WO2020234686A1

WO2020234686A1 - Motor and motor control apparatus

Info

Publication number: WO2020234686A1
Application number: PCT/IB2020/054422
Authority: WO
Inventors: Linda Caroline LEE
Original assignee: Lee Linda Caroline
Priority date: 2019-05-11
Filing date: 2020-05-11
Publication date: 2020-11-26

Abstract

A method and apparatus for controlling operation of a BLDC, the BLDC comprises a plurality of stator windings, and the stator windings are isolated or isolatable so that a stator current flowing in one stator winding is independent of another stator current flowing in any other stator windings of the stator. The method includes sending at least a probing voltage pulse and at least a drive voltage pulse to the stator during a time slot and to detect an event for commutation. An event to be utilized to trigger commutation is when the rate of rise of drive current first becomes faster than the rate of rise of probing pulse current.

Description

MOTOR AND MOTOR CONTROL APPARATUS

Field

[01 ] The present disclosure relates to brushless DC (BLDC) motors, as well as methods and apparatus for driving BLDC motors.

Background

[02] Brushless DC motors have been recognized as a useful type of motors which can meet modern power efficiency requirements, especially in requirements where power rating of a motor is limited. As BLDC motors do not have brush commutators, novel means of electronic communication is required.

Disclosure

[03] A method and apparatus for controlling the operation of a BLDC is disclosed. The example BLDC comprises a plurality of stator windings. The stator windings are isolated or isolatable so that a stator current flowing in one stator winding is independent of another stator current flowing in any other stator windings of the stator. This method includes sending at least a probing voltage pulse and at least a drive voltage pulse to the stator during a time slot and to detect an event for commutation. An example event to trigger commutation is when the time taken by the drive current to reach a reference level is shorter than the time taken by the probing pulse current to reach the same reference level. An example event to be utilized to trigger commutation is when the rate of rise of drive current first becomes faster than the rate of rise of probing pulse current.

[04] According to the disclosure, there is disclosed a motor control apparatus comprising a controller and a switching circuitry for controlling operation of a BLDC motor. The motor comprises a stator and a rotor surrounded by the stator, and the stator comprises a plurality of circumferentially distributed stator windings. The switching circuitry is for making electrical connection between a BLDC motor and a power supply. The controller is to execute stored instructions to operate the switching circuitry to facilitate supply of stator power and stator current from the power supply to the motor. The stator current comprises probing current and drive current. The stator power comprises probing power and drive power. The switching circuitry comprises a plurality of electronically switchable networks, and each switchable network is assigned to operate a stator winding as a corresponding stator winding and is in exclusive electrical connection therewith. Adjacent stator windings of the stator are electrically isolatable from each other by switching operation of a switchable network. The controller on execution of stored instructions is to operate the switching circuitry to facilitate flow of drive power into the stator, to operate the switching circuitry to facilitate flow of probing signals into the stator, to monitor electrical responses of the stator in response to the probing signals and in response to the drive power, to determine occurrence of a commutation signal signifying commutation with reference to the electrical responses, and to trigger commutation upon detection of the commutation signal.

Figures

[05] The present disclosure is described by way of example with reference to the accompanying figures, in which:

Figure 1 is a schematic diagram of a BLDC motor having an example plurality of three isolated stator windings,

Figure 1A is a block diagram depicting an example BLDC motor assembly comprising a BLDC and a motor controller 100,

Figure 2 is a schematic diagram showing stator windings of the BLDC motor of Figure 1 connected to a DC supply rail by means of example drive circuitry,

Figures 2A to 2F are schematic circuit diagrams depicted flow of stator currents through an example embodiment of an example assembly of Figure 2,

Figure 3 is a schematic diagram depicting stator current waveforms at different times of operation within a drive current cycle of a set of stator windings,

Figure 4A is a schematic diagram depicting stator current waveforms immediately before and after commutation according to the disclosure, and

Figure 4B shows the stator responses of a stator winding to drive power pulses D and probing power pulses P of the example of Figures 2A to 2F.

Description

[06] An example brushless DC motor comprises a motor shaft, a rotor on the motor shaft, a stator, and a motor housing. The motor shaft defines a motor axis and the rotor is to rotate about the motor axis. The motor axis and the rotor axis are coaxial. The rotor comprises a rotor body on which a plurality of permanent magnetic polar surfaces is disposed. The permanent magnetic polar surfaces have opposite magnetic polarities and permanent magnetic polar surfaces having opposite magnetic polarities magnetic polarities are distributed alternately around an outer periphery of the rotor. The magnetic polar surfaces and the stator windings are oppositely facing and are in physical proximity thereto. The magnetic polar surfaces of the rotor (which are oppositely facing the stator windings), are radially the outermost surfaces of the rotor which collectively define a rotor outermost surface. Each stator winding has a stator winding inner surface which is oppositely facing the rotor and is radially inward-facing. The stator winding inner surface is a radially inner-most surface of the stator winding. The stator winding inner surfaces of the plurality of stator windings collectively define stator windings inner-most surface. A circumferential air gap of substantially uniform thickness is formed between the rotor outermost surface and the stator winding innermost surface.

[07] Each magnetic polar surface may be a salient polar surface protruding from the rotor body and an air gap is disposed between adjacent magnetic polar surfaces of opposite magnetic polarities. Alternatively, the magnetic polar surfaces having opposite magnetic polarities are alternately disposed to form a continuous outer peripheral surface of the rotor. The rotor may comprise a rotor magnet or a plurality of rotor magnets mounted on the motor shaft. Each rotor magnet is a piece of permanent magnet extending along an axial direction and having a magnetic polar surface extending in the axial direction and radially-facing the stator, the axial direction being parallel to but radially offset from the rotor axis. Each radially-facing magnetic polar surface of a rotor magnet is rotatable between a first angular position when the magnetic polar surface is directly and oppositely facing the magnetic polar surface of a stator winding and a second angular position when the magnetic polar surface of the permanent magnet is 180-degree from the first configuration. The rotor is mechanically supported by a rotor bearing or a pair of axially distributed rotor bearings and the motor shaft defines a motor axis. During operations, the motor shaft is to rotate about the motor axis to output driving power. The motor housing typically comprises a metal casing which is configured as a metal shell to define a cylindrical motor windings compartment. In low-power applications, the motor housing can be a plastic casing.

