Method for controlling brushless permanent magnet motor

Abstract

A method of controlling a brushless permanent magnet motor includes exciting a phase winding of the motor by applying a voltage to the phase winding using an inverter of the motor, and turning off the inverter at a first time interval. The method includes sequentially exciting the phase windings of the electric machine for a plurality of excitation cycles within a second time interval subsequent to the first time interval. The starting point of each excitation period is spaced from the zero crossing point of the back electromotive force induced in the phase winding by an advance period. The method includes determining an advance period such that the advance period varies within a second time interval.

Description

Technical Field

[0001] The invention relates to a method for controlling a brushless permanent magnet motor. Background Art

[0002] It is generally desirable to improve electric motors, such as brushless motors, in a variety of ways. For example, improvements may be needed in size, weight, power density, manufacturing cost, efficiency, reliability, and noise. Summary of the invention

[0003] According to a first aspect of the present invention, a method for controlling a brushless permanent magnet motor is provided, the method comprising: exciting a phase winding of a motor by applying a voltage to the phase winding using an inverter of the motor; turning off the inverter in a first time interval; sequentially exciting the phase winding of the motor in a plurality of excitation cycles in a second time interval after the first time interval, wherein a starting point of each excitation cycle is separated from a zero-crossing point of a back electromotive force induced in the phase winding by an advance period; and determining the advance period so that the advance period varies in the second time interval.

[0004] It may be desirable to shut down the inverter for a short period during operation to enable resynchronization of the sensorless method of determining the rotor position of the BLPM motor. However, it has been found that after the period when the inverter is shut down, the peak value of the phase current flowing through the phase windings may exceed the phase current peak value normally expected during steady-state operation of the BLPM motor. This may cause software or hardware current trips, may cause problems accelerating the BLPM motor to steady-state speed or maintaining the BLPM motor at steady-state speed, and may even cause current peaks high enough to damage the BLPM motor hardware.

[0005] By determining the lead period such that the lead period varies within the second time interval, the current flowing through the phase winding can be more efficiently managed during the second time interval, i.e., when the motor is restarted after the first time interval in which the inverter is shut down, relative to an approach that uses a fixed lead period within the second time interval. For example, by varying the lead period within the second time interval, i.e., such that the excitation cycle is advanced or delayed by different amounts relative to the zero crossing of the back EMF induced in the phase winding within the second time interval, the peak value of the phase current induced in the phase winding can be reduced relative to the peak value of the phase current experienced by an approach that uses a fixed lead period for each excitation cycle within the second time interval.

[0006] The lead period may include positive values, such as causing the excitation cycle to start before the corresponding zero crossing of the back EMF induced in the phase winding, or may include negative values, such as causing the excitation to start after the corresponding zero crossing of the back EMF induced in the phase winding. In the case where the lead period includes negative values, they may be referred to as delay periods.

[0007] The second time interval may comprise a predetermined length and / or a predetermined number of excitation cycles and may, for example, have a sufficient length and / or a sufficient number of excitation cycles to return the current peak induced in the phase winding to an amplitude normally expected in steady-state operation after the inverter is shut down in the first time interval.

[0008] The second time interval may include no more than 20 excitation cycles, or no more than 10 excitation cycles. This may provide a sufficient number of excitation cycles for the current peak to normalize after the first time interval.

[0009] The method may include determining the lead period such that the magnitude of the lead period increases within the second time interval. This may help reduce the current peak to a level typically expected in steady state operation. The method may include determining the lead period such that the magnitude of the lead period increases substantially linearly within the second time interval. This may provide a relatively smooth transition of the current peak within the second time interval.

[0010] The method may include determining the lead time based at least in part on a reference lead time, the reference lead time being based at least in part on one or more of an operating 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 lead time may provide a starting point for calculating the lead time. The reference lead time may be obtained from a memory, such as from a lookup table or the like. The reference lead time may be calculated by a controller of the brushless permanent magnet motor.

[0011] The method may include determining the lead time such that the lead time converges to the reference lead time at the end of the second time interval. This may provide sufficient time for the current peak to stabilize after the first time interval before resuming use of the reference lead time.

