a vehicle and does not rotate during use. The coils themselves are formed on tooth laminations 235 which together with the drive arrangement 231 and rear portion 230 form the stator 252.

A rotor 240 comprises a front portion 220 and a cylindrical portion 221 forming a cover, which substantially surrounds the stator 252. The rotor includes a plurality of magnets 242 arranged around the inside of the cylindrical portion 221. The magnets are thus in close proximity to the coils on the assembly 231 so that magnetic fields generated by the coils in the assembly 231 generate a force on the magnets 242 arranged around the inside of the cylindrical portion 221 of the rotor 240 thereby causing the rotor 240 to rotate.

The rotor 240 is attached 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 233 of the wall 230 of the stator 252 and also to a central portion 225 of the housing wall 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 a significant advantage in that a wheel rim and lyre can then be fixed to the rotor 240 at the central portion 225 using the normal wheel bolts to fix the wheel rim to the central portion of the rotor 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 into the bearing block itself. The rotor also includes a focussing ring and magnets 227 for position sensing discussed later.

Figure 4 shows an exploded view of the same assembly as Figure 3 from the opposite side showing the stator 252 comprising the rear stator wall 230 and coil and electronics assembly 231. The rotor 240 comprises the outer rotor wall 220 and circumferential wall 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.

Additionally shown in Figure 3 are circuit boards 80 carrying control electronics described later. Due to their kite shape these circuit boards can be referred to as kite boards. Additionally in Figures 3 and 4 a V shaped seal 350 is provided between the circumferential wall 221 of the rotor and the outer edge of the stator housing 230, again described in detail later. Further, in Figure 4, a magnetic ring 227 comprising a commutation focusing ring and a plurality of magnets is provided for the purpose of indicating the position of the rotor with respect to the stator to a series of sensors arranged on the circuit boards 80 of the stator 252. This is also described in greater detail later.

Figure 5 schematically shows an example of an electric motor in accordance with an embodiment of this invention. In this example, the motor is generally circular.

The motor 40 in this example is a three phase motor. Again, it will be appreciated that motors according to this invention can include an arbitrary number of phases (N = 1, 2, 3...). Being a three phase motor, the motor 40 includes three coil sets. In this example, each coil set includes two coil sub-sets.

The coil sub-sets of each coil set are labelled 44, 46 and 48, respectively. The coil sub-sets 44, 46 and 48 are arranged around a periphery of the motor 40. In this example, each coil sub-set is positioned opposite the other coil sub-set in that coil set, although such an arrangement is not strictly essential to the working of the invention. Each coil sub-set includes one or more coils, as described below in relation to Figure 6.

The motor 40 can include a rotor (not shown in Figure 5) positioned around the coils as previously disclosed in Figures 3 and 4. The rotor may typically comprise one or more permanent magnets arranged to rotate such that their poles sweep across the ends of the coils of the motor 40. Appropriate switching of currents in the coils of the coil sub-sets allows synchronized attraction and repulsion of the poles of the permanent magnet of the rotor to produce the rotating action of the motor 40. It will be appreciated that Figure 5 is highly schematic and, in practice, the coil sub-sets will be arranged at the outer periphery of the stator with the rotor magnets surrounding the coils.

Each coil set 44, 46, 48 includes one or more coils. As shown in Figure 6, in the present example, there is a single coil per coil sub-set. An example with more than one coil per coil sub-set is described below in relation to Figures 7 and 8. Where more than one coil is provided in a given coil sub-set, these coils can generally be wound in opposite directions such that the magnetic field produced by each coil is in an anti-parallel configuration with respect to the magnetic field in an adjacent coil. As described above, appropriate switching of the current in the coils causes the permanent magnets of the rotor to rotate.

As shown in Figure 5, in accordance with an embodiment of this invention, the coil or coils of each coil sub-set can be connected to a separate control device 80. In Figure 5, it is schematically shown that each coil sub-set is connected to the terminals 54, 56, 58 of respective control devices 80.

