CN219960371U - Power tool - Google Patents

Abstract

The utility model discloses a power tool, comprising: a motor for driving the power tool to work; the motor comprises a stator core, wherein the stator core comprises magnetic poles and a magnetic yoke part, and the magnetic poles comprise pole shoes, pole body parts for connecting the pole shoes and the magnetic yoke part and pole tips at the top of the pole shoes; a pole groove is formed between the pole shoe and the magnetic yoke part in a surrounding way; the ratio of the pole shoe width to the pole body width is more than or equal to 1.8 and less than or equal to 4. The power tool and the motor have good performance, long service life and excellent reversing performance, and the motor is applied to the power tool, so that the holding comfort is ensured, and the performance and the service life of the tool can be improved.

Description

Power tool

Technical Field

The present utility model relates to a power tool.

Background

A power tool is generally a mechanized tool that uses a motor as power and drives a working head through a transmission mechanism to perform work. In order to make the tool structure light and convenient to use, the motor is required to be smaller in size and have larger power and longer service life.

Disclosure of Invention

In order to solve the defects in the prior art, the utility model aims to provide a power tool and a motor with high performance, long service life and small size.

In order to achieve the above object, the present utility model adopts the following technical scheme:

a power tool, comprising: a motor for driving the power tool to work; the motor comprises a stator core, wherein the stator core comprises magnetic poles and a magnetic yoke part, and the magnetic poles comprise pole shoes, pole body parts connecting the pole shoes and the magnetic yoke part and pole tips at the top of the pole shoes; a pole groove is formed between the pole shoe and the magnetic yoke part in a surrounding manner; the ratio of the pole shoe width to the pole body width is greater than or equal to 1.8 and less than or equal to 4.

In some embodiments, the ratio of the pole shoe width to the pole body width is equal to 2.0 and less than or equal to 4.

In some embodiments, the stator core has an outer diameter of 60mm or less.

In some embodiments, the stator core has an outer diameter of 55mm or less.

In some embodiments, the stator core has a flat width of 55mm or less.

In some embodiments, the stator core has a flat width of 45mm or less.

In some embodiments, the stator core has a flat width of 43mm or less.

In some embodiments, the motor includes a rotor core disposed inside the stator, the rotor core and the stator core being spaced apart, the spacing being an air gap; the length of the air gap is less than or equal to 2mm.

In some embodiments, the air gap length is 1mm or less.

In some embodiments, the air gap length is less than or equal to 0.5mm.

In some embodiments, the width of the yoke portion is 4.5mm or less.

In some embodiments, the width of the yoke portion is 4mm or less.

In some embodiments, the power tool includes a housing for housing the motor, the housing having an outer circumference greater than or equal to 150mm and less than or equal to 200mm.

In some embodiments, the pole groove depth is 5mm or more and 20mm or less.

In some embodiments, the pole groove depth is 8mm or greater and 16mm or less.

In some embodiments, the cross-sectional circumference of the pole slot is 25mm or more and 50mm or less.

In some embodiments, the cross-sectional circumference of the pole slot is 28mm or more and 45mm or less.

In some embodiments, the cross-sectional circumference of the pole slot is 30mm or more and 40mm or less.

In some embodiments, the motor includes a rotor core disposed inside the stator, the rotor core and the stator core being spaced apart, the spacing being an air gap; the air gap length is unchanged.

In some embodiments, the motor includes a rotor core disposed inside the stator, the rotor core and the stator core being spaced apart, the spacing being an air gap; the length of the air gap near the pole tip increases.

In some embodiments, a chamfer is formed near the pole tip, the pole piece includes a first side and a second side, the first side of the pole piece extends along a curve other than the chamfer, and the chamfer has an included angle with the curve at the first side.

In some embodiments, there is an edge air gap between the end of the chamfer segment and the outer periphery of the rotor core, the difference between the edge maximum air gap and the air gap being less than or equal to 2mm.

In some embodiments, the edge maximum air gap differs from the air gap by less than or equal to 1mm.

In some embodiments, the pole piece includes a first side and a second side, the first side extending along a curve that is eccentric with respect to a curve of the rotor core outer periphery.

In some embodiments, the pole piece includes a first side and a second side, the first side extending along a curve that tapers away from the outer periphery of the rotor core from the pole body to the pole tip.

