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 for an excitation period, where exciting the phase winding includes applying a voltage to the phase winding, and freewheeling the phase winding for a first freewheeling period and a second freewheeling period. The method includes commutation of the phase windings at a commutation time measured relative to a zero crossing of a back electromotive force induced in the phase windings. The first freewheeling period begins at the end of the excitation period, and the second freewheeling period causes a zero crossing point of the back electromotive force induced in the phase winding to occur during the second freewheeling period.

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 terms of size, weight, power density, manufacturing cost, efficiency, reliability, and noise. Summary of the invention

[0003] According to a first aspect of the present invention, there is provided a method for controlling a brushless permanent magnet motor, the method comprising: exciting a phase winding of the motor during an excitation cycle, wherein exciting the phase winding comprises applying a voltage to the phase winding; freewheeling the phase winding during a first freewheeling cycle and a second freewheeling cycle; and commutating the phase winding at a commutation time measured relative to a zero-crossing point of a back electromotive force induced in the phase winding; wherein the first freewheeling cycle starts at the end of the excitation cycle, and the second freewheeling cycle causes the zero-crossing point of the back electromotive force induced in the phase winding to occur during the second freewheeling cycle.

[0004] By performing a second freewheeling cycle near the zero-crossing point of the back EMF induced in the phase winding, demagnetization of the stator core of the motor can be achieved. In particular, the back EMF induced in the phase winding is the derivative of the flux linkage in the stator core, so when the back EMF induced in the phase winding is zero, the flux linkage is at a peak value, which means that the flux density of the stator core is at a maximum value. By freewheeling around the zero-crossing point of the back EMF, a magnetic flux opposite to the magnetic flux of the stator core can be generated in the phase winding, thereby reducing the flux density in the stator core and reducing the iron loss associated with the motor (iron loss is proportional to the square of the flux density).

[0005] The motor may include an inverter for applying a voltage to the phase winding, the inverter may include a first pair of switches and a second pair of switches, the first and second pairs of switches may be movable between a plurality of switch configurations, and the freewheeling of the second freewheeling cycle may include placing the inverter in a first switch configuration in which one pair of the first and second pairs of switches is closed and the other pair of the second and first pairs of switches is open.

[0006] By freewheeling by closing one of the first and second pairs of switches and opening the other of the second and first pairs of switches, i.e., by using two closed switches, efficiency can be improved relative to an arrangement in which freewheeling occurs by using a switch configuration in which one of a pair of switches is open and the other of the pair of switches is closed.

[0007] For example, where the switches of the first and second pairs of switches include transistors, in an arrangement where freewheeling occurs by utilizing a switch configuration where one of the pair of transistors is open and the other of the pair of transistors is closed, current may flow through the body diode of the open transistor to provide freewheeling. However, this may result in greater losses than an arrangement where current flows through two closed transistors to provide freewheeling, for example due to a voltage drop across the body diode, resulting in reduced efficiency.

[0008] Furthermore, it has been found that the freewheeling in the second freewheeling period may result in the current induced in the phase winding being symmetrical about the zero crossing of the back electromotive force. This symmetry may enable the second freewheeling period to be applied to a given time period and may avoid the need to monitor the current flowing through the phase winding to know when to end the second freewheeling period. In particular, for a given starting point, it may be possible to predict where the second freewheeling period will end, thereby avoiding the need to monitor the current in the phase winding to know when to end the second freewheeling period.

[0009] The first and second pairs of switches may include a high-side and a low-side switch pair. Freewheeling may include freewheeling around the high-side switch pair or freewheeling around the low-side switch pair.

[0010] The method may include performing zero current clamping between the end of the first freewheeling period and the beginning of the second freewheeling period. When the current in the phase winding decays to zero at the end of the first freewheeling period, by performing zero current clamping, current can be suppressed from flowing through the phase winding in a direction opposite to the back electromotive force induced in the phase winding at this time, which would otherwise generate negative torque, thereby suppressing motor acceleration.

