GB2478361A - Electric motor torque control using temperature input signal - Google Patents

Abstract

An electric motor control system comprises a controller 100 having a temperature signal input to receive a temperature signal indicating the temperature of a component (e.g. coil 10, coil switch 80 or permanent magnet 11) of the motor 1 from temperature sensors 12, 14, 82. The controller has an output to control the torque provided by the electric motor. When the temperature is above a threshold value the controller varies the maximum torque that may be delivered by the motor as a continuous function of the temperature indicated by the temperature signal from the component. The electric motor may be used in an electric vehicle, particularly where the motor is mounted within the wheel of the vehicle.

Description

METHOD AND APPARATUS FOR ELECTRIC MOTOR CONTROL

FIELD OF THE INVENTION

The invention relates to electric motors and in particular to methods and apparatus for control of electric motors for use in electric vehicles.

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.

A three phase motor is a well-known arrangement but it will now be described for completeness. The motor comprises three coils 10 connected in a so-called Y configuration. A three phase supply 20 is connected to be coils, with each phase of the supply connected to each respective coil. The phases of the supply are 120° shifted in phase with respect to one another. As a result, a circulating magnetic field which rotates as shown by the arrows is created. Permanent magnets which may be mounted inside or outside of the coils are thereby caused to rotate with respect to the coils. The rotating part (the rotor) may be the coils or the permanent magnets and the static part (the stator) will be the opposite.

Figure 2 shows a 3 phase motor 40 having eight coils in each phase. 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. Each coil sub-set can be connected to a respective control device.

The terminals for each coil sub-set of each coil set are labelled 54, 56 and 58, respectively in Figure 2. To supply current to each phase, the coils of a given phase may be connected together to a single power source. Alternatively, each coil subset may have a completely separate control device. As is known to the skilled person, a ring of magnets is arranged either inside or outside of the coils.

For a three phase electric motor, the switching system is usually a three phase bridge circuit including a number of switches. Typical power electronic switches including the Metal Oxide Silicon Field Effect Transistor (MOSFET) and the Insulated Gate Bipolar Transistor (IGBT) exhibit two principal losses: switching losses and conduction losses.

When running, a motor has energy losses which are generally dominated by copper losses that increase proportionally to coil temperature and torque squared, as well as other losses in the power devices, electromagnetics etc. This imposes serious thermal management problems for the motor as the losses manifest themselves as heat energy within the motor. Extracting this heat without elevating the temperature of the device above its safe operating level becomes the limiting factor in what power the device can handle.

Inefficiencies in electric motors result in heat energy that must be dissipated to the surroundings. If heat losses are greater than heat dissipation then the motor temperature will rise. Traditionally this results in two motor power/torque ratings -peak and continuous. The peak rating is the maximum motor performance when the temperature is below a predetermined critical value, this however is unsustainable, and so a continuous rating is specified, which is the maximum performance the motor can sustain without exceeding the critical temperature value.

Prescribing a single figure for an electric machine's continuous rating or peak performance duration results in a performance compromise. The continuous rating or peak duration must be given for the worst-case environmental conditions where the motor can be expected to run continuously, this means that when external conditions are more favourable, the motor is operating at a point somewhat less than its thermal limits could otherwise tolerate. With reference specifically to vehicle traction motors, they must work reliably in widely varying conditions. If traction motors were designated ratings for 70C ambient the vehicle would potentially lose an unacceptable amount of performance at lower temperatures. In contrast, if a motor was designated ratings for a lower temperature in order to improve performance, motor damage is likely to result in less favourable conditions.

A known way of protecting a motor is the so-called I squared T method (12T). This method is described in "Motor Control Tech Tips volume 3, issue 20 February 2004, Hitachi of America". In brief, this technique continuously monitors the motor winding current values and evaluates the overload energy in real-time.

The overload energy is calculated as lp2T -lc2T, where Ip is the peak current, Ic is the continuous current and T is the duration of the transient in seconds.

Accordingly, the overload energy is measured directly from the current and the duration of a transient. When the evaluated 121 value exceeds a predetermined value, a protection circuit limits the current supply to the motor. Note that this control method requires a defined continuous limit, which, as discussed previously, can lead to peak performance compromises.

