Electric motor

Abstract

Electric motor, comprising: a rotor (20) with a shaft (22), a rotor core (24) which is fixed to the shaft (22) and has n teeth (26), a commutator, which is attached to the shaft (22) adjacent to the rotor core (24) and m segments Z1-Z m (30), and rotor winding units (36), which are wound around the teeth (26) and connected to the segments (30); a stator (40) configured to form 2P magnetic poles and magnetically coupled to the rotor (20); and brushes (60) arranged to be in sliding contact with the commutator, where P is an integer greater than 1 and where m and n are even integers greater than P, wherein each of the rotor winding units (36) is connected to a pair of adjacent segments (30) and at least one of the rotor winding units (36) comprises at least two coils connected in series, and wherein each such coil comprises at least two sub-coils connected directly in series and separated from each other by at least one tooth (26), and that a first sub-coil and a last sub-coil of the coil are each connected to a pair of segments (30), wherein the rotor (20) comprises m rotor winding units R1-R m (36) and each rotor winding unit R k (36) is connected to a pair of adjacent segments Z k and Z k+1 (30), is formed of P+1 coils when k is an integer multiple of m / P, and is formed of P coils when k is not an integer multiple of m / P, where 1≤k≤m-1.

Description

FIELD OF THE INVENTION

[0001] The invention relates to an electric motor and in particular to a wound rotor of an electric motor. BACKGROUND OF THE INVENTION

[0002] A single-phase series motor is also known as a universal motor because it can operate with both alternating current and direct current. It is a single-phase motor with a wound stator and a wound rotor, whose rotor windings are connected in series with the stator windings via brushes. Due to its characteristic high speed, small size, low weight, high starting torque, convenient speed control, and series excitation, the universal motor is often used to drive low-power devices that require high speed and must be small and lightweight, such as kitchen mixers, vacuum cleaners, household sewing machines, power tools, etc.

[0003] In the Fig. 5 and Fig. 6, a conventional universal motor of the known type has a rotor and a stator. The rotor has a shaft 22, a commutator (not shown) attached to the shaft, a rotor core 24 attached to the shaft adjacent to the commutator and having a plurality of rotor poles 26, and rotor windings (not shown) wound around the rotor poles and connected to the commutator. The stator has a stator core 42 with a yoke of a rectangular configuration and two teeth 43 (stator poles) extending inward from two opposite sides of the yoke, on which stator windings 44 are wound. The motor also has brushes (not shown) in sliding contact with the commutator to transmit electric current to the rotor windings. When the stator windings are electrified, two magnetic poles of opposite polarity are formed at the stator poles, forming two magnetic circuits.Each magnetic circuit passes through the two stator poles, through one half of the rotor and one side of the yoke, so the path is relatively long, as shown in . Fig. 6. Furthermore, both ends of each rotor winding are usually connected to the two segments of the commutator. The commutation performance of the motor is not good, especially when the number of turns of the rotor windings is relatively large.

[0004] JP 2005 - 269 781 A discloses a rotor for an electric motor.

[0005] The invention aims to shorten the stator magnetic circuit in an electric motor and to improve the commutation performance of the motor.

[0006] To achieve this object, the invention provides an electric motor according to claim 1. Advantageous embodiments are the subject of the subclaims, the following description and the figures. OVERVIEW OF THE INVENTION

[0007] Accordingly, the present invention provides an electric motor comprising: a rotor having a shaft, a rotor core having a plurality of teeth and fixed to the shaft, a commutator having a plurality of segments and fixed to the shaft adjacent to the rotor core, and rotor winding units wrapped around the teeth and connected to the segments; a stator magnetically coupled to the rotor; and brushes arranged in sliding contact with the commutator; wherein each of the rotor winding units is connected to a pair of adjacent segments, and at least one of the rotor winding units comprises at least two coils connected in series, and each coil has at least two sub-coils connected directly in series and separated from each other by at least one tooth, and wherein a first sub-coil and a last sub-coil of the coil are each connected to a pair of segments.The stator is configured to form 2P magnetic poles. The commutator has m segments Z1-Zm, and the rotor has n teeth, where P is an integer greater than 1 and where m and n are even integers greater than P. The rotor has m rotor winding units R1-Rm, and each rotor winding unit Rk is connected to a pair of adjacent segments Zk and Zk+1, is formed of P+1 coils when k is an integer multiple of m / P, and is formed of P coils when k is not an integer multiple of m / P, where 1≤k≤m-1.

[0008] Preferably, when k ≠ m / P, a pair of segments connected by each coil of a rotor winding unit Rk has substantially the same polarity.

