Operationally reliable brushless DC motor

Abstract

The present invention relates to a brushless DC motor, in particular for an actuator unit of an implant, preferably for a heart assist system, comprising a stator and a rotor, the stator having a hollow cylindrical ironless winding, and a rotor rotatable relative to the stator, the rotor comprising a shaft with p permanent magnet pole pairs, the winding having n three-phase systems separated from one another. For a number of permanent magnet pole pairs p=1, the number of three-phase systems separated from one another is 2, and for a number of permanent magnet pole pairs p>1, the number of three-phase systems separated from one another corresponds to either an integer divisor of p (wherein the integer divisor is not equal to 1), or to the number p, or to twice the number p of permanent magnet pole pairs, wherein the n three-phase systems separated from one another in the hollow cylindrical ironless winding are arranged spatially offset from one another by an angle of 360° / n.

Description

[0001] describe

[0002] The present invention relates to a brushless DC motor, in particular for an actuator unit of an implant, preferably for a heart assist system, comprising a stator and a rotor, the stator having a hollow cylindrical ironless winding and the rotor being rotatable relative to the stator and comprising a shaft having p permanent magnet pole pairs, wherein the winding has n three-phase systems separated from one another.

[0003] A brushless DC motor has a hollow, cylindrical, ironless winding that is supplied with current from a three-phase system and is arranged in the stator because there is no brush system to transfer current to the rotating winding. Therefore, a pair of permanent magnets must rotate with the winding relative to the stator. The material of this pair of permanent magnets typically includes a neodymium-iron-boron alloy to achieve a sufficiently high power density. The shaft provided with this pair of permanent magnets is typically mounted using preloaded ball bearings to achieve a long service life. To achieve even higher power density, the rotor can be provided with several pairs of permanent magnets. Brushless DC motors require electronic commutation to operate, which in principle mimics the brush system of conventional DC motors. As a result, a speed-torque behavior similar to that of conventional DC motors with high starting torque and high dynamics is achieved. The main advantages of brushless DC motors are longer service life and higher speeds, which are not limited by the mechanical commutation system. Specifically, the hollow cylindrical ironless winding enables a brushless DC motor with reduced iron losses, low friction and low heat losses, resulting in an extremely efficient motor that achieves high acceleration and short response time due to its low inertia, which is particularly suitable for use with actuator units for implants and is particularly advantageous for heart assist pumps.

[0004] The use of brushless DC motors has increased steadily over the past two decades as the electronics required for operation have become cheaper and smaller, and also due to the increased use and expansion of fields where operationally reliable motors with redundancy or increased reliability are essential.

[0005] EP754365B1 discloses a redundant electric motor comprising a rotor having two magnetic poles arranged axially one behind the other, and a stator having two separate windings arranged axially one behind the other. These windings can be controlled independently and are each associated with a pair of permanent magnet poles. DE3140034A1 describes a redundant brushless DC motor in which the slotted stator winding consists of four strands, the commutation sensor system includes four sensor sets, and the rotor has four pairs of magnetic poles. This redundant motor thus includes four independently controllable motor units, enabling a high level of redundancy but requiring a large installation footprint and a large number of individual components. EP2922180A1 discloses another redundant brushless DC motor in which the stator windings have several independently operable winding strands and several independent sensor sets for detecting the rotor's rotational position and for electronic commutation. The coils are designed as bifilar windings, comprising at least two independently controllable winding strands made from a single conductor. Although bifilar coils consist of two parallel winding wires that can be powered by different voltage sources, there's a high probability that, in the event of a fault, the directly adjacent redundant winding system will also be affected by an increase in operating temperature or a localized weakening due to production technology. In the event of a short circuit, the affected winding wire exerts a magnetic effect on the adjacent bifilar wire, causing it to be affected by the reduced magnetic flux caused by the opposing short-circuit current, just like the affected winding wire, and thus can only participate in the electromechanical energy conversion to a limited extent. Consequently, bifilar coils have a low level of redundancy, with a high negative flyback effect that prevents the short-circuit current from being shut off in the remaining windings. In addition to the long-standing use of redundant electric motors in the aviation industry, recent years have seen the development of numerous implantable medical devices that must be operated electrically in a fail-safe manner, such as heart assist pumps. Reliable, uninterrupted operation is particularly essential for these types of ventricular assist devices, as an interruption or significant degradation in performance can have life-threatening consequences. However, in addition to the redundancy of the implant's drive unit, the small footprint and low weight of the drive unit are now essential for use in implants. The invention is therefore based on the object of providing an operationally reliable brushless DC motor which, while having a light and compact design, has sufficient redundancy to maintain operation at least to a limited extent in the event of a partial fault.