[0011] Freewheeling during the first freewheeling period may include placing the inverter in a first switching configuration, and performing zero current clamping may include placing the inverter in a second switching configuration in which the first and second pairs of switches are open. Freewheeling during the first freewheeling period using the first switching configuration may provide higher efficiency than, for example, an arrangement in which freewheeling occurs using a switching configuration in which one of a pair of switches is open and the other of the pair of switches is closed.

[0012] Freewheeling in the first freewheeling cycle may include placing the inverter in a first switch configuration, and placing the inverter in a subsequent switch configuration, wherein one of the first and second pairs of switches is open, and the other pair of the second and first pairs of switches has an open switch and a closed switch, and performing zero current clamping may include placing the inverter in the subsequent switch configuration. Freewheeling in the first freewheeling cycle using the first switch configuration may provide a higher efficiency than an arrangement in which freewheeling occurs, for example, by using a switch configuration in which one of the pair of switches is open and the other of the pair of switches is closed. However, switching to the subsequent switch configuration during the first freewheeling cycle may allow zero current clamping to occur naturally at the end of the first freewheeling cycle. In particular, in the above-mentioned subsequent switch configuration, once the current transitions from the previous freewheeling state through its zero crossing point, the current may be prohibited from flowing through the open switch. This natural current clamping in the subsequent configuration may avoid the need to time the end of the first freewheeling cycle to manually apply current clamping, for example by disconnecting the first and second pairs of switches at the determined end of the first freewheeling cycle, and may ensure that there is no delay in the current clamping, which would otherwise result in negative torque generation as described above.

[0013] Freewheeling of the second freewheeling period may include placing the inverter only in the first switching configuration. This may result in increased efficiency relative to an arrangement where freewheeling occurs by utilizing a switching configuration where one of a pair of switches is open and the other of the pair of switches is closed.

[0014] Freewheeling of the second freewheeling period may include placing the inverter in a first switching configuration, and placing the inverter in a subsequent switching configuration in which one of the first and second pairs of switches is open, and the other of the second and first pairs of switches has one open switch and one closed switch. As previously described, this may provide increased efficiency of the first switching configuration relative to the subsequent configuration, while allowing current clamping to occur naturally at the end of the second freewheeling period given the subsequent switching configuration, thereby avoiding the need to time the end of the second freewheeling period, and / or avoiding the need to monitor phase current during the second freewheeling period to know when to end the second freewheeling period.

[0015] The electric machine may include an inverter for applying a voltage to a phase winding, the inverter may include a first pair of switches and a second pair of switches, the first and second pairs of switches being movable between a plurality of switch configurations, and freewheeling of a second freewheeling period may include placing the inverter in a first switch configuration in which one of the first and second pairs of switches is open and the other of the second and first pairs of switches has one open switch and one closed switch. This may allow current clamping to occur naturally at the end of the second freewheeling period given the first switch configuration, thereby avoiding the need to time the end of the second freewheeling period, and / or avoiding the need to monitor the phase current during the second freewheeling period to know when to end the second freewheeling period.

[0016] Freewheeling in the first freewheeling cycle may include placing the inverter in a second switch configuration, wherein 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 includes placing the inverter in the first switch configuration. Freewheeling in the first freewheeling cycle using the second switch configuration may provide higher efficiency than, for example, an arrangement in which freewheeling occurs using the first switch configuration, in which one of the pair of switches is open and the other of the pair of switches is closed. However, switching to the first switch configuration during the first freewheeling cycle may cause zero current clamping to occur naturally at the end of the first freewheeling cycle. Specifically, in the above-mentioned first switch configuration, once the current transitions from the previous freewheeling state through its zero crossing point, the current may be prohibited from flowing through the open switch. This natural current clamping in the first switch configuration may avoid the need to time the end of the first freewheeling cycle to manually apply current clamping, for example by disconnecting the first and second pairs of switches at the determined end of the first freewheeling cycle, and may ensure that there is no delay in the current clamping, which would otherwise result in negative torque generation as described above.

[0017] According to a second aspect of the present invention, there is provided a brushless permanent magnet motor comprising a phase winding and a controller, wherein the controller is configured to perform the method according to the first aspect of the present invention.

[0018] The brushless permanent magnet motor may include an inverter including first and second pairs of switches, the first pair of switches may include a pair of high-side switches, and the second pair of switches may include a pair of low-side switches. Freewheeling in the second freewheeling period may include freewheeling using the second pair of switches.

