There is a general desire to improve electric machines, such as brushless motors, in a number of ways. For example, improvements may be desired in terms of size, weight, power density, manufacturing cost, efficiency, reliability, and noise. of the Invention

According to a first aspect of the present invention there is provided a method of controlling a brushless permanent magnet motor, the method comprising: exciting a phase winding of the motor by applying a voltage to the phase winding using an inverter of the motor; turning off the inverter for a first time interval; sequentially exciting the phase winding of the motor for a plurality of excitation periods over a second time interval subsequent to the first time interval, wherein a start point of each excitation period is spaced from a zero-crossing of back EMF induced in the phase winding by an advance period; and determining the advance periods such that the advance periods vary within the second time interval.

Turning off the inverter for a brief time period during operation may be desirable to enable resynchronisation of a sensorless method of determining a position of a rotor of the brushless permanent magnet motor. However, it has been found that following a time period in which the inverter is turned off, peaks of phase current flowing through the phase winding can exceed the value of peaks of phase current that would typically be expected during steady-state operation of the brushless permanent magnet motor. This may lead to software or hardware current trips, may lead to issues with accelerating the brushless permanent magnet motor to, or maintaining the brushless permanent magnet motor at, a steady-state speed, and may even result in current peaks high enough to damage the hardware of the brushless permanent magnet motor.

By determining the advance periods such that the advance periods vary within the second time interval, current flowing through the phase winding may be managed more efficiently during the second time interval, i.e. when the motor is restarted following the first time interval in which the inverter is turned off, relative to a method where a fixed advance period is used within the second time interval. For example, by varying the advance periods within the second time interval, i.e. such that the excitation periods are advanced or retarded relative to zerocrossings of back EMF induced in the phase winding by differing amounts within the second time interval, peaks in phase current induced in the phase winding may be reduced relative to peaks in phase current experienced utilising a method in which a fixed advance period is used for each excitation period within the second time interval.

Advance periods may comprise positive values, for example such that an excitation period begins in advance of the respective zero-crossing of back EMF induced in the phase winding, or may comprise negative value, for example such that an excitation begins after the respective zero-crossing of back EMF induced in the phase winding. Where advance periods comprise negative values, they may instead be referred to as retard periods.

The second time interval may comprise a pre-determined length and/or a predetermined number of excitation periods, and may, for example, be of a sufficient length and/or a sufficient number of excitation periods for peaks in the current induced in the phase winding to return to a magnitude typically expected in steady-state operation following the turn-off of the inverter in the first time interval.

The second time interval may comprise no more than 20 excitation periods, or no more than 10 excitation periods. This may provide a sufficient number of excitation periods for current peaks to normalise following the first time interval.

The method may comprise determining the advance periods such that magnitudes of the advance periods increase within the second time interval. This may aid with reducing current peaks to a level typically expected in steady-state operation. The method may comprise determining the advance periods such that magnitudes of the advance periods increase generally linearly within the second time interval. This may provide a relatively smooth transition in current peaks within the second time interval.

The method may comprise determining the advance periods based at least in part on a reference advance period, the reference advance period based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating. The reference advance period may provide a starting point upon which the advance periods may be calculated. The reference advance period may be obtained from memory, for example from a lookup table or the like. The reference advance period may be calculated by a controller of the brushless permanent magnet motor.

The method may comprise determining the advance periods such that the advance periods converge toward the reference advance period toward an end of the second interval. This may provide sufficient time for current peaks to stabilise following the first time interval before reverting to using the reference advance period. The method may comprise determining an initial advance period associated with an initial excitation period within the second time interval based at least in part on a modified version of the reference advance period, the modified version of the reference advance period comprising the reference advance period adjusted by a scaling factor based at least in part on a speed of the motor. A magnitude of current peaks may be greatest at the initial excitation period within the second time interval. By scaling the reference advance period based on the speed of the motor, the initial current peak associated with the initial excitation period within the second time interval may be reduced relative to a method where no adjustment of the initial advance period is made relative to the reference advance period.