The motor has good performance, long service life and excellent reversing performance, and can be applied to a power tool, so that the holding comfort is ensured, and the performance and the service life of the tool can be improved.

Drawings

FIG. 1 is a schematic view of a power tool of the present utility model;

FIG. 2 is a schematic diagram of a prior art structure;

FIG. 3 is a schematic cross-sectional view of a portion of the structure of a motor according to a first embodiment of the present utility model;

FIG. 4 is a partial schematic view of FIG. 3;

FIG. 5 is a partial schematic view of a second embodiment;

FIG. 6 is a partial schematic view of a third embodiment;

fig. 7 is a schematic representation of the commutation performance of the motor of the present utility model as a function of the pole arc angle of the pole pieces.

Detailed Description

In order to make the technical problems solved by the present utility model, the technical solutions adopted and the technical effects achieved more clear, the technical solutions of the embodiments of the present utility model will be described in further detail below with reference to the accompanying drawings, and it is obvious that the described embodiments are only some embodiments of the present utility model, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the utility model without making any inventive effort, are intended to be within the scope of the utility model.

In the description of the present utility model, unless explicitly stated and limited otherwise, the terms "connected," "connected," and "fixed" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally formed; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communicated with the inside of two elements or the interaction relationship of the two elements. The specific meaning of the above terms in the present utility model will be understood in specific cases by those of ordinary skill in the art.

In the present utility model, unless expressly stated or limited otherwise, a first feature "above" or "below" a second feature may include both the first and second features being in direct contact, as well as the first and second features not being in direct contact but being in contact with each other through additional features therebetween. Moreover, a first feature being "above," "over" and "on" a second feature includes the first feature being directly above and obliquely above the second feature, or simply indicating that the first feature is higher in level than the second feature. The first feature being "under", "below" and "beneath" the second feature includes the first feature being directly under and obliquely below the second feature, or simply means that the first feature is less level than the second feature.

Those of ordinary skill in the art will understand that relative terms (e.g., "about," "approximately," "generally," "substantially," etc.) used in connection with a quantity or condition are intended to encompass the stated value and have the meaning dictated by the context (e.g., the term includes at least the degree of error associated with measurement of the particular value, tolerances (e.g., manufacturing, assembly, use) associated with the particular value, etc.). Such terms should also be considered to disclose a range defined by the absolute values of the two endpoints. Relative terms may refer to a percentage (e.g., 1%,5%,10% or more) of the indicated value plus or minus. Of course, numerical values that do not take relative terms should also be construed as having particular values within tolerances.

The present utility model is applied to a power tool 1, as shown in fig. 1, in an embodiment of the present utility model, the power tool is a hand-held power tool, such as an angle grinder, an electric circular saw, a straight grinder, a percussion drill, a screwdriver, a multifunctional tool, etc., and other hand-held power tools with similar structures can be considered as a modification of the angle grinder in fig. 1, and will not be repeated herein. The power tool 1 has a hollow housing in which a motor 10 is accommodated. The housing of the power tool 1 is also connected to a work piece, which is connected to the output shaft of the motor 10 via a transmission mechanism so that the motor 10 provides power to the work piece.

The motor 10 may be a brush or brushless motor, or may be an ac, dc or ac-dc motor, without limitation. The motor 10 may be an electrically driven motor, in this embodiment the motor 10 is a single phase series motor. The motor 10 includes a stator core 100 and a rotor core 200, and in the present embodiment, an inner rotor type motor is employed. Rotor core 200 and stator core 100 are typically lamination stacks formed by axially stacking and riveting or welding a suitable number of metal laminations, and the main component is iron, and therefore may also be referred to as a core, specifically, magnetically conductive silicon steel sheets. The stator core 100 may be a split stator or a unitary stator, and when a unitary stator, each lamination in the axial direction is hollow and unitary, rather than being spliced from a plurality of small laminations. Typically, the thickness of the individual metallic laminations is fixed, and the axial length of the stator core 100 is the total thickness of the stack of a suitable number of laminations. The outer profile of the unitary stator may be configured to be circular, oval or oblate, oblong or other shape suitable for receipt in a housing. The motor stator is composed of a stator core and an exciting winding sleeved on a magnetic pole, and the motor stator is used for generating exciting magnetic flux. The rotor (also called an armature) is composed of a rotor core, a rotor winding, a commutator, a bearing, a fan and the like.

