GB2456349A

GB2456349A - Hub motor with individually controlled stator coils provides safe braking

Info

Publication number: GB2456349A
Application number: GB0800586A
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
Also published as: GB0800586D0

Abstract

An electric motor includes coil 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 include more than one winding (74A, 74B, 740, fig.8; fig.9). The motor also includes a plurality of control devices (80, fig.10) housed within the casing of the motor (80, fig.3). Each control device is connected to a respective individually energised coil for current control. Each control device (80) controls the switching of voltage applied to its associated coil in response to a determination of the torque required from the motor, without use of current sensors. An arrangement is disclosed for mounting the motor in the hub of a vehicle wheel. The motor may provide regenerative braking. 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 plugging may be used at very low speeds. A separate mechanical braking arrangement may not be needed.

Description

ELECTRIC MOTOR CONTROL

FIELD OF THE INVENTION

The invention relates to controlling electric motors, in particular to current sensing and control in in-wheel or hub electric motors.

BACKGROUND OF THE INVENTION

Known electric motor systems typically include a motor and a control unit for controlling power to the motor. Known types of electric motor include the induction motor, brushless permanent magnet motor, switched reluctance motor and synchronous slip ring machine. Three phase electric motors are the most common kind of electric motor available.

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.

A

As shown in Figure 2, each coil sub-set can include one or more coils. In particular, Figure 2 shows the coils 24A, 24B in one of the coil sub-sets 14. In this example, there are two coils per coil sub-set. The two coils are wound in the opposite directions, and are interconnected so that the current flowing in each coil is substantially the same. As the poles of the rotor 12 sweep across the coils 24A, 24B, switching of the current in the coils 24A, 24B can produce the appropriate magnetic field for attracting and repelling the rotor for continued rotation thereof. The magnetic field produced by the two oppositely wound coils 24A, 24B is referred to as belonging to the same phase of this three phase motor. Every third coil sub-set arranged around the periphery of the motor 10 produces a magnetic field having a common phase. The coils and the interconnections may typically comprise a single piece of wire (e.g. copper wire) running around the periphery of the motor and wound into coils at the appropriate locations.

For a three phase electric motor, the switching system is almost invariably a three phase bridge circuit including a number of switches. Such switching systems require current sensing circuitry in at least two of the three coils in order to determine the current flowing in the coil, and hence the magnetic field produced by the coil. The measured current is then used by the motor control circuitry in a closed loop to determine how to subsequently adjust the voltage applied to the coil depending upon requirements.

Because each coil is independently powered via the connections 34, 36, and 38, it is necessary for at least two of the circuits 34, 36, and 38 to have a separate current sensor. This allows the voltage applied to each coil set 34, 36, 38 to be controlled depending upon the actual current flowing in the coil sets 34 and subsets 14, 16, 18. Accordingly as the number of separate control circuits increases it is desirable to avoid the need for separate current sensing apparatus for each circuit.

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, the applied voltage and the motor back emf. The PWM control then sequentially modulates the applied voltage so that the current in the winding matches the desired value so that precise control of the current is achieved.

SUMMARY OF THE INVENTION

Aspects of the invention are defined in the accompanying claims.

According to an aspect of the invention, there is provided an electric motor. 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 the instantaneous voltage applied to the coils of that respective coil sub-set. Each control device may operate without requiring an input synchronisation signal. A key aspect of the present invention is that it operates in an entirely open loop manner -that is without the need for any feedback from current sensors. Thus the control device is pre equipped with all of the known characteristics of the motor and control device such that for any given input or torque demand, the system knows precisely what voltage to apply at any instant so that the resulting current is the most optimum current for that instant. The control devices include means for monitoring a back EMF within the coils of that coil sub-set. This allows the current within a coil-subset to be determined so that the control device can adjust voltage applied to the coil subset in response to the monitored back EMF and determined current in the coil sub-set. This allows for high-speed power control without the need for additional current sensors to be included for each coil-subset.

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. 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 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.

The control devices can operate independently of one another because each control device comprises sufficient logic to determine the position of the rotor and so to apply the appropriate voltage to control the current in the respective coil subset. The control devices can receive a demand signal from an external device, such as a brake pedal sensor, and apply appropriate coil control based on the coil characteristics, the position of the rotor and the demand signal.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in detail, by way of example only, with reference 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; 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 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. For the avoidance of doubt, the vanous aspects of the invention are equally applicable to an electric generator having the same arrangement. In addition, some of the aspects of the invention are applicable to an arrangement having the rotor centrally mounted within radially surrounding coils.

