The present invention relates to an electric motor and a method of braking using an electric motor.

With increased interest being placed in environmentally friendly vehicles there has, perhaps unsurprisingly, been a corresponding increase in interest in the use of electric vehicles .

Electric vehicles typically use an electric motor to provide both drive for the vehicle and regenerative braking for stopping the vehicle. To effect regenerative braking rotary motion of drive wheels connected to an electric motor is converted into electric energy, which involves consumption of kinetic energy and provides a braking force to the drive wheels by applying a braking torque in an opposite direction to the rotation of the drive wheels. To allow electric motor braking torque to be used to stop a vehicle, without the need for friction brakes, it is necessary to prevent the vehicles electric motor rotating in an opposite direction once the vehicle has come to a halt. Consequently, it is necessary for braking torque generated by an electric motor to be set at a value to hold a vehicle stationary .

One mechanism used for determining a zero speed condition, and hence when to remove a braking torque, is to identify the point when the direction of rotation of the motor changes. However, noise associated with a signal indicative of direction can cause an overshoot condition that causes an opposite torque to be applied resulting in instability and an electric motor oscillating when the zero speed condition is approached.

If a speed signal is used to identify a zero speed

condition, due to signal filtering delays the motor can reach a zero speed and then start to rotate in the opposite direction before braking torque is removed, which can also result in instability and the electric motor oscillating when the zero speed condition is approached.

Another braking solution that has been adopted is a

technique commonly known as plug braking, where the armature windings of an electric motor are short circuited when braking is required. The short circuiting of electric motor armature windings causes an alternating electrical field generated in the armature windings to freeze.

For an induction motor, where the braking current in the rotor is induced by the rotation of the rotor in a fixed stator field, plug braking can act as a suitable braking technique .

However, for a permanent magnet synchronous motor, where the rotor magnetic field is fixed by permanent magnets mounted on the rotor, the freezing of the stator field can cause a large cogging torque to occur as the stator and rotor magnetic fields come in and out of phase if, as a result of its own inertia, the rotor continues to rotate once the stator field has been frozen.

It is desirable to improve this situation. In accordance with an aspect of the present invention there is provided an electric motor and a method of providing a braking torque according to the accompanying claims . This provides the advantage of allowing a progressive braking torque to be applied to an electric motor, where the braking torque varies based upon the relative movement of the electric motors stator and rotor, thereby minimising the risk of the electric motor oscillating when the zero speed condition is approached.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which :

Figure 1 illustrates an exploded view of an in-wheel

electric motor;

Figure 2 is an exploded view of the motor of Figure 1 from an alternative angle;

Figure 3 schematically shows an example arrangement of coil sets for an electric motor according to an embodiment of the present invention;

Figure 4 illustrates a three phase stator current complex space vector;

Figure 5 illustrates a three phase stator current reference frame with a d, q rotating reference frame;

Figure 6 illustrates an electro-magnetic phase diagram representing the electrical and magnetic phase angles for a first stage of a braking mode according to an embodiment of the present invention; Figures 7a and 7b illustrate an electro-magnetic phase diagrams representing the electrical and magnetic phase angles for a second stage of a braking mode according to an embodiment of the present invention;

Figure 8 illustrates an electro-magnetic phase diagram representing the electrical and magnetic phase angles for a third stage of a braking mode according to an embodiment of the present invention;

Figure 9 illustrates an electrical phase diagram representing the electrical and magnetic phase angles according to an embodiment of the present invention;

Figure 10 illustrates method steps according to an embodiment of the present invention;

Figure 11 illustrates method steps according to an embodiment of the present invention;

Figure 12 schematically illustrates the coils sub-sets of an electric motor according to an embodiment of the present invention that are configured in a wye configuration;

Figure 13 schematically illustrates the coils sub-sets of an electric motor according to an embodiment of the present invention that are configured in a delta configuration; Figure 14 schematically shows an example of a control device in accordance with an embodiment of the present invention;

Figure 15 is a circuit diagram of the switching arrangement. The embodiment of the invention described is a permanent magnet synchronous electric motor for use in a wheel of a vehicle, that is to say an in-wheel electric motor. However, as would be appreciated by a person skilled in the art, the invention is applicable to other types of permanent magnet synchronous electric motors. 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. 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 1 and 2. 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 1, 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 which together with the drive arrangement 231 and rear portion 230 form the stator 252.

Although not shown, also mounted to the stator are a plurality of capacitor circuit boards for providing capacitance between the electric motor and the voltage supply to reduce voltage line drop. 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. 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. Figure 2 shows an exploded view of the same assembly as Figure 1 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 223 at the central portions of the rotor and stator walls.

