Brushless DC motor

Abstract

A brushless DC motor comprising a rotor including circumferentially spaced permanent magnets, a first stator including a number of first poles having respective first windings, a second stator including a number of second poles having respective second windings and a controller configured to control application of drive currents to the first and second windings to thereby apply a torque to the rotor to cause rotation of the rotor, wherein the first and second stators are configured to provide a phase offset so that drive currents are applied to at least some of the second windings when switching application of drive currents between the first windings to thereby maintain torque on the rotor.

Description

BRUSHLESS DC MOTORBackground of the Invention

[0001] The present invention relates to a brushless DC motor, and in one particular example, a brushless DC motor including phase offset stators that can provide improved efficiency and / or reduced torque ripple.Description of the Prior Art

[0002] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgement or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0003] Brushless DC (BLDC) motors have been utilized since the mid-20th century, significantly transforming industries owing to their operational efficiency, long life, low noise, better speed-torque characteristics and reliability. Unlike brushed motors, these motors relocate commutation outside the motor, employing electronic switches for this function. While electronic switching enhances motor performance, representing a primary advantage over brushed counterparts, a persistent challenge remains. The transition of current between different motor phases by the motor driver prompts voltage and current drops during switching intervals, resulting in intermittent reductions in torque known as torque ripple. This phenomenon induces motor vibration during operation and escalates power consumption. Torque ripple denotes abrupt declines in torque, correlating with reductions in motor speed. Lower speeds correspondingly elevate current consumption. Furthermore, commutation switching necessitate increased current drawn to maintain driver specified speeds.

[0004] Given the increasing prevalence of BLDC motors across various applications encompassing both stationary and mobile machines, mitigating the aforementioned issues assumes paramount importance. Such applications include, amongst other uses, VCDs, DVDs, CDs, electric bicycles, vehicles, vacuum cleaners, mixers, hair dryers, video cameras, household fans, air conditioners and refrigerators, in automobiles (automobile air conditioners,wipers, electric doors, airbags, electric seats), industrial robots, extruder drive motors and feed drives for CNC machine tools and all drones related application. Substantial efforts have been invested in enhancing motor performance and efficiency. Some researchers have focused on motor structural modifications, while others have concentrated on refining motor drivers. In another example, slot-less BLDC motors have been developed, and whilst these demonstrate lower torque ripple compared with conventional BLDC motors, their torque density is also significantly less than that of conventional BLDC motors, which in turn has limited their uptake. Accordingly, despite these endeavours yielding some successes, unresolved challenges persist concerning motor vibration and torque ripple.

[0005] US8203296 describes a 2-phase BLDC motor driven by a trapezoidal waveform. For one-half of the motor rotation period T, the phase is driven by the trapezoidal waveform and for the other half-period, the coil remains undriven. An up down counter is operable to increment at a first frequency fl and to decrement at a second frequency f2. Incrementing operation is initiated at the start of the driven period of the waveform and stopped at the start of the down slope of the waveform. Decrementing operation is initiated at the start of the down slope of the waveform and stopped at the end of the down slope. The ratio of frequencies f 1 :f2 is used to measure the relative duration of the slope to the driven period and is selected to mirror the desired ratio of slope duration: driven period duration.

[0006] US8212508 describes a three phase BLDC motor in which the rotor position is monitored by detecting the zero crossing points of the induced back EMF signals BEMF_U, BEMF_V, BEMF_W in the phase windings U, V, W. The three back EMF signals are 120° out of phase with each other. In order to accurately monitor the back EMF in a phase winding, the driving waveform for each phase U, V, W includes an undriven period P close to the expected zero crossing point. The period P can be a preset part of the driving waveform or can be an interruption of the normal driving waveform in response to suitable interrupt signals. In order to determine the zero crossing points of each back EMF signal, two (or more) samples of the back EMF are taken during the undriven period P and used to interpolate the back EMF signal to determine the zero crossing point.