[0019] The brushless permanent magnet motor may include a single-phase brushless permanent magnet motor.

[0020] A brushless permanent magnet motor can include a current sensor located on the low side of the inverter. A brushless permanent magnet motor can include two current sensors located on the low side of the inverter. This allows for direct monitoring of the phase current during freewheeling.

[0021] According to a third aspect of the invention there is provided a data carrier comprising machine readable instructions for operating one or more controllers of a brushless permanent magnet motor to execute machine readable instructions according to the first aspect of the invention.

[0022] According to a fourth 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.

[0023] According to a fifth 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.

[0024] 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

[0025] Figure 1 is a first schematic diagram showing a motor system;

[0026] Figure 2 It is shown Figure 1 A second schematic diagram of a motor system;

[0027] Figure 3 Yes means Figure 1 and 2 A table of the switching states of the motor system;

[0028] Figure 4 is a flow chart illustrating a method according to the present invention;

[0029] Figure 5 is through the use of Figure 4 A first schematic diagram of a current waveform obtained by the method;

[0030] Figure 6 is through the use of Figure 4 a second schematic diagram of a current waveform obtained by the method, and a switching configuration of an inverter of the motor system;

[0031] Figure 7 is through the use of Figure 4 a third schematic diagram of a current waveform obtained by the method, and a switching configuration of an inverter of the motor system;

[0032] Figure 8 is through the use of Figure 4 a fourth schematic diagram of a current waveform obtained by the method, and a switching configuration of an inverter of a motor system;

[0033] Fig. 9 Yes Figure 1 and 2 A schematic diagram of a motor system of a vacuum cleaner; and

[0034] Fig.10Yes Figure 1 and 2 Schematic diagram of a hair care appliance with a motor system. DETAILED DESCRIPTION

[0035] 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.

[0036] 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.

[0037] 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 .

[0038] The filter 24 includes a link capacitor C1 which smoothes relatively high frequency ripple caused by switching of the inverter 26 .

[0039] The inverter 26 includes a full bridge of four power switches Q1-Q4 that couple the phase winding 22 to the voltage rail. Each switch Q1-Q4 includes a freewheeling diode. Figure 2 As shown, switches Q1 and Q3 comprise a pair of high-side switches, and switches Q2 and Q4 comprise a pair of low-side switches.

[0040] 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 .

[0041] The current sensor 30 includes 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 envisioned, such as by using multiple shunt resistors.

[0042] 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 .

[0043] 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.

[0044] 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.

[0045] 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.

[0046] Figure 3 The enabled states of switches Q1-Q4 in response to control signals S1-S4 output by controller 34 are summarized and may be referred to herein as switch configurations. Hereinafter, the terms "set" and "clear" will be used to indicate that a signal has been pulled to logic high and logic low, respectively. Figure 3It 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 be turned on in both directions. Therefore, the controller 34 closes the two low-side switches Q2 and Q4 during freewheeling, so that the current flows through the switches Q2 and Q4 instead of the less efficient diodes.

[0047] 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).

[0048] During normal operation, such as in a steady-state mode, appropriate control of switches Q1-Q4 can be used to drive rotor 18 at speeds up to or exceeding 100 krpm. In particular, phase windings 22 can be sequentially excited and freewheeled, with commutation of phase windings 22 occurring between successive excitations of phase windings 22.

[0049] When the rotor 18 is driven, the back EMF induced in the phase winding 22 is the derivative of the flux linkage in the stator 20, so when the back EMF induced in the phase winding 22 is zero, the flux linkage is at a peak value, which means that the flux density of the stator 20 is at a maximum value. This may result in relatively high iron losses associated with the motor 14, which are proportional to the square of the flux density.

[0050] Figure 4 , a flow chart illustrating a method 100 for mitigating such iron losses. The method 100 includes exciting 102 a phase winding 22 of an electric machine 14 during an excitation period E1, wherein exciting the phase winding 22 includes applying a voltage to the phase winding 22. The method 100 includes freewheeling 104 the phase winding 22 during a first freewheeling period FW1 and a second freewheeling period FW2, and commutating 106 the phase winding 22 at a commutation time measured relative to a zero crossing of a back electromotive force induced in the phase winding 22. The first freewheeling period FW1 begins at the end of the excitation period E1, and the second freewheeling period FW2 causes a zero crossing of the back electromotive force induced in the phase winding 22 to occur during the second freewheeling period FW2.

