WO2023214149A1

WO2023214149A1 - A method of controlling a brushless permanent magnet motor

Info

Publication number: WO2023214149A1
Application number: PCT/GB2023/051127
Authority: WO
Inventors: Mate Horvat
Original assignee: Dyson Technology Limited
Priority date: 2022-05-05
Filing date: 2023-04-28
Publication date: 2023-11-09
Also published as: GB2618357B; GB2618357A; CN119137851A

Abstract

A method of controlling a brushless permanent magnet motor includes exciting a phase winding of the motor for an excitation period, wherein exciting the phase winding comprises applying a voltage to the phase winding, and freewheeling the phase winding for a first freewheel period and a second freewheel period. The method includes commutating the phase winding at a commutation time measured relative to a zero-crossing of back EMF induced in the phase winding. The first freewheel period starts at an end of the excitation period, and the second freewheel period is such that the zero-crossing of back EMF induced in the phase winding occurs during the second freewheel period.

Description

A METHOD OF CONTROLLING A BRUSHLESS PERMANENT MAGNET

MOTOR

Field of the Invention

The present invention relates to a method of controlling a brushless permanent magnet motor.

of the Invention

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.

Summary 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 for an excitation period, wherein exciting the phase winding comprises applying a voltage to the phase winding; freewheeling the phase winding for a first freewheel period and a second freewheel period; and commutating the phase winding at a commutation time measured relative to a zero-crossing of back EMF induced in the phase winding; wherein the first freewheel period starts at an end of the excitation period, and the second freewheel period is such that the zero-crossing of back EMF induced in the phase winding occurs during the second freewheel period.

By performing the second freewheel period about the zero-crossing of back EMF induced in the phase winding, defluxing of a stator core of the motor may be achieved. In particular, the back EMF induced in the phase winding is a derivative of the flux linkage in the stator core, and so when the back EMF induced in the phase winding is zero, the flux linkage is at a peak, which means a flux density of the stator core is at a maximum. By freewheeling around the back EMF zerocrossing, magnetic flux can be created in the phase winding that opposes the magnetic flux of the stator core, thereby reducing magnetic flux density in the stator core and reducing iron losses associated with the motor (with iron losses being proportional to the square of the flux density).

The motor may comprise an inverter for applying the voltage to the phase winding, the inverter may comprise a first pair of switches and a second pair of switches, the first and second pairs of switches movable between a plurality of switch configurations, and freewheeling for the second freewheel period may comprise placing the inverter in a first switch configuration where one of the first and second pairs of switches is closed and the other of the second and first pairs of switches is open.

By freewheeling where one of the first and second pairs of switches is closed and the other of the second and first pairs of switches is open, i.e. by freewheel using two closed switches, efficiency may be improved relative to an arrangement where freewheeling takes place by utilising a switch configuration where one of a pair of switches is open and the other of the pair of switches is closed.

For example, where the switches of the first and second pairs of switches comprise transistors, in an arrangement where freewheeling takes place by utilising a switch configuration where one of a pair of transistors is open and the other of the pair of transistors is closed, current may flow through a body diode of the open transistor to provide freewheeling. This may, however, result in greater losses than an arrangement in which current flows through two closed transistors to provide freewheeling, for example due to the voltage drop across the body diode, leading to reduced efficiency. Furthermore, it has been found that freewheeling in the second freewheel period may lead to symmetry of current induced in the phase winding about the zerocrossing of back EMF. Such symmetry may enable the second freewheel period to be applied for a given time period, and may avoid the need to monitor current flowing through the phase winding to know when to end the second freewheel period. In particular, for a given start point it may be possible to predict where the second freewheel period will end, thereby avoiding the need to monitor current in the phase winding to know when to end the second freewheel period.

The first and second pairs of switches may comprise high- and low- side pairs of switches. Freewheeling may comprise freewheeling around either the high-side pair of switches or freewheeling around the low-side pair of switches.

The method may comprise performing zero-current clamping between an end of the first freewheel period and a start of the second freewheel period. By performing zero-current clamping when the current in the phase winding decays to zero at the end of the first freewheel period, current may be inhibited from flowing through the phase winding in an opposite direction to the back EMF induced in the phase winding at that time, which would otherwise produce a negative torque, thereby inhibiting acceleration of the motor.