The method may comprise determining the advance periods based at least in part on a number of excitation periods within the second time interval. This may enable a gradual reduction in magnitude of current peaks across the second time interval.

The method may comprises determining the advance periods based at least in part on a reference excitation period duration, the reference excitation period duration based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating. The reference excitation period duration may provide a starting point upon which the advance periods may be calculated. The reference excitation period duration may be obtained from memory, for example from a lookup table or the like. The reference excitation period duration may be calculated by a controller of the brushless permanent magnet motor. The advance periods may be determined based at least in part on a percentage of a duration of the reference excitation period duration. The method may comprise determining the initial advance period associated with the initial excitation period within the second time interval to comprise a negative advance period. This may provide a greater reduction in an initial current peak associated with the initial excitation period compared to, for example, a method where the initial advance period comprises a positive value. By utilising a negative advance period, i.e. such that the initial excitation period begins after the relevant zero-crossing of back EMF induced in the phase winding, current may have less time to rise than a method where a positive advance period is utilised.

The method may comprise determining subsequent advance periods associated with subsequent excitation periods within the second time interval to comprise positive advance periods. This may enable sufficient current to be driven into the phase winding to obtain a desired motor speed, whilst utilising the initial negative advance period to inhibit the initial current peak associated with the initial excitation period.

The method may comprise determining durations of excitation periods within the second time interval such that durations of excitation periods within the time interval vary. This may facilitate management of current peaks associated with the excitation periods, for example with adjustment of the duration of excitation periods being utilised to limit the amount of current flowing through the phase winding.

The method may comprise determining a duration of an initial excitation period within the second time interval to be different to durations of remaining excitation periods within the second time interval. This may allow for mitigation of what would be the initial, typically highest, current peak within the second time interval, whilst using a constant excitation duration period for the remainder of the second time interval. The method may comprise determining a duration of an initial excitation period within the second time interval to be less than durations of remaining excitation periods within the second time interval. This may allow for mitigation of what would be the initial, typically highest, current peak within the second time interval, whilst using a constant excitation duration period for the remainder of the second time interval.

The method may comprise determining the duration of the initial excitation period based at least in part on the reference advance period, the reference advance period based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating. The reference advance period may provide a starting point upon which the duration of the initial excitation period may be calculated. The reference advance period may be obtained from memory, for example from a lookup table or the like. The reference advance period may be calculated by a controller of the brushless permanent magnet motor.

The method may comprise determining the duration of the initial excitation period based on the initial advance period associated with the initial excitation period. In such a manner both the duration of the excitation period and the timing at which the excitation period begins may be controlled to mitigate for current peaks that may typically be experienced following the first time interval.

The remaining excitation periods within the second time interval may comprise the same duration. This may simplify the method compared to a method where more than two different durations of excitation period are used within the second time interval. The method may comprise commutating the phase winding between sequential excitation periods within the second time interval. Tus the motor may still be accelerated or kept at a desired speed within the second time interval.

According to a second aspect of the present invention there is provided a brushless permanent magnet motor comprising a phase winding, an inverter, and a controller configured to perform a method according to the first aspect of the present invention.

According to a third aspect of the present invention there is provided a vacuum cleaner comprising a brushless permanent magnet motor according to the second aspect of the present invention.

According to a fourth aspect of the present invention there is provided a haircare appliance comprising a brushless permanent magnet motor according to the second aspect of the present invention.

Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate.

Brief Description of the Drawings

Figure 1 is a first schematic view illustrating a motor system;

Figure 2 is a second schematic view illustrating the motor system of Figure 1 ;

Figure 3 is a table indicating switching states of the motor system of Figures 1 and 2;

Figure 4 is a flow diagram illustrating a method according to the present invention; Figure 5 is a first schematic illustration of current waveforms obtained via use of the method of Figure 4;

Figure 6 is a second schematic illustration of current waveforms obtained via use of the method of Figure 4;

Figure 7 is a schematic illustration of a vacuum cleaner comprising the motor system of Figures 1 and 2; and

Figure 8 is a schematic illustration of a haircare appliance comprising the motor system of Figures 1 and 2.