[0051] By performing a second freewheeling cycle FW2 near the zero-crossing point of the back electromotive force induced in the phase winding 22, demagnetization of the stator 20 of the motor 14 can be achieved. Specifically, by freewheeling around the zero-crossing point of the back electromotive force, a magnetic flux opposite to the magnetic flux of the stator 20 can be generated in the phase winding 22, thereby reducing the magnetic flux density in the stator 20 and reducing the iron loss associated with the motor 14.

[0052] Figure 5 An exemplary current waveform 200 and back-EMF waveform 202 according to the method 100 are schematically illustrated.

[0053] 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, so that a current is driven through the phase winding 22 and increases during a first excitation period E1. Here, the switches Q1-Q4 are in a switching configuration in which one of a pair of high-side switches (i.e., Q1) is turned on, the other of the pair of high-side switches (i.e., Q3) is turned off, and one of a pair of low-side switches (i.e., Q4) is turned on, and the other of the pair of low-side switches (i.e., Q2) is turned off.

[0054] When the excitation period E1 expires, or when the current flowing through the phase winding 22 reaches a threshold, the phase winding 22 freewheels in a first freewheeling period FW1. Freewheeling in the first freewheeling period FW1 can occur in a variety of ways, which will be described in more detail below.

[0055] When the current in the phase winding 22 decays to zero, the first freewheeling period FW1 ends, and zero current clamping is performed in the first clamping period C1. This can simultaneously suppress the current with the opposite polarity to the back electromotive force from flowing in the phase winding 22, which would otherwise result in the generation of negative torque. It should be understood that zero current clamping can be achieved by different switch configurations of the inverter 26, which will be described in more detail below.

[0056] At the end of the first clamping period C1, the phase winding 22 freewheels in the second freewheeling period FW2. The freewheeling in the second freewheeling period FW2 can occur in a variety of ways, which will be described in more detail below. The second freewheeling period FW2 is timed so that the zero crossing of the back electromotive force induced in the phase winding 22 occurs during the second freewheeling period FW2, and as Figure 5 As shown, the second freewheeling period FW2 is generally symmetrical around the zero crossing point of the back electromotive force induced in the phase winding 22. The sensorless method of estimating the zero crossing point of the back electromotive force induced in the phase winding 22 can estimate the future zero crossing point, and thus the end of the first clamping period C1 / the beginning of the second freewheeling period FW2 can be determined relative to the estimated future zero crossing point of the back electromotive force induced in the phase winding 22.

[0057] At the end of the second freewheeling period FW2, zero current clamping is performed within the second clamping period C2. This can simultaneously inhibit the current of opposite polarity to the back electromotive force from flowing in the phase winding 22, which would otherwise result in the generation of negative torque. It will be appreciated again that zero current clamping can be achieved by different switch configurations of the inverter 26, which will be described in more detail below.

[0058] At the end of the second clamping period C2, the phase winding 22 is commutated, and by closing switches Q3 and Q2, i.e., by exciting the phase winding 22 from right to left, a voltage is applied to the phase winding 22, so that a current is driven through the phase winding 22 and increases during the second excitation period E2. Here, the switches Q1-Q4 are in a switching configuration in which one of the pair of high-side switches (i.e., Q3) is turned on, the other of the pair of high-side switches (i.e., Q1) is turned off, and one of the pair of low-side switches (i.e., Q2) is turned on, and the other of the pair of low-side switches (i.e., Q4) is turned off.

[0059] The sequence of energizing, freewheeling, and clamping as described above may be repeated at multiple energizations of the phase winding 22 as needed to properly drive the motor 14 .

[0060] As described above, various switch configurations of the inverter 26 may be used to implement freewheeling and zero current clamping.

[0061] Figure 6 A first set of switch configurations for the above-described method 100 is schematically illustrated, with arrows illustrating current flow around the inverter 26 .

