Electric synchronous motor for a power tool

Abstract

The present disclosure provides an electric synchronous motor (1) for a power tool. The motor (1) includes a rotor (2) having at least one pair of circumferential polar alterations (7), a slotless stator yoke (6) of ferromagnetic material, and a stator (4) comprising three, or a multiple of three, concentrically wound coils (5) arranged in a non-overlapping manner inside the stator yoke (6). Each coil (5) has a phase cable (9) arranged to feed the coil with current and a y-point cable (8), with the y-point cables (8) connecting the coils (5) together in a y-point. The phase cable (9) and the y-point cable (8) of each coil (5) are arranged in axially opposite ends of the stator (4).

Description

[0001] ELECTRIC SYNCHRONOUS MOTOR FOR A POWER TOOL

[0002] FieldSummary of the invention

[0003] The present disclosure relates to electric motors for power tools, and more particularly to an electric synchronous motor and a power tool comprising such a motor.

[0004] BackgroundSummary of the invention

[0005] Slotless electrical permanent magnet synchronous motors, in contrast to their slotted counterparts, can operate at high rotational speeds. High speed operation results not only in high efficiency and power density, but low weight as well. Therefore, slotless permanent magnet machines are preferred in industrial power tools.

[0006] The conventional way of making a slotless motor for such an application is to use a 2-pole rotor inside a tubular stator yoke, which internally carries a three-phase winding comprising three coils. The commonly used winding configuration is the so-called overlapping winding technique, characterized by overlapping of the end-windings in a complex manner.

[0007] Particularly, the combination of an overlapping winding technique and a slotless stator design results in a complex assembly process.

[0008] WO 2018 / 202410 Al describes a synchronous electric motor for power tools comprising three concentrically wound coils that are inserted individually in the stator yoke in a non-overlapping fashion. This may be referred to as a concentrated winding technique. By using such concentrated winding technique, the assembly process can be simplified, with higher quality and reduced costs as consequence. It would be advantageous to achieve a motor design that can even further facilitate the assembly process while still providing good motor performance.

[0009] Summary of the invention

[0010] In a first aspect, an electric synchronous motor for a power tool is provided. The motor includes: a rotor having at least one pair of circumferential polar alterations; a slotless stator yoke of ferromagnetic material; and a stator comprising three, or a multiple of three, concentrically wound coils arranged in a non-overlapping manner inside the stator yoke, each coil having a phase cable arranged to feed the coil with current and a y-point cable, the y-point cables connecting the coils together in a y-point; wherein the phase cable and the y-point cable of each coil are arranged in axially opposite ends of the stator.

[0011] In motors for power tools, the so-called y-point connection (also called star connection) is typically used instead of the D (delta) connection. When using the D connection, any small asymmetry in the coils may cause circulating currents which in turn causes power loss. Therefore, the Y connection is preferably used in motors for power tools. In such Y connection, the conductors of the coils may in one of their ends be connected together in a y-point, which may serve as a neutral. This end of the conductor may be referred to as the y-point cable. The opposite end of the conductor of each coil may be connected to the current feed. This end of the conductor may be referred to as the phase cable.

[0012] The arrangement of the phase cables and y-point cables in axially opposite ends of the stator simplifies the assembly process of the motor and improves manufacturability by reducing crowding of cables at a single end of the stator. By separating the phase cable bundle and the y-point cable bundle to opposite ends of the stator, the assembly process becomes more straightforward and less prone to errors. This separation reduces the complexity of cable management at each end, allowing for easier access and manipulation during assembly. The reduced crowding of cables at a single end minimizes the risk of short circuits or insulation damage that could occur when too many cables are bundled together in a limited space. Additionally, this configuration allows for better space utilization in the motor. In conventional designs where all connections are at one end, valuable space is often left unused at the opposite end. By distributing the connections, both ends of the motor can be efficiently utilized. This is especially advantageous in high power density applications, such as power tools, where compact design is highly desired. The improved space utilization allows for potentially shorter overall motor length or provides additional room for other components, contributing to the overall efficiency and power-to-size ratio of the motor.

[0013] In conventional motors having all phase and y-point cables in the same end, the number of winding turns in each coil will be an integer. Hence, each half of the coil will have equal number of conductors. By instead having the phase cables and the y-point cables in opposite ends of the stator, (roughly) half of the coil may have one conductor less than the other half. This may be referred to as fractional windings and has been studied to a limited extent in conventional slotted motors with overlapping windings, where it resulted in an unacceptable amount of torque ripple and vibrations. The inventors have realized that such fractional windings, contrary to the expectations, can be applied in a slotless motor with concentrated (non-overlapping) windings without causing unacceptable levels of torque ripple and vibrations. Hence, with the present invention, the assembly and space benefits of having the phase cables and y-point cables in opposite ends can be achieved while still providing a good motor performance.