Detailed Description of the Invention

A motor system, generally designated 10, is shown in Figures 1 and 2. The motor system 10 is powered by a DC power supply 12, for example a battery, and comprises a brushless permanent magnet motor 14 and a control circuit 16. It will be recognised by a person skilled in the art that the methods of the present invention may be equally applicable to a motor system powered by an AC power supply, with appropriate modification of the circuitry, for example to include a rectifier.

The motor 14 comprises a four-pole permanent-magnet rotor 18 that rotates relative to a four-pole stator 20. Although shown here as a four-pole permanent magnet rotor, it will be appreciated that the present invention may be applicable to motors having differing numbers of poles, for example eight poles. Conductive wires wound about the stator 20 are coupled together to form a single-phase winding 22. Whilst described here as a single-phase motor, it will be recognised by a person skilled in the art that the teachings of the present application may also be applicable to multiphase, for example three-phase, motors. The control circuit 16 comprises a filter 24, an inverter 26, a gate driver module 28, a current sensor 30, a first voltage sensor 32, a second voltage sensor 33, and a controller 34.

The filter 24 comprises a link capacitor C1 that smooths the relatively high- frequency ripple that arises from switching of the inverter 26.

The inverter 26 comprises a full bridge of four power switches Q1-Q4 that couple the phase winding 22 to the voltage rails. Each of the switches Q1 -Q4 includes a freewheel diode.

The gate driver module 28 drives the opening and closing of the switches Q1 -Q4 in response to control signals received from the controller 34.

The current sensor 30 comprises a shunt resistor R1 located between the inverter and the zero-volt rail. The voltage across the current sensor 30 provides a measure of the current in the phase winding 22 when connected to the power supply 12. The voltage across the current sensor 30 is output to the controller 34 as signal, l_SENSE. It will be recognised that in this embodiment it is not possible to measure current in the phase winding 22 during freewheeling, but that alternative embodiments where this is possible, for example via the use of a plurality of shunt resistors, are also envisaged.

The first voltage sensor 32 comprises a voltage divider in the form of resistors R2 and R3, located between the DC voltage rail and the zero-volt rail. The voltage sensor outputs a signal, V_DC, to the controller 34 that represents a scaled-down measure of the supply voltage provided by the power supply 12.

The second voltage sensor 33 comprises a pair of voltage dividers constituted by resistors R4, R5, R6, and R7, that are connected either side of the phase winding 22. The second voltage sensor 33 provides a signal indicative of back EMF induced in the phase winding 22 to the controller, as bEMF.

The controller 34 comprises a microcontroller having a processor, a memory device, and a plurality of peripherals (e.g. ADC, comparators, timers etc.). In an alternative embodiment, the controller 34 may comprise a state machine. The memory device stores instructions for execution by the processor, as well as control parameters that are employed by the processor during operation. The controller 34 is responsible for controlling the operation of the motor 14 and generates four control signals S1 -S4 for controlling each of the four power switches Q1 -Q4. The control signals are output to the gate driver module 28, which in response drives the opening and closing of the switches Q1-Q4.

During normal operation, the controller 34 estimates the position of the rotor 18 using a sensorless control scheme, ie without the use of a Hall sensor or the like, by using software to estimate a waveform indicative of back EMF induced in the phase winding 22 via the signals V_DC and l_SENSE. In particular, zerocrossings of back EFM induced in the phase winding 22 can be estimated to estimate aligned positions of the rotor 18. The details of such a control scheme will not be described here for the sake of brevity, but can be found, for example, in published GB patent application GB2582612. Another sensorless control scheme that utilises hardware components to estimate back EMF induced in the phase winding 22 is disclosed in published PCT patent application WO2013132247A1 . With knowledge of the position of the rotor 18 in normal operation, the controller 34 generates the control signals S1 -S4.