[0006] According to the invention, this object is met because for a number p=1 of permanent magnet pole pairs, the number of three-phase systems separated from one another is 2, and for a number p>1 of permanent magnet pole pairs, the number of three-phase systems separated from one another corresponds to either an integer divisor of p (wherein the integer divisor is not equal to 1), or to the number p, or to twice the number p of permanent magnet pole pairs (14), wherein n three-phase systems separated from one another in the hollow cylindrical ironless winding are arranged in a manner spatially offset from one another by an angle of 360° / n. An integer divisor of p is a natural number which, when multiplied by another natural number, gives p. For example, if p=6, the numbers 1, 2, 3 and 6 are integer divisors of p. Thus, since the divisor according to the invention is not equal to 1, 2, 3 or 6 three-phase systems separated from one another can be provided. The spatially offset arrangement of a plurality of three-phase systems separated from one another (i.e. electrically isolated from one another) in the winding strands results in a segmentation of the hollow cylindrical ironless winding arranged around the rotor and thus enables a fault-tolerant winding connection of the brushless DC motor. For a number p>1 of permanent magnet pole pairs, at least one separate three-phase system can be associated with one or more pairs of magnetic poles consisting of a north pole and a south pole, thereby achieving decoupling of these magnetic circuits so that in the event of a short circuit in one of these separate three-phase systems, there is no magnetic effect or a reduced magnetic effect on the other three-phase systems.

[0007] Since two three-phase systems can also be arranged opposite a pair of magnetic poles, embodiments of the DC motor with a single pair of magnetic poles are also possible, but preferably at least p = 2 permanent magnet pole pairs are provided on the shaft, wherein the number n of three-phase systems separated from one another corresponds to an integer divisor of p, where this integer divisor is not equal to 1, or corresponds to the number p of permanent magnet pole pairs. Accordingly, the three-phase systems arranged spatially offset by an angle of 360° / n are each arranged diametrically opposite one or more pole pairs. In the case of a rotor having only one pair of permanent magnet poles (p = 1 or p > 2) and twice the number of three-phase systems (n = 2×p), these three-phase systems separated from one another are each arranged diametrically opposite one magnetic pole.

[0008] It is particularly advantageous to arrange at least p = 2 permanent magnet pole pairs on the shaft, and the number n of separated three-phase systems corresponds exactly to the number p of permanent magnet pole pairs spatially offset by an angle of 360° / n from one another in the hollow cylindrical ironless winding. In this case, the three-phase systems spatially offset by an angle of 360° / n are each arranged directly opposite one or more pole pairs. This structural configuration of the brushless DC motor enables a high level of robustness against unforeseen faults during operation of the motor and a good degree of winding stability even during emergency operation despite faulty or disconnected winding sections. Although this configuration can achieve a high level of redundancy and fail-safety for the brushless DC motor, conventional processes for manufacturing, testing, and quality control of the motor can be used, as can conventional components for power electronics and commutation logic. Compared to redundant electric motors known from the prior art with bifilar windings in single coils of conventional three-phase windings or five-phase or seven-phase windings, the configuration of the brushless DC motor according to the invention makes it possible to dispense with the special and therefore cost-intensive production of the windings as well as the use of special power electronics and commutation logic.

[0009] The use of at least two spatially separated three-phase systems in the hollow cylindrical ironless winding of the brushless DC motor according to the invention not only enables a small and compact motor configuration with a short installation length and small diameter while at the same time having a high nominal power, but also enables a high level of redundancy against faults in the winding system caused, for example, by increased operating temperature or local weakening of the winding due to production technology. For this reason, the brushless DC motor is ideally suited for use as a drive unit for an implant in an implantable heart assist system for long-term blood circulation assistance of the human heart. With such an implantable heart assist system, possible faults in the coils of the winding, as well as faults in or connections to the power supply line, can lead to a failure of the heart assist system and thus to the death of the patient.