Accordingly, the coils of corresponding coil sub-sets within a given coil set are not connected in series. Instead, each coil sub-set is individually controlled and powered. The connections to the control device and the coils of each coil sub-set can be formed using, for example, a single piece of wire (e.g. copper wire) as is shown schematically in Figure 6. There are numerous advantageous to providing individual power control for the coils of each coil sub-set.

By providing individual power control for the coils of each coil sub-set, and by using a larger number of turns per coil than would be achievable using a motor in which the coils of each coil sub-set are connected in series, the total inductance of the motor can be greatly increased. In turn, this allows far lower current to be passed through each coil sub-set whereby switching devices having a lower power rating can be used for current control. Accordingly, switching devices which are, cheaper, lighter and less bulky can be used to operate the motor.

The use of lower currents also reduces heat dissipation problems and lowers switching losses due to the faster speed of the smaller switching devices which can be employed. The fact that smaller switching devices can operate at higher frequencies allows for finer and more responsive motor control. Indeed, torque adjustment can take place on the basis in a highly responsive manner, with adjustments being able to be made within a single PWM period. A typical PWM period according to an embodiment of the invention is approximately 50 is.

Another advantage of the use of smaller switching devices is that they can be located proximal the coils which they control. In prior electric motors, where relatively large switching devices have been employed to co,MroI the operation of coil sub-sets connected in series, the control device is sufficiently large that it cannot be included with the other motor components (e.g. stator, rotor, etc.) but instead has been provided separately. In contrast, since small switching devices can be used, in accordance with an embodiment of this invention the switching devices and the control devices in which those switching devices are incorporated can be located in, for example the same housing/casing as the other motor components. Further detail regarding an example of a control device incorporating switching devices is given below in relation to Figures 10 and 11.

Figures 7 and 8 show another example arrangement for a motor 40 in accordance with an embodiment of this invention. The motor 40 shown in Figure is a three phase motor. The motor therefore has three coil sets. In this example, each coil set includes eight coil sub-sets. The coil sub-sets of each coil set are labelled 44, 46 and 48, respectively in Figure 7. In common with the example described above in relation to Figure 5, each coil set includes pairs of coil sub-sets which are arranged opposite each other around the periphery of the motor 40. Again, however, it should be noted that there is no express need for each coil sub-set to have a corresponding coil sub-set located opposite from it on the opposite side of the periphery of the motor 40.

As described above in relation to Figure 7, each coil sub-set can be connected to a respective control device 80. The terminals for each coil sub-set of each coil set are labelled 54, 56 and 58, respectively in Figure 7. While the arrangement shown in Figure 7 includes a larger number of coil sub-sets than, for example, the arrangement shown in Figure 3, this does not significantly increase the size and bulk of the switching means which are used to operate the motor, as would be the case if the increased number of coil sub-sets were connected together in series. Instead, it is merely necessary to provide an additional control device 80 incorporating relatively small switching devices as described above for each additional coil sub-set. As described above, these control devices 80 are sufficiently small such that they can be located adjacent to their corresponding coil sub-sets within, for example, the same casing as the motor 40.

As described above, each coil sub-set can include one or more coils. In this example, each coil sub-set includes three coils as is shown schematically in Figure 8. In Figure 8, these three coils are labelled 74A, 74B and 74C. The three coils 74A, 74B and 74C are alternately wound such that each coil produces a magnetic field which is anti-parallel with its adjacent coil/s for a given direction of current flow. As described above, as the permanent magnets of the rotor of the motor 40 sweep across the ends of the coils 74A, 74B and 74C, appropriate switching of the currents in the coils can be used to create the desired forces for providing an impulse to the rotor. As is shown schematically in Figure 6, each coil in a coil sub-set can be wound in series.