Freewheeling in the first freewheel period may comprise placing the inverter in the first switch configuration, and performing zero-current clamping may comprise placing the inverter in a second switch configuration where the first and second pairs of switches are open. Freewheeling in the first freewheel period using the first switch configuration may provide greater efficiency than, for example, an arrangement where freewheeling takes place by utilising a switch configuration where one of a pair of switches is open and the other of the pair of switches is closed. Freewheeling in the first freewheel period may comprise placing the inverter in the first switch configuration, and placing the inverter in a subsequent switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed, performing zero-current clamping may comprise placing the inverter in the subsequent switch configuration. Freewheeling in the first freewheel period using the first switch configuration may provide greater efficiency than, for example, an arrangement where freewheeling takes place by utilising a switch configuration where one of a pair of switches is open and the other of the pair of switches is closed. However, transitioning to the subsequent switch configuration during the first freewheeling period may enable zero-current clamping to occur naturally at the end of the first freewheel period. In particular, in the subsequent switch configuration described above, current may be inhibited from flowing through the open switch once the current transitions past its zero-crossing from the previous freewheeling state. Such natural current clamping in the subsequent configuration may avoid the need to time the end of the first freewheel period to manually apply current clamping, eg by switching off both of the first and second pairs of switches at a determined end of the first freewheel period, and may ensure that there is no delay in current clamping that could otherwise lead to negative torque generation as previously described.

Freewheeling for the second freewheel period may comprise only placing the inverter in the first switch configuration. This may result in increased efficiency relative to an arrangement where freewheeling takes place by utilising a switch configuration where one of a pair of switches is open and the other of the pair of switches is closed.

Freewheeling for the second freewheel period may comprise placing the inverter in the first switch configuration, and placing the inverter in a subsequent switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed. This may provide the increased efficiency of the first switch configuration relative to the subsequent configuration, as previously discussed, whilst allowing for current clamping to happen naturally, given the subsequent switch configuration, at the end of the second freewheel period, thereby avoiding the need to time the end of the second freewheel period, and/or avoiding the need to monitor the phase current during the second freewheel period, to know when to end the second freewheel period.

The motor may comprise an inverter for applying the voltage to the phase winding, the inverter may comprise a first pair of switches and a second pair of switches, the first and second pairs of switches movable between a plurality of switch configurations, and freewheeling for the second freewheel period may comprises placing the inverter in a first switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed. This may allow for current clamping to happen naturally, given the first switch configuration, at the end of the second freewheel period, thereby avoiding the need to time the end of the second freewheel period, and/or avoiding the need to monitor the phase current during the second freewheel period, to know when to end the second freewheel period.

Freewheeling in the first freewheel period may comprise placing the inverter in a second switch configuration where one of the first and second pairs of switches is closed and the other of the second and first pairs of switches is open, and placing the inverter in the first switch configuration, and performing zero-current clamping comprises placing the inverter in the first switch configuration. Freewheeling in the first freewheel period using the second switch configuration may provide greater efficiency than, for example, an arrangement where freewheeling takes place by utilising the first switch configuration where one of a pair of switches is open and the other of the pair of switches is closed. However, transitioning to the first switch configuration during the first freewheeling period may enable zero-current clamping to occur naturally at the end of the first freewheel period. In particular, in the first switch configuration described above, current may be inhibited from flowing through the open switch once the current transitions past its zero-crossing from the previous freewheeling state. Such natural current clamping in the first switch configuration may avoid the need to time the end of the first freewheel period to manually apply current clamping, eg by switching off both of the first and second pairs of switches at a determined end of the first freewheel period, and may ensure that there is no delay in current clamping that could otherwise lead to negative torque generation as previously described.

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

The brushless permanent magnet motor may comprise an inverter comprising first and second pairs of switches, the first pair of switches may comprise a pair of high-side switches, and the second pair of switches may comprise a pair of low-side switches. Freewheeling in the second freewheel period may comprise freewheeling using the second pair of switches.

The brushless permanent magnet motor may comprise a single phase brushless permanent magnet motor.

The brushless permanent magnet motor may comprise a current sensor located on a low side of the inverter. The brushless permanent magnet motor may comprise two current sensors located on a low side of the inverter. This may enable the phase current to be monitored directly during freewheeling. According to a third aspect of the present invention there is provided a data carrier comprising machine-readable instructions for the operation of one or more controllers of a brushless permanent magnet motor to perform the according to the first aspect of the present invention.