Additionally shown in Figure 1 are control devices 80, otherwise known as motor drive circuits, which, as described below, includes an inverter. Additionally in Figures 1 and 2 a V shaped seal 350 is provided between the circumferential wall 221 of the rotor and the outer edge of the stator housing 230. Further, in Figure 2, the 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 control devices 80 of the stator 252.

Figure 3 schematically shows an example of an electric motor 40 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 includes eight coil sets 60 with each coil set 60 having three coil sub-sets 61, 62, 63 that are coupled to a respective control device 64, where each control device 64 and respective coil sub-sets form a three phase logical or sub electric motor that can be controlled independently of the other sub motors. The control devices 64 drive their respective sub motor with a three phase voltage supply, thereby allowing the respective coil sub-sets to generate a rotating magnetic field. Although the present embodiment describes each coil set 60 as having three coil sub-sets 61, 62, 63, the present invention is not limited by this and it would be appreciated that each coil set 60 could have two or more coil sub-sets. Equally, although the present embodiment describes an electric motor having eight coil sets 60 (i.e. eight sub motors) the motor could have two or more coil sets with associated control devices (i.e. two or more sub motors) . The motor 40 can include a rotor (not shown in Figure 3) 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 1 and 2. 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 61, 62, 63 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 3 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.

As stated above, each control device includes a three phase bridge inverter which, as is well known to a person skilled in the art, contains six switches. The three phase bridge inverter is coupled to the three subset coils of a coil set 60 to form a three phase electric motor configuration. Accordingly, as stated above, the motor includes eight three phase sub-motors, where each three phase sub-motor includes a control device 64 coupled to the three sub-set coils of a coil set 60.

Each three phase bridge inverter is arranged to provide PWM voltage control across the respective coil sub-sets 61, 62, 63 to generate a current and provide a required torque for the respective sub-motors.

For a given coil set the three phase bridge switches of a control device 64 are arranged to apply a single voltage phase across each of the coil sub-sets 61, 62, 63.

The phase angle of the resulting current flow in each coil sub-set is separated by 120 degrees, as represented in Figure 4 by the three axis A, B, C.

The sinusoidal voltage waveforms generated by the control devices 64 are created using a Space Vector modulation technique known as Field Orientation Control, where the rotor flux and stator currents are represented by respective vectors .

As illustrated in Figure 4, currents ia, ib, ic represent the instantaneous current in the stator coils in the A, B, and C axis of a three phase current reference frame, where the stator current vector is defined by ⁵ s = ^£ s + ®i t> , where a = e(i*2*pi/3) . Field Oriented Control is based on projections that transform a three phase time and speed dependent system into a two co-ordinate time invariant system, where a stator current component is aligned with a quadrature axis q and a magnetic flux component is aligned with a direct axis d.

Field Orientated Control algorithms utilize Clarke and Park transforms to transform the three phase voltage and current of a three phase motor into a two dimension co-ordinate system.

The Clarke transforms use the three phase current ia, ib, and ic to calculate currents in the two phase orthogonal stator axis ia and ίβ. A Park transformation is then used to transform the two fixed co-ordinate stator axis ia and ίβ to a two co-ordinate time invariant system id and iq, which defines a d, q rotating reference frame. Figure 5 illustrates the relationship of the stator current in the d, q rotating reference frame with respect to the two phase orthogonal stator axis ia and ίβ and the a, b and c stationary reference frame.

Under normal drive conditions the rotor phase angle 9 _r , which is defined by the rotor magnetic flux vector Ψ _κ , and the stator electrical phase angle 9 _e are aligned with the d- axis, thereby maintaining synchronization between the rotor phase angle 9 _r and the stator electrical phase angle 9 _e . For the purposes of the present embodiment, the rotor phase angle 9 _r is measured using the rotor commutation magnets and position sensors mounted on the control devices 80, as is well known to a person skilled in the art.

To obtain maximum torque for a given current flow the stator current vector I _s is set at substantially 90 degrees relative to the stator electrical phase angle θ _θ and aligned with the q-axis. To obtain maximum drive torque when driving the electric motor the stator current I _s leads the rotor magnetic flux vector Ψ _κ by 90 degrees. To obtain maximum braking torque the stator current vector I _s lags the rotor magnetic flux vector Ψ _κ by 90 degrees, that is to say the rotor magnetic flux vector Ψ _κ is arranged to rotate away from the stator current vector I _s with the stator current vector I _s following the magnetic flux vector Ψ _κ by 90 degrees.

During a braking manoeuvre, to avoid a braking torque being applied to the rotor that results in the associated vehicle moving in an opposite direction once the vehicle has stopped moving, the phase angle between the stator electrical phase angle 9 _e and the rotor phase angle 9 _r becomes de- synchronised, as described in detail below.