The reason that the coils 74A, 74B and 74C within each subset are wound in opposite directions to give antiparallel magnetic fields can be understood with respect to Figure 9 which shows the arrangement of the magnets 242 on the rotor surrounding the coils 44, 46 and 48 of the stator. For simplicity, the arrangement is shown as a linear arrangement of magnets and coils, but it will be understood that in the embodiment of the invention described the coils will be arranged around the periphery of the stator with the magnets arranged around the inside of the circumference of the rotor, as already described.

The magnets 242 are arranged with alternate magnetic polarity towards the coil subsets 44, 46 and 48. Each subset of three coils 74A, 74B and 74C thus presents alternate magnetic fields to the alternate pole faces of the magnets.

Thus, when the left-hand coil of a subset has a repelling force against a North Pole of one of the magnets, the adjacent central coil will have a repelling force against a South Pole of the magnets and so on.

As shown schematically in Figure 9, the ratio of magnets to coils is eight magnets to nine coils. The advantage of this arrangement is that the magnets and coils will never perfectly align. If such perfect alignment occurred, then the motor could rest in a position in which no forces could be applied between the coils and the magnets to give a clear direction as to which sense the motor should turn. By arranging for a different number of coils and magnets around the motor, there would always be a resultant force in a particular direction whatever position the rotor and motor come to rest.

A particular benefit of the independent control of the coil subsets by the separate control devices is that a larger than normal number of phases can be arranged. For example, rather than a three phase motor, as described in Figure 7, higher numbers of phases such as twenty-four phase or thirty-six phase are possible with different numbers of magnets and coils. Ratios of coils to magnets, such as eighteen coils to sixteen magnets, thirty-six coils to thirty-two magnets and so on, are perfectly possible. Indeed, the preferred arrangement, as shown in Figures 3 and 4 is to provide 24 separate control "kite" boards 80, each controlling three coils in a sub-set. Thereby providing a twenty-four phase motor.

The use of a multiphase arrangement, such as twenty-four phases, provides a number of advantages. The individual coils within each sub-set can have a larger inductance than arrangements with lower numbers of phases because each control circuit does not have to control large numbers of coils (which would require controlling a large aggregate inductance). A high number of phases also provides for lower levels of ripple current. By this it is meant that the profile of the current required to operate the motor undulates substantially less than the profile from, say a three-phase motor. Accordingly, lower levels of capacitance are also needed inside the motor. The high number of phases also minimise the potential for high voltage transients resulting from the need to transfer large currents quickly through the supply line. As the ripple is lower, the impact of the supply cabling inductance is lower and hence there is a reduction in voltage transient levels. When used in a braking arrangement (described later), this is a major advantage, as in hard braking conditions, several hundred kilowatts need to be transferred over several seconds and the multiphase arrangement reduces the risk of high voltage transients in this situation.

The relative arrangement of magnets and coils, shown in Figure 9 can be repeated twice, three times, four times or indeed as many times as appropriate around 360 mechanical degrees of the rotor and stator arrangement. The larger the number of separate sub-sets of coils with independent phases, the lower the likelihood of high voltage transients or significant voltage ripple.

In accordance with an embodiment of this invention, a plurality of coil sub-sets with individual power control can be positioned adjacent each other in the motor. In one such example, three coils such as those shown in Figure 8 could be provided adjacent each other in a motor but would not be connected in series to the same control device 80. Instead, each coil would have its on control device 80.

Where individual power control is provided for each coil sub-set, the associated control devices can be operated to run the motor at a reduced power rating. This can be done, for example, by powering down the coils of a selection of the coil sub-sets.

-14 -Figure 10 shows an example of a control device 80 in accordance with an embodiment of this invention. As described above, the control device 80 includes a number of switches which may typically comprise one or more semiconductor devices. The control device 80 shown in Figure 10 includes a printed circuit board 82 upon which a number of components are mounted. The circuit board 82 includes means for fixing the control device 80 within the motor, for example, adjacent to the coil sub-set which it the controls -directly to the cooling plate. In the illustrated example, these means include apertures 84 through which screws or suchlike can pass. In this example, the printed circuit board is substantially wedge-shaped. This shape allows multiple control device 80 to be located adjacent each other within the motor, forming a fan-like arrangement.