According to a fourth 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 fifth 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

of the

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 first schematic illustration of a current waveform obtained via use of the method of Figure 4; Figure 6 is a second schematic illustration of a current waveform obtained via use of the method of Figure 4, alongside switch configurations of an inverter of the motor system;

Figure 7 is a third schematic illustration of a current waveform obtained via use of the method of Figure 4, alongside switch configurations of an inverter of the motor system;

Figure 8 is a fourth schematic illustration of a current waveform obtained via use of the method of Figure 4, alongside switch configurations of an inverter of the motor system;

Figure 9 is a schematic illustration of a vacuum cleaner comprising the motor system of Figures 1 and 2; and

Figure 10 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. As illustrated in Figure 2, the switches Q1 and Q3 comprise a pair of high-side switches, and the switches Q2 and Q4 comprise a pair of low- side switches.

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, and such allowed states may be referred to as switch configurations here. 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 the rotor 18 is driven, the back EMF induced in the phase winding 22 is a derivative of the flux linkage in the stator 20, and so when the back EMF induced in the phase winding 22 is zero, the flux linkage is at a peak, which means a flux density of the stator 20 is at a maximum. This can lead to relatively high iron losses associated with the motor 14, with iron losses being proportional to the square of the flux density.

A method 100 to mitigate for such iron losses is illustrated in the flow diagram of Figure 4. The method 100 comprises exciting 102 the phase winding 22 of the motor 14 for an excitation period E1 , wherein exciting the phase winding 22 comprises applying a voltage to the phase winding 22. The method 100 comprises freewheeling 104 the phase winding 22 for a first freewheel period FW1 and a second freewheel period FW2, and commutating 106 the phase winding 22 at a commutation time measured relative to a zero-crossing of back EMF induced in the phase winding 22. The first freewheel period FW1 starts at an end of the excitation period E1 , and the second freewheel period FW2 is such that the zero-crossing of back EMF induced in the phase winding 22 occurs during the second freewheel period FW2.

By performing the second freewheel period FW2 about the zero-crossing of back EMF induced in the phase winding 22, defluxing of the stator 20 of the motor 14 may be achieved. In particular, by freewheeling around the back EMF zerocrossing, magnetic flux can be created in the phase winding 22 that opposes the magnetic flux of the stator 20, thereby reducing magnetic flux density in the stator 20 and reducing iron losses associated with the motor 14.

An exemplary current waveform 200 and back EMF waveform 202 in accordance with the method 100 are illustrated schematically in Figure 5. Initially, a voltage is applied to the phase winding 22 by closing switches Q1 and Q4, i.e. by exciting the phase winding 22 from left-to-right, such that current is driven through the phase winding 22 and increases during the first excitation period E1 . Here the switches Q1 -Q4 are in a switch configuration where one of the pair of high-side switches, i.e. Q1 , is on and the other of the pair of high-side switches, i.e. Q3, is off, and one of the pair of low-side switches, i.e. Q4, is on and the other of the pair of low-side switches, i.e. Q2, is off.

When the excitation period E1 has expired, or when the current flowing through the phase winding 22 reaches a threshold value, the phase winding 22 is freewheeled for the first freewheel period FW1. Freewheeling in the first freewheel period FW1 can take place in a number of ways, as will be described in more detail hereafter.

The first freewheel period FW1 ends when the current in the phase winding 22 decays to zero, and zero-current clamping is performed for a first clamping period C1 . This may inhibit current of an opposite polarity to the back EMF from flowing in the phase winding 22 at the same time, which would otherwise result in negative torque generation. It will be appreciated that zero-current clamping can be achieved via different switch configurations of the inverter 26, as will be described in more detail hereafter.

At the end of the first clamping period C1 , the phase winding 22 is freewheeled for the second freewheel period FW2. Freewheeling in the second freewheel period FW2 can take place in a number of ways, as will be described in more detail hereafter. The second freewheel period FW2 is timed such that the zerocrossing of back EMF induced in the phase winding 22 occurs during the second freewheel period FW2, and as illustrated in Figure 5 the second freewheel period FW2 is generally symmetric around the zero-crossing of back EMF induced in the phase winding 22. Sensorless methods of estimating zero-crossings of back EMF induced in the phase winding 22 can estimate future zero-crossings, and hence the end of the first clamping period C1 / the start of the second freewheel period FW2 can be determined relative to an estimated future zero-crossing of back EMF induced in the phase winding 22.

At the end of the second freewheeling period FW2, zero-current clamping is performed for a second clamping period C2. This may inhibit current of an opposite polarity to the back EMF from flowing in the phase winding 22 at the same time, which would otherwise result in negative torque generation. It will again be appreciated that zero-current clamping can be achieved via different switch configurations of the inverter 26, as will be described in more detail hereafter.