A further way of controlling a motor is described in US 2008/031 5814. The arrangements described measure the temperature of a motor and, if the temperature exceeds a specified value, a torque control unit reduces the torque control value to limit the torque output from the motor. The torque is reduced with time until a specified level is reached. A control unit then checks to see if the temperature threshold is still exceeded and, if so, the torque control unit continues to reduce the torque.

We have appreciated the need for proper thermal management of an electric motor. We have particularly noted the need for such thermal management in electric motors that are mounted within a wheel of an electric vehicle.

SUMMARY OF THE INVENTION

The invention is defined in the claims to which reference is now directed.

In broad terms, the invention relates to a control method and apparatus for controlling an electric motor of an electric vehicle taking into account the temperature of one or more components within the electric motor. In particular, there is provided an electric motor control system comprising a controller having a temperature signal input, and an output to control the torque provided by the electric motor, the controller being configured to receive a temperature signal indicative of the temperature of at least one component of the electric motor, and configured such that when the temperature signal indicates that the temperature of the at least one component is above a threshold value the controller varies the torque that may be delivered by the motor in accordance with a predetermined relationship between torque and the temperature indicated by the temperature signal from the at least one component.

An embodiment of the invention has the advantage of not requiring a prescribed limit for either the maximum time a motor may provide a particular torque output, or the continuous torque output itself. This allows the motor to always use the cooling system most effectively with resulting performance and life benefits. This embodiment results in a dynamically changing maximum torque duration and a dynamically changing continuous rating, which are the result of the control strategy, rather than an input to it.

The manner in which the torque delivered is varied as a function of temperature may be achieved in a number of ways.

Preferably, the maximum torque is varied as a linear function of the temperature of the at least one component between the threshold temperature and a maximum temperature. This ensures that the thermally critical components within a motor neither under-use their cooling means (which would reduce performance) nor do they exceed their maximum temperatures (which would result in unnecessary damage). In this arrangement, if the operator is demanding the maximum torque then the torque delivered will be reduced as soon as the threshold temperature is reached.

In another preferred embodiment, in distinction to limiting the maximum torque, the control method and apparatus is configured to apply a torque scaling factor (as a function of temperature) after the threshold temperature has been exceeded, thus resulting in a torque reduction and driver awareness of the motor temperature without regard to the amplitude of the actual torque demand. In this arrangement, the torque delivered will be scaled (reduced) in comparison to the torque demanded if the component is above the threshold temperature, even if the maximum torque is not currently demanded.

In all such arrangements, a threshold temperature is used to govern the point at which the torque is reduced in a controlled way, the reduction being related to the temperature of the component. For the avoidance of doubt, the threshold and/or maximum temperatures may be fixed values chosen for the component or may be variable in some manner. One such manner of variation is a so called "overload condition" in which the threshold and/or maximum temperatures may be increased for a period of time and then reduced again in a controlled manner to allow for temporary increases in peak torque duration, for example when overtaking. The embodiments may also be used with a motor in a braking mode and the techniques described apply equally to such an arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figure 1 schematically shows an example arrangement for a three phase motor; Figure 2 schematically shows a three phase motor with eight coil subsets for each phase; Figure 3 schematically shows a control system and electric motor comprising the invention; Figure 4 is a graph of maximum motor torque as a function of temperature; Figure 5 is a graph showing one example of how the motor current is controlled as a result of the function shown in figure 4; Figure 5A shows how the threshold and maximum temperatures shown in figure 4 may vary with respect to time before, during and after an overload" condition.

Figure 6 is a graph of the heat power in/out of a coil against coil temperature and maximum torque when controlled using the arrangement shown in figure 4, for typical heat rejection conditions; Figure 7 is a flowchart showing the process implemented in a control circuit that limits maximum torque demand; Figure 8 is a flowchart of the process for temperature sensing of multiple components; and Figure 9 is an exploded perspective view of a motor embodying the invention.

DETAILED DESCRIPTION

An embodiment of the invention comprises a control system for controlling the torque that may be delivered by an electric motor driving an electric vehicle.