[0007] US8299661 describes a rotor of an electric brushless motor that is configured to be light weight and prevent vibrations generated during an operation of the motor to be transferred tothe shaft of the rotor. The rotor includes a shaft elongated in a rotational axis, a single body magnet comprising alternately magnetized portions, and a vibration absorption portion interposed between the shaft and single body magnet. The vibration absorption portion absorbs vibrations generated during the operation of the motor and can include an elastic or a nonelastic material. The rotor further includes a non-elastic portion inhibiting the expansion of the vibration absorption portion when the vibration portion is elastic.

[0008] US8633662 describes a control method for a sensor-less, brushless, three-phase DC motor. The effects of commutation on the motor may be minimized using a sinusoidal current drive on each electromagnet. The “off” times and / or the “on” times of the drive transistors controlling the electromagnets in a full “H-bridge” configuration drive scheme may be delayed. By overlapping the drive signals to the electromagnets with respect to a commutation command, the effects of switching between electromagnets may be minimized. In addition, the “on” and “off’ times may also be adjusted during the overlapping to further ensure that the coils continuously conduct current, and that the current does not change direction during the switching. The delays, and hence the overlap times of the coil drive signals may be dynamically controlled, for example by using digital timers, making the response predictable and easily controlled. The present position of the rotor in the motor may be determined using Hall sensors configured in the motor, or it may be determined using the un-energized electromagnets in a motor without Hall sensors.

[0009] US8638014 describes a brushless DC motor including a rotor, a magnet provided to the rotor, a pair of bearings to rotatably support the rotor, a stator assembly that at least partly surrounds the rotor and magnet thereof and adapted to control movement of the rotor, and a bearing tube having an exterior surface and an interior surface that defines a tube interior. The stator assembly is provided along the exterior surface of the tube and the bearings are provided along the interior surface of the tube to support the rotor and magnet within the tube interior. The motor has sample application for use in PAP devices for delivery of positive airway pressure therapy for users or patients.

[0010] US9853580 describes a method to reduce acoustic noise in a cooling fan motor. The method includes reducing the slew rate of a PWM (pulse width modulation) voltage waveform applied to energize a coil of the fan motor. This slew rate reduction results in lower mechanicalvibrations and acoustic noise in the fan motor. In one embodiment, the slew rate reduction is performed during startup of the fan motor, when the motor is spinning slowly and there is little air flow noise. In another embodiment, the slew rate reduction is not performed during high speed operation of the fan motor, when the fan motor is spinning very fast and air flow noise masks the motor noise. In one embodiment, there is variable slew rate control depending on the speed of the fan motor.

[0011] US 10498188 describes a motor equipped with dampers for reducing the vibration and noise of a rotator. The motor includes: a stator block configured such that a core around is fastened and installed in the upper outer portion of the inside of a cover; a printed circuit board (PCB) fastened and installed below the stator block inside a motor housing; a shaft installed inside the stator block; and a rotor configured to include a magnet; wherein the motor housing includes an upper cover and a lower cover, a plurality of fastening portions is formed on the upper cover, and a plurality of fastening depressions is formed on the lower cover; and wherein the motor further includes dampers configured to receive and accommodate the fastening portions, to be inserted over side surfaces of the base portion of the stator block, and to cancel vibration and noise.

[0012] US20220173675A1 describes a method for the optimized operation of a BLDC motor with a BLDC motor assembly that includes the BLDC motor; — a control and evaluation unit; — a data memory; — a current regulator; — a rotor angle sensor; and — a torque evaluator. The BLDC motor includes a stator and a rotor. The rotor includes excitation magnets, and the stator has stator coils. A current is applied to the stator coils. The size of the current is defined in a value table depending on the rotor angle. The current values in the value table are continually optimized in accordance with a deviation between a setpoint torque and a determined actual torque.Summary of the Present Invention

[0013] In one broad form, an aspect of the present invention seeks to provide a brushless DC motor comprising: a rotor including circumferentially spaced permanent magnets; a first stator including a number of first poles having respective first windings; a second stator including a number of second poles having respective second windings; and a controller configured tocontrol application of drive currents to the first and second windings to thereby apply a torque to the rotor to cause rotation of the rotor, wherein the first and second stators are configured to provide a phase offset so that drive currents are applied to at least some of the second windings when switching application of drive currents between the first windings to thereby maintain torque on the rotor.