At the end of the second clamping period C2, the phase winding 22 is commutated, and a voltage is applied to the phase winding 22 by closing switches Q3 and Q2, i.e. by exciting the phase winding 22 from right-to-left, such that current is driven through the phase winding 22 and increases during a second excitation period E2. Here the switches Q1 -Q4 are in a switch configuration where one of the pair of high-side switches, i.e. Q3, is on and the other of the pair of high-side switches, i.e. Q1 , is off, and one of the pair of low-side switches, i.e. Q2, is on and the other of the pair of low-side switches, i.e. Q4, is off.

The sequence of excitation, freewheeling, and clamping described above can be repeated over a number of excitations of the phase winding 22 as desired to appropriately drive the motor 14.

As noted above, a variety of switch configurations of the inverter 26 can be utilised to achieve freewheeling and zero-current clamping.

A first set of switch configurations for use in the method 100 described above is illustrated schematically in Figure 6, where arrows illustrate flow of current around the inverter 26. As above, the first excitation period E1 is achieved by applying a voltage to the phase winding 22 by closing switches Q1 and Q4, i.e. by exciting the phase winding 22 from left-to-right, such that current is driven through the phase winding 22 and increases during the first excitation period E1. Here the switches Q1 -Q4 are in a switch configuration where one of the pair of high-side switches, i.e. Q1 , is on and the other of the pair of high-side switches, i.e. Q3, is off, and one of the pair of low-side switches, i.e. Q4, is on and the other of the pair of low-side switches, i.e. Q2, is off. This is indicated as switch configuration 1 in Figure 6.

The first freewheel period FW1 is achieved by opening switches Q1 and Q3, and closing switches Q2 and Q4, i.e. by so-called body freewheeling or dual device freewheeling. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low- side switches, i.e. Q2 and Q4, are closed. This is indicated as switch configuration 2 in Figure 6. Dual device freewheeling may provide greater efficiency than, for example, so-called single device freewheeling or diode freewheeling, as a greater voltage drop may be experienced across a diode of a respective power switch in comparison with a voltage drop across a body of a respective power switch.

The first clamping period C1 is achieved by opening all switches, i.e. all of Q1- Q4, of the inverter 26, such that the inverter 26 is turned off. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are open. This is indicated as switch configuration 3 in Figure 6.

The second freewheel period FW2 is achieved by opening switches Q1 and Q3, and closing switches Q2 and Q4, i.e. by so-called body freewheeling or dual device freewheeling. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are closed. This is indicated as switch configuration 4 in Figure 6. As above, dual device freewheeling may provide greater efficiency than, for example, so-called single device freewheeling or diode freewheeling, as a greater voltage drop may be experienced across a diode of a respective power switch in comparison with a voltage drop across a body of a respective power switch.

As indicated by the arrows in Figure 6, current flows in the opposite direction around the low-side of the inverter 26 in the second freewheel period FW2 when compared to the first freewheel period FW1 .

The second clamping period C2 is achieved by opening all switches, i.e. all of Q1 -Q4, of the inverter 26, such that the inverter 26 is turned off. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are open. This is indicated as switch configuration 5 in Figure 6.

A second set of switch configurations for use in the method 100 described above is illustrated schematically in Figure 7, where arrows illustrate flow of current around the inverter 26.

As above, the first excitation period E1 is achieved by applying a voltage to the phase winding 22 by closing switches Q1 and Q4, i.e. by exciting the phase winding 22 from left-to-right, such that current is driven through the phase winding 22 and increases during the first excitation period E1. Here the switches Q1 -Q4 are in a switch configuration where one of the pair of high-side switches, i.e. Q1 , is on and the other of the pair of high-side switches, i.e. Q3, is off, and one of the pair of low-side switches, i.e. Q4, is on and the other of the pair of low-side switches, i.e. Q2, is off. This is indicated as switch configuration 1 in Figure 7. The first freewheel period FW1 is initially achieved by opening switches Q1 and Q3, and closing switches Q2 and Q4, i.e. by so-called body freewheeling or dual device freewheeling. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are closed. This is indicated as switch configuration 2 in Figure 7.

After a pre-determined amount of the first freewheel period FW1 has elapsed, the low-side switch Q2 is opened, such that so-called single device freewheeling or diode freewheeling is performed. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q4, is closed, and the other of the pair of low side switches, i.e. Q2, is open. This is indicated as switch configuration 3 in Figure 7.