[0012] The method may include determining an initial lead time associated with an initial excitation period in a second time interval based at least in part on a modification of a reference lead time, the modification of the reference lead time including adjusting the reference lead time by a scaling factor based at least in part on a speed of the motor. The magnitude of the current peak may be greatest during the initial excitation period in the second time interval. By scaling the reference lead time based on the speed of the motor, the initial current peak associated with the initial excitation period in the second time interval may be reduced relative to a method where the reference lead time is not adjusted for the initial lead time.

[0013] The method may include determining the advance period based at least in part on the number of excitation cycles within the second time interval.This may cause the magnitude of the current peak to gradually decrease over the second time interval.

[0014] The method may include determining an advance period based at least in part on a reference excitation cycle duration, the reference excitation cycle duration being based at least in part on one or more of an operating 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 cycle duration may provide a starting point for calculating the advance period. The reference excitation cycle duration may be obtained from a memory, such as from a lookup table or the like. The reference excitation cycle duration may be calculated by a controller of the brushless permanent magnet motor. The advance period may be determined based at least in part on a percentage of the reference excitation cycle duration.

[0015] The method may include determining an initial lead period associated with the initial excitation cycle within the second time interval to include a negative lead period. This may provide a greater reduction in the initial current peak associated with the initial excitation cycle than, for example, a method where the initial lead period includes a positive value. By utilizing a negative lead period, i.e. causing the initial excitation cycle to start after a relevant zero crossing of the back electromotive force induced in the phase winding, the current may have less rise time than a method utilizing a positive lead period.

[0016] The method may include determining a subsequent lead period associated with a subsequent excitation cycle within the second time interval to include a positive lead period. This may enable sufficient current to be driven into the phase winding to achieve the desired motor speed while utilizing an initial negative lead period to suppress an initial current peak associated with the initial excitation cycle.

[0017] The method may include determining the duration of the excitation period within the second time interval such that the duration of the excitation period within the time interval varies. This may help manage current peaks associated with the excitation period, for example, by limiting the amount of current flowing through the phase winding using adjustment of the duration of the excitation period.

[0018] The method may include determining that the duration of an initial excitation period in the second time interval is different from the duration of the remaining excitation periods in the second time interval. This may allow mitigation of an initial current peak, typically the highest current peak, in the second time interval while using a constant excitation duration for the remainder of the second time interval.

[0019] The method may include determining that the duration of an initial excitation period in the second time interval is less than the duration of the remaining excitation periods in the second time interval. This may allow mitigation of an initial current peak, typically the highest current peak, in the second time interval while using a constant excitation duration for the remainder of the second time interval.

[0020] The method may include determining a duration of an initial excitation period based at least in part on a reference lead time, the reference lead time being based at least in part on one or more of an operating 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 lead time may provide a starting point based on which the duration of the initial excitation period may be calculated. The reference lead time may be obtained from a memory, such as from a lookup table or the like. The reference lead time may be calculated by a controller of the brushless permanent magnet motor.

[0021] The method may include determining a duration of the initial excitation period based on an initial lead time associated with the initial excitation period. In this way, the duration of the excitation period and the time at which the excitation period begins may be controlled to mitigate current peaks typically experienced after the first time interval.

[0022] The remaining activation periods within the second time interval may comprise the same duration. This may simplify the method compared to methods using more than two activation periods of different duration within the second time interval.

[0023] The method may include commutating the phase winding between consecutive excitation cycles within a second time interval. During the second time interval, the motor may still be accelerated or maintained at a desired speed.

[0024] 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, wherein the controller is configured to perform the method according to the first aspect of the present invention.

[0025] According to a third aspect of the present invention, there is provided a vacuum cleaner comprising the brushless permanent magnet motor according to the second aspect of the present invention.

[0026] According to a fourth aspect of the present invention, there is provided a hair care appliance comprising a brushless permanent magnet motor according to the second aspect of the present invention.