The invention may also be embodied in an electric motor or in a vehicle using such an electric motor. The invention is particularly applicable to the type of electric motor mounted within the wheel of a vehicle.

Figure 3 shows an electric motor and control system embodying the invention. As previously described in relation to figure 1, a 3 phase motor 1 comprises coils 10 arranged as a standard three phase configuration in a so-called Y configuration. Whilst a three-phase arrangement is preferred, any number of phases could be used. In addition, a different number of coils could be used for example using eight coil subsets for each phase as in figure 2. To supply current to the coils, each coil is connected to a respective control circuit 80. The individual control circuits 80 are connected to a controller 100. The controller 100 is connected to a demand device 102, typically a control pedal such as an accelerator pedal or brake pedal.

In operation, a user requests acceleration using the demand device 102 which asserts a signal to the controller 100. The controller includes hardware and software to interpret the demand signal and consequently to request a particular current to be supplied by each of the control devices 80. The control devices then supply an appropriate level of current to each of the respective coil groups.

Almost all electronic control units for electric motors today operate by some formof 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. Whilst the switch is closed, the current rises in the motor winding at a rate dictated by its inductance, resistance and the applied voltage. If a duty cycle of less than 100% is required, the closed period is followed by an open period, in which the current decays by freewheeling' through a diode. This switch cycling occurs with enough frequency so that precise control of the current is achieved. The control circuits 80 use this PWM technique to supply the appropriate current to each coil. Each control circuit comprises a transistor switching arrangement, typically a FET switch or IGBT as previously described.

The temperature control aspects of the control system will now be described. As can be seen in figure 3, each of the coils 10 has an associated temperature sensor 12 connected by lines 16 to input 17 of the controller 100. In addition, each of the control circuits 80 has an associated temperature sensor 82 which connects via lines 84 to an input 85 of the controller. Lastly, the permanent magnets, here shown schematically by the dotted line 11, have a temperature sensor or 14 connected via a line 22 to an input 23 of the controller.

By measuring the thermal state of critical motor components (e.g. transistors, coils, magnets etc.), the torque demand that is passed to the motor is reduced as a function of temperature after a certain temperature 01 of the respective component has been exceeded, to zero torque at an upper critical value 9max' which the component therefore cannot exceed. The motor operating envelope when controlled using this strategy is shown in figure 4 As an example, for a motor coil, if the maximum motor torque is 800 Nm, 91 = 170C and Omax 180C, the function that determines the maximum torque demand as a function of coil temperature, , is 800 w/zereO «=170 f(°)= 80x(18O-9) where17O<9<l80.

0 where9 »= 180 The function may be implemented in the controller in a variety of ways, but preferably comprises a lookup table storing the direct relationship between the temperature and the torque that may be provided by the motor. Alternatively, the torque could be computed as a function of the temperature. As can be seen in figure 4, full torque may be provided by the motor up until the threshold temperature is reached. At this point, the maximum torque is reduced as a function of the temperature, in this case a linear relationship between the relative maximum torque and the temperature between a threshold temperature and a maximum temperature. For the avoidance of doubt, other functions could be possible having a predetermined relationship between specific temperature and maximum torque values. By reducing torque linearly based on temperature the torque and temperature will be stabilised, thereby avoiding the risk that torque will reduce to zero as can occur in prior known systems.

Between temperatures 81 and Omax two potential control strategies shall now be described with reference to figure 4 which shows the maximum motor torque as a function of temperature by lines A and B, and lines of constant torque demand C -K, where F -H and I -K are the result of two different control strategies applied after threshold temperature 01.