[08] When in motor operations, a plurality of stator windings is activated at the same time to generate rotating torques to rotate the rotor about the rotor axis. Adjacent rotor-facing polar surfaces of the plurality of activated stator windings typically have opposite magnetic polarities. In some embodiments, adjacent rotor-facing polar surfaces of the plurality of activated stator windings typically have same magnetic polarity. The magnetic polar surfaces of the stator windings and the corresponding magnetic polar surfaces of the stator may be arranged into a push-push, push-pull or pull-pull relationship. An activated stator winding means the stator winding is being activated by a DC driving current having a magnitude which is sufficient to drive the rotor into rotation. Because the magnetic polar surfaces of the rotor are due to magnetic fields of permanent magnets, no commutation or brush is required to change the magnetic polarity of the magnetic polar surfaces of the rotor and hence the DC motor is a brushless motor. The activating current passing through a stator winding has current directions or current polarities which change with time and has a zero-current duration during which no activating current passes through the stator winding.

[09] A circumferentially extending air gap is maintained between the rotor and the stator. The air gap defines a minimum radial clearance between the rotor and stator so that the rotor can freely rotate about the rotor axis not obstructed by the stator or the stator windings. During operations, magnetic flux generated by current flowing through the stator windings extends across the radial air gap to magnetically interact with the rotor magnets and drives the rotor into rotation.

[010] An example stator winding comprises a plurality of stator windings. The stator windings are spatially distributed around an inner periphery of the motor housing to circumferentially surround the rotor. In some embodiments, each stator winding occupies a circumferential portion of the inner periphery and the spatial extent of adjacent stator windings may or may not overlap. Each stator winding comprises a first electrical terminal, a second electrical terminal and a coil of wound wire defining a magnetic axis, the wound-wire coil electrically interconnecting the first and second electrical terminals. Each coil winding comprises a magnetic polar surface which is proximal to and radially opposite-facing the rotor. The magnetic polar surface has a first magnetic polarity when a direct current of a first current direction flows through the winding and a second magnetic polarity when a direct current of a second current direction flows through the winding, the first current direction being opposite to the second current direction and the first magnetic polarity being opposite to the second magnetic polarity. Each stator winding is optionally a closed winding or a two-terminal winding so that the amplitude of current flowing into the winding through the first terminal and the current flowing out of the winding through the second terminal are the same. In a two-terminal winding, there is no shunt path to current to enter or leave the winding except at the first terminal or the second terminal.

[01 1 ] An example BLDC motor comprises a stator having an example plurality of three stator windings R, S, T, as depicted in Figure 1. The three example stator windings R, S, T are referred to, respectively, as a first stator winding R, a second stator winding 5, and a third stator winding T for ease of reference. The stator windings are isolatable stator windings with no permanent inter winding electrical connection, although the stator windings are electrically connectible to form different groups of stator windings at different times of operations. The stator windings are arranged in a predetermined spatial order. In this example, the stator windings R, S, T are disposed in a clockwise manner around the inner periphery of the motor housing. Each stator winding comprises a first terminal R_{l t} S_{l t} T_{l t} a second terminal R₂ , S₂ , T₂ , and a winding coil interconnecting the first terminal R_lt S_lt T and the second terminal R₂, S₂, T₂.

[012] As a convenient reference, a stator current entering the stator windings at the first terminal and exiting at the second terminal is assigned a first current direction or a first current flow direction and a stator current entering the stator windings at the second terminal and exiting at the first terminal is assigned a second current direction or a second current flow direction which is opposite to the first direction. The first current direction can be clockwise or anticlockwise, depending on the order of the terminals with respect to the motor axis. When the first current direction is clockwise, the second current direction is anti-clockwise and vice versa. The example R, S, T stator windings are ordered sequentially in a clockwise direction.

[013] The stator windings are circumferentially distributed around an inner periphery of the motor housing to surround the rotor. Each stator winding is spatially distributed for a circumferential extent along the inner periphery of the motor housing and the circumferential extent defines an angular extent with respect to the motor axis. The example BLDC has an example plurality of three stator windings and each stator has an angular extent of no more than 120 degrees.

[014] Each stator winding is connected to a driving power source so that the entire driving current coming from the power source either:

a) enters the stator winding through the first terminal, transits through the winding in a first current direction, and exits through the second terminal, or

b) enters the stator winding through the second terminal, transits through the winding in a second current direction opposite to the first current direction, and exits through the first terminal.

[015] The stator windings have no permanent inter-winding electrical connection, such that adjacent stator windings do not have a permanently connected common node, which is an inter winding node of electrical connection. An inter-winding node is temporarily formed by switching operations of a drive network, which is in turn operated by a drive controller. In example embodiments, the plurality of stator windings are stand-alone windings each having two terminals and no intra-winding terminal between the two terminals. Where the stator windings consist of two-terminal stator windings only, stator current flowing in a stator winding will not flow into another winding via an electrical path which is intermediate the two terminals, which are a first terminal and a second terminal.

[016] An example BLDC motor assembly 10 comprises the BLDC motor 100 and a motor controller 120, as depicted in Figure 1A. The motor controller comprises drive circuitry 140 and a drive controller 160 to control operation of the drive circuitry 140 so that predetermined combinations of winding currents are to flow through the stator windings in a sequence to drive the rotor to rotate in a predetermined direction and speed, as well as facilitating commutation control.