The physical arrangement of the embodying assembly is best understood with respect to Figures 3 and 4, showing a 24-phase motor. 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 tyre 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. A first advantage of this arrangement is that the whole assembly may be simply retrofitted to an existing vehicle by removing the wheel, bearing block and any other components such as the braking arrangement. The existing bearing block can then fitted inside the assembly and the whole arrangement fitted to the vehicle on the stator side and the normal rim and wheel fitted to the rotor so that the rim and wheel surrounds the whole motor assembly. Accordingly, retrofitting to existing vehicles becomes very simple.

A second advantage is that there are no forces for supporting the vehicle on the outside of the rotor 240, particularly on the circumferential wall 221 carrying the magnets on the inside circumference. This is because the forces for carrying the vehicle are transmitted directly from the suspension fixed to one side of the bearing block (via the central portion of the stator wall) to the central portion of the wheel surrounding the rotor fixed to the other side of the bearing block (via the central portion of the rotor wall). This means that the circumferential wall 221 of the rotor is not subject to any forces that could deform the wall thereby causing misalignment of the magnets. No complicated bearing arrangement is needed to maintain alignment of the circumferential rotor wall.

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.

Coil Control: Example No. I 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. However, it will be appreciated that embodiments of this invention can employ other topologies. For example a linear arrangement of coils for producing linear movement is envisaged.

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 in the centre of the circle defined by the positioning of the various coils of the motor, thereby to allow rotation of the rotor within the rotating magnetic field produced by the coils. Preferably, though, the rotor is arranged 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 voltages applied to 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.

Since there is no need to run connecting wires around the periphery of the motor providing series interconnections for the coils of each coil sub-set, less wire is used in manufacturing the motor. This reduces manufacturing costs as well as reducing the complexity of the motor construction. The reduction in wire also reduces conduction losses.

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 j.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 control the operation of coil sub-sets connected in series, the control device is sufficiently large that it can not 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.

Coil Control: Example No. 2 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 penphery 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 voltage applied to 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.

N-phase electric motor 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 minimize 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.

Reduced coil subset operation By way of example, in Figure 7 some of the coil sub-sets are highlighted with a i'. If these coil sub-sets were to be powered down, the motor would still be able to operate, albeit with reduced performance. In this way, the power output of the motor can be adjusted in accordance with the requirements of a given application. In one example, where the motor is used in a vehicle such as a car, -12 --Is powering down of some of the coil sub-sets can be used to adjust the performance of the car. In the example shown in Figure 7, if each of the coil sub-sets indicated with an were powered down, the remaining coil sub-sets would result in a configuration similar to that shown in Figure 5, although of course there are three coils per coil sub-set as opposed to the single coil per coil sub-set shown in Figure 5.

Powering down of one or more of the coil sub-sets has the further benefit that in the event of a failure of one of the coil sub-sets, other coil sub-sets in the motor 40 can be powered down resulting in continued operation of the motor 40 in a manner which retains a balanced magnetic field profile around the periphery of the motor for appropriate multiphase operation. In contrast, in prior systems involving series interconnection of the coils of the coil sub-sets, a failure in the coils or interconnections associated with any given coil set is likely to be catastrophic and highly dangerous, given the large currents involved. Moreover, a failure anywhere within the coils or interconnections between the coils of a given coil set would result in the motor not being able to continue functioning in any way whatsoever.

In summary, individual power control for the coil sub-sets in accordance with an embodiment of this invention allows independent powering up and or powering down of selected coil sub-sets in order to react to differing powering requirements and/or malfunctions or failures within the coil sub-sets.

Control Circuitry 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 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 II, 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.

In an alternative and preferred embodiment, each control unit can operate independently, without the need of a central control device. For example, each control unit can have independent sensors to detect a position of a rotor of the motor, which would dispense with the need to provide synchronisation signals of the kind described above. Instead, each control unit would receive a demand signal enabling it to control the voltage applied to its associated coils in isolation.

It is stressed that the preferred embodiment does not require any form of central control device for the operation of each wheel incorporating a motor.