[0010] An advantageous embodiment provides that for each phase of the electrically isolated three-phase system, at least k=2 single coils are connected in series, wherein the product k×n of k single coils and n separated phase systems corresponds to twice the number of permanent magnet pole pairs, and wherein the spatial angle of the series-connected single coils of the respective phase of the separated three-phase system is 360° / n / k. In this embodiment, the total number of stator single coils thus corresponds to twice the number of permanent magnet pole pairs or the total number of rotor poles, respectively, so that the spatial angle between two single coils corresponds to the spatial angle between two rotor poles. The spatially separated arrangement of the single coils increases the fault tolerance of the motor in the event of winding-level faults and thereby improves the availability of the motor as a drive unit for implants requiring a high level of fault reliability. The DC motor is preferably configured as a four-pole permanently excited synchronous motor having two electrically isolated three-phase systems, wherein each phase of the three-phase system includes two single coils connected in series and arranged at a spatial angle of 90°. Such a four-pole synchronous motor achieves a high level of redundancy even with a small installation size and low manufacturing complexity. In order to achieve the highest possible turn density of the hollow cylindrical ironless winding, the single coils of the three-phase system are provided with a rhombus winding, which is also called a diamond winding.

[0011] To increase fault tolerance, two separate single coils connected in series for one phase of the at least two three-phase systems can be electrically connected in opposing winding directions within the hollow cylindrical ironless winding. The connection of the two single coils in opposing winding directions results in magnetic coil coupling with the north and south poles, thereby reducing winding currents during emergency operation.

[0012] A convenient configuration provides for the axial positions of the at least two three-phase systems, which are separated from one another, to overlap relative to the shaft at least in certain areas. The overlapping arrangement of the axial positions of the at least two three-phase systems, which are electrically separated from one another in the axial extension of the shaft, enables a compact design of the brushless DC motor while having a short overall length. It is sufficient for the magnetic fields of the three-phase systems to act on a common section of the shaft over a certain axial length. The magnetic fields of the three-phase systems, which extend perpendicular to the axial direction of the shaft, preferably act essentially only on the common section of the shaft, thereby achieving a very short motor length with a high level of redundancy.

[0013] A useful embodiment provides for connecting the individual coils of separate three-phase systems in a star connection, wherein the neutral points of at least two separate three-phase systems are preferably connected to each other. Using a star connection for the electrical coupling of the individual coils of the corresponding three-phase systems not only enables a higher torque constant to be achieved but also avoids undesirable circulating currents in the windings. These neutral points can be easily led out of the hollow cylindrical ironless winding individually or as a common neutral point and connected to the power electronics.

[0014] Alternatively, the individual coils of the three-phase systems that are separated from one another can be connected in series, thereby creating an electrical coupling of the individual three-phase systems, known as a delta connection. These delta connections can then be connected to power electronics. Alternatively, the individual coils of the three-phase systems that are separated from one another of the brushless DC motor according to the present invention can be connected to one another in a single polygonal connection. This delta connection or polygonal connection of these individual coils of several three-phase systems enables a higher speed constant of the motor and, therefore, a reduction in the required supply voltage.

[0015] Advantageously, a separate electronic commutator, preferably an electronic block commutator, can be provided for each of the three-phase systems that are separated from one another. As a result, despite the use of two three-phase systems that are electrically and spatially separated from one another, conventional commutators, which are also used for standard motors, can be employed. Consequently, the development and production costs of redundant brushless DC motors can be reduced. Furthermore, when separate electronic commutators are employed for each of these separate three-phase systems, provided that these commutators are equipped with their own voltage circuits, in the event of a commutator failure, there is only minimal interference between the three-phase systems. Consequently, the fault tolerance of the DC motor according to the invention can be improved, and increased redundancy can be achieved.

[0016] Furthermore, the stator may include a yoke, preferably a laminated iron core, arranged around the hollow cylindrical ironless winding. The yoke reduces eddy current losses and correspondingly improves the power density of the DC motor. A laminated iron package or an iron-nickel metal package may be arranged around the hollow cylindrical ironless winding for a yoke of the highest possible quality.