Figure 3 summarises the allowed states of the switches Q1-Q4 in response to the control signals S1 -S4 output by the controller 34. Hereafter, the terms 'set and 'clear' will be used to indicate that a signal has been pulled logically high and low respectively. As can be seen from Figure 3, the controller 34 sets S1 and S4, and clears S2 and S3 in order to excite the phase winding 22 from left to right. Conversely, the controller 34 sets S2 and S3, and clears S1 and S4 in order to excite the phase winding 22 from right to left. The controller 34 clears S1 and S3, and sets S2 and S4 in order to freewheel the phase winding 22. Freewheeling enables current in the phase winding 22 to re-circulate around the low-side loop of the inverter 26. In the present embodiment, the power switches Q1-Q4 are capable of conducting in both directions. Accordingly, the controller 34 closes both low-side switches Q2,Q4 during freewheeling such that current flows through the switches Q2,Q4 rather than the less efficient diodes.

Conceivably, the inverter 26 may comprise power switches that conduct in a single direction only. In this instance, the controller 34 would clear S1 , S2 and S3, and set S4 so as to freewheel the phase winding 22 from left to right. The controller 34 would then clear S1 , S3 and S4, and set S2 in order to freewheel the phase winding 22 from right to left. Current in the low-side loop of the inverter 26 then flows down through the closed low-side switch (e.g. Q4) and up through the diode of the open low-side switch (e.g. Q2).

Appropriate control of the switches Q1 -Q4 can be used to drive the rotor 18 at speeds up to or in excess of Okrpm during normal operation, for example in a steady-state mode. In particular, the phase winding 22 can be excited and freewheeled sequentially, with commutation of the phase winding 22 occurring between successive excitations of the phase winding 22.

When exciting and freewheeling the phase winding 22 using a sensorless control scheme, it may be desirable to occasionally directly monitor the back EMF induced in the phase winding 22 such that a zero-crossing of the back EMF induced in the phase winding 22 can be directly observed, and an aligned position of the rotor 18 can be directly determined. This may help to “resynchronise” a sensorless control scheme used by the controller 34, for example to mitigate for any errors that may have occurred during estimation of the zero-crossings of the back EMF induced in the phase winding 22. To achieve this, the signal bEMF from the voltage sensor 33 is periodically monitored by turning switches Q1 -Q4 off, i.e., by turning off the inverter 26, and when the voltage measured by the voltage sensor 33 transitions from negative to positive or positive to negative a back EMF zero-crossing is deemed to occur. A period in which the inverter 26 is turned off may be referred to as a first time interval herein.

To maintain the rotor 18 at a steady-state speed, or to continue acceleration of the rotor 18, it may be desirable for the first time interval to be relatively short, and for excitation of the phase winding 22 to resume after expiry of the first time interval. However, it has been found that following a time period in which the inverter 26 is turned off, peaks of phase current flowing through the phase winding 18 can exceed the value of peaks of phase current that would typically be expected during steady-state operation of the brushless permanent magnet motor 14. This may lead to software or hardware current trips, may lead to issues with accelerating the brushless permanent magnet motor 14 to, or maintaining the brushless permanent magnet motor 14 at, a steady-state speed, and may even result in current peaks high enough to damage the hardware of the brushless permanent magnet motor 14.

A method 100 to mitigate for such peaks in current is illustrated in the flow diagram of Figure 4. The method 100 comprises exciting 102 the phase winding 22 of the motor 14 by applying a voltage to the phase winding 22 using the inverter 26 of the motor 14, and turning off 104 the inverter 26 for a first time interval T1. The method 100 comprises sequentially exciting 106 the phase winding 22 of the motor 14 for a plurality of excitation periods over a second time interval T2 subsequent to the first time interval T1 , wherein a start point of each excitation period is spaced from a zero-crossing of back EMF induced in the phase winding 22 by an advance period. The method 100 comprises determining 108 the advance periods such that the advance periods vary within the second time interval T2.