Preferably, each motor is self-contained and, within each motor, the control circuits 80 are self-contained and depend upon nothing other than a torque demand signal to operate. This means that the elements are able to continue to function and to deliver demanded torque levels, irrespective of any other failures within the total drive system. In a system incorporating a plurality of wheels each having a motor, each motor incorporates all the intelligence needed to manage its actions. Each motor understands its position on the vehicle and controls its actions accordingly. Preferably, each motor is further provided with information regarding the other motors such as the speed, torque and status and are based on each motor's knowledge of its position on the vehicle and the state and status of the other motors it can determine the optimum level of torque that it should apply for a given demanded torque. Even without this other information, though, the motor can continue to respond to a demanded torque.

Other control signals such as power up/power down control signals can also be sent/received via the interconnects. These signals can also include signals for adjusting/defining the voltage pulses applied by the control device 80 to the coils of its associated coil sub-set for powering the motor.

For example, in accordance with an embodiment of this invention, means can be provided for monitoring a back EMF within the coil or coils of a coil sub-set. The task of emulating a motor with series connected coil sub-sets as described above is complicated by virtue of the back EMF associated with the motor. In a series connected system, the back EMFs are also in series and this gives rise to a smooth sine wave back EMF profile. Accordingly, in a series configuration the sinusoidal back EMF minimises the bandwidth required from the drive electronics when controlling the current in the coils.

In contrast, the reduced number of coil sub-sets connected in series in accordance with an embodiment of this invention can result in a non sinusoidal back EMF. Accordingly a more agile control system is desirable in order to ensure that the currents in the coils remain sinusoidal, or more often remain in a form that closely matches the back emf.

According to an embodiment of this invention, near instantaneous compensation can be provided for back EMF and further adjusting for any variations in a system dc supply voltage. The means for measuring the back EMP can include a current sense device fitted to provide feedback of the actual current flowing in the coil or coils of each coil sub-set. In one example, a simple series resistor of suitably low value in series with the switching devices can be employed. For example, in one embodiment two resistors can be provided in the bottom emitter of a H" bridge power stage.

As the back EMF changes with rotor angle and rotor velocity, this results in a change in the rate of change of current in the coil. This rate of change of current can be detected across a resistor or other current sense device as a change in voltage. This change can then be differentiated to produce a voltage which is proportional to the back EMF.

Similarly, the supply voltage can be applied to a capacitor at the start of each PWM period. The resulting voltage ramp can be added to the back EMF signal and combined as a feed forward term to modify the current PWM period up or down. Thus both supply variation and back EMF changes substantially instantly adjust the PWM period and hence voltage applied to the coil, resulting in rapid adjustment of coil current to follow the demanded value.

Embodiments of the invention control the motor in a number of different ways, without the need for current sensors in the coil subset, as described below.

Open-loop voltage control With open loop voltage control, embodiments of the invention include a control device that is pre-programmed with the motor characteristics.

Embodiments of the invention may be hard wired into the circuitry, with the use of appropriate logic and software, or with a suitable pre-programmed chip.

The characteristics are stored in the form of specific values, specific relationships, specific constants and multi dimensional look up tables of instant by instant values.

The specific values comprise the motor coil resistance, transistor volt drop, diode volt drop, etc. Specific relationships comprise the motor inductance variation with rotor angle, magnet flux variation with temperature, coil resistance with temperature, motor torque constant variation with torque, transistor and diode volt drop variation with temperature etc. The multi dimensional values stored in the look up table are specific voltage waveforms for rising and falling quadrants, as well as the different stored waveforms required depending upon whether the motor is operating in the generating or regenerative braking mode. These voltage values are complex values based on motor type measured back emf versus rotor angle, together with adjustments made to compensate for rotor speed and coil inductance variations with rotor angle. The battery supply voltage level is a dynamic variable also used in computing the instantaneous required voltage.

Thus in this preferred embodiment there is no sensor input other than rotor angle position. In particular, there are no current sensors. In this way embodiments of the invention are able to determine what voltage needs to be -17-applied to the coil based on the torque required from the motor by the user without the need for current sensing apparatus in the coil. This embodiment has the advantage that a current sensor is not needed for each coil subset. In particular, for electric motors with a large number of phases, it is particularly advantageous to not need current sensing apparatus in each coil. This avoids the need for additional hardware which reduces the efficiency of the motor and necessarily increases its weight.

Partly derived and partly measured back emf The second method of motor control without current sensors in the coil-subsets is the derived and partly measured back emf embodiment. In this embodiment, the back emf is determined by the following method.