The first strategy limits the motor maximum torque only. Consider the lines of constant torque demand C-E and F-H. For a demand of less than 100%, the motor torque demand is passed without change until the temperature is such that the line B is reached. For example, consider a torque demand of 75%. II this torque demand would result in the temperature rising above 9max tthe motor will operate along lines C and F until the temperature climbs above the intersection of lines F and B, at which point the torque demand shall be reduced in accordance with the function that defines line B, and a continuously sustainable torque shall be reached at a temperature below °max An advantage of the maximum torque limitation approach is that the motor provides the demanded torque as long as a critical component is not operating outside of line B even though it may be operating at a temperature above 91, The second example strategy applies a scaling factor to the demanded torque. Consider lines of constant torque demand C-E and I-K. This method scales down the demanded torque between 91 and 9max, using this strategy, a demand of 75% results in the motor operation following line C to line I. Again assuming that a demand of 75% of full torque would result in a critical component rising in temperature to above 9rnax, the motor will operate along line I and a continuously sustainable torque is reached at a temperature below 9max An advantage of the scaling factor approach is that regardless of the torque demanded, it will be scaled down after the threshold temperature has been exceeded. For example, this would be useful in giving the driver of a car a physical change in accelerator pedal and vehicle feel, which would act as a warning that the drive motor(s) are hot, whilst also providing the same benefits of the maximum torque limitation approach when the driver demands 100%.

Figure 5 shows how torque (current) and coil temperature vary during a typical 100% torque demand event, where this control strategy is applied with respect to motor coil temperature, and 81 = 170C and Oma = 180C. A brief description of the figure follows. An initial increase in temperature is shown, with a reduction in torque, when a threshold temperature has been reached (0-12s), then an overload event where an increase in torque is allowed to occur at the detriment of motor temperature (12-20s), and then the transition from the overload condition back to the normal' condition (20-28.5s). Note that the driver can be assumed to be demanding 100% torque for this entire figure and therefore both the maximum torque and torque scaling factor control strategies will result in the same plots.

Initially at time T=0, as the coil temperature is below the threshold, maximum current is provided to the coils in order to provide maximum torque. However, we assume that the driving conditions are such that the temperature of the components being measured, in this case the coil, steadily increases until it reaches the threshold temperature of 170°C. At this point, the torque that may be provided is reduced in accordance with the function shown in figure 4, so that the coil current is also reduced until the temperature is maintained below 8max Consequently, the torque and coil temperature are stabilised at a torque rating that is suited to the heat rejection capability of the cooling system at the given nominal ambient conditions. Although figure 5 illustrates variation of torque based on coil temperature, other component temperature values could be used or a combination of different electric motor component temperature values. As shown in the figure, there is a momentary overshoot caused by thermal resistances between the coil and the temperature transducer, before settling to a continuous limit appropriate to the conditions that does not allow the coil temperature to exceed the maximum temperature.

In addition to measuring the temperature of individual components as the mechanism to control the maximum torque generated, other factors may be used within the controller such as the time for which a particular torque has been demanded or the manner in which the torque is demanded (such as a rate of change of torque demand). For example, the motor will still be physically capable of producing full torque if the coil temperature is between 170-180C and above if required. Different lookup tables from figure 4 could be used if, for example, driver accelerator pedal effort was measured to be unusually high, in which case excursions to, for example, a coil temperature of 190-200C may be allowed. This is shown in figure 5 at a time 12 seconds. In this "overload condition", the result is a further burst of full torque, trading this increased performance for a rise in coil temperature. After the overload temperature threshold has been reached at approximately 15s, the coil current is reduced until it steadies at a continuous limit between 190C and 200C. No further time at maximum torque may be demanded due to the increased rate of motor thermal damage, but this new continuous limit may be demanded for as long as the conditions for overload are still met.