[0014] In one embodiment the first stator has a greater axial length than the second stator.

[0015] In one embodiment the first stator is a primary stator and the second stator is a secondary stator.

[0016] In one embodiment at least some of the second windings are rotationally offset relative to the first windings.

[0017] In one embodiment rotor includes one or more sets of permanent magnets aligned with each other.

[0018] In one embodiment the motor includes the same number of first and second windings.

[0019] In one embodiment the first and second windings have one of: the same number of wires; different numbers of wires; the same number of wire turns; and, different numbers of wire turns.

[0020] In one embodiment the first windings have more wires than the second windings and each wire on the second winding has a greater number of wire turns than each wire in the first winding.

[0021] In one embodiment the controller is configured to apply drive currents to the first and second windings that have one of: the same pulse widths; and, different pulse widths.

[0022] In one embodiment the controller is configured to apply drive currents to the second windings for at least some of the time when drive currents are not applied to the first windings.

[0023] In one embodiment the first stator is axially offset from the second stator.

[0024] In one embodiment the drive currents are controlled to reduce torque ripple.

[0025] In one embodiment the controller includes: one or more sensors configured to detect a rotational angle of the rotor; and, switches configured to selectively apply drive currents to the windings; and, a processing device configured to control the switches to apply drive currents in accordance with the rotational angle of the rotor.

[0026] In one embodiment the controller includes: first switches configured to selectively apply drive currents to the first windings; and, second switches configured to selectively apply drive currents to the second windings.

[0027] In one embodiment the motor includes n first and n second windings, and wherein the first and second windings are rotationally offset by an angle a of one of: 36072n ± 20%; 36072n ± 15%; 36072n ± 10%; 36072n ± 5%; and, 36072n.

[0028] In one embodiment the controller is configured to apply drive currents with: a pulse width of 360° / Ml, where Ml is a length of magnets in the rotor; a period of Mn I 2, where Mn is a number of magnets in the rotor; a switching delay of Ml 12; and, a switching delay for the second windings of a + Ml, where a is a rotational offset between the first and second windings.

[0029] In one embodiment a number of wire turns in the second windings is given by: Ns= % RR * k * Nf where: Ns= number of wire turns in second windings; %RR = required percentage reduction in ripple; k = motor coefficient; Nf= number of wire turns in first windings.

[0030] It will be appreciated that the broad forms of the invention and their respective features can be used in conjunction and / or independently, and reference to separate broad forms is not intended to be limiting. Furthermore, it will be appreciated that features of the method can be performed using the system or apparatus and that features of the system or apparatus can be implemented using the method.Brief Description of the Drawings

[0031] Various examples and embodiments of the present invention will now be described with reference to the accompanying drawings, in which: -

[0032] Figure 1A is a schematic end view of an example of a conventional prior art brushless DC motor;

[0033] Figure IB is a schematic end view of the stator of the conventional brushless DC motor of Figure IB;

[0034] Figure 1C is a schematic side view of the stator of Figure IB;

[0035] Figure 2A is a schematic end view of first and second stators of an example of a brushless DC motor;

[0036] Figure 2B is a schematic side view of the stators of Figure 2A;

[0037] Figures 3A to 3D are schematic side, front, isometric and cross-sectional views of an example of a primary stator;

[0038] Figures 3E to 3H are schematic side, front, isometric and cross-sectional views of an example of a secondary stator;

[0039] Figures 31 to 3M are schematic side, front, rear, isometric and cross-sectional views of an example of a rotor;

[0040] Figures 3N to 3Q are schematic side, front, isometric and cross-sectional views of an example of a secondary stator;

[0041] Figures 3R to 3U are schematic side, isometric and first and second cross-sectional views of an example of a stator assembly;

[0042] Figures 3V to 3Y are schematic side, isometric and first and second cross-sectional views of an example of a motor;

[0043] Figure 4 is a circuit diagram of an example of a motor controller for a primary motor;