[0027] Where appropriate, optional features of one aspect of the invention may be equally applied to other aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a first schematic diagram showing a motor system;

[0029] Figure 2 It is shown Figure 1 A second schematic diagram of a motor system;

[0030] Figure 3 Yes means Figure 1 and 2 A table of the switching states of the motor system;

[0031] Figure 4 is a flow chart illustrating a method according to the present invention;

[0032] Figure 5 is through the use of Figure 4 A first schematic diagram of a current waveform obtained by the method;

[0033] Figure 6 is through the use of Figure 4 A second schematic diagram of a current waveform obtained by the method;

[0034] Figure 7 Yes Figure 1 and 2 A schematic diagram of a motor system of a vacuum cleaner; and

[0035] Figure 8 Yes Figure 1 and 2 Schematic diagram of a hair care appliance with a motor system. DETAILED DESCRIPTION

[0036] exist Figure 1 and Figure 2 1 shows a motor system, generally designated 10. The motor system 10 is powered by a DC power source 12, such as a battery, and includes a brushless permanent magnet motor 14 and a control circuit 16. Those skilled in the art will recognize that the method of the present invention is equally applicable to motor systems powered by an AC power source with appropriate modifications to the circuit, such as by including a rectifier.

[0037] The motor 14 includes a four-pole permanent magnet rotor 18 that rotates relative to a four-pole stator 20. Although a four-pole permanent magnet rotor is shown here, it should be understood that the present invention can be applied to motors with different pole numbers, such as eight poles. The wires wound on the stator 20 are connected together to form a single-phase winding 22. Although described here as a single-phase motor, those skilled in the art will recognize that the teachings of the present application can also be applied to multi-phase motors, such as three-phase motors.

[0038] The control circuit 16 includes 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 .

[0039] The filter 24 includes a link capacitor C1 that smoothes relatively high frequency ripple caused by the switching of the inverter 26 .

[0040] The inverter 26 includes a full bridge of four power switches Q1-Q4 coupling the phase winding 22 to a voltage rail. Each switch Q1-Q4 includes a freewheeling diode.

[0041] 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 .

[0042] The current sensor 30 comprises a shunt resistor R1 located between the inverter and the zero volt rail. When connected to the power supply 12, the voltage across the current sensor 30 provides a measure of the current in the phase winding 22. The voltage across the current sensor 30 is output as a signal I_SENSE to the controller 34. It will be appreciated that in this embodiment, it is not possible to measure the current in the phase winding 22 during freewheeling, but possible alternative embodiments are also envisaged, for example by using multiple shunt resistors.

[0043] The first voltage sensor 32 comprises a voltage divider in the form of resistors R2 and R3 between the DC voltage rail and the zero volt rail. The voltage sensor outputs a signal V_DC to the controller 34 which represents a scaled down measurement of the supply voltage provided by the power supply 12 .

[0044] The second voltage sensor 33 includes a pair of voltage dividers formed by resistors R4, R5, R6 and R7, which are connected across the phase winding 22. The second voltage sensor 33 provides a signal bEMF representing the back electromotive force induced in the phase winding 22 to the controller.

[0045] The controller 34 includes a microcontroller having a processor, a storage device, and a plurality of peripheral devices (e.g., ADC, comparator, timer, etc.). In an alternative embodiment, the controller 34 may include a state machine. The storage device stores instructions executed by the processor, as well as control parameters used 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.

[0046] During normal operation, the controller 34 estimates the position of the rotor 18 using a sensorless control scheme, i.e., without using Hall sensors or the like, by using software to estimate the waveform indicating the back EMF induced in the phase winding 22 via the signals V_DC and I_SENSE. In particular, the zero crossings of the back EMF induced in the phase winding 22 can be estimated to estimate the aligned position of the rotor 18. For the sake of brevity, the details of this control scheme will not be described here, but can be found in, for example, published UK patent application GB2582612. Another sensorless control scheme is disclosed in published PCT patent application WO2013132247A1, which utilizes hardware components to estimate the back EMF induced in the phase winding 22. Knowing the position of the rotor 18 in normal operation, the controller 34 generates control signals S1-S4.

[0047] Figure 3 The enabled states of switches Q1-Q4 in response to control signals S1-S4 output by controller 34 are summarized. Hereinafter, the terms "set" and "clear" will be used to indicate that a signal has been pulled to a logic high and logic low, respectively. Figure 3 It can be seen that the controller 34 sets S1 and S4 and clears S2 and S3 to excite the phase winding 22 from left to right. Conversely, the controller 34 sets S2 and S3 and clears S1 and S4 to excite the phase winding 22 from right to left. The controller 34 clears S1 and S3 and sets S2 and S4 to freewheel the phase winding 22. Freewheeling enables the current in the phase winding 22 to recirculate around the low-side loop of the inverter 26. In this embodiment, the power switches Q1-Q4 can conduct in both directions. Therefore, the controller 34 closes the two low-side switches Q2, Q4 during freewheeling, so that the current flows through the switches Q2, Q4 instead of the less efficient diodes.