[017] An example drive circuity comprises a plurality of drive networks H_R, H_s, H_T for controlling the flow of drive current in the plurality of stator windings, as depicted in Figure 2. In example embodiments such as the present, each drive network is dedicated to facilitate flow of stator current through a stator winding so that the number of drive networks and the number of stator windings are same or equal. Since a drive network is dedicated for a specific stator winding, each stator winding has a corresponding or associated drive network and vice versa. An example drive network comprises a pair of supply-connection terminals for connection to the power supply and a pair of stator-connection terminals for connection with the terminals of a stator winding. The pair of stator-connection terminals comprises a first terminal and a second terminal. When current flows from the first terminal into a stator winding and flows out of the stator winding from the second terminal, the current is said to flow in a first direction. When current flows from the second terminal into a stator winding and flows out of the stator winding from the first terminal, the current is said to flow in a second direction, which is opposite to the first direction.

[018] An example drive network comprises a plurality of switches. The switches may be electronic switches, for example MOSFET or IGBT switches, electro-mechanical switches such as relays, or their combination. The switches are configured to define a plurality of possible switchable current paths. Each switchable current path is activated to become operational when some of the plurality of switches are selectively operated, for example operated by the drive controller. The plurality of possible switchable current paths may be unipolar paths. A unipolar path is one in which current flows in only one direction, but not another or opposite direction.

[019] The drive network comprises a plurality of switchable current paths. The switchable current paths are defined by the operation states of the plurality of switches. The plurality of switchable current paths includes a first switchable path when the plurality of switches is in a first switching configuration state and a second switchable path when the plurality of switches is in a second switching configuration state. When the drive network is in the first switching configuration state, a first set of switches forming a first combination is in a conductive or ON state to define the first switchable path to facilitate flow of current through the stator winding in the first direction. When the drive network is in the second switching configuration state, a second set of switches forming a second combination (different to the first combination) is in a conductive or ON state to define the second switchable path to facilitate flow of current through the stator winding in the second direction. The drive network is operable in a third switching configuration state in which state the drive network forms a high impedance path which is to block flow of current between the power supply and the stator winding so that no drive current is to flow into the stator winding through the drive network. The switchable paths are formed by selective operation of the switches, for example, by operation of the drive controller.

[020] The drive controller is to control the drive circuitry whereby the flow paths of currents in the plurality of stator windings are controlled and changed. An example motor controller is a solid- state controller, for example, a microprocessor-based or an FPGA-based controller board or 1C (integrated circuit). The 1C may be an application specific 1C (ASIC) with built-in control instructions to control operations of the drive circuitry. A power bridge, which is a single-direction configuration of an H-bridge motor controller, is an example drive network suitable for the present disclosure.

[021 ] In operation, only one switchable current path of a drive network is to be activated at any one time. The time during which a switchable current path of a drive network is activated and operational is referred to as a conduction window. The conduction window of the switchable current path defines a conduction period. During a conduction window, a stator current is to flow from the power source into an associated stator winding through a drive network.

[022] Example operations of an example motor and motor controller are described herein with reference to a time axis, which is labelled with a time original t₀ and a plurality of time slots T_s. Each time slot is a time period reserved for supply of power into the stator, and is also referred to as a supply time period. A supply time period in examples herein has a fixed or predetermined length of time, but the supply time periods may have variable lengths without loss of generality. A supply time period may have an associated conduction window. A conduction window is allocated within a supply time period. A conduction window defines a stator conduction period which is usually shorter than a supply time period. In some embodiments, a conduction period may have the same time span of a supply time period.

[023] Example operations of an example BLDC are described herein with reference to an example duration of time. The example duration of time is a time span corresponding to an example duration of motor operation time. The example duration of motor operation time comprises an initial time and a plurality of time slots immediately following the initial time. The initial time is labelled as time zero (t₀) or t = 0 in the time axis as a convenience reference. The time slots are arranged consecutively to form a series of time slot in which the component time slots are in sequence. Each time slot has a prescribed or predetermined width of time, and the plurality of time slots forms a series of time slots, with no time gap and no overlap in time between adjacent time slots.

[024] The motor, which is connected to the power supply by the drive circuitry, may be in a stand by mode or in an operational mode which is also referred to as drive mode. When in the drive mode, drive current sufficient to rotate the rotor is to flow through the drive circuitry into the stator windings. The magnetic fields generated in the stator windings as a result of drive current flowing through the stator windings interact with the permanent magnetic fields of the rotor and generate a rotation torque which is sufficient to drive the rotor to rotate. When in the stand-by mode, the rotor is stationary at a stand-still position. When the motor changes from the stand-by mode to the drive mode, the rotor accelerates from zero RPM (revolution-per-minute) and gradually increases its speed towards a rated speed.

[025] The drive controller is to operate the drive circuitry to control flow of power from the power supply to the motor. The power supply is typically a direct current (DC) power source, for example, a constant voltage DC source such as a battery in low-power applications, but can be other types of power source such as a switching mode power supply without loss of generality. In some embodiments, the power source may be an AC (alternating current) source with rectifying circuitry to support DC operations. The power which is flowed into motor is typically characterized by the phase and amplitude of the stator current which flows into the motor and/or the winding voltages across the component stator windings. The stator current at a specific time is the total current which flows into the stator and comprises component currents which flow in the individual stator windings during the specific time. A component current which flows in an individual stator winding is referred to as a winding current herein.