[0044] Figure 5 is a circuit diagram of an example of a motor controller for a secondary motor;

[0045] Figure 6 is a graph of an example of a comparison of motor speed for a brushless DC motor and a prior art motor;

[0046] Figure 7 is a graph of an example of a comparison of motor vibrations for a brushless DC motor and a conventional brushless DC motor;

[0047] Figures 8A to 8C are graphs of examples of a comparison of motor current draw for a brushless DC motor and a conventional brushless DC motor at different operating speeds; and,

[0048] Figures 9A and 9B are graphs of an example of simulated comparisons of motor torque and speed for a brushless DC motor and a conventional brushless DC motor;

[0049] Figures 10A to 10C are graphs example simulations of comparisons of motor torque, speed and torque at 1000RPM for a bi-layer motor and a conventional motor;

[0050] Figures 11A and 11B are graphs of simulated back-EMF and current waveforms for a bi-layer motor;

[0051] Figure 12 is a graph showing experimental current waveforms of the bi-layer motor; and,

[0052] Figures 13 A and 13B are graphs showing a comparison of vibrations measured for a bi- layer motor at 10,000 RPM along X and Y directions respectively; and,

[0053] Figures 13C and 13D are graphs showing a comparison of vibrations measured for a conventional motor at 10,000 RPM along X and Y directions respectively.Detailed Description of the Preferred Embodiments

[0054] An example of a conventional prior art brushless DC motor will now be described with reference to Figures 1A to 1C.

[0055] In this example, the motor 100 includes a stator 110 including a number of poles 111 (only some of which are labelled for clarity), around which windings (not shown) are provided. The windings typically include a number of turns (also referred to as coils), and may include multiple wires, depending on the particular motor configuration. A rotor 130 is provided, which includes a number of permanent magnets 131 (only some of which are labelled for clarity) circumferentially spaced around the rotor.

[0056] In use, a controller (not shown) selectively applies drive currents to the windings to generate magnetic fields, which are switched between the windings, so that the magnetic fields effectively progressively move circumferentially around the stator. The fields in turn interactwith the permanent magnets 131 in the rotor, thereby causing the rotor 130 to rotate. The controller can adjust the phase and amplitude of the current pulses applied to the windings, to thereby control the speed and torque of the motor.

[0057] It will be appreciated by persons skilled in the art that in the arrangement shown in Figures 1A to 1C the rotor is positioned outwardly of the stator, which is referred to as an outrunner configuration. Inrunner and axial configurations, in which the rotor is inward of, or axially offset from the stator, are also known. As these function in a broadly similar manner, these will not be described in any further detail. Additionally, in the current example, the motor includes twelve poles 111 on the stator 110 and fourteen magnets 131 on the rotor 130, but it will be appreciated that different number of poles and magnets can be used, and this is one particular embodiment used for the purpose of illustration only.

[0058] As described above, the transition of current between different stator windings results in voltage and current drops during switching intervals, resulting in intermittent reductions in torque known as torque ripple.

[0059] An improved DC brushless motor that attempts to mitigate torque ripple and / or provide other benefits will now be described in more detail with reference to Figures 2A and 2B, which show an alternative stator configuration.

[0060] Specifically, in this example, the single stator 110 of the motor of Figure 1A is replaced with two stators 210, 220. Both stators again include poles 211, 221 allowing respective first and second windings to be provided on the first and second stators. In this example, the stators are aligned on a common axis, and positioned adjacent to each along the axis, but it will be appreciated that other arrangements could be used.

[0061] In use, a controller is provided which is configured to control application of drive currents to the first and second windings to thereby apply a torque to the rotor to cause rotation of the rotor. Specifically, the first and second stators are configured to provide a phase offset so that drive currents are applied to at least some of the second windings when switching application of drive currents between the first windings to thereby maintain torque on the rotor. Thus, as drive currents are switched between the different first windings, the fields generated by the first windings will decay, before new fields are generated. In the intervening time, drivecurrents applied to the second windings generate respective fields, ensuring that torque is applied more consistently to the rotor.