By determining the advance periods such that the advance periods vary within the second time interval T2, current flowing through the phase winding 22 may be managed more efficiently during the second time interval T2, i.e. when the motor 14 is restarted following the first time interval T1 in which the inverter 26 is turned off, relative to a method where a fixed advance period is used within the second time interval T2. For example, by varying the advance periods within the second time interval T2, i.e. such that the excitation periods are advanced or retarded relative to zero-crossings of back EMF induced in the phase winding 22 by differing amounts within the second time interval T2, peaks in phase current induced in the phase winding 22 may be reduced relative to peaks in phase current experienced utilising a method in which a fixed advance period is used for each excitation period within the second time interval.

Exemplary current waveforms 200 in accordance with an implementation of the method 100 are shown schematically in Figure 5, with reference current waveforms 202 typically experienced absent use of the method 100 illustrated for reference. Here the back EMF 204 induced in the phase winding 22 is illustrated as varying sinusoidally over time. During the first time interval T1 , the current 200 present in the phase winding 22 is zero, as the inverter 26 is turned off, whilst the back EMF 204 is monitored by monitoring the signal bEMF from the voltage sensor 33.

Then, in the second time interval T2 that follows the first time interval T1 , the phase winding 22 is excited and freewheeled sequentially, with freewheeling occurring between a peak in the current 200 shown in Figure 5 for a given excitation and commutation of the phase winding 22. As noted in accordance with the method 100, a start point of each excitation period in the second time interval T2 is spaced from a zero-crossing of back EMF 204 induced in the phase winding 22 by an advance period ADV.

As seen in Figure 5, an initial advance period ADV(1 ) for an initial excitation period 206 within the second time interval T2 has a negative value, such that a start of the initial excitation period 206 is retarded relative to the corresponding zero-crossing of back EMF 204. Here the initial advance period ADV(1 ) is -7.3ps, and the initial excitation period 206 has a duration EXC(1 ).

A second advance period ADV(2) for a second excitation period 208 within the second time interval T2 has a positive value, such that a start of the second excitation period 208 is advanced relative to the corresponding zero-crossing of back EMF 204. Here the second advance period ADV(2) is 1.7ps, and the second excitation period 208 has a duration EXC(2). As can be seen from Figure 5, the duration EXC(1 ) of the initial excitation period 206 is less than the duration EXC(2) of the second excitation period 208.

The remaining advance periods ADV(3)-ADV(8) within the second time interval T2 also have positive values, such that starts of the third through eighth 210,212,214,216,218,2220 excitation periods are advanced relative to the corresponding zero-crossings of back EMF 204. The third through eighth advanced periods ADV(3)-ADV(8) have values of 2.7ps, 3.6ps, 4.6ps, 5.5ps, 6.5ps, and 7.4ps, respectively. Thus it can be seen that the advance periods increase, in a generally consistent manner, from the second advance period ADV(2) to the eighth advance period ADV(8). The third through eighth 210,212,214,216,218,2200 excitation periods have durations EXC(3)-EXC(8) equal to the duration EXC(2) of the second excitation period 208.

A reference advance period ADV(REF) is also illustrated in Figure 5. The reference advance period ADV(REF) is the advance period that would typically be applied during steady-state operation, and can be obtained via a look-up table, or via calculation, based on any of a running speed of the brushless permanent magnet motor 14, a DC link voltage to be applied to the brushless permanent magnet motor 14, and a power mode in which the brushless permanent magnet motor 14 is operating. As can be seen in Figure 5, the advance periods ADV(2)- ADV(8) within the second time interval T2 converge towards the reference advance period ADV(REF), with the reference advance period ADV(REF) being utilised after expiry of the second time interval T2.

Similarly, a reference excitation period duration EXC(REF) is utilised for the durations EXC(2)-EXC(8) of the second through eighth 208,210,212,214,216,218,220 excitation periods within the second time interval. The reference excitation period duration EXC(REF) is the excitation period duration that would typically be applied during steady-state operation, and can be obtained via a look-up table, or via calculation, based on any of a running speed of the brushless permanent magnet motor 14, a DC link voltage to be applied to the brushless permanent magnet motor 14, and a power mode in which the brushless permanent magnet motor 14 is operating.