Mounted on the printed circuit board 82 of the control device 80 there can be provided terminals 86 for receiving wires to send and receive signals from a 92 control device as described below.

In the example shown in Figure 10, the control device 80 includes a number of switches 88. The switches can include semiconductor devices such as MOSFETs or IGBTs. In the present example, the switches comprise IGBTs.

Any suitable known switching circuit can be employed for controlling the current within the coils of the coil sub-set associated with the control device 80. One well known example of such a switching circuit is the H-bridge circuit. Such a circuit requires four switching devices such as those shown in Figure 10. The wires (e.g. copper wires) of the coil sub-sets can be connected directly to the switching devices 88 as appropriate, and interconnections between the switching devices 88 can be formed on the printed circuit board 82. Since the switching devices 88 can be located adjacent the coil sub-sets as described above, termination of the wires of the coil sub-sets at the switching devices 88 is made easier.

As shown in Figure 11, the control device includes semiconductor switches arranged in an H-bridge arrangement. The H-bridge is of course known to those skilled in the art and comprises four separate semiconductor switches 88 connected to a voltage supply (here 300 volts) and to ground. The coils of each sub-coil are connected across the terminals 81 and 83. Here a sub-coil 44 is shown connected across the terminals. Simplistically, to operate the motor and supply a voltage in one direction, switches 88A and 88D are closed and the other switch is left open, so that a circuit is made with current in one direction. To -15-operate the motor this current direction is changed in harmony with the alternating magnetic polarity passing the coil. To change the direction of rotation of the motor, the timing and polarity of the current flow in the coil is changed to cause the resulting forces in the opposite direction. The direction of current flow in the coil is reversed when switches 88B and 88C are closed and the other two switches are left open. In practice, the technique of pulse width modulating is used to pulse width modulate the signal applied to the gate of the semiconductor switches to control the voltage applied to the coils. The braking arrangement operates in a manner not known in the prior art and will be described after describing the overall control arrangement.

As shown in Figure 12, a common control device 92 can be used to coordinate the operations of the multiple control devices 80 provided in the motor.

In prior motors, in which synchronization of the magnetic fields produced by the coils of each coil sub-set is automatically achieved by virtue of the fact that they are connected in series. However, where separate power control is provided for each coil sub-set, automatic synchronization of this kind does not occur.

Accordingly, in accordance with an embodiment of this invention, a common control device 92 such as that shown in Figure 12 can be provided to ensure correct emulation of a polyphase system incorporating series-connected coils. As described above in relation to Figure 11, terminals 86 can be provided at the multiple control devices 80 to allow interconnections 90 to be formed between the multiple control devices 80 and the common control device 92.

The interconnections 90 can pass signals between the common control device 92 and the control devices 80 such as timing/synchronization signals for appropriate emulation of a polyphase series-connected system.

A particular feature of the embodiment of the invention that allows the motor to provide the full braking torque is the use of staggered PWM switching for the purpose of current control. As already described, when operating in a motor mode, the voltage across the coils is controlled by PWM switching by the switches in control circuits 80. When in braking mode, the PWM switching is arranged so that the switches within a given wheel provide staggered switching such that the current profile delivered by each coil through the switches is offset in relation to the current profile from other switches within the wheel. As a result, the sum of such currents has an approximately DC form with a current ripple.

-16 -To achieve the staggered switching, each control device communicates with other control devices within the motor to establish a timing pattern to evenly distribute the switching over continuous rotation of the wheel. The communication may be by inter control device communication across a network. Alternatively the control devices can have their PWM generators synchronised by an off board device such as the common control device 92. A further possibility is that the timing of the switching is established with respect to some other parameter such as the position of the rotor, back EMF from the coils and so on.