The stator core 100 of the present embodiment includes an inner side surface 100a facing the rotor core 200 and an outer side surface 100b distant from the rotor core 200, the inner side surface 100a enclosing a hollow region 100c in which the rotor core 200 is disposed. The outer side surface 100b of the stator core 100 has an outer diameter D and a flat width D, and most of the hand-held power tools are small and medium-sized tools, and the housing space has a high restriction on the size of the motor 10. For the structural design of the stator core 100 with a given outer diameter of the motor 10, when the outer diameter of the corresponding stator core 100 is fixed, larger output power can be generated by increasing the outer diameter of an armature (a combination of a rotor core and windings), but an excessively large armature outer diameter reduces the wire window area where the stator exciting windings are placed and the width of a stator yoke portion, so that the motor efficiency is reduced, the temperature rise of the stator windings is improved, the magnetic pole leakage is increased, the width of the stator yoke portion is reduced, the magnetic circuit of the yoke portion is saturated (the magnetic pole leakage is increased, the width of the stator yoke portion is reduced, and the magnetic circuit saturation of the yoke portion is changed into poor motor commutation). Accordingly, while sizing and structural design of the stator core 100 is contemplated to provide sufficient space for the excitation windings, the effective magnetic flux is also maximized to provide the motor 10 with a reasonable magnetic flux density distribution and sufficient space for the stator windings.

In the power tool 1 of the present utility model, the outer diameter D of the stator core 100 is 60mm or less, and in particular, the outer diameter D of the stator core 100 is 55mm or less, and when applied to the angle grinder shown in fig. 1, a stator core having an outer diameter of 50mm may be used. The flat width d is less than or equal to 55mm, especially less than or equal to 45mm or 43mm. The power tool 1 includes a housing for housing the motor 10, and the outer circumference of the housing may be 150mm or more and 200mm or less. Taking the angle grinder shown in fig. 1 as an example, for the angle grinder and the like, the motor 10 is disposed in the housing of the holding portion, and when the diameter of the motor 10 is sufficiently small, the outer circumference of the housing may be more than or equal to 160mm and less than or equal to 190mm, in particular, in consideration of the wall thickness of the housing of the power tool 1 and the design requirements of the stator positioning ribs. Can fully meet the requirements of a user on the thin handle of the hand-held power tool for ergonomic holding comfort. The larger axial length of the motor 10 can improve the output power of the motor, but the too large length can cause too long axial flow passage of the motor 10 and poor heat dissipation of the motor, and also can deteriorate the vibration and the service life of the carbon brush of the motor, and according to different tool performance requirements, the axial length of the combination body of the stator core 100 and the rotor core 200 of the motor 10 is more than or equal to 40mm and less than or equal to 60mm, and the longer the length, the higher the power of the motor 10.

As shown in fig. 3 and 4, the first magnetic pole 110 and the second magnetic pole are disposed opposite to each other on the inner side surface 100 a. Specifically, in the present embodiment, the inner side surface 100a is a smooth continuous curve at a portion other than the magnetic poles, and the smoothness refers to the shape of the curve without protrusions, and not to the smoothness of the stator core 100 in terms of material, and the continuous smooth curve can increase more winding area. The stator core 100 further includes a yoke portion 140, which may also be referred to as a stator ring portion, and the yoke portion 140 forms a hollow region 100c around the outside of the rotor core 200. The width L3 of the yoke portion 140 may be 4.5mm or less, and particularly 4.0mm or less. The reduction in the width of the yoke portion 140 provides a larger space for the windings and the armature, but the reduction in the main magnetic flux deteriorates motor performance and commutation, and is designed to be balanced and optimized, in this embodiment, the ratio of the outer diameter of the rotor core 200 to the outer diameter of the stator core 100 is 0.63 or more.

Taking the first magnetic pole 110 as an example, the first magnetic pole 110 includes a pole body 111, a pole piece 112, and a pole tip 113. The pole body 111 is formed on the inner surface 100a and extends toward the hollow region 100c. Pole piece 112 is formed at the end of pole body 111 with first and second pole pieces, respectively, symmetrical, pole piece 112 extending in one direction and at least partially surrounding the outer circumference of rotor core 200, with a gap between pole piece 112 and the outer circumference of rotor core 200, which is air gap 120. The length L2 of the air gap 120 may be 2mm or less, particularly 0mm or less and 1mm or less, and further, particularly 0mm or less and 0.5mm or less.