[0062] In one example, this is achieved by having the stators are configured so that at least some of the second windings on the second stator are rotationally offset relative to the first windings on the first stator. This introduces the required phase offset, so that fields generated by the second windings can generate a torque when torque from the first windings is minimised. In this arrangement, a single set of permanent magnets can be provided on the rotor, with the fields generated by the stators interacting with the same permanent magnets. However, it will be appreciated that an alternative approach would be to have each stator cooperate with respective first and second permanent magnets on the rotor.

[0063] In any event, the above described arrangement introduces a second stator that allows fields to be generated that are offset in phase to those generated by the windings on the first stator, in turn allowing the rotor to be exposed to more consistent magnetic fields, which in turn can help reduce torque ripple and associated vibrations and / or make the motor more efficient, as will be described in more detail below.

[0064] A number of further features will now be described.

[0065] In one example, the first stator has a greater axial length than the second stator. This is not essential, and alternatively the first and second stators could have an identical size. The second windings are intended primarily to generate magnetic fields to compensate for the reduced torque during switching of the drive currents to the first windings, as well as to increase the overall torque generated by the motor. Accordingly, the fields generated by the second windings do not need to be as strong, and consequently the second stator can be smaller. In this example, the first stator is acting as a primary stator and the second stator is acting as a secondary (or auxiliary) stator.

[0066] Typically the rotor includes a single set of magnets, with the fields generated by the first and second windings both interacting with the same magnets. However, it will be appreciated that this is not essential, and in another example, the rotor could include respective one or more sets of permanent magnets aligned with each other, which interact with the fields generated by the first and second windings, respectively. In this example, the motor effectivelyincludes a primary rotor and a second rotor, defined by the respective set of magnets, with the primary and secondary rotors being mechanically connected, so that they rotate in conjunction.

[0067] In one example, the first and second windings are rotationally offset, to introduce the necessary phase offset when the resulting magnetic fields interact with the permanent magnets on the rotor. However, it will be appreciated that the offset could be achieved in any suitable manner.

[0068] In one example, such as the example shown in Figures 2 A and 2B, the motor can include the same number of first and second windings. Thus, in the example, both the first and second stators include twelve poles, and will therefore include twelve first and second windings, respectively.

[0069] The first and second windings can incorporate the same number of wires and / or wire turns. However, this is not essentially, and alternatively different number of wires or wire turns could be used. In particular, as the second stator is typically smaller there are typically less wires or wire turns, to allow these to be physically accommodated by potentially smaller poles, and also as less resulting field strength is required. Thus, this could result in a configuration in which the first windings have more wires than the second windings and each wire on the second winding has a greater / equal / lesser number of wire turns than each wire in the first winding. For example, the first windings might include six wires with an eight-turn configuration, whilst the second windings can include two wires with fifteen turns, with the wires potentially including different diameters.

[0070] The controller is typically configured to apply drive currents to the first and second windings that have the same or different pulse widths, with the pulse widths depending for example on the size of the permanent rotor magnets or motor speed.

[0071] In one example, the controller is configured to apply drive currents to the second windings for at least some of the time when drive currents are not applied to the first windings. Thus, as a first winding is de-energised but before another first winding is energised, second drive currents could be applied to a second winding provided at an intervening position, to ensure rotational torque is maintained. In one particular example, the first and second drive currents could be applied so there is at least some overlap, so drive current is applied to thesecond winding before drive current ceases being applied to the first winding that is being deenergised. The switching can be specifically configured to ensure the drive currents are used to reduce torque ripple, including potentially eliminating any torque ripple.

[0072] In one example, the controller includes one or more sensors configured to detect a rotational angle of the rotor, switches configured to selectively apply drive currents to the windings and a processing device, such as a micro controller, configured to control the switches to apply drive currents in accordance with the rotational angle of the rotor. Typically the controller includes first switches configured to selectively apply drive currents to the first windings and second switches configured to selectively apply drive currents to the second windings. Thus, it will be appreciated that this is broadly similar to the configuration of controllers used in conventional brushless DC motors, albeit with additional switches to allow drive currents to be selectively applied to the second windings. It will also be appreciated that sensor-less controller configurations can also be implemented.