It is particularly important to note that the timing pattern of the PMW switching, including a slight delay between the switching of the various switching devices in the motor, avoids peaks in current delivered to a power source in a manner not previously known. This kind of spreading of the switching events can be coordinated locally at the individual control devices 80 or could alternatively be coordinated by the common control device 92 using adjusted timing signals sent via the interconnections 90.

Although the control devices 80 described in this application can provide individual power control for the coils of each coil sub-set in a motor, and although this may be achieved using various kinds of switching devices and arrangements, the control device system cells can be coupled to a common power source such as a DC power supply. The connection may be referred to as a power connector, a DC bus, a battery connection, as supply line or the like. A particularly useful arrangement for the DC power supply is to provide a circular bus bar. Because the control circuit 80 are arranged in a ring, the DC power feed may also be arranged as a ring. This provides increased safety in that there is a current path around each side of the ring (in the same way as a domestic ring main) and so breakage of the DC supply at one point will not prevent power reaching the control circuits. In addition, because current can flow from the source power supply to each control circuit by two routes through the circular bus bar, the current demand on the bus bar is halved.

A number of the features already described provide a significant advantage when implemented in a motor within a vehicle wheel in providing a safe mechanism for applying a braking force and thereby avoid the need for a separate mechanical braking arrangement. The motor itself can provide the -17-braking force and thereby return energy to the power supply, such that this arrangement may be termed "regenerative braking. When operating in this mode, the motor is acting as a generator.

The braking arrangement makes use of the considerable redundancy built into the motor assembly as a whole. The fact that each separate coil sub-set 44, shown in Figures 7 and 8, is independently controlled by a switching circuit 80 means that one or more of the switching circuits may fail without resulting in a total loss of braking force. In the same way that the motor is able to operate with reduced power when providing a driving force by intentionally switching some of the switching circuits to be inoperable, the motor can operate with a slight reduction in braking force if one or more of the switching circuits fail. This redundancy is inherent in the design already described but makes the motor a very effective arrangement for use in a vehicle, as it can replace both the drive and braking arrangement.

A further reason why the motor assembly can provide an effective braking arrangement is in relation to the handling of power. As already mentioned, the use of multiple independently controlled coils means that the current through each coil when operating in a generating mode need not be as high as the current through an equivalent arrangement with fewer phases. It is, therefore, simpler to deliver the power generated by the coils back to the power source.

To ensure safe operation of the braking arrangement, even in the event of failure of the power source, the circuitry 80 for each individual coil sub-set is itself powered by an electricity supply derived from the wheel itself. As the wheel rotates, it generates a current as the magnets pass the coils. If the power supply fails, this current is used to supply power to the switches 80.

A further redundancy measure is in providing separate physical sensors connected to the brake pedal (or other mechanical brake arrangement) of the vehicle, one sensor for each wheel. For example, in a typical four-wheeled car, four separate brake sensor arrangements would be physically coupled to the brake pedal with four separate cables going to the four separate motors.

Accordingly, one or more of these separate electrical sensors connected to the mechanical brake pedal or, indeed, the separate cables could fail and still one or more of the wheels will be controlled to operate a braking force. By virtue of the ability of the control units to communicate with each other, software features allow the failure of any sensor or it's cable to have no effect on the motor operation. This is achieved by each motor being able to arbitrate the sensor information and use the sensor data from the other motors if it's sensor data is disparate with the other three sensors.

A yet further redundancy measure is the use of a so-called dump resistor.

In the event of failure of the power supply, the energy generated by the wheel, when providing a braking force, needs to be dissipated. To do this, a resistance is provided through which the electrical power generated by the wheel may be dissipated as heat. The use of the multiphase design with separate electrical switching of each sub-coil allows the use of distributed resistance, so that each sub-coil may dissipate its power across a resistance and the dump resistance as a whole may therefore be distributed around the wheel. This ensures that the heat thereby generated can be evenly dissipated through the mass of the wheel and the cooling arrangement.