[0014] For example, such fractional windings may be expressed as each coil having N plus one half number of conductor winding turns, wherein N is an integer.

[0015] It will be appreciated that in the present context, the term "one half" is to be broadly interpreted as roughly one half, I.e., the extra "half" of the winding turns is to be understood as approximately one half. For example, the conductor of the extra half may extend at least along the long side of the coil (i.e. in the axial direction of the stator). For example, one of the two long sides of the coil may have one more conductor as compared to the other half. This fractional winding configuration, when combined with the slotless nonoverlapping arrangement of the coils, may thus reduce torque ripple and vibrations that may be associated with fractional windings in conventional motors.

[0016] The motor described in the present application is slotless and aimed at providing a smooth and continuous torque output, which is particularly critical in precision-driven power tools. In contrast to slotted machines, a slotless motor inherently eliminates cogging torque, thereby enabling uniform torque production throughout the rotor’s rotation. This makes slotless motors especially suitable where stable and predictable performance is required.

[0017] Moreover, in such applications, it is desirable that the torque be as even as possible over the full angular range of the rotor, i.e., throughout the entire revolution. Irregularities in torque output, such as torque ripple, may lead to reduced tool precision, increased vibration, and user fatigue. Therefore, the inventors have realized that the use of fractional windings in the present slotless configuration is particularly advantageous, as it allows the design benefits of axial cable separation to be realized without compromising the required torque uniformity.

[0018] According to an embodiment, the electric synchronous motor may be arranged to be fed with a voltage below 50 V.

[0019] This may be referred to as a low voltage application and may be used for handheld battery-powered tools. In such applications (below 50 V), where thicker cables are typically used due to higher current requirements, the arrangement of the phase cables and y-point cables in opposite ends of the stator may provide an improved utilization of the available space, resulting in improved ease of assembly and higher power density.

[0020] Alternatively, the electric synchronous motor may be arranged to be fed with a voltage over 300 V, such as from the mains. This may be referred to as a high voltage application. This high voltage configuration may allow for increased power output, making it suitable for more demanding applications or fixed power tools.

[0021] According to an embodiment, the phase cable and y-point cable of each coil may be arranged in safety spaces provided at both axially opposite ends of the stator. This arrangement may utilize the safety spaces at both ends of the stator, which often are required in high voltage applications, improving space efficiency while maintaining necessary safety standards for high voltage applications. Hence, the configuration may take advantage of the safety spaces that may be required at both ends of the stator, effectively using what would otherwise be empty space to accommodate the separated connections. For example, each safety space may comprise a support structure in a non-conducting material (such as plastic). The support structure may e.g. be in a similar shape as the stator yoke and extend out from the axial end of the stator yoke so as to make sure a safety distance from the stator to surrounding components is kept.

[0022] According to an embodiment, the rotor of the electric synchronous motor may comprise no less and no more than two pairs of circumferential polar alterations.

[0023] This four-pole configuration may improve the motor's performance characteristics for power tool applications, balancing power output and efficiency.

[0024] According to an embodiment, a power tool comprising the electric synchronous motor of the first aspect (or any of the above-described embodiments) is provided.

[0025] According to an embodiment, the power tool may be a tightening tool.

[0026] Brief description of the drawings

[0027] Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:

[0028] FIG. 1 illustrates a longitudinal section view of an electric synchronous motor, according to aspects of the present disclosure. FIG. 2 shows a cross-sectional view of the motor taken at B-B in FIG. 1.

[0029] FIG. 3 depicts a schematic diagram of an electrical circuit for the motor, according to aspects of the present disclosure.

[0030] FIG.4 shows a prior art coil arrangement in a motor.

[0031] FIG. 5 illustrates a coil arrangement of the motor of FIGS. 1 and 2, according to an embodiment of the present disclosure.

[0032] FIGs. 6A and 6B are graphs illustrating performance characteristics of an example electric synchronous motor of the present disclosure.

[0033] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted. Like reference numerals refer to like elements throughout the description.

[0034] Detailed description of embodiments

[0035] Referring to Figures 1 and 2, an electric synchronous motor 1 for a power tool comprising a tubular rotor 2 having at least one pair of circumferential polar alterations 7 (i.e. at least one pole pair) is shown. The motor 1 further comprises a tubular slotless stator yoke 6 of ferromagnetic material. A stator 4 is provided comprising three, or a multiple of three, concentrically wound coils 5 arranged in a non-overlapping manner inside the stator yoke 6.