[0062] As described 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 so 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 switching configuration in which one of the pair of high-side switches (i.e., Q1) is turned on, the other of the pair of high-side switches (i.e., Q3) is turned off, and one of the pair of low-side switches (i.e., Q4) is turned on, and the other of the pair of low-side switches (i.e., Q2) is turned off. This is Figure 6 denoted as switch configuration 1.

[0063] The first freewheeling 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 switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open and each of a pair of low-side switches (i.e., Q2 and Q4) is closed. This Figure 6denoted as switch configuration 2. Dual device freewheeling may provide higher efficiency than, for example, so-called single device freewheeling or diode freewheeling, because a larger voltage drop may be experienced across the diode of the corresponding power switch compared to the voltage drop across the body of the corresponding power switch.

[0064] The first clamping cycle C1 is achieved by turning off all switches of the inverter 26, i.e., all Q1-Q4, so that the inverter 26 is turned off. Here, the switches are in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is turned off and each of a pair of low-side switches (i.e., Q2 and Q4) is turned off. This Figure 6 denoted as switch configuration 3.

[0065] The second freewheeling 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 switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open and each of a pair of low-side switches (i.e., Q2 and Q4) is closed. This Figure 6 denoted as switch configuration 4. As described above, dual device freewheeling may provide higher efficiency than, for example, so-called single device freewheeling or diode freewheeling because a larger voltage drop may be experienced across the diode of the corresponding power switch compared to the voltage drop across the body of the corresponding power switch.

[0066] like Figure 6 As shown by the middle arrow, in the second freewheeling period FW2 , the current flows around the low side of the inverter 26 in the opposite direction compared to the first freewheeling period FW1 .

[0067] The second clamping period C2 is achieved by turning off all switches of the inverter 26, i.e., all Q1-Q4, so that the inverter 26 is turned off. Here, the switches are in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is turned off and each of a pair of low-side switches (i.e., Q2 and Q4) is turned off. This Figure 6 is represented as switch configuration 5.

[0068] Figure 7 A second set of switch configurations for the above-described method 100 is schematically illustrated, with arrows illustrating current flow around the inverter 26 .

[0069] As described above, the first excitation cycle 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 so that current is driven through the phase winding 22 and increases during the first excitation cycle E1. Here, the switches Q1-Q4 are in a switching configuration in which one of the pair of high-side switches (i.e., Q1) is turned on, the other of the pair of high-side switches (i.e., Q3) is turned off, and one of the pair of low-side switches (i.e., Q4) is turned on, and the other of the pair of low-side switches (i.e., Q2) is turned off. This is Figure 7 denoted as switch configuration 1.

[0070] The first freewheeling 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 switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open and each of a pair of low-side switches (i.e., Q2 and Q4) is closed. This Figure 7 denoted as switch configuration 2.

[0071] After a predetermined amount of the first freewheeling period FW1 has elapsed, the low-side switch Q2 is turned off, thereby performing so-called single-device freewheeling or diode freewheeling. Here, the switch is in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is turned off, one of a pair of low-side switches (i.e., Q4) is closed, and the other of a pair of low-side switches (i.e., Q2) is turned off. This Figure 7 denoted as switch configuration 3.

[0072] Although dual-device freewheeling may be more efficient than single-device freewheeling, single-device freewheeling may achieve a natural zero-current clamp when the polarity of the current induced in the phase winding 22 changes, which may avoid the need to shut down 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 from flowing in an opposite second direction around the low side of the inverter 26, thereby achieving zero-current clamping.

[0073] therefore, Figure 7 The first clamping cycle C1 in the example of is achieved by maintaining the inverter 26 in a switching configuration in which each of the pair of high-side switches (i.e., Q1 and Q3) is 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. Figure 7 is represented as switch configuration 4.

[0074] The second freewheeling 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 switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open and each of a pair of low-side switches (i.e., Q2 and Q4) is closed. This Figure 7 is represented as switch configuration 5.