During a braking manoeuvre, for example after a driver of the vehicle has applied pressure to a brake pedal, when the speed of rotation of the electric motor rotor falls below a pre-determined speed, the control devices are arranged to enter a zero braking mode. Although the present embodiment describes the brake operation being controlled by the respective sub motor control devices, the brake operation can be controlled in other parts of the drive system, for example a central controller.

In the zero braking mode, while the wheel, and hence the electric motor rotor, is moving and the rotor magnetic flux vector Ψ _κ continues to rotate away from the stator current vector I _s , the stator electrical phase angle 9 _e is set equal to the rotor phase angle 9 _r to maintain 90° of separation and hence provide maximum braking torque. In this configuration the stator electrical phase angle 9 _e and rotor phase angle 9 _r are synchronised. Figure 6 illustrates a phase diagram of the electric motor when operating in this stage of the zero braking mode, where the electric motor is rotating in a clockwise direction with both the rotor and stator fields synchronised.

When, as a result of the braking torque, the rotor is nearly at rest the braking torque will eventually cause the

magnetic flux vector Ψ _κ to start to rotate in an opposite direction. That is to say, the magnetic flux vector Ψ _κ starts to rotate in the direction of the applied braking torque. Upon the occurrence of this event the stator electrical phase angle 9 _e and rotor phase angle 9 _r become de-synchronised with the stator electrical phase angle 9 _e being frozen and the magnetic flux vector Ψ _κ being allowed to rotate towards the stator current vector I _s , as

illustrated in Figures 7a and 7b. As a consequence, the angle between the rotor magnetic flux vector Ψ _κ and the stator current vector I _s becomes less than 90 degrees. The sign of the stator current vector I _s is also frozen, which in the embodiment shown in figure 7a and 7b is along the positive q-axis. As the difference in phase angle between the magnetic flux vector Ψ _κ and the stator current vector I _s reduces from 90° to 0°, the force interaction between the rotor and stator fields moves from tangential to radial. This results in a reduction in braking torque applied to the rotor, which reduces to zero when the magnetic flux vector Ψ _κ and the stator current vector I _s become aligned, as illustrated in Figure 8. If a load torque exists on the rotor that results in the magnetic flux vector Ψ _κ rotating past the stator current vector I _s , the rotor magnetic flux vector Ψ _κ is initially allowed to rotate past the stator current vector I _s . The further the rotor magnetic flux vector Ψ _κ rotates from the stator current vector I _s , the greater the attractive

tangential force between them, until it reaches a peak when the difference in phase angle between magnetic flux vector R and the stator current vector I _s reaches 90°. In this case, the brake torque has naturally changed direction, whilst the sign of the stator current vector I _s remains the same .

If the phase angle between the magnetic flux vector Ψ _κ and the stator current vector I _s reaches 90 degrees and the rotor continues to rotate, the control device is arranged to allow the stator current vector I _s to move with it to maintain 90° of separation, and hence maximum braking torque is applied. In this case, 9 _e = 9 _r - 180°, and the rotor magnetic flux vector Ψ _κ is aligned with the negative d-axis. That is to say, the rotor phase angle 9 _r and the stator phase angle 9 _e have become synchronised again but with the phase shifted by 180°. As the speed of rotation of the rotor increases above another predetermined limit, the control device leaves the zero braking mode and returns to normal braking mode. In normal braking mode, the sign of I _s is calculated such that a braking torque that opposes the direction of rotation is produced. That is to say, the stator phase angle 9 _e is once again set equal to the rotor phase angle 9 _r in order to align the rotor magnetic flux vector Ψ _κ with the d-axis of the d-q reference frame, thereby establishing

synchronisation and phase alignment between the rotor phase angle 9 _r and stator phase angle 9 _e again, as illustrated in Figure 9.

Accordingly, by changing the stator phase angle 9 _e by 180° this is equivalent to changing the sign of the stator current vector, thereby resulting in a seamless transition between normal and the zero braking mode.

The process steps associated with the zero braking mode are illustrated in Figures 10 and 11.

Figure 10 illustrates the steps for determining whether the electric motor should be operating in the zero braking mode, where if the electric motor is operating in a braking mode a determination is made as to the speed of the rotor. For the purposes of the present embodiment the predetermined speed for transitioning from the normal braking mode to the zero braking mode is 10 rpm with the predetermined speed for transitioning from the zero braking mode to the normal braking mode being 15 rpm, however any speeds can be

selected. Accordingly, if the speed of the rotor is

determined to be less than 10 rpm the zero braking mode is initiated. Once the rotor speed has increased above 15 rpm the zero braking mode is exited.

Once in zero braking mode a determination is made as which direction braking torque is being applied.