[0073] In one specific example, the motor includes n first and n second windings which are rotationally offset by an angle a, which is one of 36072ft ± 20%, 36072ft ± 15%, 36072ft ± 10%, 36072ft ± 5% or approximately 36072ft. In one specific example, for a motor having a coil arrangement including twelve windings, arranged in a three phase configuration, with four windings per phase, the rotational offset a is between 12.3° and 16.5°.

[0074] In one specific example, the controller is configured to apply drive currents in accordance with the following approximate values:• a pulse width of 360° / Ml, where Ml is a length of magnets in the rotor;• a period of Mn 12, where Mn is a number of magnets in the rotor;• a switching delay of Ml I , and,• a switching delay for the second windings of a + Ml, where a is a rotational offset between the first and second windings.

[0075] Similarly, in one specific example, a number of wire turns in the second windings is given by:Ns= % RR * k * Nf where: Ns= number of wire turns in second windings%RR = required percentage reduction in ripple k = motor coefficientNf= number of wire turns in first windings

[0076] The motor coefficient accounts for the effects of the airgap and other factors and so is motor specific, but in one example, is 1.22.

[0077] However, it will be appreciated that the above values are for the purpose of illustration and alternative values could be used depending on the configuration of the motor.

[0078] Accordingly, the above described arrangement introduces an additional stator, which acts as an auxiliary motor integrated within the primary motor structure to address torque ripple issues. Specifically, in one example, this auxiliary motor serves to mitigate the torque ripples observed in the main motor. By adopting this approach, the torque ripple problem within the motor can be effectively resolved, subsequently not only eliminating motor vibration attributed to this issue, but also the motor torque will increase and the motor current consumption will decrease significantly (around 20% for a test discussed below).

[0079] The auxiliary stator configuration can vary, accommodating radial, axial, or hybrid configurations. The auxiliary stator can be positioned below, between, or above the primary stator, and typically incorporates a mechanical phase angle ensuring the placement of auxiliary stator poles between the main stator poles.

[0080] Although the diameter of the auxiliary stator can be different from the diameter of the main stator, having the same diameter as the main stator's diameter is recommended. The height of the auxiliary stator is determined by the requisite torque output necessary for compensating the main motor's torque ripple.

[0081] For the examples described above, the motor includes only one auxiliary stator but depending different motor structures, several auxiliary stators could be used.

[0082] As also mentioned above, the motor could also employ an auxiliary rotor, which can function as an extension of the main rotor or exist as a separate entity, and which can interact with the secondary stator.

[0083] In any event, the presence of the secondary stator in effect creates a second motor that is phase offset from the primary motor, but which can otherwise share identical rotor and stator pole configurations with the main motor but can differ in coil wire-turn counts and wire diameters. Notably, the winding pattern can repeat that of the main motor. For instance, in a motor featuring 3 phases, 12 stator poles, and 14 rotor poles, the auxiliary motor can maintain the same specifications. However, variations exist in the coil configurations; for instance, if the main motor stator coils might consist of 6 wires with an 8 -turn configuration, the auxiliary motor's coils may utilize 2 wires with 15 turns, potentially of differing diameters.

[0084] Further examples of components are used to physically construct motors are shown in Figures 3A to 3D, with resulting stator assembly and motor being shown in Figures 3R to 3U and 3V to 3Y. In these examples, primary and secondary stators 310, 320 are shown, each having respective first and second windings. A rotor 330 is provided, including permanent magnets 331 circumferentially spaced around an inner surface, as well as an axle 332, which in use is supported by a stator base 340 including a bearing assembly.

[0085] An example circuit diagram for a motor controller is shown in Figures 4 and 5. In this example, the driver includes 12 sets of switches, with 6 sets dedicated to the main motor shown in Figure 4 to drive phases A to C, and an additional 3 sets serving the auxiliary motor shown in Figure 4, to drive phases D to F. In this example, the controller is configured as a three phase motor controller, for a motor including 12 stator poles, and 14 rotor poles, although it will be appreciated other arrangements could be used. For example, separate or combined drivers could be used, and where separate drivers are used, these would ideally from the same brand and with different nominal powers (lower nominal power for the auxiliary motor as it does not need a driver as powerful as the one needed for the main motor).