[0075] After a predetermined amount of the second freewheeling period FW2 has passed, the low-side switch Q4 is turned off, thereby performing so-called single-device freewheeling or diode freewheeling. Here, the switch is in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is turned off, one of a pair of low-side switches (i.e., Q2) is closed, and the other of a pair of low-side switches (i.e., Q4) is turned off. This Figure 7 is represented as switch configuration 6. Figure 7 It can be seen that between switch configurations 3 and 6 , current flows in opposite directions on the low side of inverter 26 .

[0076] Then, by keeping the inverter 26 in the switching configuration Figure 7 In the second clamping cycle C2 in the example of FIG. 1 , in this switching configuration, each of the pair of high-side switches (i.e., Q1 and Q3) is 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. Figure 7 denoted as switch configuration 7.

[0077] Figure 8 A third set of switch configurations for the above-described method 100 is schematically illustrated, with arrows illustrating current flow around the inverter 26 .

[0078] As described 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 so 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 switching configuration in which one of the pair of high-side switches (i.e., Q1) is turned on, the other of the pair of high-side switches (i.e., Q3) is turned off, and one of the pair of low-side switches (i.e., Q4) is turned on, and the other of the pair of low-side switches (i.e., Q2) is turned off. This is Figure 8 denoted as switch configuration 1.

[0079] The first freewheeling 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 switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open and each of a pair of low-side switches (i.e., Q2 and Q4) is closed. This Figure 8 denoted as switch configuration 2.

[0080] After a predetermined amount of the first freewheeling period FW1 has elapsed, the low-side switch Q2 is turned off, thereby performing so-called single-device freewheeling or diode freewheeling. Here, the switch is in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open, one of a pair of low-side switches (i.e., Q4) is closed, and the other of a pair of low-side switches (i.e., Q2) is open. This Figure 8 3 is the switch configuration.

[0081] Then, by keeping the inverter 26 in the switching configuration Figure 8 In the first clamping cycle C1 of the example of FIG. 1 , in this switching configuration, each of the pair of high-side switches (i.e., Q1 and Q3) is 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. Figure 8 is represented as switch configuration 4.

[0082] The second freewheeling period FW2 is achieved by closing switch Q2 and opening switch Q4, i.e., by switching the direction of freewheeling of a single device or diode. Here, the switches are in a switching configuration in which each of a pair of high-side switches (i.e., Q1 and Q3) is open, one of a pair of low-side switches (i.e., Q2) is closed, and the other of a pair of low-side switches (i.e., Q4) is open. This is Figure 8 is represented as switch configuration 5.

[0083] Then, by keeping the inverter 26 in the switching configuration Figure 7 In the second clamping cycle C2 in the example of FIG. 1 , in this switching configuration, each of the pair of high-side switches (i.e., Q1 and Q3) is 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. Figure 8 is represented as switch configuration 6.

[0084] Although described above with respect to low-side freewheeling, it will be appreciated that the techniques used herein may also be implemented in conjunction with high-side freewheeling where appropriate. Furthermore, although described above with respect to continuous excitation cycles, it will be appreciated that the techniques used herein may be used in conjunction with excitation schemes utilizing separate excitation cycles, such as utilizing first and second excitation pulses separated by an intermediate freewheeling cycle.

[0085] exist Figure 6 , 7 In each of the switch configurations of 8 and 8, the second freewheeling period is applied so that the zero crossing point of the back electromotive force induced in the phase winding 22 occurs during the second freewheeling period FW2.

[0086] By performing the second freewheeling period FW2 near the zero-crossing point of the back electromotive force induced in the phase winding 22, demagnetization of the stator 20 of the motor 14 can be achieved. Specifically, by freewheeling around the zero-crossing point of the back electromotive force, a magnetic flux opposite to the magnetic flux of the stator 20 can be generated in the phase winding 22, thereby reducing the magnetic flux density in the stator 20 and reducing the iron loss associated with the motor 14. This can lead to more efficient operation of the motor 14, as well as more efficient operation of products using the motor 14.

[0087] Fig. 9 A vacuum cleaner 300 comprising a brushless permanent magnet motor 14 is schematically shown. Fig.10 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 during an excitation cycle, wherein exciting the phase winding comprises applying a voltage to the phase winding; Freewheeling the phase winding in a first freewheeling period and a second freewheeling period; and commutating the phase winding at a commutation time measured relative to a zero crossing of a back electromotive force induced in the phase winding; The first freewheeling period starts at the end of the excitation period, and the second freewheeling period causes a zero crossing point of the back electromotive force induced in the phase winding to occur during the second freewheeling period.