[0009] Preferably, a rotor winding unit Rm connected to a pair of adjacent segments Zm and Z1 is formed of P-1 coils.

[0010] Preferably, when 1≤k≤m / P-1, the coils of the rotor winding unit Rk have the same winding direction.

[0011] Preferably, when 1≤k≤(m / P-2), the rotor winding units Rk and Rk+2 have the same winding direction, and the rotor winding units Rk and Rk+1 have an opposite winding direction.

[0012] Preferably, the ratio of the number of segments to the number of teeth is 1, 2 or 3.

[0013] Preferably, each of the coils comprises two sub-coils connected directly in series and separated from each other by a tooth.

[0014] Preferably, the at least two secondary coils have the same winding direction.

[0015] Preferably, the stator comprises a stator core with a yoke having at least two first sections with two main poles extending therefrom and at least two second sections with two auxiliary poles extending therefrom, wherein the at least two main poles and the at least two auxiliary poles are arranged alternately in the circumferential direction of the stator core and the at least two main poles carry stator windings wound thereon and wherein the stator windings are configured such that, upon electrification thereof, at least two main magnetic poles of the same polarity are formed at the at least two main poles and at least two induced magnetic poles of the same polarity, which is opposite to the polarity of the main magnetic poles, are formed at the at least two auxiliary poles.

[0016] Preferably, the auxiliary poles do not carry a stator winding.

[0017] Alternatively, the auxiliary poles carry stator windings, whereby the number of turns of the stator winding at the auxiliary poles is less than the number of turns of the stator winding at the main poles.

[0018] Preferably, the first sections of the yoke are narrower than the second sections of the yoke.

[0019] Preferably, the ratio of an outer diameter of the rotor to a minimum outer size of the stator is greater than 7:10. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] A preferred embodiment of the invention will now be described by way of example, with reference to the figures of the accompanying drawings. Identical structures, elements, or parts that appear in more than one figure bear the same reference numerals in all the figures in which they appear. The dimensions of components and features illustrated in the figures are generally chosen for convenience of illustration and are not necessarily to scale. The figures are listed below. Fig. 1 is a schematic sectional view of an electric motor according to a preferred embodiment of the present invention; Fig. 2 is a magnetic path diagram for the motor of Fig. 1; Fig. 3 shows a simplified winding scheme for the motor of Fig. 1; Fig. 4 is a table showing the winding scheme of Fig. 3; Fig. 5 is a schematic sectional view of a conventional universal motor; and Fig. 6 is a magnetic path diagram for the motor of Fig. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] A single-phase series motor according to a preferred embodiment of the present invention is shown in Fig. 1 shown. Fig. Figure 1 is a schematic cross-sectional view of the motor, with the rotor windings omitted. The motor has a rotor 20 and a stator 40 magnetically coupled to the rotor 20.

[0022] The rotor 20 includes a shaft 22, a rotor core 24 attached to the shaft 22, and a commutator (not shown) attached to the shaft 22 adjacent to the rotor core 24. The rotor core 24 is formed by laminations stacked one on top of the other in the axial direction of the rotor. The rotor core 24 has n teeth 26, and m rotor winding units 36 are wound around the teeth 26. A winding slot 28 is formed between every two adjacent teeth 26. The commutator has m segments (also called bars) 30. The rotor winding units 36 are wound in the winding slots 28 of the rotor core 24 and are connected to the segments 30 of the commutator. In this embodiment, m is 36 and n is 18.

[0023] The stator 40 has a stator core 42 and stator windings 44. The stator core 42 is formed by laminations stacked in the axial direction of the motor and has a yoke with two first sections 46 and two second sections 48. The first sections 46 and the second sections 48 are arranged alternately in the circumferential direction of the stator core 42. Two main poles 50 and two auxiliary poles 52 extend inward from the first sections 46 and the second sections 48, respectively. The stator windings 44 are wound around the main poles 50, while the auxiliary poles 52 do not carry any windings. The motor also has two brushes 60 that are fixed with respect to the stator 40 such that they are in sliding contact with the commutator to supply current to the rotor winding units 36 via the commutator. The stator core 42 is symmetrical.The main poles 50 and the auxiliary poles 52 are salient poles and each have a neck 54 extending inwardly from the yoke and a pole piece 56 extending circumferentially from the neck 54. Each pole piece 56 forms a continuous curved surface facing the rotor 20. Together, the pole pieces form a discontinuous cylindrical wall.