[026] The example motor is a multi-phase BLDC motor comprising a plurality of stator windings, the stator windings being electrically isolatable from each other. The stator windings can be selectively connected, for example, by electronic switching by the controller, so that stator current can flow across the stator along one or a selected plurality of stator windings among the plurality of stator windings at a selected time or any selected times. By selective switching operation of the drive circuitry to selectively isolate and/or connect the stator windings from the power supply, a winding current may flow through a stator winding in a first direction, a second direction or not flow through the stator winding. [027] In example embodiments such as the present, the controller is to selectively activate the switches of the drive circuitry to form a first current path so that a winding current is to flow from the power supply through the stator winding in a first current direction, to form a second current path so that a winding current is to flow from the power supply through the stator winding in a second current direction opposite to the first direction, or to form no current path so that no winding current is to flow through the stator winding.

[028] For example, to form a selected switchable current path, the drive controller is to close a pre-determined or selected set of switches of the drive network to activate a selected current path.

[029] For example, when the first switchable current path is to be activated, the drive controller is to close a first set of switches which defines the first switchable current path to allow a current to flow in the first direction through the stator winding. When the second switchable current path is to be activated, the drive controller is to close a second set of switches which defines the second switchable current path to allow a current to flow in the second direction (opposite to the first direction) through the stator winding; and when no current is to flow through the drive network, all or at least some of the switches which define a current path are to be opened.

[030] During operations, the drive controller is to operate the drive network so that one set of the activatable or available switchable current flow paths of the drive network H_R, H_s, H_T is activated during a first conduction time window to facilitate stator current flow, while another set of the available switchable current flow paths is to be activated at a second conduction time window which is at a time duration different to and non-overlapping with the first conduction time window. In the present example, a set of switchable current flow paths comprises a flow of drive current through two stator windings at one drive portion of a time slot and a flow of probe current through two stator windings at one probe portion of the time slot, wherein the probe portion and the drive portion are at different times of the time slot. During the time slot, all the three stator windings have current flow, although drive current flows through two of the three stator winding and probe current flows through two of the three stator windings such that one of the stator windings has both drive and current flow, one of the stator windings has only drive current flow and one of the stator windings has only probe current flow in the example where the BLDC is a three-phase motor comprising or consisting of three stator windings. In some embodiments, a set of switchable current flow paths comprises one stator winding or more than two stator windings. In some embodiments such as the present, the drive network is non-conductive outside the first and second conduction windows.

[031 ] For example, the switches H_R1 and H_R4, H_S1 and H_S4, H_T1 and H_T4 form a first current flow path and the switches H_R3 and H

H_T3 and //_r2,form a second current path of the drive network H_R, H_s, H_T respectively.

[032] In example motor operations, the plurality of drive networks H_R, H_s, H_T is to operate in synchronization so that their time slots are aligned in the time domain. When time slots are aligned in the time domain, the boundaries or borders of the respective times slots are aligned in the time domain.

[033] In example operations, a selected plurality of the plurality of stator windings is to be activated in an activated mode during a time slot. When the selected plurality of stator windings is activated, stator power is to flow into the selected plurality of stator windings at the same time or same times in the same time slot. A stator winding is activated when a current path connecting the stator winding and the power supply is activated without loss of generality. A stator winding may be in a probing mode during which probing power is to flow or a drive mode during which drive power is to flow when in the activated mode.

[034] In example embodiments, drive power in the selected plurality of activated stator windings flows in different directions across the stator windings. Where two stator windings are activated to operate in the drive mode, drive power is to flow in opposite directions through the stator windings.

[035] In example embodiments, probe power in the selected plurality of activated stator windings flows in different directions across the stator windings. Where two stator windings are activated to operate in the probing mode, probing power is to flow in opposite directions through the stator windings.

[036] Table 1 shows example allocation or distribution of flow of drive power in selected plurality of stator windings.

[037] Table 1 -Drive power

[038] Referring to Table 1 , a plus (+) sign means that drive power (and more specifically, drive current) is to flow in the first direction from terminal 1 to terminal 2 of the stator winding and that the drive voltage at terminal 1 is higher than the drive voltage at terminal 2, a minus (-) sign means drive power (and more specifically, drive current) is to flow in the second direction from terminal 2 to terminal 1 of the stator winding and that the drive voltage at terminal 2 is higher than the drive voltage at terminal 1 , and“0” means no drive power (and no drive current) is to flow between terminal 1 and terminal 2 of the stator winding.

[039] Each combination of drive power among the selected plurality of stator windings is referred to as a drive power group (or“group” in short”).

[040] When in group 1 conditions, drive current in the first direction is to flow through the first stator winding R, drive current in the second direction is to flow through the third stator winding T, and no drive current is to flow through the second stator winding S.

[041 ] When in group 2 conditions, drive current in the first direction is to flow through the second stator winding S, drive current in the second direction is to flow through the third stator winding T, and no drive current is to flow through the first stator winding R.

[042] When in group 3 conditions, drive current in the first direction is to flow through the second stator winding S, drive current in the second direction is to flow through the first stator winding R, and no drive current is to flow through the third stator winding T.

[043] When in group 4 conditions, drive current in the first direction is to flow through the third stator winding T, drive current in the second direction is to flow through the first stator winding R, and no drive current is to flow through the second stator winding S.

[044] When in group 5 conditions, drive current in the first direction is to flow through the third stator winding T, drive current in the second direction is to flow through the second stator winding S, and no drive current is to flow through the first stator winding R.

[045] When in group 6 conditions, drive current in the first direction is to flow through the first stator winding R, drive current in the second direction is to flow through the second stator.

[046] The example BLDC motor has an example plurality of three isolatable stator windings and a selected plurality of two stator windings is to be activated at the same time. The number of drive power groups or drive current groups that can be formed is equal to P, where n is the total number of stator windings, m is the number of stator windings in conduction during a time slot and P is a sign representing operation by permutation, and the drive current in two of the stator windings are in opposite direction.