Referring again to Figure 11, the mode of operation of the switch 80 for each coil sub-set 44 is as follows when in a braking mode. The upper switches 88A and 88B are opened and switch 88C operated in on / off pwm mode to control the voltage generated by the coil. As the magnet passes the coil sub-set 44, the voltage at connection point 83 rises. When the switch 88C is then opened as part of the pwm process, the voltage at point 83 rises to maintain the coil current and so energy is returned to the power supply (via the diode across switch 88B). This arrangement effectively uses the coils of the motor itself as the inductor in a boost form of DC-to-DC converter. The switching of the controls in the H bridge circuit controls the DC voltage that is provided back to the power source.

The boost type dc / dc converter switching strategy employed for regenerative braking has a further distinct advantage in that it reduces battery loading. In known systems regenerative mode operates by switching the top switches to provide the battery volts in series with the motor coil and its back emf.

This requires the current to be established through the battery. Hence even though the coil is generating, it depletes the battery state of charge by virtue of its current having to flow through the battery in the discharge direction. By employing the DC-to DC converter arrangement described above, the coil -19-establishes its current locally by an effective short circuit across the coil, created by the bottom switches. When the generated current is established it is then directed back to the battery in the charge direction. So whilst both regimes collect the transient energy when the bottom switch turns off in the normal pwm sequence, the conventional system consumes battery current whilst establishing the generated current flow, whereas the arrangement here described consumes no battery current.

When the voltage generated by the coil falls below say four volts, the current can no longer flow due to the voltage dropped across the switches or diodes used within the H bridge circuit. In the embodiment, a voltage of approximately 1.75 volts per mile per hour is generated and so at speeds below 3 miles per hour, this situation arises. At this speed, the switching strategy changes to a form of DC plugging. In DC plugging the phase of all voltages is arranged to be the same. This common phase of all voltages results in the removal of rotation force and the application of a static force. The static force attempts to hold the rotor in one position. Thus normal pwm control is used but with each coil subset having it's applied voltage in phase with all others. This DC mode of operation is particularly beneficial at low speeds, as it ensures safe stopping of the vehicle. When the vehicle has come to a complete rest, the vehicle will stay at rest, as any movement of the rotor is resisted by the static field. There is thus no risk that the motor would accidentally move forwards or backwards.

The dump resistor arrangement already described may also be used in the event that the battery is simply full and energy needs to be dissipated when braking. If the voltage across the supply goes over a given threshold then energy may be switched to the dump resistor.

Embodiments of this invention can provide a highly reliable motor or generator, at least in part due the separateness of the power control for the coil sub-sets as described above. Accordingly, a motor or generator according to this invention is particularly suited to applications in which a high degree of reliability is required. A further safety feature, particularly beneficial when incorporated in a vehicle, is that the motor can supply power not only to the switches within the motor, but also to remote aspects of a whole system, including a master controller processor, shown as common control device 92, in Figure 12, and to -20-other sensors, such as the break pedal sensor. In this way, even if there is a total failure of power supply within the vehicle, the braking arrangement can still operate.

Claims

1. An electric vehicle comprising a plurality of wheels, each wheel having an in-wheel electric motor, the electric motor of each wheel being operable in a braking mode and each electric motor comprising: one or more separate coil sets arranged to produce a magnetic field of the motor, each coil set comprising a plurality of coil sub-sets, each coil sub-set comprising one or more coils, the magnetic field produced by the coils in each coil set having a substantially common phase; a plurality of control devices each coupled to a respective coil sub-set for independently controlling a current in the one or more coils of said respective coil sub-set; and a common power connection between the control devices and a power source; wherein each control device comprises switches arranged to switch the coils in the braking mode using pulse width modulation switching so arranged that the current profile delivered by each control device to the common power connection is offset in relation to other control devices so that full braking torque may be provided by the electric motors whilst avoiding current peaks on the common power connection.

2. The electric vehicle of claim 1, further comprising a sensor arranged to detect the position of a rotor of the motor to generate a position signal, wherein each control device is operable to control current in a respective coil sub-set using the position signal.