[0024] In the circumferential direction, the necks of the main poles 50 are narrower than the necks of the auxiliary poles 52. In the radial direction, the first sections 46 are narrower than the second sections 48, and the distance between the first sections 46 and the shoes 56 of the main poles 50 is greater than the distance between the second sections 48 and the shoes 56 of the auxiliary poles 52. This creates a larger space between the first sections 46 and the main poles 50 for accommodating the stator windings 44. It should be understood that the aforementioned circumferential direction and radial direction do not only apply to round or cylindrical structures, but that other shapes such as square or oval shapes are also included within the scope of the present invention.

[0025] When the stator windings 44 are energized, two main magnetic poles of the same polarity are formed by the stator windings 44 at the two main poles 50, and two induced magnetic poles of the same polarity, which are opposite to the polarity of the main magnetic poles, are formed at the two auxiliary poles 52. That is, four magnetic poles and four magnetic paths are formed, as shown in Fig. 2. Each magnetic path passes through a main pole 50, through the stator yoke, through an auxiliary pole 52, through the air gap between the auxiliary pole 52 and the rotor 20, through the rotor 20, and through the air gap between the main pole 50 and the rotor 20.

[0026] In the embodiment, the rotor 20 has an outer diameter D, the stator 40 has a minimum outer dimension Y, and the ratio of the outer diameter D to the minimum outer dimension Y is greater than 7:10. The outer diameter of the rotor 20 is called the outer diameter of the rotor core 24. The minimum outer dimension of the stator 40 is called the distance between two intersection points between a straight line extending through the center of the stator 40 and the periphery of the stator core 42. The outer diameter D of the rotor 20 is slightly smaller than the diameter of a circle defined by the curved surfaces of the pole pieces of poles 50 and 52, and the ratio of the circle diameter to the minimum outer diameter of the rotor 20 is also greater than 7:10. It should be understood that the distance between the main poles 50 and the rotor core 42 can be the same as or different from the distance between the auxiliary poles 52 and the rotor core 42. If the two distances are different, the curved surfaces of the pieces of the main poles 50 and the curved surfaces of the pieces of the auxiliary poles 52 define two circles with different diameters. The circle diameter in this configuration is called the diameter of the smaller circle.

[0027] In the preferred embodiment, the auxiliary poles 52 project inward from the inside of the second sections 48 and are thus formed as salient poles. Alternatively, the auxiliary poles 52 may be formed as non-salient poles and recessed relative to the inside of the second sections 48. If the auxiliary poles 52 are salient poles, they may also carry stator windings, and the stator windings on the auxiliary poles 52 have fewer turns than the stator windings 44 carried by the main poles 50.

[0028] The preferred winding pattern of the rotor winding units in the present invention will now be generally described. For ease of description, the segments (or bars) 30 of the commutator are designated Z1-Z m indicated, the winding slots 28 of the rotor core 24 are indicated by S1-S n indicated, the rotor winding units 36 of the rotor 20 are indicated by R1-R mand the number of stator magnet poles is 2P. The rod Z k is connected via a rotor winding unit R k electrically with the rod Z k+1 If k is not an integer multiple of m / P, the rotor winding unit R k formed from P coils connected in series. If k is an integer multiple of m / P, the rotor winding unit is R k formed from P+1 coils connected in series. The rod Z m is formed by a rotor winding unit R m electrically with the rod Z l next to the bar Z m connected. Each coil W i the rotor winding units 36 is connected to two bars 30 and comprises a first secondary coil W ia and a second secondary coil W ib , which is connected to the first secondary coil W ia is connected directly in series and from the first sub-coil W iaseparated by at least one tooth. Two sub-coils of each coil have the same winding direction, which is also the winding direction of the coil. The first sub-coil W ia and the second sub-coil W ib are respectively connected to the two bars. Preferably, the number of teeth q, which is the number of teeth around which the secondary coil is wound, of the first secondary coil W ia equal to that of the second sub-coil W ib The number of teeth q is an integer and satisfies the equation: |qn / 2P| <1, where n / 2P is the pole pitch. In the above description, P is an integer greater than 1, m and n are even integers greater than P, 1≤k≤m-1, and 1≤i≤m. Preferably, the ratio of m to n is 2, q is an integer less than n / 2P, and the ratio of m to n can be 1, 2, or 3.