[0036] The rotor 2 may for example comprise no less and no more than two pairs of circumferential polar alterations 7 (i.e., the rotor 2 may be a four-pole rotor). For example, the circumferential polar alterations 7 may be formed by permanent magnets 3. The permanent magnets 3 may be arranged in an alternating polarity configuration around the circumference of the rotor 2.

[0037] This configuration with two pairs of alternating polar directions 7 may provide improved torque characteristics and reduced cogging torque compared to a two-pole design. In some cases, the four-pole arrangement may allow for a more compact motor design while maintaining high power output, which may be particularly advantageous for power tool applications where size and weight are important considerations.

[0038] The stator yoke 6 surrounds the coils 5 and provides a return path for the magnetic flux generated by the coils 5. The slotless design of the stator yoke 6 may contribute to reduced cogging torque and improved efficiency of the motor 1.

[0039] Each coil 5 of the stator 4 is wound concentrically and in a non-overlapping fashion.

[0040] The motor 1 may be configured with various numbers of coils 5 depending on the specific application requirements. For example, the stator 4 may comprise three coils 5 for a three-phase motor configuration, or multiples of three coils 5. Figure 3 depicts a schematic diagram of an example of an electrical circuit illustrating the y-point connection principle that may be used in the electric synchronous motor 1. In this configuration, the motor comprises three coils A, B, C, representing the three phases of the motor windings. Each coil A, B, C has two ends, one connected to the inverter output and the other connected to a common point, forming the y-point or star connection.

[0041] The inverter, shown on the left side of the diagram, has three output terminals. Each of these terminals is connected to one end of a corresponding coil A, B, C, providing the phase current to drive the motor. These connections may be referred to as the phase cables of the coils A, B, C.

[0042] The opposite ends of all three coils A, B, C are joined together at a single point, forming the y-point connection. This common connection point may serve as a neutral point in the motor circuit. The cables leading to this common point may be referred to as the y-point cables.

[0043] In some cases, the y-point may be left floating, while in other implementations, it may be connected to a neutral point of the power supply or to ground, depending on the specific motor control strategy and application requirements. Figure 5 illustrates one of the coils 5 of the stator 4. Each coil 5 of the stator 4 comprises a phase cable 9 and a y-point cable 8. The phase cable 9 and the y-point cable 8 may be the respective ends of the conductor of the coil 5. Hence, each coil 5 may comprise a conductor (cable) that is wound a number of turns. One end of the conductor is the phase cable 9 and the other end of the conductor is the y-point cable 8. The y-point cables 8 connect the coils 5 together in a y-point configuration. For example, the y-point cables 8 of the coils 5 may be soldered together.

[0044] As illustrated in Figure 5, the phase cable 9 and the y-point cable 8 of each coil 5 are arranged in axially opposite ends of the stator 4. In other words, the phase cables 9 of the coils 5 are arranged in a first axial end of the stator 4 and the y-point cables 8 are arranged in a second axial end of the stator 4, opposite to the first end. Hence, the phase cable 9 (which may be an end portion of the conductor of the coil 5) may lead / extend out from the coil 5 in one axial end of the stator 4 while the y-point cable 8 may lead / extend out from the coil 5 in the opposite axial end of the stator 4. This arrangement differs from the prior art coil configuration shown in Figure 4, where both the phase cable 53 and y-point cable 52 of a coil 51 are arranged in the same axial end of the stator.

[0045] For example, the phase cable 9 and y-point cable 8 of each coil 5 may be arranged in safety spaces provided at the axially opposite ends of the stator 4. These safety spaces may provide electrical isolation and protection for the coil 5 and the conductor ends 9, 8, which may be particularly beneficial in high-voltage applications.

[0046] The stator coils 5 may have fractional windings, where one half of the coil 5 has one winding less than the other half. For example, as illustrated in Figure 5, the coil 5 comprises two halves denoted C and D. Half C of the coil 5 may have one more conductor as compared to half D. This fractional winding configuration is a consequence of the y-point cable 8 and the phase cable 9 leading out from the coil 5 in opposite axial ends. Each coil 5 may e.g. comprise N plus one half number of conductor winding turns, where N is an integer. For example, the coil 5 may have 10.5 winding turns, where N equals 10. The last half winding turn of the coil 5 illustrated in Figure 5 is in the left half C.

[0047] The fractional winding configuration creates an asymmetry in the coil 5, with one half of the coil 5 having one more conductor turn than the other half.