3. The electric vehicle of claim 2, wherein each control device has an associated sensor to detect the position of the rotor to generate a position signal for the respective control device.

4. The electric vehicle of any preceding claim, wherein each coil sub-set comprises a plurality of adjacent coils. -22-

5. The electric vehicle of claim 1, wherein at least one of the control devices comprises means for monitoring a back EMF within the coils of that coil sub-set, and wherein the control device is operable to adjust a pulse of the pulsed switching in response to the monitored back EMF.

6. The electric vehicle of claim 5, wherein the monitoring of the back EMF is used to determine the position of the rotor to thereby control adjust a pulse of the pulsed switching.

7. The electric vehicle of claim 5 or 6, wherein adjusting a pulse of the pulsed voltage in response to the monitored back EMF comprises adjusting a width of the pulse.

8. The electric vehicle of claim 5 or 6, wherein adjusting a pulse of the pulsed voltage in response to the monitored back EMF comprises adjusting a timing of the pulse.

9. The electric vehicle of any preceding claim, further comprising an interconnection between control devices, wherein the control devices are arranged to stagger switching with respect to one another by communication on the interconnection.

10. The electric vehicle of any preceding claim, comprising a common control device configured to coordinate the operation of the plurality of control devices to ensure staggered switching of the switches.

11. The electric vehicle of claim 9, wherein the common control device is configured to coordinate the operation of the plurality of control devices to control the current in the one or more coils of each respective coil sub-set such that each coil set produces a magnetic field having a substantially common phase.

12. The electric vehicle of claim 10 comprising a plurality of separate coil sets, wherein the common control device is configured to coordinate the plurality of control devices to provide polyphase current emulation within the coils.

-23 - 13. The electric vehicle of claim 17, wherein the plurality of control devices are configured to provide staggered switching of the currents in the coils of the motor within a polyphase cycle of the motor.

14. The electric vehicle of any preceding claim, wherein each control device is coupled to receive power generated by coils of the motor, whereby each control device continues to receive power in the event of failure of a common dc power supply.

15. The electric vehicle of claim 14 and an external device comprising at least brake sensor, wherein the coils of the motor are coupled to the brake sensor to supply power generated by coils to the brake sensor.

17. The electric vehicle of claim 24 or 25 and an external device comprising at least an external controller, wherein the coils of the motor are coupled to the external controller to supply power generated by coils to the external controller.

18. The electric vehicle of any preceding claim wherein each control device comprises an H-bridge switching arrangement having first and second switches coupled to a first side of the dc supply and third and fourth switches coupled to a second side of the dc supply and configurable so that, in the regenerative braking mode, the first and second switches on the first side of the dc supply are opened, the third switch on the second side of the dc supply is closed and the fourth switch on the second side of the dc supply repeatedly opened and closed.

19. The electric vehicle of claim 18, wherein the fourth switch is repeatedly opened and closed by pulse width modulation.

20. The electric vehicle of claim 18 or 19, further comprising a diode arrangement such that, when the fourth switch is repeatedly opened and closed, a voltage is applied from the coil sub-set to the dc supply via the diode to regenerate the dc supply.

21. The electric vehicle of claim 18, 19 or 20, further configurable so that, when the speed of the motor drops below a given value, the control devices are operable, in a non-regenerative mode, to apply voltages to the coil sets in a common phase.

22. The electric vehicle of any of claims 18 to 21, further comprising a dump resistor coupled to the control device and configured to receive current from the control device in the event that power cannot be returned to the DC supply.

23. The electric vehicle of claim 22, wherein the dump resistor is configured to receive current in the event that the DC supply is a battery that is full, the voltage across the supply goes over a given threshold or the DC supply fails.

24. An electric motor as described in any preceding claim.

25. An in-wheel electric motor for a road vehicle capable of providing the full braking torque for a vehicle.

26. A vehicle comprising a plurality of wheels each with in-wheel electric motors, the electric motors capable of providing the full braking torque for the vehicle.

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