[0048] It is contemplated that the inverter 26 may include power switches that conduct only in one direction. In this case, the controller 34 will clear S1, S2, and S3 and set S4 to freewheel the phase winding 22 from left to right. The controller 34 will then clear S1, S3, and S4 and set S2 to freewheel the phase winding 22 from right to left. The current in the low-side loop of the inverter 26 then flows downward through the closed low-side switch (e.g., Q4) and upward through the diode of the open low-side switch (e.g., Q2).

[0049] During normal operation, such as in a steady-state mode, appropriate control of switches Q1-Q4 may be used to drive rotor 18 at speeds up to or exceeding 100 krpm. In particular, phase windings 22 may be sequentially energized and freewheeled, with commutation of phase windings 22 occurring between subsequent energizations of phase windings 22.

[0050] When a sensorless control scheme is used to energize and freewheel the phase winding 22, it may be desirable to occasionally directly monitor the back EMF induced in the phase winding 22 so that the zero crossings of the back EMF induced in the phase winding 22 can be directly observed and the aligned position of the rotor 18 can be directly determined. This may help to “resynchronize” the sensorless control scheme used by the controller 34, such as to mitigate any errors that may occur during the estimation of the zero crossings of the back EMF induced in the phase winding 22.

[0051] To achieve this, by turning off switches Q1-Q4, i.e., by turning off inverter 26, the signal bEMF from voltage sensor 33 is periodically monitored, and when the voltage measured by voltage sensor 33 transitions from negative to positive or from positive to negative, the zero crossing of the back EMF is considered to occur. The time period during which inverter 26 is turned off may be referred to herein as a first time interval.

[0052] In order to maintain the rotor 18 at a steady-state speed, or to continue the acceleration of the rotor 18, it may be desirable for the first time interval to be relatively short, and it may be desirable to resume energizing the phase winding 22 after the first time interval has expired. However, it has been found that after the period of time when the inverter 26 is off, the peak value of the phase current flowing through the phase winding 18 may exceed the peak value of the phase current typically expected during steady-state operation of the brushless permanent magnet motor 14. This may cause software or hardware current trips, may cause problems accelerating the brushless permanent magnet motor 14 to a steady-state speed or maintaining the brushless permanent magnet motor 14 at a steady-state speed, and may even cause current peaks high enough to damage the hardware of the brushless permanent magnet motor 14.

[0053] Figure 4 1 shows a method 100 for mitigating such current peaks. The method 100 includes exciting 102 a phase winding 22 of the motor 14 by applying a voltage to the phase winding 22 using an inverter 26 of the motor 14, and turning off 104 the inverter 26 at a first time interval T1. The method 100 includes sequentially exciting 106 the phase winding 22 of the motor 14 for a plurality of excitation cycles at a second time interval T2 after the first time interval T1, wherein a starting point of each excitation cycle is spaced apart from a zero crossing of a back electromotive force induced in the phase winding 22 by an advance period. The method 100 includes determining 108 the advance period such that the advance period varies within the second time interval T2.

[0054] By determining the lead period so that the lead period varies within the second time interval T2, the current flowing through the phase winding 22 can be more effectively managed during the second time interval T2, i.e., when the motor 14 is restarted after the first time interval T1 in which the inverter 26 is shut down, relative to an approach that uses a fixed lead period within the second time interval T2. For example, by varying the lead period within the second time interval T2, i.e., such that the excitation cycle is advanced or delayed by different amounts relative to the zero crossing of the back EMF induced in the phase winding 22 within the second time interval T2, the peak value of the phase current induced in the phase winding 22 can be reduced relative to the peak value of the phase current experienced by an approach that utilizes a fixed lead period for each excitation cycle within the second time interval.