[0048] The motor 1 utilizes non-overlapping windings in combination with fractional windings. Thanks to the non-overlapping windings, the fractional windings do not produce unacceptably high levels of torque ripple and vibrations in the motor 1. When combined with the fractional winding configuration, the motor 1 may exhibit reduced cogging torque and improved overall performance.

[0049] The motor 1 may in different embodiments be designed to operate at various voltage levels. For low-voltage applications, the motor 1 may be arranged to be fed with a voltage below 50 V, such as from a battery. In such cases, the coils 5 may use thicker conductors to handle the higher current requirements.

[0050] Alternatively, for high-voltage applications, the motor 1 may be arranged to be fed with a voltage over 300 V, such as from the mains. In these high-voltage configurations, the safety spaces at the axially opposite ends of the stator 4 may be particularly important for ensuring proper electrical isolation.

[0051] In some aspects, the electric synchronous motor may be incorporated into a power tool. The enablement of a compact design and high-power output of the motor make it well-suited for various power tool applications. For instance, the motor may be integrated into handheld power tools such as drills, saws, or grinders.

[0052] The power tool incorporating the electric synchronous motor may be a tightening tool. The tightening tool may be used in assembly lines where precise torque control is important.

[0053] Figures 6A and 6B are graphs illustrating simulated performance characteristics of an example electric synchronous motor 1 in which the phase cable 9 and the y-point cable 8 of each coil 5 are arranged at axially opposite ends of the stator 4, as illustrated in Figure 5. These results are compared with a conventional motor configuration in which both the phase and y-point cables are located on the same axial end of the stator, as illustrated in Figure 4.

[0054] Figure 6A shows a plot of normalized torque as a function of the rotor angle over one third of an electrical cycle. The trace labeled " Moved Y-point, asymmetric" corresponds to the configuration with fractional windings and axially separated cable exits. The trace labeled " Standard" represents the conventional coil configuration. As can be observed, the torque ripple amplitude and profile of the " Moved Y-point, asymmetric" motor is substantially identical to that of the standard configuration. This confirms that the described cable arrangement and fractional winding do not introduce any significant increase in torque ripple, despite the inherent coil asymmetry.

[0055] Figure 6B illustrates the radial force acting on the stator at peak torque, also plotted as a function of rotor angle over one third of an electrical cycle. The radial force is a key indicator of potential mechanical vibrations and magnetic unbalance. Again, the two configurations demonstrate nearly identical radial force profiles, indicating that the rearrangement of coil terminations and the use of fractional windings in the slotless, non-overlapping winding architecture does not adversely affect mechanical stability or dynamic balance.

[0056] Together, these results confirm that the design choices of the present motor 1 provide the intended manufacturing and spatial advantages without sacrificing torque quality or introducing detrimental dynamic effects.

[0057] Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0058] It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable modification and alteration of the above devices or methods for purposes of describing the aforementioned aspects, but one of ordinary skill in the art can recognize that many further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

CLAIMS1. An electric synchronous motor (1) for a power tool, the motor comprising:a rotor (2) having at least one pair of circumferential polar alterations (7);a slotless stator yoke (6) of ferromagnetic material; and a stator (4) comprising three, or a multiple of three, concentrically wound coils (5) arranged in a non-overlapping manner inside the stator yoke (6), each coil (5) having a phase cable (9) arranged to feed the coil with current and a y-point cable (8), the y-point cables (8) connecting the coils (5) together in a y-point;wherein the phase cable (9) and the y-point cable (8) of each coil (5) are arranged in axially opposite ends of the stator (4).

2. The electric synchronous motor (1) as defined in claim 1, wherein each coil (5) has N plus one half number of conductor winding turns, wherein N is an integer.

3. The electric synchronous motor (1) as defined in claim 1 or 2, wherein the motor (1) is arranged to be fed with a voltage below 50 V.

4. The electric synchronous motor (1) as defined in claim 1 or 2, wherein the motor (1) is arranged to be fed with a voltage over 300 V.

5. The electric synchronous motor (1) as defined any one of the preceding claims, wherein the phase cable (9) and y-point cable (8) of each coil (5) are arranged in safety spaces provided at the axially opposite ends of the stator (4).

6. The electric synchronous motor (1) as defined in any one of the preceding claims, wherein the rotor (2) comprises no less and no more than two pairs of circumferential polar alterations (7).

7. A power tool comprising the electric synchronous motor (1) of any of the preceding claims.

8. The power tool as defined in claim 7, wherein the power tool is a tightening tool.

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