[0086] In one example, the switching time and pulse width, based on the rotor position is given by:• Pulse width: 17.14 degrees (360 / AfZ).• Period: 51.4 degrees (Afn / 2).• Switching delay: 8.57 degrees ( Z / 2).• Switching delay for the Auxiliary coils excitation: 32.5 degrees

[0087] In order to assess the effectiveness of the above described arrangement, a test rig was used to perform measurements of motor performance. An example of the test rig used a motor attached to and driving a propeller, such as a propeller for a UAV (Unmanned Aerial Vehicle). Sensors are used to monitor current used by the motor, as well as torque ripple and motor vibration. Results of comparative testing between the motor and a prior art arrangement are shown in Figures 6 to 9.

[0088] As shown in Figure 6, for a motor with no auxiliary (second) stator 602, the motor speed is lower than for a motor with an auxiliary (second) stator 601 for a given driver pulse width. This demonstrates that the motor with an auxiliary (second) stator has a higher rotational speed for the same applied driver pulse width.

[0089] Figure 7 demonstrates that at a given driver pulse width, the motor with no auxiliary (second) stator 702 shows greater physical vibrations than an equivalent motor with an auxiliary (second) stator 701.

[0090] Figures 8A to 8C show that at different rotational speed, the motor with no auxiliary (second) stator 802 requires an increased current than an equivalent motor with an auxiliary (second) stator 801. From this, it is apparent the additional stator can lead to a 20% improvement in efficiency.

[0091] Figures 9A and 9B show simulated torque and speed comparison between a motor incorporating a second stator (referred to as a bi-layer motor) and a conventional single stator motors in Ansys Maxwell software. The simulations are for drone-sized BLDC motors (equivalent to a 2506 motor) with a speed starting from zero and increasing. These results highlight the motor with no auxiliary (second) stator 902 demonstrates greater torque ripple and requires an increased current than an equivalent motor with an auxiliary (second) stator 901.

[0092] Further simulations were performed in order to model the effectiveness of simulated scooter motor components. Figures 10A and 10B are simulations of comparisons of torque and speed for a conventional single stator electric scooter BLDC motor (XM6352EA-15 motor) and an equivalent bi-layer motor 1001, when the speed starts from zero and increases. Figure 10C shows torques for the same motors when rotating at a fixed speed of "1000 rpm".As shown, the bi-layer motor 1001 demonstrates a peak to peak torque variation of 0.7 N-m compared to 1.7 N-m for the conventional single stator motor 1002, meaning the bi-layer motor shows a 60% reduction in torque ripple. Similar values of 0.45 N-m and 1.7 N-m, leading to a 74% reduction in torque ripple occur at 1000 RPM. Furthermore, the conventional motor 1002 demonstrates significant speed waveform fluctuations caused by torque ripple, as shown at 1003.

[0093] Figure 11A shows example back-EMF waveforms 1100A, 1100B, 1100C, HOOD, 1100E, HOOF, for phases A-C for the primary coils and D-F for the secondary coils, with similar current waveforms 1101A, 1101B, 1101C, 1101D, 1101E, 1101F, being shown in Figure 1 IB, based on a single coil turn with a 12 V input.

[0094] Figure 12 shows experimental current waveforms of the bi-layer motor based on a modified Xing 2506 motor, including Phase A 1200A and Phase D 1200D at 2000 rpm, captured using an oscilloscope, with the labelled peaks showing a phase shift delay for the primary motor.

[0095] Figures 13A and 13B are graphs showing vibrations measured for a drone-sized bi-layer motor (equivalent to the 2506 motor) at 10,000 RPM along X and Y directions respectively, with corresponding graphs for a conventional motor (a 2506 motor) being shown in Figures 13C and 13D. The graphs highlight significantly reduced and more consistent vibrations for the bi-layer motor.