[0055] Figure 5 An exemplary current waveform 200 according to an embodiment of the method 100 is schematically shown, wherein a reference current waveform 202 is typically experienced without using the method 100, as a reference. Here, the back electromotive force 204 induced in the phase winding 22 is shown as varying sinusoidally with time. During the first time interval T1, since the inverter 26 is turned off, the current 200 present in the phase winding 22 is zero, while the back electromotive force 204 is monitored by monitoring the signal bEMF from the voltage sensor 33.

[0056] Then, in a second time interval T2 after the first time interval T1, the phase winding 22 is excited and freewheeling in sequence, and the freewheeling occurs at Figure 5 As shown, the peak value of the current 200 for a given excitation and the commutation of the phase winding 22 are spaced apart by an advance period ADV from the starting point of each excitation cycle in the second time interval T2 and the zero crossing point of the back electromotive force 204 induced in the phase winding 22.

[0057] like Figure 5 As shown, the initial advance period ADV(1) of the initial excitation cycle 206 in the second time interval T2 has a negative value, so that the start of the initial excitation cycle 206 is delayed relative to the zero crossing point of the corresponding back electromotive force 204. Here, the initial advance period ADV(1) is -7.3 μs, and the initial excitation cycle 206 has a duration EXC(1).

[0058] The second advance period ADV(2) of the second excitation cycle 208 within the second time interval T2 has a positive value, so that the start of the second excitation cycle 208 is advanced relative to the corresponding zero crossing point of the back electromotive force 204. Here, the second advance period ADV(2) is 1.7 μs, and the second excitation cycle 208 has a duration EXC(2). Figure 5 It can be seen that the duration EXC( 1 ) of the initial excitation period 206 is shorter than the duration EXC( 2 ) of the second excitation period 208 .

[0059] The remaining advance periods ADV(3)-ADV(8) within the second time interval T2 also have positive values, so that the start of the third to eighth excitation cycles 210, 212, 214, 216, 218, 220 is advanced relative to the corresponding zero crossing points of the back electromotive force 204. The third to eighth advance periods ADV(3)-ADV(8) have values ​​of 2.7μs, 3.6μs, 4.6μs, 5.5μs, 6.5μs and 7.4μs, respectively. It can therefore be seen that the advance period increases from the second advance period ADV(2) to the eighth advance period ADV(8) in a generally consistent manner. The durations EXC(3)-EXC(8) of the third to eighth excitation cycles 210, 212, 214, 216, 218, 220 are equal to the duration EXC(2) of the second excitation cycle 208.

[0060] Reference lead time ADV(REF) is also Figure 5 The reference advance period ADV(REF) is an advance period generally applied during steady-state operation and can be obtained by a lookup table or by calculation based on any one of the operating speed of the brushless permanent magnet motor 14, the DC link voltage to be applied to the brushless permanent magnet motor 14, and the power mode in which the brushless permanent magnet motor 14 operates. Figure 5As shown, the lead times ADV(2)-ADV(8) within the second time interval T2 converge to the reference lead time ADV(REF), which is used after the second time interval T2 expires.

[0061] Similarly, the reference excitation cycle duration EXC(REF) is used for the durations EXC(2)-EXC(8) of the second to eighth excitation cycles 208, 210, 212, 214, 216, 218, 220 within the second time interval. The reference excitation cycle duration EXC(REF) is an excitation cycle duration typically applied during steady-state operation and can be obtained by a lookup table or by calculation based on any one of the operating speed of the brushless permanent magnet motor 14, the DC link voltage to be applied to the brushless permanent magnet motor 14, and the power mode in which the brushless permanent magnet motor 14 operates.

[0062] By implementing Figure 4 and Figure 5 The illustrated method 100 may reduce current peaks and may gradually transition to current peak levels typically expected during steady-state operation of the motor 14 relative to an arrangement that uses the reference advance period ADV(REF), for example, immediately after the first time interval T1.