[0017] A specific embodiment provides that the air gap between the rotor and the stator allows a fluid to flow through the air gap, in particular, allows human blood to flow through the air gap, wherein the circumferential air gap is preferably greater than 15% of the rotor radius, in particular greater than 25% of the rotor radius. Such a large air gap between the rotor and the stator allows for easy use as a drive unit for an implantable heart assist system and its integration into the patient's blood flow.

[0018] Non-limiting embodiments of the present invention will be described in more detail below using the accompanying exemplary drawings, in which:

[0019] Figure 1 shows a schematic exploded view of a brushless DC motor according to the present invention,

[0020] Figure 2 Shown Figure 1 Schematic diagram of the coil connection of a DC motor with two pairs of permanent magnet poles and two three-phase systems,

[0021] Figure 3 Shown Figure 2 Schematic diagram of the independent magnetic flux circuit, where each phase has two single coils,

[0022] Figure 4 Shown according to Figure 3 Schematic diagram of the independent magnetic flux with internal short circuits between turns, and

[0023] Figure 5 Shown Figure 2 Schematic diagram of the winding configuration of the circuit, where each phase has two single coils.

[0024] It applies to the following embodiments in which similar components are designated by similar reference numerals. In case a figure contains a reference numeral which is not specified in the associated figure description, reference is made to the preceding or following figure description.

[0025] First, refer to Figure 1 The general structure of a brushless DC motor 1 according to the present invention is described below. The main components of the brushless DC motor 1 are a rotatable rotor 2 having permanent magnets 3 directly connected to a shaft 4, the rotor 2 being mounted in the stator 5 so as to be rotatable, the stator having a hollow cylindrical ironless winding 6 and a yoke 7 arranged around the winding 6 and connected to a housing 8. The yoke 7 comprises a laminated iron core for reducing iron losses due to the rotating permanent magnets 3 of the rotor 2. A printed board 10 provides electrical connections from the winding 6 to the associated power electronics via connecting wires 9. Sensors (e.g. Hall sensors) can also be arranged on the printed board 10 and scan the position of the permanent magnets 3 that rotate together with the shaft 4.

[0026] The shaft 4, on which the permanent magnets 3 are mounted, is mounted for rotation on two preloaded ball bearings 11. Two balancing rings 12 arranged between the ball bearings 11 and the permanent magnets 3 enable dynamic balancing of the rotor, as material can be selectively removed from the two balancing rings 12. Balancing the rotor with the balancing rings 12 reduces vibration and noise, thereby correspondingly extending the service life of the ball bearings 11 and the entire motor 1, particularly at the high speeds achievable by the brushless DC motor 1. The rotor 2, mounted in the housing 8, can be secured on the face side by a bearing flange 13.

[0027] Figure 2 Shown in Figure 1 The DC motor 1 in FIG. 1 is a hollow cylindrical ironless winding 6 connected to a permanent magnet 3. The permanent magnet 3 comprises two pairs of permanent magnet poles 14 and two three-phase systems 15 of the winding 6, which are electrically isolated from each other and are connected to the single phases P1, P3 and P5 or P2, P4 and P6, respectively. In addition to the permanent magnets 3 having four poles, the single phases P1, P3 and P5 and P2, P4 and P6 each have two single coils 16 connected in series and arranged in the winding 6 at a spatial angle of 90° offset from each other. A large air gap 17 is provided between the stator 5 with the winding 6 and the rotor 2 with the permanent magnets 3, which allows the flow of fluids, in particular human blood. Furthermore, the individual phases P1, P3 and P5 as well as P2, P4 and P6 of the respective three-phase system 15 are each electrically connected to one another in a star connection, wherein the individual neutral points 18 of the two three-phase systems 15 are merged with one another and led out of the winding 6 as a common neutral point 19. Figure 2 As an alternative to the star connection shown in , the two three-phase systems 15 can also be connected to form a six-phase polygonal circuit without a separate neutral point 18 or a common neutral point 19 .