[047] When there is a plurality of K drive power groups, each drive power group, identified as group 1, 2 is expected to cover an angular extent of L degrees, where L equals 360 /K degrees or thereabout. In the example of six different drive power groups, each drive power group is to cover approximately sixty degrees of rotation. [048] The plurality of K drive power groups form a drive power group cycle and the drive power cycle is to repeat immediately as a next cycle after the last cycle has finished. The drive power groups forming the cycles are in the same sequential order, that is, each drive power cycle comprises the K drive power groups ordered in the same sequence, for example, in the sequence of 1, 2, ... K. The drive power cycles are to repeat cyclically unless and until changed by instructions of the drive controller.

[049] Each drive power group has a specific combination of stator drive powers of specific drive power directions in specific stator windings. The drive power directions in the specific stator windings are selected to provide a more even torque for smoother motor drive operations, and the specific drive power directions in the stator windings are set or determined with reference to the spatial distribution of the permanent magnets of the rotor relative to the specific stator windings. The drive power groups each having drive powers of opposite drive power directions in the selected plurality of the stator windings are for example only. In some embodiments, the drive power directions in the stator windings may or may include the same power directions.

[050] The controller is to operate the drive circuitry to control flow of drive power into the stator so that the drive power conditions of the motor change sequentially among a plurality of predefined drive power groups.

[051 ] The motor is to operate under the drive power conditions of a specific drive power group until the controller changes the operation conditions to those of the next drive power group. While operating under a specific drive power group, the drive power group conditions are to repeat in a plurality of time slots, adjacent time slots are in contiguity with no time gap and the time slots in totally define a drive period comprising a plurality of time slots.

[052] While each drive power group is expected to operate for approximately L degrees, the exact time to change to the next drive power group is dependent on torque conditions to promote smooth drives and timing for commutation to the next drive power group needs to be determined.

[053] In example embodiments, the controller is to execute stored instructions so that drive power is to flow into the stator as a plurality of drive power pulses in a plurality of time slots. The drive power pulses are to flow in the same combination of stator windings while under a specific power drive group condition, although the duration of the power pulses may vary due to change in rotor loading.

[054] The controller is to execute stored instructions so that drive power is to flow into the stator under a specific drive power group conditions and the flow is to continue for a plurality of time slots. The controller is to switch over to the next group of drive power conditions when a commutation signal is detected.

[055] An example implementation of the drive circuitry 140 comprises a plurality of H-bridges H_R, H_s, H_j, as shown in Figures 2A to 2F. Each H-bridge is connected between a stator winding and the power supply rail to function as a drive circuitry and comprises an example plurality of four semiconductor power switches SW_± to SW₄. Each H-bridge comprises a first branch and a second branch. The first branch comprises a first upstream switch SW and a first downstream switch SW₂ which are connected in series and comprises a first inter-switch junction J_± . The second branch comprises a second upstream switch SW₃ and a second downstream switch SW₄ which are connected in series and comprises a second inter-switch junction /₂. The first and second terminals of a stator winding are connected to the first and second inter-switch junctions respectively. The example plurality of switches is arranged and configured to be operable to form two predefined current paths. The predefined current paths comprise a first current path which is defined by the switches SW_± and SW₄ and which facilitates a flow of stator current in the first direction (+).The predefined current paths comprise a second current path which is an alternative path to the first current path. The alternative second current path is defined by the switches SW₃ and SW₂ and this second current path is to facilitate a flow of stator current in the second direction (-)

[056] The switches of the H-bridges H_R, H_s, H_T as labelled on Figures 2A to 2F correspond to the switches SW_± and SW₄ by the below Table 2 for convenience.

[057] Table 2

[058] For example, when the drive circuitry is to operate under group one drive current conditions, as depicted in Table 1 , the drive controller is to switch on the first upstream switch SW_± and the second downstream switch (or the fourth switch) SW₄ of the first H-bridge H_R to form a first current path so that current from the supply is to flow through the first stator winding in a first direction (+) through the switches SW_± and SW₄. At the same time, the switches SW₂ and SW₃ of H_R are turned off to prevent flow of current through the second current path in the second direction (-). All the switches SW₁ to SW^ of the second H-bridge H_s are turned off so that there is no drive current to flow through the second H-bridge H_s. Switches SW₂ and SW₃ of the third H-bridge H_T are turned on to form the second current path so that current from the supply is to flow through the third stator winding in the second direction (-) through the switches SW₂ and sw_3.

[059] Operation of the drive circuitry to facilitate other drive current conditions set out in Table 1 follows the same pattern and applies mutatis mutandis without loss of generality.

[060] As a further example, when the drive circuitry is to operate under group two drive current conditions, as depicted in Table 1 , the drive controller is to switch on the first upstream switch SW_± and the second downstream switch (or the fourth switch) SW₄ of the second H-bridge H_s to form a first current path so that current from the supply is to flow through the second stator winding in a first direction (+) through the switches SW_± and SW₄. At the same time, the switches SW₂ and SW₃ of H_s are turned off to prevent flow of current through the second current path in the second direction (-). All the switches SW_± to SW₄ of the first H-bridge H_R are turned off so that there is no drive current to flow through the first H-bridge H_R. Switches SW₂ and SW₃ of the third H-bridge H_T are turned on to form the second current path so that current from the supply is to flow through the third stator winding in the second direction (-) through the switches SW₂ and sw_3.

[061 ] The motor is to operate for a plurality of times slots while under one drive current group conditions before proceeding to operate under the next drive current group. For example, the motor is to operate for a plurality of times slots while under group 1 drive current conditions. At the end of group 1 drive current operations, group 2 drive current operations will take over. The motor is to operate for a plurality of times slots while under group 2 drive current conditions. At the end of group 2 drive current operations, group 3 drive current operations will take over, and the change over is to continue according to the same pattern and repeat.