[0096] Accordingly, it will be appreciated that the above described results demonstrate that incorporating a second stator with a phase offset relative to the main stator can lead to improvements in motor operation, including, but not necessary limited to one or more of, reduced torque ripple, reduced physical vibrations, improved efficiency and a higher rotational speed.

[0097] Whilst the above experiments were performed using smaller motors, for example used in UAVs or similar, it will be appreciated that the techniques described herein are scalable could be applied to smaller or larger motors, including motors of up to a few hundred kW or higher in power, such as those used in electric vehicles or other similar applications.

[0098] Throughout this specification and claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers. As used herein and unless otherwise stated, the term "approximately" means ±20%.

[0099] Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:1) A brushless DC motor comprising: a) a rotor including circumferentially spaced permanent magnets; b) a first stator including a number of first poles having respective first windings; c) a second stator including a number of second poles having respective second windings; and, d) a controller configured to control application of drive currents to the first and second windings to thereby apply a torque to the rotor to cause rotation of the rotor, wherein the first and second stators are configured to provide a phase offset so that drive currents are applied to at least some of the second windings when switching application of drive currents between the first windings to thereby maintain torque on the rotor.2) The motor according to claim 1, wherein the first stator has a greater axial length than the second stator.3) The motor according to claim 1 or claim 2, wherein the first stator is a primary stator and the second stator is a secondary stator.4) The motor according to any one of the claims 1 to 3, wherein at least some of the second windings are rotationally offset relative to the first windings.5) The motor according to any one of the claims 1 to 4, wherein rotor includes one or more sets of permanent magnets aligned with each other.6) The motor according to any one of the claims 1 to 5, wherein the motor includes the same number of first and second windings.7) The motor according to any one of the claims 1 to 6, wherein the first and second windings have one of: a) the same number of wires; b) different numbers of wires; c) the same number of wire turns; and, d) different numbers of wire turns.8) The motor according to any one of the claims 1 to 6, wherein the first windings have more wires than the second windings and each wire on the second winding has a greater number of wire turns than each wire in the first winding.9) The motor according to any one of the claims 1 to 8, wherein the controller is configured to apply drive currents to the first and second windings that have one of:a) the same pulse widths; and, b) different pulse widths.10) The motor according to any one of the claims 1 to 9, wherein the controller is configured to apply drive currents to the second windings for at least some of the time when drive currents are not applied to the first windings.11)The motor according to any one of the claims 1 to 10, wherein the first stator is axially offset from the second stator.12) The motor according to any one of the claims 1 to 11, wherein the drive currents are controlled to reduce torque ripple.13) The motor according to any one of the claims 1 to 12, wherein the controller includes: a) one or more sensors configured to detect a rotational angle of the rotor; and, b) switches configured to selectively apply drive currents to the windings; and, c) a processing device configured to control the switches to apply drive currents in accordance with the rotational angle of the rotor.14) The motor according to claim 13, wherein the controller includes: a) first switches configured to selectively apply drive currents to the first windings; and, b) second switches configured to selectively apply drive currents to the second windings.15)The motor according to any one of the claims 1 to 14, wherein the motor includes n first and n second windings, and wherein the first and second windings are rotationally offset by an angle a of one of: a) 36072n ± 20%; b) 36072n ± 15%; c) 36072n ± 10%; d) 36072n ± 5%; and, e) 36072n.16) The motor according to any one of the claims 1 to 15, wherein the controller is configured to apply drive currents with: a) a pulse width of 360° / Ml, where Ml is a length of magnets in the rotor; b) a period of Mn 12, where Mn is a number of magnets in the rotor; c) a switching delay of Ml I , and, d) a switching delay for the second windings of a + Ml, where a is a rotational offset between the first and second windings.)The motor according to any one of the claims 1 to 16, wherein a number of wire turns in the second windings is given by:Ns= % RR * k * Nf where: Ns= number of wire turns in second windings%RR = required percentage reduction in ripple k = motor coefficientNf= number of wire turns in first windings

Images