[0063] The initial excitation cycle 206 within the second time interval T2 may be the excitation cycle in which an excessive current peak is most likely to occur after the first time interval T1 expires. By delaying the start of the initial excitation cycle 206 and reducing the duration EXC(1) of the initial excitation cycle 206 relative to the reference excitation cycle duration EXC(REF), the current level induced in the phase winding 22 may be reduced relative to the arrangement in which the reference advance period ADV(REF) and the reference excitation cycle duration EXC(REF) are used for the initial excitation cycle. The expected current peak levels of the remaining excitation cycles 208, 210, 212, 214, 216, 218, 220 within the second time interval T2 may be lower than the expected current peak levels of the initial excitation cycle 206, and therefore the reference excitation cycle duration EXC(REF) may be used for the remaining excitation cycles 208, 210, 212, 214, 216, 218, 220 within the second time interval T2.

[0064] Although in Figure 5 In the example of FIG. 8, 8 excitation cycles are shown, but in practice the number of excitation cycles within the second time interval T2 may vary. In some examples, after the first interval T1, the current peak may require less than 8 excitation cycles to stabilize to a level typically expected during steady-state operation. Typically, the second time interval may include 20 or fewer excitation cycles.

[0065] In some examples, such as Figure 5 For example, the reference advance period ADV(REF) and the reference excitation period EXC(REF) may be used to determine one or more of the advance period and the duration of a given excitation period within the second time interval T2.

[0066] for Figure 5 For the second to eighth excitation cycles 208, 210, 212, 214, 216, 218, 220, the following relationships apply:

[0067] ADV(n)=ADV(REF)-ADV_offset(n)

[0068]

[0069] maximum_offset=EXC(REF)*offset_exc_ratio

[0070] Here, ADV(REF) is the reference lead time discussed previously, and EXC(REF) is the reference excitation period discussed previously. The value offset_exc_ratio is the ratio of the excitation period by which the lead time is offset relative to the reference lead time ADV(REF). Figure 5 In the example of , the value offset_exc_ratio is 15%. The value N is the number of excitation cycles in the second time interval T2 (here 8), and n indicates the number of the current excitation cycle in the second time interval T2 for which the advance period is calculated.

[0071] As described above, the initial excitation cycle 206 in the second time interval T2 may be the excitation cycle in which an excessive current peak is most likely to occur after the first time interval T1 expires. Therefore, the relationship of the initial excitation cycle 206 in the second time interval T2 may be different from the relationship of the second to eighth excitation cycles 208, 210, 212, 214, 216, 218, 220 described above, and the relationship of the initial excitation cycle is as follows.

[0072] ADV(1)=ADV_scaled-ADV_offset(1)

[0073] ifADV(REF)≥0:ADV_scaled=(ADV(REF)-ADV_offset_extra)*scale_factor

[0074] ifADV(REF)<0:ADV_scaled=(ADV(REF)-ADV_offset_extra)*(1+scale_factor)

[0075]

[0076] scale_factor=sf_low+speed_ratio*(sf_high-sf_low)

[0077]

[0078] maximun_offset=EXC(REF)*offset_exc_ratio

[0079] EXC(1)=EXC(REF)-((ADV(REF)-ADV_offset(1)-ADV(1))

[0080] Here, sf_low is the lead time scaling factor at the lower speed reference, and sf_high is the lead time scaling factor at the higher speed reference. These values ​​can be obtained through simulation and / or experimentation. Figure 5 In the example, the value of sf_low is 0 and the value of sf_high is 0.5. Here s_low is the lower reference speed, s_high is the higher reference speed, and Motor_speed is the current motor speed. Figure 5 In the example, the value of s_low is 80krpm, the value of s_high is 150krpm, and the value of Motor_speed is 110krpm. Figure 5 The value of ADV(REF) is 8.4μs and the value of EXC(REF) is 51μs.

[0081] for Figure 5 Given the example of FIG. 1 , it can be seen that the current peak of the initial excitation cycle 206 is lower than the current peak of the corresponding reference current waveform 202 and is delayed relative to that current peak, which is generally expected without using the method 100. In contrast, if the reference advance period ADV(REF) is used throughout the second time interval T2, the current peak may occur earlier and at a higher level for those excitation cycles closer to the expiration of the first time interval T1. This may be more pronounced for different operating conditions of the motor 14, such as during Figure 6 Another example is shown in , where the motor 14 is running at 150 krpm, the DC link voltage is 37 V, and the output power is 1209 W. Here it can be seen that when the method 100 is not used, the negative current peak experienced at the beginning of the second time interval T2 is significantly larger than when the method 100 is used.