In this example, the conditions for overload are removed at 20secs, this may be the driver's high pedal effort being released, or a time period elapsing, but it is important to note that in this illustration the driver is still demanding 100% torque. The motor coil now has a temperature debt' which must be dealt with in a controlled manner for reasons of motor life. The only way to reduce the temperature of the coil is to reduce the coil current (and hence torque), even though the driver may still be applying a 100% torque demand. Between 20 <t < 27 seconds, the threshold and maximum temperatures are steadily reduced back to the initial conditions of 170 and 180C respectively. This results in a reduction in torque and consequently cooling of the motor. At t 27 seconds, the temperature of the motor has been reduced over the 7 second period, and the threshold and maximum temperatures are fixed at 170 and 180 C. The so called overload condition is further shown by the torque temperature and time graph of figure 5A. As already explained in relation to figure 4, when a threshold temperature is reached, the maximum torque that may be demanded is reduced linearly as a function of temperature. This function shown in figure 4 can be seen at the time origin of the three dimensional graph of figure 5A. During the normal operation mode, shown as the first section of time, this relationship stands. However, when an overload condition is reached, the threshold and maximum temperatures are increased above the normal operating values, thereby allowing a higher torque value to be demanded until a new steady state temperature is reached between the increased threshold and maximum torque values. This would be particularly useful when a driver wishes to "accelerate out of trouble", allowing a burst of full -10-torque even though the motors may already be operating at a continuous limit at a temperature between the threshold and maximum temperatures. After a resulting period of time, the overfoad condition finishes and the threshold and maximum temperatures are reduced in a controlled manner in the controlled transient stage back down to the normal threshold and maximum temperature values in a normal operation mode. During this controlled transient stage, the maximum torque will be gradually reduced until the motor settles at a temperature between the original threshold and maximum temperature values.

In the overload mode shown in figure 5A, the transition between normal mode and overload and back again may be sudden or linear as shown or could be some other curve to transition between the modes.

Figure 6 illustrates how this control strategy allows a motor to find' a continuous limit in accordance with the cooling capability at that moment in time.

What is shown is how a heat power -vs-coil temperature plot may look for a typical wheel-motor coil when at maximum demand and controlled using this strategy. The elbow in the power loss curve at 170C is due to the change in maximum demanded torque at this temperature. The heat extraction curve is set by heat rejection rates. Where the two curves cross is where the continuous motor rating lies. In this case -174C, which as shown in Figure 4 corresponds to a torque of approximately 0.5 (relative units). When cooling conditions give higher heat rejection rates, the heat extraction curve will move upwards' and vice versa when the external conditions are less favourable. This changes the intersection point of the curves and therefore the continuous rating of the motor.

Accordingly, the embodiment of the invention ensures that the electric motor can operate appropriately in different environments.

Accordingly, differing look up curves will be provided for each of the components for which temperature is a critical factor in motor performance, life, or otherwise. A look up curve similar to that used for the coils may be provided for the FET switches, but typically at a lower temperature so that, for example, the maximum torque that may be demanded is reduced linearly down to 0 if the FET temperature reaches 1100 C. Similarly, a look up table may be provided for the permanent magnets which would be similar to the lookup table for the coils.

Again, the particular temperature at which the torque would be reduced to 0 will depend upon the temperature at which the permanent magnets would either be damaged or reduced in efficiency or lifespan.

The method by which the controller would limit the maximum torque based on the temperature measurements is as follows, as shown in figure 7.

The temperature of the various components within the motor, in particular the coils the switches and the permanent magnets, is measured by the various temperature sensors and provided at inputs of the controller 100. The controller monitors the temperature of each component and compares the temperature to the corresponding look up table. If the temperature reaches the respective threshold temperature for the component being monitored, then the controller determines if the torque demanded (Tdemin) is greater than the permitted torque based on the function of the temperature f(Dc). If the torque demand is greater than the permitted torque, the output torque demand signal is set to equal the permitted torque Tdemout = f(Dc). The maximum torque is directly related to the maximum current and so by controlling the maximum current the maximum torque that may be demanded of the motor is controlled. In this way, the controller will reduce the current that can be demanded of the control circuits 80. If the temperature is below the threshold value, or the torque demanded is below the allowed torque based on the function of the temperature, then the torque demanded is provided as the torque demand signal to the motor.

The embodiment of the invention allows the temperature of at least one component to cause the maximum torque to be reduced as described above.

The preferred embodiment allows the temperature of multiple components within the motor to restrict maximum torque or to result in the downscaling of torque in a similar manner. The logic for multiple components is shown in figure 8. The controller comprises a function for each of the components being monitored, here shown as multiple look up tables LUT, each of which determine the torque demand that is passed to the motor as described above in relation to figure 7. Further logic then passes the most conservative (the lowest) torque demand to the motor. In this way the logic is extensible so that multiple components may be measured and any one of those components may cause the maximum torque that may be demanded to be reduced.