[0028] Figure 3 Shown for Figure 2 Schematic diagram of the individual magnetic fluxes of a DC motor 1 according to the invention in a star connection is shown in FIG. Here again, the preferred embodiment of a DC motor 1 with four-pole permanent magnets 3 and two three-phase systems 15 is used as the basis, each phase having two individual coils 16 connected in series and arranged at a spatial angle of 90° relative to one another in the winding 6. Figure 3 The diagram shows the individual magnetic fluxes Φ1 to Φ4 of four single coils 16 of two associated single phases of two three-phase systems 15 separated from one another (i.e. offset by 180°). The two single coils 16 of the respective single phases are electrically connected in opposite winding directions, resulting in coupling of the magnetic coils to the north and south poles.

[0029] Figure 4 The schematic diagram in FIG shows a diagram of the individual magnetic fluxes Φ1 to Φ4 of the four single coils 16 for a faulty operating state, wherein the single coils 16 of the winding 6 are short-circuited due to an internal winding fault and ideally do not allow any magnetic flux. Figure 3In contrast to the undisturbed operating state of DC motor 1 in FIG. 1 , a single coil 16 of single-phase P1 is short-circuited in an unforeseen manner due to an internal fault between the turns, resulting in the formation of a short-circuit current that counteracts the rotor field. Magnetic fluxes Φ1 and Φ2 are forced to leave the short-circuited single coil 16 of single-phase P1 and must therefore close tangentially, i.e., in the circumferential direction, in air gap 17 between the surface of rotor 2 and the inner diameter of winding 6. Magnetic fluxes Φ1 and Φ2 thus only form leakage flux components that are no longer detected by the winding system and can therefore no longer contribute to electromechanical power conversion.

[0030] The magnetic coil flux composed of the magnetic flux Φ1 and Φ2 of the single-phase P1 is reduced by 3 / 4 to 1 / 4 due to the short circuit between the turns. However, the magnetic coil flux in the single-phase P2 of the opposite arrangement of the winding 6 shows Figure 3 The flux that prevails in the fault-free state is reduced by only 1 / 4 to 3 / 4. After disconnecting the short-circuited single phase P1, the single phase P2 can still continue to operate, with a flux reduction of 25%. This relatively small damage is due to the fact that the directly adjacent pole pairs are associated with the single phase. After disconnecting the faulty single phase, 10 / 12 (i.e., approximately 83% of the winding system) can still make a power contribution. Therefore, in emergency operation, the winding current and the copper losses corresponding to the square of the current can also be reduced. Due to the higher degree of efficiency during emergency operation, the DC motor 1 according to the invention can be classified as significantly more fault-tolerant than conventional motors with a simple three-phase winding 6. With the DC motor 1 according to the invention, starting from a standstill during emergency operation is still possible.

[0031] The multi-phase nature of the DC motor 1 according to the present invention, resulting in smaller voltage differences between adjacent single coils 16, reduces current flow between adjacent single phases P1 to P6 during emergency operation with internal short circuits between turns. Emergency operation with existing internal short circuits between turns therefore results in lower parasitic losses, and the DC motor 1 according to the present invention therefore offers improved efficiency during emergency operation. Compared to bifilar windings 6, in the event of an internal short circuit between turns in the DC motor 1 according to the present invention, magnetic flux can still form over more than half the circumference of the winding 6, which allows for slightly lower voltage induction in the remaining unaffected single coils 16.

[0032] Figure 2 The winding arrangement of the DC motor 1 according to the present invention is shown in FIG. Figure 5, a schematic diagram of a winding 6 with two single coils 16 for each single phase P1 to P6 is shown. The single phases P1 to P6 of two separate three-phase systems 15 are connected to one another in a star connection, wherein the neutral points 18 of at least two separate three-phase systems 15 are connected to one another and can be led out of the winding 6 as a common neutral point 19 and connected to the power electronics. The single coils 16 of the two three-phase systems 15 are configured in a diamond-shaped winding configuration to achieve the highest possible turn density of the hollow-cylindrical ironless winding 6. This winding configuration is also known as diamond winding. The two single coils 16 of the respective single phases P1 to P6 are electrically connected in opposite winding directions in the hollow-cylindrical ironless winding, resulting in coupling of the magnetic coils to the north and south poles and, thus, increased fault tolerance of the DC motor 1 according to the present invention.