[0082] Therefore, by using method 100, that is, by changing the advance period within the second time interval T2, so that the excitation cycle is advanced or delayed by different amounts relative to the zero crossing point of the back electromotive force induced in the phase winding 22 within the second time interval T2, the peak value of the phase current induced in the phase winding 22 can be reduced relative to the peak value of the phase current experienced by the method using a fixed advance period for each excitation cycle within the second time interval T2.

[0083] Figure 7 A vacuum cleaner 300 comprising a brushless permanent magnet motor 14 is schematically shown. Figure 8 A hair care appliance 400 comprising a brushless permanent magnet motor 14 is schematically shown.

Claims

1. A method for controlling a brushless permanent magnet motor, the method comprising: exciting a phase winding of the electric machine by applying a voltage to the phase winding using an inverter of the electric machine; shutting down the inverter during a first time interval; sequentially exciting the phase windings of the motor in a plurality of excitation cycles in a second time interval after the first time interval, wherein a starting point of each excitation cycle is spaced apart from a zero crossing point of a back electromotive force induced in the phase winding by an advance period; and The lead time is determined such that the lead time varies within the second time interval. 2 . The method of claim 1 , wherein the method comprises determining the lead time such that the magnitude of the lead time increases within the second time interval.

3. A method according to claim 1 or 2, wherein the second time interval comprises no more than 20 excitation cycles.

4. A method according to any one of the preceding claims, wherein the method includes determining the lead time based at least in part on a reference lead time, the reference lead time being based at least in part on one or more of an operating speed of the brushless permanent magnet motor, a DC link voltage applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor operates. 5 . The method of claim 4 , wherein the method comprises determining the lead time such that the lead time converges to the reference lead time at the end of the second time interval.

6. A method according to claim 4 or 5, wherein the method includes determining an initial lead time associated with an initial excitation cycle within the second time interval based at least in part on a modification of the reference lead time, the modification of the reference lead time including adjusting the reference lead time by a proportional factor based at least in part on the speed of the motor.

7. A method according to any preceding claim, wherein the method comprises determining the lead time based at least in part on the number of activation cycles within the second time interval.

8. A method according to any of the preceding claims, wherein the method includes determining the lead period based at least in part on a reference excitation cycle duration, the reference excitation cycle duration being based at least in part on one or more of an operating speed of the brushless permanent magnet motor, a DC link voltage applied to the brushless permanent magnet motor, and a power mode in which the brushless permanent magnet motor operates.

9. A method according to any preceding claim, wherein the method comprises determining an initial lead time associated with an initial excitation cycle within the second time interval to include a negative lead time.

10. A method according to claim 9, wherein the method comprises determining a subsequent lead time associated with a subsequent activation cycle within the second time interval to comprise a positive lead time.

11. A method according to any one of the preceding claims, wherein the method comprises determining the duration of the activation period within the second time interval such that the duration of the activation period within the second time interval varies.

12. A method according to claim 11, wherein the method comprises determining that the duration of an initial activation period in the second time interval is different from the duration of the remaining activation periods in the second time interval.

13. A method according to claim 11 or 12, wherein the method comprises determining that the duration of an initial activation period in the second time interval is less than the duration of the remaining activation periods in the second time interval.

14. A method according to claim 12 or 13, wherein the method includes determining the duration of the initial excitation period based at least in part on the reference lead time, and the reference lead time is based at least in part on one or more of the operating speed of the brushless permanent magnet motor, the DC link voltage applied to the brushless permanent magnet motor, and the power mode in which the brushless permanent magnet motor operates.

15. The method according to any one of claims 12 to 14, wherein the remaining activation periods within the second time interval comprise the same duration.

16. A method according to any one of claims 12 to 15, wherein the method comprises determining the duration of the initial activation period based on an initial lead time associated with the initial activation period.

17. A method according to any preceding claim, wherein the method comprises commutating the phase winding between consecutive excitation cycles within the second time interval.

18. A brushless permanent magnet electric machine comprising phase windings, an inverter and a controller, the controller being configured to perform the method according to any one of the preceding claims.

19. A vacuum cleaner comprising the brushless permanent magnet motor according to claim 18.

20. A hair care appliance comprising the brushless permanent magnet motor according to claim 18.

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