The physical arrangement of the embodying assembly is best understood with respect to Figure 9. The assembly can be described as a motor with built in -12-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 9, 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 teeth are shown as a ring 235 and fits to the assembly 231. 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.

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 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 223 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 embodiment of the invention may use various types of temperature sensors for the different components. For example, within the windings of one or more of the coils, a thermocouple may be used to measure the temperature of the coils. The temperature of each individual coil may be taken, the temperature of a representative coil in each of the three phases or simply the temperature of a single coil in the whole motor. The temperature of the magnets could also be measured in various ways. A thermocouple may be used on one or more of the magnets and connected via a rotating electrical or wireless link to the controller.

Alternatively, other noncontact solutions may be used such as measuring the flux from the magnets as an indirect measure of the temperature or by remote temperature measurements such as using a laser. The temperature of each control circuit 80, or a single representative control circuit 80 may similarly be measured, typically using a thermocouple or other measurement device such as thermistors.

-14 -

Claims (16)

1. SCLAIMS1. An electric motor control system comprising a controller having a temperature signal input, and an output to control the torque provided by the electric motor, the controller being configured to receive a temperature signal indicative of the temperature of at least one component of the electric motor, and configured such that when the temperature signal indicates that the temperature of the at least one component is above a threshold value the controller varies the torque that may be delivered by the motor in accordance with a predetermined relationship between torque and the temperature indicated by the temperature signal from the at least one component.
2. 2. An electric motor control system according to claim 1, wherein the controller limits the motor maximum torque between the component threshold temperature and the component maximum temperature
3. 3. An electric motor control system according to claim 1, wherein the controller applies a torque scaling factor between the component threshold temperature and the component maximum temperature
4. 4. An electric motor control system according to any preceding claim, wherein at least one component is one of a coil, a coil switch or a permanent magnet of the motor.
5. 5. An electric motor control system according to any preceding claim, wherein the controller is configured to vary the torque that may be delivered as a continuous function of the temperature.
6. 6. An electric motor control system according to claim 5, wherein the continuous function of the temperature signal includes reducing the maximum torque from a maximum value down to zero between the threshold temperature of the component and a maximum temperature for the component.
7. 7. An electric motor control system according to claim 6, wherein the function of the temperature signal includes linearly reducing the maximum torque down to zero between the threshold temperature of the component and the maximum temperature for the component.
8. 8. An electric motor control system according to any preceding claim, wherein the function of the temperature includes allowing the component threshold and maximum temperatures to be changed for a period of time.
9. 9. An electric motor control system according to claim 8, wherein the controller has a further input, and wherein the function of the temperature includes allowing the component threshold and maximum temperatures to be changed for a period of time in accordance with the further input.
10. 10. An electric motor control system according to claim 9, wherein the further input indicates the force on an accelerator control, and wherein the function of the temperature allows the component threshold and maximum temperatures to be increased for a period of time in the event that the input indicates a high force on the accelerator control.
11. 11. An electric motor control system according to claims 8, 9 or 10, wherein the component threshold and maximum temperatures are changed as a function of time.
12. 12. An electric motor control system according to claim 11, wherein the function of time includes increasing the threshold and maximum temperatures from initial values for a first period of time and then continuously decreasing the threshold and maximum temperature values back to the initial values after a second period of time.
13. 13. An electric motor control system according to any preceding claim, wherein the controller is configured to receive a temperature signal from each of two or more different components of the electric motor, wherein the function of the temperature signal is different for each of the two or more different components.
14. 14. An eiectric motor comprising a plurality of coils within a stator, a plurality of magnets within a rotor, a temperature sensor to produce a temperature signal from each of the two or more different components and a controller according to any preceding claim.
15. 15. An electric motor according to claim 14, wherein the motor is a multi-phase motor and the controller is arranged to assert a torque demand signal to a current control circuit of each phase.
16. 16. An electric motor vehicle comprising a plurality of wheels, at least one wheel having mounted therein an electric motor according to claim 14 or 15. -17-