[0033] Reference Signs List

[0034] 1 DC motor

[0035] 2 rotors

[0036] 3 permanent magnets

[0037] 4-axis rod

[0038] 5 Stator

[0039] 6 Winding

[0040] 7 Magnetic yoke

[0041] 8 Housing

[0042] 9 connecting wires

[0043] 10 Printing board

[0044] 11 Ball bearings

[0045] 12 Gimbal

[0046] 13 Bearing flange

[0047] 14 Pole Pairs

[0048] 15 Three-phase system

[0049] 16 single coil

[0050] 17 Air Gap

[0051] 18 Neutral point

[0052] 19 Common Neutral Point

[0053] P1-P6 single phase

[0054] Φ1-Φ4 magnetic flux

Claims

1. A brushless DC motor (1) for an actuator unit of an implant, comprising a stator (5) having a hollow cylindrical ironless winding and a rotor (2), the stator being rotatable relative to the stator (5) and comprising a shaft (4) having p pairs of permanent magnet poles (14), wherein the winding (6) has at least n=2 three-phase systems separated from one another, It is characterized by: At least a number p=2 of permanent magnet pole (14) pairs are provided, and the n three-phase systems (15) are separated from one another, - For a number p>1 of permanent magnet poles (14), n - an integer divisor corresponding to p, where said integer divisor is not equal to 1, or - corresponds to the quantity p, or - corresponds to twice the number p of pairs of permanent magnet poles (14), wherein the n three-phase systems (15) separated from one another in the hollow cylindrical ironless winding (6) are arranged in a manner spatially offset from one another by an angle of 360° / n, wherein for each phase of the three-phase system electrically separated from one another, at least a number k=2 of single coils (16) are connected in series, the product k·n of k single coils (16) and the n separated phase systems corresponds to twice the number of pairs of permanent magnet poles (14), and wherein the spatial angle of the single coils (16) connected in series of the respective phases of the three-phase system (15) separated from one another is 360° / n / k.

2. The brushless DC motor (1) according to claim 1, characterized in that The number n of three-phase systems (15) separated from one another corresponds to an integer divisor of p, wherein the integer divisor is not equal to 1, or corresponds to the number p of pairs of permanent magnet poles (14).

3. The brushless DC motor (2) according to claim 2, characterized in that The number n of three-phase systems ( 15 ) separated from one another corresponds exactly to the number p of pairs of permanent magnet poles ( 14 ).

4. The brushless DC motor (1) according to claim 1, characterized in that Two single coils (16) of a phase connected in series are electrically connected in the opposite winding direction in the hollow cylindrical ironless winding (6).

5. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that The axial positions of the at least two three-phase systems (15) separated from each other overlap at least in certain areas with respect to the shaft (4).

6. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that The single coils (16) of the three-phase systems (15) separated from each other are connected to each other in a star connection.

7. The brushless DC motor (1) according to claim 6, characterized in that Neutral points (18) of the at least two three-phase systems separated from each other are connected to each other.

8. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that The single coils (16) of the three-phase systems (15) separated from each other are connected in series with each other, thereby producing an electrical coupling of the individual three-phase systems (15), which is called a delta connection.

9. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that The single coils of the three-phase systems (15) of the brushless DC motor (1) that are separated from each other are connected to each other in a single polygonal connection.

10. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that For each of the three-phase systems (15) separated from each other, a separate electronic commutator is provided.

11. The brushless DC motor (1) according to claim 10, characterized in that The electronic commutator is an electronic block commutator.

12. The brushless DC motor (1) according to one of claims 1 to 4, characterized in that The stator (5) includes a yoke (7).

13. The brushless DC motor (1) according to claim 12, characterized in that The yoke (7) is a laminated iron package arranged around the hollow cylindrical ironless winding (6).

14. The brushless DC motor according to claim 1, wherein A gap (17) between the rotor (2) and the stator (5) allows fluid to flow through the gap (17).

15. The brushless DC motor (1) according to claim 14, characterized in that The fluid is human blood.

16. The brushless DC motor (1) according to claim 14, characterized in that The gap (17) is greater than 15% of the radius of the rotor (2).

17. The brushless DC motor (1) according to claim 16, characterized in that The gap (17) is greater than 25% of the radius of the rotor (2).

18. The brushless DC motor (1) according to claim 1, characterized in that The implant is a heart assist system.

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