[062] The change-over from one drive current group to another drive current group is referred to as commutation, or electronic communication to distinguish from brushed commutation. Commutation is to occur when the torque generated by the drive current group is no longer sufficient or effective.

[063] An example train of voltage pulses resulting from operation of the drive controller operating on the example drive circuitry under an example drive current group is depicted in Figure 3. The voltage pulses comprise a plurality of probing voltages and a plurality of drive voltages. The voltage pulses are substantially rectangular voltage pulses since the example supply rail is at a constant DC voltage. The voltage pulses are arranged in groups and an example group comprises a probing voltage preceding a drive voltage, with drive voltage separating from the probing voltage by a predetermined time. The drive voltage has a much longer duration than the probing voltage and is for generating drive current in the stator winding.

[064] As depicted in Figure 3, the drive current, which is show immediately below a corresponding drive voltage pulse in that figure, rise with time and the rise of drive current is to follow the rise of the drive voltage pulse. An example plurality of three drive voltage pulses D is depicted in Figure 3. The drive voltage pulse D1 is a first drive voltage pulse in the drive cycle under a particular drive current condition of Table 1. The third voltage pulse D3 is the last drive voltage pulse of the drive cycle. The second voltage pulse D2 is the drive voltage pulse that occurs at a time between D1 and D3. Each of the drive voltage pulse has a duration of T_D in time determined by the controller and the drive voltage pulses of Figure 3 are of the same width in time.

[065] It is observed from Figure 3 that the rise time (t₂, t₄, t₆) of the stator drive current progressively shortens with increases of time distance away from the time origin of a drive cycle. For example, the rise time t₂ it takes the drive current in response to the corresponding drive voltage D1 to reach a threshold current _hr_eshom is longer than the rise time t₄ it takes the drive current in response to the corresponding drive voltage pulse D2 to reach the threshold current I _thr_eshold. , where D2 is a driving voltage period after the driving voltage period D1. The rise time t₄ it takes the drive current in response to the corresponding drive voltage D2 to reach the threshold current I _thr_eshold is longer than the rise time t₆ it takes the drive current in response to the corresponding drive voltage DN to reach the threshold current I _thr_eshold’ where DN is a driving voltage period after the driving voltage periods D1 and D2. It is observed from Figure 3 that the rise time (t₄, t₃, t_X) of the stator probe current progressively lengthens with increases of time distance away from the time origin of a drive cycle. For example, the rise time t₄ it takes the drive current in response to the corresponding probe voltage P1 to reach a threshold current t_hr_eshold is shorter than the rise time t₃ it takes the drive current in response to the corresponding drive voltage P2 to reach the threshold current I _thr_eshold, where P2 is a second probing voltage period after the first probing voltage period P1. The rise time t₃ it takes the probe current in response to the corresponding probe voltage pulse P2 to reach the threshold current _hr_eshom is shorter than the rise time t_X it takes the probe current in response to the corresponding probe voltage PN to reach the threshold current /_threshoid, where PN is a probing voltage period after the probing voltage periods P1 and P2. As depicted in Figure 3, t₂ > t_c during the first time slot containing the drive pulse D1 , t₄ > t₃ during the second time slot containing the drive pulse D2, and t_X > t₆ during the Nth time slot containing the drive pulse DN. The reversal in rise-time between the drive current and current, that is, from T_D > t_R to T_D < t_R indicates an example event to trigger commutation.

[066] Upon studies, it is noted that the differences in drive current rise time are due to the change in inductance of the stator winding due to change in magnetic flux interaction with the permanent magnets on the rotor.

[067] The total flux linkage of a stator winding of a BLDC motor can be expressed as: ^_{stator winding} X _{ma net} T Li, where X _magnet is flux linkage due to the permanent magnets on the rotor, Li is flux linkage dur to drive current and L is the inductance of the energized stator winding. The total flux linkage of a stator winding of a BLDC motor has non-linear characteristics due to magnetic saturation.

[068] It is noted that inductance and flux linkage are also affected by stator current directions. Where drive current i⁺ in the stator winding is to generate flux having same direction as the immediately adjacent permanent magnet, the inductance L⁺ can be expressed as L⁺ =

[069] Where drive current i in the stator winding is to generate flux having opposite direction as the immediately adjacent permanent magnet, the inductance LT can be expressed as:

[070] _w|-,_ere A+is change in flux linkage due to _t ⁺ and

AX^~ s change in flux linkage due to i^~.

[071 ] The flux linkage change AX⁺, which is due to t⁺, is smaller than AX^~, and inductance L⁺ is smaller than L^~ . These flux change properties are used to facilitate determination of time for commutation from one drive current condition group to next.

[072] In order to determine time for drive power group commutation, probing voltage pulses are used. Example probing power (and therefore probing currents) associated with the drive currents of Table 1 are set out in Table 3 below. [073] Table 3 probe voltage

[074] The same convention and nomenclature of Table 1 are used for this Table 3. For example, when in group 1 drive current condition, no probe voltage is to flow in the first stator winding R, a probe current in the first direction or positive direction is to flow in the second stator winding 5, and a probe current P in the second direction or negative direction is to flow in the third stator winding T.

[075] Combining Tables 1 and 3, the probe and drive voltages under the example plurality of drive current conditions are combined and set out in Table 4 below.

[076] Table 4 probe and drive current combined

[077] When commutation is to take place so that the drive current group changes from one group to the next group, the drive current in only one of the two conducting stator windings is switched.

[078] For example, the first stator winding R and the third stator winding T are conductive with drive currents in opposite directions when under group 1 condition. To change to the next group (group 2 conditions), the drive current in the first stator winding R is to change from the first current direction (+) to no drive current flow (0) while the drive current condition in the third stator remains unchanged, that is, remains to have drive current flow in the second direction (-). At the same time, the second stator winding 5 changes from no drive current flow to drive current flow in the first direction (+).