The pole piece 112 has a first side 112a and a second side 112b, the first side 112a facing the hollow region 100c and the second side facing the pole slot 130. The extreme end of the pole piece 112 forms a pole tip 113. A pole slot 130 is formed between the pole piece 112 and the yoke portion 140.

In the prior art stator structure shown in fig. 2, the pole shoes extend in a substantially straight direction, and in this structure, only shorter pole shoes and shallower pole slots tend to be formed. In this embodiment, the pole piece 112 extends substantially along a curve, meaning that at least a portion of at least one of the first side 112a and the second side 112b of the pole piece 112 extends along a curve.

The arc of the pole piece 112 is α, also referred to as the angle of the first side 112a (pole arc) of the pole piece, which is the magnitude of the central angle that the arc length of the arc in which the pole piece 112 is located has, which also represents the extension length of the pole piece 112. The radian α of the pole shoe 112 is greater than or equal to 120 ° and less than or equal to 155 °, in particular, α may be greater than or equal to 125 ° and less than or equal to 148 °, and further, 128 °, 130 °, 132 °, 135 °, 140 ° and the like may be selected. Fig. 7 is a comparison of motor commutation performance for different pole shoe angles at a given rated input power, with the pole shoe 112 arc varying between 120 ° and 155 °, the commutation spark loss decreasing slowly with increasing arc, the commutation spark loss being minimal at an arc α=140°, and the commutation spark loss increasing rapidly at an arc α > 140 °. The oxide film loss increases with the arc α, and the oxide film loss increases gradually and then rapidly, and the arc α=135° of the pole piece 113 is a critical point for the oxide film loss transition. Therefore, combining the two losses, the arc α of the pole piece 112 is usable when 120 ° or more and 155 ° or less, and the combined value of the two losses is small particularly when α is 125 ° or more and 148 ° or less. The increase in the arc α of the pole piece 112 can increase the depth of the pole slot 130 to some extent. The greater the arc of the pole piece 112, the higher the utilization of motor material, the greater the air gap flux of one pole, and the greater the electromagnetic torque. Excessive pole shoe curvature can result in reduced pole-to-pole distance, increased leakage of the main pole, reduced commutation area, and increased likelihood of main flux entering the commutation area, resulting in increased sparking and degraded commutation. The stator field coil windings are affected by the pole tip position, and the larger pole shoe radians have large slot areas and coil spaces.

The pole piece 112 has a pole piece width c, which is defined as the horizontal distance from the pole tip of the first pole piece to the outside of the pole tip of the second pole piece. The pole body 111 has a pole body width b. To describe the depth of the pole slot 130, the ratio of the pole shoe width c to the pole body width b is defined as the pole slot depth coefficient. The stator core 110 has a pole groove depth coefficient of 1.8 or more and 4 or less, and particularly 2.0 or more and 4 or less. The pole slot 130 also has a slot depth a, which refers to the vertical distance from the top of the pole tip 113 to the bottom of the pole slot 130, of 5mm or more and 20mm or less, and particularly 8mm or more and 16mm or less.

Specifically, the pole groove 130 refers to an area surrounded by the second side 112b and the yoke portion 140, in which windings are provided. In order to clarify the circumference of the pole slot 130, it is necessary to clearly define the area to which the pole slot 130 is directed. In the present embodiment, the cross-sectional perimeter M of the pole slot 130 is the solid length (without dotted line) of the region S in cross-section, the first end of the region S starting from the top of the pole tip 113, and the second end of the region S being the intersection point where the tangent to the top of the pole tip 113 intersects the horizontal line of the yoke portion 140. The cross-sectional perimeter M is a distance from the first end to the second end, and when the outer diameter D of the stator core 100 is 60mm or less, the cross-sectional perimeter M is 25mm or more and 50mm or less, and particularly 28mm or less and 45mm or less, preferably 30mm or less and 40mm or less. In other embodiments, the circumference of the pole slot 130 may also be determined based on the area covered by the winding, and when the circumference is determined in this manner, the circumference of the pole slot 130 is at least not less than in this embodiment, since the winding would typically go beyond the pole slot 130 into the hollow area 110 c.