By implementing the method 100 as illustrated in Figure 4 and Figure 5, current peaks may be reduced relative to, for example, an arrangement where the reference advance period ADV(REF) is immediately used following the first time interval T 1 , and may reduce current peaks and enable a gradual transition toward the level of current peaks typically expected during steady-state operation of the motor 14.

The initial excitation period 206 within the second time interval T2 can be an excitation period where excessive current peaks are most likely to occur following expiry of the first time interval T1. By retarding a start of the initial excitation period 206, and reducing the duration EXC(1 ) of the initial excitation period 206 relative to the reference excitation period duration EXC(REF), a level of current induced in the phase winding 22 may be reduced relative to an arrangement where the reference advance period ADV(REF) and the reference excitation period duration EXC(REF) are utilised for the initial excitation period. A level of current peak expected for the remaining excitation periods 208,210,212,214,216,218,220 within the second time interval T2 may be lower than that expected for the initial excitation period 206, and hence the reference excitation period duration EXC(REF) can be utilised for the remaining excitation periods 208,210,212,214,216,218,220 within the second time interval T2.

Whilst there are 8 excitation periods shown in the example of Figure 5, in practice the number of excitation periods within the second time interval T2 may vary. In some examples it may take fewer than 8 excitation periods for the current peaks to stabilise, following the first interval T1 , to a level typically expected during steady state operation. In general, the second time interval can comprise 20 or less excitation periods.

In some examples, such as the example of Figure 5, the reference advance period ADV(REF) and the reference excitation period duration EXC(REF) can be utilised to determine one or more of an advance period and a duration for a given excitation period within the second time interval T2.

For the second through eighth excitation periods 208,210,212,214,216,218,220 of Figure 5, the following relationships apply:

Here ADV(REF) is the reference advance period previously discussed, and EXC(REF) is the reference excitation period duration previously discussed. The value offset_exc_ratio is the ratio of the excitation period duration by which the advance period will be offset relative the reference advance period ADV(REF). In the example of Figure 5, offset_exc_ratio takes a value of 15%. The value N is the number of excitation periods within the second time interval T2 (here 8), and n is the current excitation period within the second time interval T2 for which an advance period is to be calculated.

As indicated above, the initial excitation period 206 within the second time interval T2 can be an excitation period where excessive current peaks are most likely to occur following expiry of the first time interval T1. Relationships for the initial excitation period 206 within the second time interval T2 can therefore differ to the relationships for the second through eighth excitation periods 208,210,212,214,216,218,220 discussed above, with the relationships for the initial excitation period shown below.

Here sf_low is an advance period scaling factor at a lower speed reference, and sf_high is an advance period scaling factor at a lower speed reference. These values may be obtained via simulation and/or experimentation, and in the example of Figure 5 sf ow has a value of 0, and sf_high has a value of 0.5. Here s_low is the lower speed reference, s_high is the higher speed reference, and Motor_speed is the current motor speed. In the example of Figure 5, s_low has a value of 80krpm, s_high has a value of 150krpm, and Motor_speed has a value of 110krpm. ADV(REF) in Figure 5 takes a value of 8.4ps, and EXC(REF) takes a value of 51 ps.

For the given example of Figure 5, it can be seen that the current peak for the initial excitation period 206 is lower than, and retarded relative to, the current peak for the corresponding reference current waveform 202 that would typically be expected absent use of the method 100. In contrast, if the reference advance period ADV(REF) were to be used throughout the second time interval T2, current peaks may occur earlier and at higher levels for those excitation periods closer to the expiry of the first time interval T1. This may be more pronounced for different operating conditions of the motor 14, with a further example illustrated in Figure 6, where the motor 14 is operating at 150krpm, with a DC link voltage of 37V, and an output power of 1209W. Here it can be seen that negative current peaks experienced toward the start of the second time interval T2 are significantly larger when not using the method 100 compared to those seen when using the method 100.