Whilst dual device freewheeling may be more efficient than single device freewheeling, single device freewheeling can achieve natural zero-current clamping at a transition in polarity of current induced in the phase winding 22, which may avoid the need to turn-off the inverter 26. In particular, the body diode of the low-side switch Q2 may allow current to flow in a first direction to achieve single device freewheeling around the low-side of the inverter 26, but may inhibit current flowing in a second, opposite, direction around the low-side of the inverter 26, thereby achieving zero-current clamping.

The first clamping period C1 in the example of Figure 7 is thereby achieved by maintaining the inverter 26 in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q4, is closed, and the other of the pair of low side switches, i.e. Q2, is open. This is indicated as switch configuration 4 in Figure 7. The second freewheel period FW2 is initially achieved by opening switches Q1 and Q3, and closing switches Q2 and Q4, i.e. by so-called body freewheeling or dual device freewheeling. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are closed. This is indicated as switch configuration 5 in Figure 7.

After a pre-determined amount of the second freewheel period FW2 has elapsed, the low-side switch Q4 is opened, such that so-called single device freewheeling or diode freewheeling is performed. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q2, is closed, and the other of the pair of low side switches, i.e. Q4, is open. This is indicated as switch configuration 6 in Figure 7. As can be seen from Figure 7, current flows in opposite directions around the low-side of the inverter 26 between switch configurations 3 and 6.

The second clamping period C2 in the example of Figure 7 is then achieved by maintaining the inverter 26 in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q2, is closed, and the other of the pair of low side switches, i.e. Q4, is open. This is indicated as switch configuration 7 in Figure 7.

A third set of switch configurations for use in the method 100 described above is illustrated schematically in Figure 8, where arrows illustrate flow of current around the inverter 26.

As above, the first excitation period E1 is achieved by applying a voltage to the phase winding 22 by closing switches Q1 and Q4, i.e. by exciting the phase winding 22 from left-to-right, such that current is driven through the phase winding 22 and increases during the first excitation period E1. Here the switches Q1 -Q4 are in a switch configuration where one of the pair of high-side switches, i.e. Q1 , is on and the other of the pair of high-side switches, i.e. Q3, is off, and one of the pair of low-side switches, i.e. Q4, is on and the other of the pair of low-side switches, i.e. Q2, is off. This is indicated as switch configuration 1 in Figure 8.

The first freewheel period FW1 is initially achieved by opening switches Q1 and Q3, and closing switches Q2 and Q4, i.e. by so-called body freewheeling or dual device freewheeling. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, and each of the pair of low-side switches, i.e. Q2 and Q4, are closed. This is indicated as switch configuration 2 in Figure 8.

After a pre-determined amount of the first freewheel period FW1 has elapsed, the low-side switch Q2 is opened, such that so-called single device freewheeling or diode freewheeling is performed. Here the switches are in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q4, is closed, and the other of the pair of low side switches, i.e. Q2, is open. This is indicated as switch configuration 3 in Figure 8.

The first clamping period C1 in the example of Figure 8 is then achieved by maintaining the inverter 26 in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q4, is closed, and the other of the pair of low side switches, i.e. Q2, is open. This is indicated as switch configuration 4 in Figure 8.

The second freewheel period FW2 is achieved by closing switch Q2 and opening switch Q4, i.e. by switching the direction of single device or diode freewheeling. Here the switches are in a switch configuration where each of the pair of high- side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q2, is closed, and the other of the pair of low side switches, i.e. Q4, is open. This is indicated as switch configuration 5 in Figure 8. The second clamping period C2 in the example of Figure 7 is then achieved by maintaining the inverter 26 in a switch configuration where each of the pair of high-side switches, i.e. Q1 and Q3, are open, one of the pair of low-side switches, i.e. Q2, is closed, and the other of the pair of low side switches, i.e. Q4, is open. This is indicated as switch configuration 6 in Figure 8.

Whilst described above in relation to low-side freewheeling, it will be appreciated that techniques used herein can also be implemented in conjunction with high- side freewheeling, where appropriate. Furthermore, although described above in relation to a continuous excitation period, it will be appreciated that techniques used herein may be utilised in conjunction with excitation schemes that utilise a split excitation period, for example with first and second excitation pulses separated by an intermediate freewheel period.

In each of the switch configurations of Figures 6, 7 and 8, the second freewheel period is applied such that a zero-crossing of back EMF induced in the phase winding 22 occurs during the second freewheel period FW2.