The depth of the pole slots 130 is larger, the stator core 100 can obtain larger slot area and larger winding area, and the motor performance can be improved. Meanwhile, the deep slot stator has smaller pole body width, each turn of the wound coil has smaller length, the coil has smaller resistance value, the stator has smaller copper loss, and an automatic winding mode is convenient to use, so that the motor has better performance. Taking angle mill as an example, after the motor of the implementation is adopted, the no-load rotating speed of the angle mill motor is more than or equal to 39000rpm, and the maximum volume power density of the motor is not less than 8.8W/cm ³ Is increased to not less than 10W/cm ³ The maximum mass power density of the angle grinding product is increased from not less than 600W/kg to not less than 700W/kg.

In this embodiment, the second side 112b of the pole piece 112 is formed with a first curved section 112b1 and a second curved section 112b2. The curvature of the surface radian of the first curved section 112b1 is different from that of the first side 112a, specifically, in this embodiment, the first side 112a is a curved surface concentric with the outer surface of the rotor core, an air gap 120 with a uniform width is formed, and the first curved section 112b1 gradually gets away from the first side 112a along the direction from the pole tip 113 to the pole body 111, that is, the width of the pole piece 112 in the first curved section 112b1 near the direction of the pole body 111 gradually increases, so as to ensure the strength of the pole piece 122. The second curved section 112b2 has the same curvature as the first side 112a, ensuring that the junction of the first curved section 112b1 and the second curved section 112b2 does not form an excessively narrow channel, thereby ensuring that the pole shoe has a reasonable magnetic flux density.

In a second similar embodiment, as shown in fig. 5, the stator core 300 includes the same parts as those of the previous embodiment, and only differences from the stator core 100 are described herein. The stator core 300 includes pole pieces 310, pole tips 330 are formed at the top of the pole pieces 310, and pole pieces 310 and yoke portions enclose pole slots 320. Pole piece 310 includes a first side 310a and a second side 310b, with first side 310a facing the rotor core and second side 310b facing pole slot 320. In this embodiment, a beveled segment 311 is formed near the pole tip 330. The first side 310a of the pole piece 310 extends generally along a curved line concentric with the outer periphery of the rotor core (except for the chamfer segment), and the chamfer segment 311 extends at the first side along a direction 301 tangential to the curved line. An edge air gap is formed between the tail end of the chamfer section and the periphery of the rotor core, the length L2' of the edge air gap is larger than the length L2 of the air gap, and the difference value between the maximum air gap of the edge and the air gap is smaller than or equal to 2mm. Further, the difference between the edge air gap and the air gap is less than or equal to 1mm or 0.5mm. The above configuration allows for a non-uniform air gap to be formed near the pole tip 330, reducing the effects of magnetic flux at the edge of the pole tip on air gap field distortion and commutation. Specifically, in the present embodiment, the second side 310b forms a first straight section, a second curved section, and a second straight section in this order from the pole tip 330 to the slot bottom, the first straight section holding and beveling section 311 having the same extension direction, the second curved section and the second straight section gradually moving away from the first side 310a in the direction from the pole tip 330 to the pole body, so that the pole piece 310 gradually increases in width. In the present embodiment, the bottom of the pole groove 320 is formed with an angular arc 321 near the pole body, and the angular arc 321 is a curved surface connecting the second side 310b and the bottom of the groove, instead of the square shape with a chamfer in the previous embodiment, so as to further increase the depth and the surface area of the pole groove 320. Meanwhile, the width L1 of the bottom of the pole shoe 310 is increased, and compared with the previous embodiment, the width L1 is increased by 25% -30%. Compared with the previous embodiment, the cross-sectional perimeter M is increased by 10% -13% with the slot depth a of the pole slot 320 and the yoke portion width unchanged. This embodiment can obtain higher motor performance.

In a similar third embodiment, as shown in fig. 6, the stator core 400 includes the same parts as the above two embodiments, and only the differences from the stator cores 100,300 are described herein. The stator core 400 includes pole pieces 410, pole tips 430 are formed at the top of the pole pieces 410, and pole pieces 410 and yoke portions enclose pole slots 420. Pole piece 410 includes a first side 410a and a second side 410b, with first side 410a facing the rotor core and second side 410b facing pole slot 420. The first side 410a of the pole piece 410 tapers away from the rotor periphery from the pole body 440 to the pole tip 430. Specifically, it is assumed that a circle concentric with the rotor core is made with a radius of the lowest point of the surface of the pole body 440 to the center of the rotor core, and this circle is denoted as equidistant air gap reference circle 401. The first side 410a extends along a curve that is an eccentric or other non-circular curve relative to the equidistant air gap reference circle 401 such that the distance of the first side 410a from the pole body 440 to the pole tip 430 increases gradually from the equidistant air gap reference circle 401. The above structure allows for a non-uniform air gap to be formed near the pole tip 330, reducing the effects of magnetic flux at the edge of the pole tip on air gap field distortion and commutation.