[079] To enhance smooth transition, one of the stator windings is to change from drive current conducting to no drive current conduction, one of the stator windings is to change from no drive current conduction to drive current conduction, and the drive current in in of the stator windings is to remain unchanged. [080] As another example, the second stator winding 5 and the third stator winding T are conductive with drive currents in opposite directions when under group 2 condition. To change to the next group (group 3 conditions), the drive current in the third stator winding T is to change from the second current direction (-) to no drive current flow (0), the drive current condition in the second stator 5 remains unchanged, that is, remains to have the drive current flowing in the same first direction (+). At the same time, the first stator winding R changes from no drive current flow to drive current flow in the second direction (-).

[081 ] When the motor is in drive operation, operations of the three stator windings are in drive synchronization with reference to the same time reference axis. For example, when the first stator is in the first time slot under the first current group condition, the second and third stator windings are also operating in the same fist time slot with the same time reference. However, the conduction windows are different among the three stator windings.

[082] For example, when in the first drive current group condition, the first stator R has a single conduction window defined for drive current D, the second stator winding 5 has a single conduction window defined for probe current P, and the third stator winding T has two conduction windows, namely, a probe current window P and a drive current window D, with conduction windows of the same type in the same time slot time aligned. The conduction windows are time aligned such that all the P pulses in the same time slot have the same time width and the beginning and end of the P pulses are aligned at the same time, and all the D pulses in the same time slot have the same time width and the beginning and end of the D pulses are aligned at the same time.

[083] As another example, when in the second drive current group condition, the first stator winding R has a single conduction window defined for probe current P, the second stator winding 5 has two conduction windows, namely, a probe current window P and a drive current window D, and the third stator winding T has a single conduction window defined for drive current D, and again with conduction windows of the same type in the same time slot time aligned.

[084] To obtain instantaneous inductance information of a stator winding in order to prepare for commutation, a plurality of probing voltages P is inserted into a corresponding plurality of stator windings while operating in one drive current group. The probing voltage pulses P are to be inserted in stator windings except one which is to change from drive current conduction to no drive current conduction. For example, in the example first drive current group, the first stator winding R is to change from drive current conduction in the first direction (+) to no drive current conduction (0) in the next group and no probe current is inserted in the time slots of R while in the first drive current group, and probing voltage pulses are inserted in the other two stator windings 5 and T.

[085] In example embodiments, a probing voltage pulse is formed at or near beginning of a time slot, while a drive voltage pulse is formed at or nearing the end of a time slot. A drive pulse is preferably disposed at end of the time slot to leave room for the drive pulse to expand towards the probing pulse or time origin of the time slot, for example, when rotor load increases and drive power needs increase.

[086] A typical probing voltage pulse is a narrow pulse while a drive pulse is a wide pulse. A drive pulse has to be a wide pulse compared to the probing pulse since the drive power conferred by a drive pulse is roughly or approximately proportional to the width of the drive pulse. A probing pulse is designed to send a probing signal to probe the instantaneous inductance or change thereof to determine communication and the width can be very narrow, for example, 2-5% the width of the time slot.

[087] Figures 4A and 4B show the stator responses of example stator windings to drive power pulses D and probing power pulses P immediately before and after commutation.

[088] At time slot N, an event of commutation has occurred. The event of commutation is taken to occur when there is a reversal in rise-time between the drive current and probe current. The reversal represents a change from T_D > t_R to T_D £ T_p, where T_D represents the time for drive current to rise to a predefined value or threshold, and t_R represents the time for probe current to rise to the predefined value or threshold. This reversal is taken as a signal indicating an event of commutation and the controller is to switch operations of the drive network to the next stator windings group.

[089] The changes in probing current rise rate (or rise time to the reference level) and drive current rise rate (or rise time to the reference level) are due to changes in instantaneous inductance of a stator winding. The variation in instantaneous inductances of a stator winding is related to the stator current by the equation: V_s = Ri +

+ e , where V_s is the stator voltage, i is the stator current, R is the stator resistance, and e is back-emf. Where there is no back-emf , t =

[090] A first occurrence of the condition that the drive current rise time is faster the probing pulse current rise time within a drive period comprising a plurality of time slots indicates a change of stator inductance due to change in relative position between the stator winding and rotor magnetic field and signifies time to commutate to compensate losing torque. In example embodiments as shown in Figure 4B, the controller is to activate commutation at the next time slot and to enter the next drive power group when the time for a current to rise to a threshold changes from T₂ > 7 to T₂ £ T_lt where T₂ is the time for the drive current to reach the threshold and T is the time for the probe current to reach the threshold.

[091 ] Upon detection of the change-over event of the stator response changing from a faster rising probing current to a faster rising drive current, the controller is to trigger commutation to the next power drive group.

[092] In example embodiments, the controller is to determine the time t_D taken by a drive current D to reach a threshold voltage time V_threshoid and the time tptaken by the probe current to reach the threshold voltage. The time can be measured by using a digital counter with reference to the rising edge of the corresponding drive voltage and the corresponding probing voltage.

[093] To determine whether a stator current has risen to the threshold voltage, comparators may be connected to the output of a current sensor in series with the stator. An example threshold voltage of 0.3V when the stator supply voltage is at 1 volt. In general, the threshold voltage may be between 5-30% of the stator supply voltage, including 5%, 10%, 15%, 20%, 25%, 30% and a range or ranges formed by combination of any of the aforesaid values.

[094] In example embodiments, the threshold voltage is monitored by means of a current sensor resistor, as depicted in Figures 2A to 2F. Irrespective of power groups, the voltage across the current sensing resistor is due to the combined probing currents flowing through the stator windings during the probing window and due to the combined drive currents flowing through the stator windings during the drive window.