By performing the second freewheel period FW2 about the zero-crossing of back EMF induced in the phase winding 22, defluxing of the stator 20 of the motor 14 may be achieved. In particular, by freewheeling around the back EMF zerocrossing, magnetic flux can be created in the phase winding 22 that opposes the magnetic flux of the stator 20, thereby reducing magnetic flux density in the stator 20 and reducing iron losses associated with the motor 14. This can result in more efficient operation of the motor 14, and more efficient operation of a product in which the motor 14 is utilised.

A vacuum cleaner 300 comprising the brushless permanent magnet motor 14 is illustrated schematically in Figure 9. A haircare appliance 400 comprising the brushless permanent magnet motor 14 is illustrated schematically in Figure 10.

Claims

1 . A method of controlling a brushless permanent magnet motor, the method comprising: exciting a phase winding of the motor for an excitation period, wherein exciting the phase winding comprises applying a voltage to the phase winding; freewheeling the phase winding for a first freewheel period and a second freewheel period; and commutating the phase winding at a commutation time measured relative to a zero-crossing of back EMF induced in the phase winding; wherein the first freewheel period starts at an end of the excitation period, and the second freewheel period is such that the zero-crossing of back EMF induced in the phase winding occurs during the second freewheel period.

2. A method as claimed in Claim 1 , wherein: the motor comprises an inverter for applying the voltage to the phase winding; the inverter comprises a first pair of switches and a second pair of switches, the first and second pairs of switches movable between a plurality of switch configurations: and freewheeling for the second freewheel period comprises placing the inverter in a first switch configuration where one of the first and second pairs of switches is closed and the other of the second and first pairs of switches is open.

3. A method as claimed in Claim 2, wherein the method comprises performing zero-current clamping between an end of the first freewheel period and a start of the second freewheel period.

4. A method as claimed in Claim 3, wherein: freewheeling in the first freewheel period comprises placing the inverter in the first switch configuration; and performing zero-current clamping comprises placing the inverter in a second switch configuration where the first and second pairs of switches are open.

5. A method as claimed in Claim 3, wherein: freewheeling in the first freewheel period comprises placing the inverter in the first switch configuration, and placing the inverter in a subsequent switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed; and performing zero-current clamping comprises placing the inverter in the subsequent switch configuration.

6. A method as claimed in any of Claims 2 to 5, wherein freewheeling for the second freewheel period comprises only placing the inverter in the first switch configuration.

7. A method as claimed in any of Claims 2 to 5, wherein freewheeling for the second freewheel period comprises placing the inverter in the first switch configuration, and placing the inverter in a subsequent switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed.

8. A method as claimed in Claim 1 , wherein: the motor comprises an inverter for applying the voltage to the phase winding; the inverter comprises a first pair of switches and a second pair of switches, the first and second pairs of switches movable between a plurality of switch configurations; and freewheeling for the second freewheel period comprises placing the inverter in a first switch configuration where one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one switch open and one switch closed.

9. A method as claimed in Claim 8, wherein the method comprises performing zero-current clamping between an end of the first freewheel period and a start of the second freewheel period.

10. A method as claimed in Claim 9, wherein: freewheeling in the first freewheel period comprises placing the inverter in a second switch configuration where one of the first and second pairs of switches is closed and the other of the second and first pairs of switches is open, and placing the inverter in the first switch configuration; and performing zero-current clamping comprises placing the inverter in the first switch configuration.

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

12. A brushless permanent magnet motor as claimed in Claim 11 , wherein the brushless permanent magnet motor comprises an inverter comprising first and second pairs of switches, the first pair of switches comprises a pair of high-side switches, and the second pair of switches comprises a pair of low-side switches.

13. A brushless permanent magnet motor as claimed in Claim 12, wherein the brushless permanent magnet motor comprises a single phase brushless permanent magnet motor.

14. A brushless permanent magnet motor as claimed in Claim 13, wherein freewheeling in the second freewheel period comprises freewheeling using the second pair of switches.

15. A brushless permanent magnet motor as claimed in any of Claims 12-14, wherein the brushless permanent magnet motor comprises a current sensor located on a low side of the inverter.

16. A brushless permanent magnet motor as claimed in any of Claims 12-15, wherein the brushless permanent magnet motor comprises two current sensors located on a low side of the inverter.

17. A data carrier comprising machine-readable instructions for the operation of one or more controllers of a brushless permanent magnet motor to perform the method as claimed in any of Claims 1 to 10.

18. A vacuum cleaner comprising a brushless permanent magnet motor as claimed in any of Claims 11 to 16.

19. A haircare appliance comprising a brushless permanent magnet motor as claimed in any of Claims 11 to 16.