Upon determining that the magnetic flux vector _R is

starting to rotate in the direction of the applied braking torque the stator phase angle 9 _e is frozen, thus freezing the stator magnetic field phase angle 9 _e and the rotor phase angle 9 _r and stator phase angle 9 _e become desynchronised . Otherwise, if the motor is braking clockwise, a. If rotor phase angle 9 _r moves > 0° from stator angle 9 _e , set the stator angle 9 _e to rotor angle 9 _r . b. If rotor phase angle 9 _r moves < -180° from

stator angle 9 _e , set the stator angle to rotor angle 9 _r + 180° .

If the motor is braking anticlockwise, c. If rotor phase angle 9 _r moves < 0° from stator angle 9 _e , set the stator angle 9 _e to rotor angle. d. If rotor phase angle 9 _r moves > 180° from stator angle 9 _e , set the stator angle 9 _e to rotor angle 9 _r - 180°.

In this way the stator current vector I _s follows the rotor magnetic flux vector _R by up to ±90°, thereby providing maximum braking torque.

When there is no load on the motor, the rotor will align with the stator field at stator phase angle 9 _e + or - 90°, depending on the direction of rotation when entering the zero braking mode.

Torque demand affects stator field strength only, that is to say the magnitude of the stator current vector, therefore changes in torque demand sign is ignored.

An embodiment of the control device will now be described with reference to different motor configurations.

Figure 12 illustrates the electric motor shown in Figure 6, where each control device bridge inverter is coupled to their respective coil sub-sets to form a wye configuration. Figure 13 illustrates the electric motor shown in Figure 6, where each control device bridge inverter is coupled to their respective coil sub-sets to form a delta configuration .

As illustrated in Figure 14, the control device 80 includes a first circuit board 83 and a second circuit board 82. Preferably the second board 82 is arranged to overlay the first circuit board 83, as illustrated in Figure 14.

The first circuit board 83 includes a plurality of switches that are arranged to apply an alternating voltage across the respective coil sub-sets. The switches can include semiconductor devices such as MOSFETs or IGBTs. In the present embodiment the switches comprise IGBT switches.

As described above, the plurality of switches are configured to form an n-phase bridge circuit. Accordingly, as is well known to a person skilled in the art, the number of switches will depend upon the number of voltage phases to be applied to the respective sub motors. In the present embodiment, in which the control devices and coil sub-sets are configured to form a three phase motor, the first circuit board 83 of the respective control devices include six switches. Although the current design shows each sub motor having a three phase construction, the sub motors can be constructed to have two or more phases.

The wires (e.g. copper wires) of the coil sub-sets can be connected directly to the switching devices as appropriate.

The second circuit board 82 includes a number of electrical components for controlling the operation of the switches mounted on the first circuit board 83. Examples of electrical components mounted on the second circuit board 82 include control logic for controlling the operation of the switches for providing PWM voltage control and interface components, such as a CAN interface chip, for allowing the control device 80 to communicate with devices external to the control device 80, such as other control devices 80 or a master controller. Typically the second control board 82 will communicate over the interface to receive torque demand requests and to transmit status information.

As mentioned above, the second circuit board 82 is arranged to be mounted on top of the first circuit board 83, where the first circuit board 83 and the second circuit board include means for being mounted within the motor 40, for example, adjacent to the coil sub-set which they control, directly to a cooling plate. In the illustrated example, these means include apertures 84 through which screws or suchlike can pass. In this example, the first circuit board 83 and the second circuit board 82 are substantially wedge- shaped. This shape allows multiple control devices 80 to be located adjacent each other within the motor, forming a fanlike arrangement. By separating the control logic from the switches this has the advantage of thermally isolating the control logic from the switches while also minimizing the impact of any electrical noise generated by the switches.

Also mounted on each of the circuit boards is a position sensor that is used for determining the position of the rotor 240, for example a hall sensor that is arranged to generate an electrical signal dependent upon the relative position of the focusing ring and magnets 227 that is mounted on the rotor 240. To determine the direction that the rotor is turning in the circuit boards preferably have two sensors that are offset by a predetermined angle so that the changes in signal from each of the sensors can be analyzed to determine both the relative position of the rotor 240 and the direction of rotation of the rotor. To allow each control device, and hence each sub motor, to operate independently of each other each circuit board has their own set of position sensors. However, a single set of position sensors could be used.

Figure 15 illustrates six switches of the first circuit board arranged in an 3 phase bridge configuration that are coupled to the coil sub-sets of a coil set that are placed in a wye configuration. The six semiconductor switches are connected to a voltage supply, for example 300 volts, and to ground. Pairs of the respective coil sub-sets are connected between two legs of the bridge circuit. Simplistically, to operate the motor and supply a voltage in one direction, the switches are operated in pairs, one in the top half of the bridge and one from a different leg in the bottom half of the bridge. Each switch carries the output current for one third of the time.

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. As described above, 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, where the PWM voltage is determined based upon a received torque demand request. The PWM voltage in turn determines the coil current and hence the produced torque.