[095] While the disclosure has been described with reference to example embodiments, it should be appreciated that the example embodiments are not to limit the scope of disclosure. For example, while a three-phase BLDC motor having three stator windings has been used as a reference, the BLDC motor may have more than three phases, that is more that more isolated stator windings without loss of generality. In some embodiments, the three stator windings may be connected to three separate power supplies so that the stator windings are isolated totally.

Claims

1 . A motor control apparatus comprising a controller and a switching circuitry for controlling operation of a BLDC motor, wherein the motor comprises a stator and a rotor surrounded by the stator, and the stator comprises a plurality of circumferentially distributed stator windings; wherein the switching circuitry is for making electrical connection between a BLDC motor and a power supply, and the controller is to execute stored instructions to operate the switching circuitry to facilitate supply of stator power and stator current from the power supply to the motor, wherein the stator current comprises probing current and drive current, and the stator power comprises probing power and drive power; wherein the switching circuitry comprises a plurality of electronically switchable networks, and each switchable network is assigned to operate a stator winding as a corresponding stator winding and is in exclusive electrical connection therewith; wherein adjacent stator windings of the stator are electrically isolatable from each other by switching operation of a switchable network; and wherein the controller on execution of stored instructions is:

- to operate the switching circuitry to facilitate flow of drive power into the stator,

- to operate the switching circuitry to facilitate flow of probing signals into the stator,

- to monitor electrical responses of the stator in response to the probing signals and in response to the drive power,

- to determine occurrence of a commutation signal signifying commutation with reference to the electrical responses, and

- to trigger commutation upon detection of the commutation signal.

2. The control apparatus according to Claim 1 , wherein the drive power is in the form of a train of drive voltage pulses and the electrical response of the stator to the drive power is in the form of a train of drive current pulses, wherein the probing signal is in the form of a train of probing voltage pulses, and the electrical response of the stator to the probing signal is in the form of a train of probing current pulses, the probing voltage pulse having same voltage magnitude as the drive voltage pulse; and wherein the controller is to monitor magnitude of the probing current pulses and magnitude of the drive current pulses, and to trigger commutation when magnitude of a drive current pulse reaches a commutation threshold before or earlier than a corresponding probing current pulse.

3. The control apparatus according to Claims 1 or 2, wherein the controller is to monitor a stator voltage to determine whether magnitude of the drive current has reached a commutation threshold has been reached, wherein the stator voltage is a voltage which is proportional to the stator current which flows through the stator.

4. The control apparatus according to any preceding Claims, wherein the stored instructions of the controller are to facilitate operation of the switching circuitry such that the stator power is to flow into the stator in a plurality of time slots, each time slot defining a time slot period; and wherein each time slot defines a drive window having a drive window period during which the drive current is to flow and a probe window having a probe window period during which the probing signal is to flow through the stator; and wherein the drive window period is substantially longer than the probe window period and separated from the probe window period by a time gap.

5. The control apparatus according to Claim 4, wherein the probe period has a duration of less than 10% of the time slot period, for example, 1 %, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10 % of the time slot period, or a range or ranges formed by combination of any of the aforesaid values.

6. The control apparatus according to Claims 4 or 5, wherein the time gap between the probe window and the drive window has a duration of 2 times the duration of the probe period or more, including 3, 4, 5, 6 times, or a range or ranges formed by combination of any of the aforesaid values.

7. The control apparatus according to any of Claims 4 to 6, wherein the probe window precedes the drive window.

8. The control apparatus according to any of Claims 4 to 7, wherein the probing voltage pulses have variable time durations.

9. The control apparatus according to any preceding Claims, wherein the controller is to operate the switching circuitry so that the stator power delivered to the stator in a time slot comprises a plurality of stator winding power types, and wherein the plurality of stator winding power types comprises a first type in which only a probe pulse is present in a time slot, a second type in which only a drive pulse is present in a time slot, and a third type in which both a probe pulse and a drive pulse is present in a time slot.

10. The control apparatus according to Claim 9, wherein the controller is to operate the switching circuitry in a plurality of power supply modes, wherein the stator power delivered to the stator in a time slot of each power supply mode comprises a first type of stator winding power to flow through one stator winding, a second type of stator winding power to flow through another stator winding, and a third type of stator winding power to flow through yet another stator winding.

1 1 . The control apparatus according to Claims 9 or 10, wherein the probe pulse of the first type and the probe pulse of the third type are to flow through different stator windings in opposite flow directions, and the drive pulse of the second type and the drive pulse of the third type are to flow through different stator windings in opposite flow directions.

12. A BLDC motor assembly comprising a BLDC motor and the control apparatus according to any preceding claim, wherein the motor comprises a stator and a rotor surrounded by the stator, and the stator comprises a plurality of circumferentially distributed stator windings; wherein adjacent stator windings of the stator are electrically isolatable from each other by switching operation of a switchable network.

13. A method of controlling operation of a BLDC motor, wherein the motor comprises a stator and a rotor surrounded by the stator, and the stator comprises a plurality of circumferentially distributed stator windings; wherein adjacent stator windings of the stator are electrically isolatable from each other by switching operation of a switchable network; and wherein the method comprises a solid-state controller on execution of stored instructions:

- to operate a switching circuitry to facilitate flow of drive power into the stator,

- to trigger commutation upon detection of the commutation signal.

14. The method according to Claim 13, wherein the electrical responses of the stator in response to the probing signals are probing current pulses and the electrical responses in response to the drive power are drive current pulses, wherein the method comprises determining occurrence of a communication event, the communication event is when the drive current pulse rises faster than the probe current pulse.

15. The method according to Claim 14, wherein the method comprises determining first occurrence within a drive period of a voltage representing the drive current pulse rises to a threshold voltage earlier than that due to the probe current pulse.