With rotor angle position information along with the supply voltage and knowledge of the motor coil inductance etc, it is possible to derive back emf by measuring the voltage across the coil at suitable instants. The voltage across the coil without the influence of back emf would be the supply volts (during a pwm on period). However, the back emf is present when the motor is rotating and this adds to or subtracts from the measured voltage. Therefore, at any instant it is, possible to determine back emf and taking account of the instantaneous speed, the period of the applied voltage can be adjusted to suit the demanded torque This allows the controller to adjust the pulse of the pulsed applied voltage in response to the back EMF without the need for an additional current sensor.

This embodiment has the advantage that additional current sensors are not required in each coil subset. Since the current in each coil sub set must be individually determined, this embodiment avoids the need for additional current sensors for each of the subsets.

Embodiments of the invention use appropriate logic and may be implemented in software or hard wired into an electric motor or can be embodied in a chip.

This is particularly advantageous in the case of a 24-phase motor as shown in figures 3 and 4, because of the large number of coil subsets.

Embodiments of the invention avoid the need for current monitoring apparatus for each of the coil subsets.

In a further example, a sense coil can be provided. Sense coils can be provided around, for example, a sub-set of coil teeth of the kind described below.

The sense coil then can be monitored at appropriate times for the back EMF voltage. This in turn can be used in a similar manner as described above to feed forward a term to adjust PWM period in mid-cycle, response to the magnitude of the back EMF.

In embodiments where each drive module generates its own PWM signal, back EMF correction thus can take place in a manner which is not synchronised with the other modules resulting in a distributed random spread spectrum.

Alternatively the control devices can have their PWM generators synchronised by an off board device such as the common control device 92.

The control device can also optionally include means for monitoring a temperature within the motor, for example within the coils sub-set associated with that control device 80. The control device can be configured automatically to respond to the temperature measurement to, for example, reduce power to the coils sub-set to avoid overheating. Alternatively, the temperature measurement can be passed onto the common control device 92 from each control device 80, whereby the common control device 92 can monitor the overall temperature within the motor and adjust the operation of the control devices 80 accordingly.

Noise Reduction In accordance with an embodiment of the invention, EMI noise can be reduced by providing for staggered switching of the switches within each control device 80. By including a slight delay between the switching of the various switching devices in the motor, a situation can be avoided in which a large number of switching events occur in a short amount of time, leading to a peak in EMI noise. Thus, the staggering of the switching within the switches 88 of the control devices 80 can spread the EMI noise associated with the switching events during operation of the motor across a wider time period thereby avoiding an EMI noise peak. 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.

Power Supply 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. 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 nng 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.

Braking Arrangement 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 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 I' 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 itsetf. 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 I 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 I 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 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 -22 -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 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.

For example, applications such as wind turbines depend for their success and take up on cost and reliability. Typical turbine systems will run for 25 years and ideally should require minimum in service down time for maintenance I breakdown etc. By incorporating the drive electronics into a compact form with compact windings, as can be achieved according to an embodiment of this invention, the total system cost can be minimised. In accordance with an embodiment of this invention, independent power control of the coil sub-sets can -23 -allow continued operation even under partial failure the system.

In particular vertical axis turbines which are recently growing in popularity due to their efficient operation can benefit from the incorporation of a motor according to this invention. This is because of the high power to wei9ht ratio which can be achieved, which allows for lower mast head mass and hence less cost for support column I structure.

Military, marine aircraft and land based drive systems are all currently less reliable than would be desired due to the dependence on single device reliability in classic 3 phase bridge topologies. Again, using a motor according to an embodiment of this invention, the reliability of such vehicles could be improved.

It will be clear from the foregoing description that electric motors generally indude a complex arrangement of interconnections and windings. As described above, the manufacture of an electric motor incorporating such features is a laborious and time consuming process. The time and effort which is required to construct an electric motor is generally exacerbated by the use of, for example, copper wire for the windings and interconnections. Wire of this kind is often relatively thick (in order to be able to handle high currents) and is difficult to manipulate. Damage to electric insulation provided on the wire can be difficult to avoid during motor construction, again due to the difficulty in manipulating the wire. Access to the relevant parts of a motor for installing the windings and interconnections is often limited and inhibited by other components of the motor.