The foregoing has shown and described the basic principles, principal features and advantages of the utility model. It will be appreciated by persons skilled in the art that the above embodiments are not intended to limit the utility model in any way, and that all technical solutions obtained by means of equivalent substitutions or equivalent transformations fall within the scope of the utility model.

Claims (25)

1. A power tool, comprising:

a motor for driving the power tool to work;

the motor is characterized by comprising a stator core, wherein the stator core comprises magnetic poles and a magnetic yoke part, and the magnetic poles comprise pole shoes, pole body parts connecting the pole shoes and the magnetic yoke part and pole tips at the top of the pole shoes; a pole groove is formed between the pole shoe and the magnetic yoke part in a surrounding manner;

the ratio of the pole shoe width to the pole body width is greater than or equal to 1.8 and less than or equal to 4.

2. The power tool of claim 1, wherein a ratio of the pole shoe width to the pole body width is equal to 2.0 and less than or equal to 4.

3. The power tool of claim 1, wherein the stator core has an outer diameter of 60mm or less.

4. A power tool according to claim 3, wherein the stator core has an outer diameter of 55mm or less.

5. The power tool of claim 1, wherein the stator core has a flat width of 55mm or less.

6. The power tool of claim 5, wherein the stator core has a flat width of 45mm or less.

7. The power tool of claim 5, wherein the stator core has a flat width of 43mm or less.

8. The power tool according to claim 1, wherein the motor includes a rotor core provided inside the stator, the rotor core and the stator core being provided at a spacing, the spacing being an air gap; the length of the air gap is less than or equal to 2mm.

9. The power tool of claim 8, wherein the air gap length is 1mm or less.

10. The power tool of claim 8, wherein the air gap length is 0.5mm or less.

11. The power tool according to claim 1, wherein the width of the yoke portion is 4.5mm or less.

12. The power tool according to claim 11, wherein the width of the yoke portion is 4mm or less.

13. The power tool according to claim 1, wherein the power tool includes a housing for housing the motor, the housing having an outer circumference of 150mm or more and 200mm or less.

14. The power tool of claim 1, wherein the pole groove depth is 5mm or more and 20mm or less.

15. The power tool of claim 14, wherein the pole groove depth is 8mm or more and 16mm or less.

16. The power tool according to claim 2, wherein the cross-sectional circumference of the pole groove is 25mm or more and 50mm or less.

17. The power tool of claim 16, wherein the cross-sectional circumference of the pole slot is 28mm or more and 45mm or less.

18. The power tool of claim 16, wherein the cross-sectional circumference of the pole groove is 30mm or more and 40mm or less.

19. The power tool according to claim 1, wherein the motor includes a rotor core provided inside the stator, the rotor core and the stator core being provided at a spacing, the spacing being an air gap; the air gap length is unchanged.

20. The power tool according to claim 1, wherein the motor includes a rotor core provided inside the stator, the rotor core and the stator core being provided at a spacing, the spacing being an air gap; the length of the air gap near the pole tip increases.

21. The power tool of claim 20, wherein a beveled segment is formed adjacent the pole tip, the pole piece including a first side and a second side, the first side of the pole piece extending along a curve other than the beveled segment.

22. The power tool of claim 21, wherein an edge air gap is provided between the end of the chamfer segment and the outer periphery of the rotor core, and wherein the difference between the edge maximum air gap and the air gap is 2mm or less.

23. The power tool of claim 22, wherein the edge maximum air gap to air gap difference is 1mm or less.

24. The power tool of claim 20, wherein the pole piece includes a first side and a second side, the first side extending along a curve that is eccentric relative to a curve of the rotor core outer periphery.

25. The power tool of claim 24, wherein the curve tapers away from the outer periphery of the rotor core from the pole body to the pole tip.