2. The method according to claim 1, wherein: The electric machine includes an inverter for applying a voltage to the phase winding; The inverter includes a first pair of switches and a second pair of switches, the first pair of switches and the second pair of switches being movable between a plurality of switch configurations; and The freewheeling of the second freewheeling period includes placing the inverter in a first switching configuration, wherein one of the first pair of switches and the second pair of switches is closed, and the other of the second pair of switches and the first pair of switches is open. 3 . The method of claim 2 , wherein the method comprises performing zero current clamping between the end of the first freewheeling period and the beginning of the second freewheeling period.

4. The method according to claim 3, wherein: Freewheeling during the first freewheeling period comprises placing the inverter in the first switching configuration; and Performing zero current clamping includes placing the inverter in a second switching configuration in which the first pair of switches and the second pair of switches are open.

5. The method according to claim 3, wherein: Freewheeling during the first freewheeling period includes placing the inverter in the first switching configuration and placing the inverter in a subsequent switching configuration in which one of the first pair of switches and the second pair of switches is open and the other of the second pair of switches and the first pair of switches has one open switch and one closed switch; and Performing zero current clamping includes placing the inverter in a subsequent switching configuration.

6. The method according to any one of claims 2 to 5, wherein freewheeling of the second freewheeling period comprises placing the inverter only in the first switching configuration.

7. A method according to any one of claims 2 to 5, wherein the freewheeling of the second freewheeling period includes placing the inverter in the first switching configuration and placing the inverter in a subsequent switching configuration, wherein one pair of the first pair of switches and the second pair of switches is open, and the other pair of the second pair of switches and the first pair of switches has an open switch and a closed switch.

8. The method of claim 1, wherein: The electric machine includes an inverter for applying a voltage to a phase winding; The inverter includes a first pair of switches and a second pair of switches, the first pair of switches and the second pair of switches being movable between a plurality of switch configurations; and Freewheeling of the second freewheeling period includes placing the inverter in a first switching configuration in which one of the first and second pairs of switches is open and the other of the second and first pairs of switches has one open switch and one closed switch. 9 . The method of claim 8 , wherein the method comprises performing zero current clamping between the end of the first freewheeling period and the beginning of the second freewheeling period.

10. The method according to claim 9, wherein: Freewheeling during the first freewheeling period includes placing the inverter in a second switching configuration, wherein one of the first pair of switches and the second pair of switches is closed, and the second pair of switches and the other of the first pair of switches are open, and placing the inverter in the first switching configuration; and Performing zero current clamping includes placing the inverter in the first switching configuration.

11. A brushless permanent magnet electric machine comprising a phase winding and a controller configured to perform a method according to any one of the preceding claims.

12. The brushless permanent magnet motor of claim 11, wherein the brushless permanent magnet motor comprises an inverter, the inverter comprising a first pair of switches and a second pair of switches, the first pair of switches comprising a pair of high-side switches, the second pair of switches comprising a pair of low-side switches.

13. The brushless permanent magnet motor of claim 12, wherein the brushless permanent magnet motor comprises a single-phase brushless permanent magnet motor.

14. The brushless permanent magnet motor of claim 13, wherein freewheeling in the second freewheeling period comprises freewheeling using the second pair of switches.

15. The brushless permanent magnet motor according to any one of claims 12-14, wherein the brushless permanent magnet motor comprises a current sensor located at the low side of the inverter.

16. The brushless permanent magnet motor according to any one of claims 12-15, wherein the brushless permanent magnet motor comprises two current sensors located at the low side of the inverter.

17. A data carrier comprising machine readable instructions for operating one or more controllers of a brushless permanent magnet electric machine to perform the method according to any one of claims 1 to 10.

18. A vacuum cleaner comprising the brushless permanent magnet motor according to any one of claims 11 to 16.

19. A hair care appliance comprising a brushless permanent magnet motor according to any one of claims 11 to 16.

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