It will be clear from the foregoing that embodiments of this invention are applicable to electric generators as well as to electric motors, due in part to the structural and conceptual similarity between the two. For example, an electric generator can benefit from separate power termination of the coils of a coils sub-set as described above. Furthermore, the coil mounting system described above is equally applicable to the construction of the arrangement of coils in a generator * and a motor.

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Claims

1. An electric motor comprising: a plurality of coils arranged to produce a magnetic field of the motor, a plurality of magnets; and a control device coupled to the coils controlling the switching of voltage applied to the coils wherein the control device is operable to adjust a pulse of the pulsed voltage in response to a determination of the torque required from the motor without use of current sensors.

2. An electric motor according to claim I in which the coils comprise 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 subset comprising one or more coils

3. An electric motor according to claim 2 in which the magnetic field produced by the coils in each coil set having a substantially common phase.

4. An electric motor according to claim 3 further comprising a plurality of control devices each coupled to a respective coil sub-set for independently controlling a current in the coils of said respective coil sub-set.

5. An electric motor according to claim 4 further comprising means for monitoring a back EMF within the coils of that coil sub-set.

6. An electric motor according to claim 5 wherein the control device adjusts the pulse of the pulsed voltage in response to the monitored back EMF.

7. An electric motor according to claim I in which each control device is operable to control current in a respective coil subset without an input synchronisation signal.

8. The electric motor of claim 1, further comprising a sensor arranged to detect the position of a rotor of the motor to generate a position signal, -25 -wherein each control device is operable to control current in a respective coil sub-set using the position signal.

9. The electric motor 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.

10. The electric motor of any preceding claim, wherein each control unit is operable to receive a demand signal and arranged to control current in a respective coil sub-set based on the demand signal.

11. The electric motor of claim 3, wherein each control unit comprises a circuit board and wherein the associated sensor is mounted on the circuit board.

12. The electric motor of claim 5, wherein the circuit board is arranged on a stator of the motor and a rotor of the motor comprises a plurality of magnets, wherein the sensor detects the position of the magnets and thereby detects the position of the rotor.

13. The electric motor of any preceding claim, wherein each coil sub-set comprises a plurality of adjacent coils.

14. The electric motor of claim 9, 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 voltage.

15. The electric motor of claim 9 or 10, wherein adjusting a pulse of the pulsed voltage in response to the monitored back EMF comprises adjusting a width of the pulse.

16. The electric motor of any preceding claim comprising a motor casing, wherein the control devices are housed within the casing.

17. The electric motor of claim 12, wherein the control devices are located adjacent their respective coil sub-sets within the motor.

-26 - 18. The electric motor of any preceding claim, wherein each control device is operable to control a temperature of its respective coil sub-set by adjusting a current within the coils of that coil sub-set.

19. The electric motor of any preceding claim, comprising a common control device configured to coordinate the operation of the plurality of control devices.

20. The electric motor of claim 15, 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.

16. The electric motor of claim 16 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.

17. The electric motor 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.

18. The electric motor of any of claims 15 to 18, wherein the common control device is operable to selectively disable one or more of the control devices to allow fractional power operation.

19. The electric motor of any preceding claim, wherein each control device is coupled to a common dc power supply.

20. The electric motor of claim 20, wherein each control device is disconnectabte from the common dc power supply in the event of a failure.

21. The electric motor of claim 21, wherein each control device includes a fuse arrangement to disconnect the control device from the common dc power supply in the event of a failure.

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22. The electric motor of any preceding daim, 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.

23. The electric motor of any preceding claim, wherein coils of the motor are connectable to control devices external to the motor to thereby supply power generated by coils to the external devices in the event of failure of a dc power supply.

24. The electric motor of claim 24 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.

25. The electric motor 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.

26. The electric motor of any preceding claim, wherein the coil sets are arranged to produce a rotating magnetic field, and wherein the motor further comprises a rotatable magnet or magnets.

27. A vehicle comprising the motor of any of claims 1 to 26.

28. An electric motor operable in a braking mode comprising: a plurality of coils arranged to produce a magnetic field of the motor; a plurality of magnets; and a control device connected to a dc supply coupled to the coils controlling the current in the coils; wherein, in braking mode, the control device is operable to adjust a pulse of the pulsed voltage in response to a determination of the torque required from the motor without the use of current sensors.

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29. An electric motor substantially as herein described with reference to the accompanying figures.

30. An electric motor operable in braking mode substantially as herein described with reference to the accompanying figures. -29 -