Thus through use of the method 100, i.e. by varying the advance periods within the second time interval T2 such that the excitation periods are advanced or retarded relative to zero-crossings of back EMF induced in the phase winding 22 by differing amounts within the second time interval T2, peaks in phase current induced in the phase winding 22 may be reduced relative to peaks in phase current experienced utilising a method in which a fixed advance period is used for each excitation period within the second time interval T2.

A vacuum cleaner 300 comprising the brushless permanent magnet motor 14 is illustrated schematically in Figure 7. A haircare appliance 400 comprising the brushless permanent magnet motor 14 is illustrated schematically in Figure 8.

Claims

1 . A method of controlling a brushless permanent magnet motor, the method comprising: exciting a phase winding of the motor by applying a voltage to the phase winding using an inverter of the motor; turning off the inverter for a first time interval; sequentially exciting the phase winding of the motor for a plurality of excitation periods over a second time interval subsequent to the first time interval, wherein a start point of each excitation period is spaced from a zero-crossing of back EMF induced in the phase winding by an advance period; and determining the advance periods such that the advance periods vary within the second time interval.

2. A method as claimed in Claim 1 , wherein the method comprises determining the advance periods such that magnitudes of the advance periods increase within the second time interval.

3. A method as claimed in Claim 1 or Claim 2, wherein the second time interval comprises no more than 20 excitation periods.

4. A method as claimed in any preceding claim, wherein the method comprises determining the advance periods based at least in part on a reference advance period, the reference advance period based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating.

5. A method as claimed in Claim 4, wherein the method comprises determining the advance periods such that the advance periods converge toward the reference advance period toward an end of the second interval.

6. A method as claimed in Claim 4 or Claim 5, wherein the method comprises determining an initial advance period associated with an initial excitation period within the second time interval based at least in part on a modified version of the reference advance period, the modified version of the reference advance period comprising the reference advance period adjusted by a scaling factor based at least in part on a speed of the motor.

7. A method as claimed in any preceding claim, wherein the method comprises determining the advance periods based at least in part on a number of excitation periods within the second time interval.

8. A method as claimed in any preceding claim, wherein the method comprises determining the advance periods based at least in part on a reference excitation period duration, the reference excitation period duration based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating.

9. A method as claimed in any preceding claim, wherein the method comprises determining an initial advance period associated with an initial excitation period within the second time interval to comprise a negative advance period.

10. A method as claimed in Claim 9, wherein the method comprises determining subsequent advance periods associated with subsequent excitation periods within the second time interval to comprise positive advance periods.

11. A method as claimed in any preceding claim, wherein the method comprises determining durations of excitation periods within the second time interval such that durations of excitation periods within the time interval vary.

12. A method as claimed in Claim 11 , wherein the method comprises determining a duration of an initial excitation period within the second time interval to be different to durations of remaining excitation periods within the second time interval.

13. A method as claimed in Claim 11 or Claim 12, wherein the method comprises determining a duration of an initial excitation period within the second time interval to be less than durations of remaining excitation periods within the second time interval.

14. A method as claimed in Claim 12 or Claim 13, wherein the method comprises determining the duration of the initial excitation period based at least in part on a reference advance period, the reference advance period based at least in part on one or more of a running speed of the brushless permanent magnet motor, a DC link voltage to be applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor is operating.

15. A method as claimed in any of Claims 12 to 14, wherein the remaining excitation periods within the second time interval comprise the same duration.

16. A method as claimed in any of Claims 12 to 15, wherein the method comprises determining the duration of the initial excitation period based on an initial advance period associated with the initial excitation period.

17. A method as claimed in any preceding claim, wherein the method comprises commutating the phase winding between sequential excitation periods within the second time interval.

18. A brushless permanent magnet motor comprising a phase winding, an inverter, and a controller configured to perform a method as claimed in any preceding claim.

19. A vacuum cleaner comprising a brushless permanent magnet motor according to Claim 18.

20. A haircare appliance comprising a brushless permanent magnet motor according to Claim 18.