[0029] If k is not an integer multiple of m / P, the path with the bars Z k and Z k+1connected rotor winding unit R k around the circumference of the rotor core 24 and is formed from P coils connected in series. Two bars connected by each coil have substantially the same polarity. Two or more bars of the same polarity in this description means that two or more bars are separated by the distance between the poles of the same polarity. As is known, for a motor with 2P stator magnetic poles and a commutator made of m bars, the distance between poles of the same polarity, measured in the number of bars, is equal to m / P. In the preferred embodiment, P is equal to 2 and m / P is equal to 18. Each rotor winding unit R k is formed by two coils connected to the rods Z k and Z k+1 are connected in series. The two coils are also connected to a common rod Z y where 1≤y≤m and y satisfies the equation |ky| =m / P or |k+1-y|=m / P.

[0030] If k is an integer multiple of m / P, the rotor winding unit R k formed of P+1 coils connected in series. Each of the first P coils is connected to two rods of essentially the same polarity, and the last coil is connected to two adjacent rods.

[0031] It is now time to Fig. 3 and Fig. 4 and the winding scheme of the rotor winding units in the preferred embodiment is described in detail. Fig. Figure 3 is a winding diagram in which the upper row represents two brushes C1 and C2 carried by the stator. The second row represents the thirty-six segments Z1-Z 36 of the commutator. The third row represents the eighteen teeth of the rotor core and the eighteen winding slots S1-S formed by the teeth 18 and the fourth row represents the four poles of the stator. Fig. 4 is a table of the winding pattern of the rotor winding units, in which the term "slot in" denotes a first winding slot in which each sub-coil of a coil is wound, and in which the term "slot out" denotes a second winding slot in which the sub-coil is wound, viewed in the winding direction of the coil.

[0032] The rod Z1 is connected to a rotor winding unit R1, which consists of the two coils W1 and W 19 is electrically connected to the rod Z2. The two coils W1 and W 19 are also available with a common rod Z 19 which has the same polarity as rod Z1. The coil W1 is connected to the rods Z1 and Z 19 connected, while the coil W 19 with the bars Z 19 and Z2. The coil W1 has the same winding direction as the coil W 19 , namely clockwise, as in Fig. 3. The first sub-coil W1a the coil W1 is wound around the teeth between the winding slots S5 and S1, and the second sub-coil W 1b The coil W1 is wound around the teeth between the winding slots S6 and S2. The first sub-coil W 19a the coil W 19 is around the teeth between the winding slots S 14 and S 10 and the second sub-coil W 19b the coil W 19 around the teeth between the winding slots S 15 and S 11 wrapped.

[0033] The rod Z2 is connected to a coil consisting of two coils W2 and W 20 The rotor winding unit R2 is electrically connected to the rod Z3. The two coils W2 and W 20 are also available with a common rod Z 20 which has the same polarity as the rod Z2. The coil W2 is connected to the rods Z2 and Z 20 connected, while the coil W 20 with the bars Z 20and Z3. Coil W2 has the same winding direction as coil W 20 , but opposite to the winding direction of the coils W1 and W 19 . The first secondary coil W 2a the coil W2 is around the teeth between the winding slots S6 and S 10 wound, and the second sub-coil W 2b the coil W2 is around the teeth between the winding slots S7 and S 11 wound. The first sub-coil W 20a the coil W 20 is around the teeth between the winding slots S 15 and S1, and the second sub-coil W 20b the coil W 20 is around the teeth between the winding slots S 16 and S2 wound.

[0034] Similar to the electrical connection of the bars Z1 and Z2, the bar Z3 is connected via a rotor winding unit R3, which consists of two coils W3 and W 21 formed, is connected to the rod Z4. The two coils W3 and W 21are also available with a common rod Z 21 which has the same polarity as rod Z3. The coil W3 is connected to the rods Z3 and Z 21 connected, while the coil W 21 with the bars Z 21 and Z4. Like the coils W1 and W 19 are also the coils W3 and W 21 wound clockwise. The first sub-coil W 3a the coil W3 is wound around the teeth between the winding slots S6 and S2, and the second sub-coil W 3b The coil W3 is wound around the teeth between the winding slots S7 and S3. The first sub-coil W 21a the coil W 21 is around the teeth between the winding slots S 15 and S 11 wound, and the second sub-coil W 21p the coil W 21 is around the teeth between the winding slots S 16 and S 12 wrapped.

[0035] Similar to the electrical connection of the bars Z2 and Z3, the bar Z4 is connected via the rotor winding unit R4, which consists of two coils W4 and W 22 is electrically connected to the rod Z5. The two coils W4 and W 22 are also available with a common rod Z 22 which has the same polarity as rod Z4. The coil W4 is connected to the rods Z4 and Z 22 connected, while the coil W 22 with the bars Z 22 and Z5. Like the coils W2 and W 20 are the coils W4 and W 22 wound counterclockwise. The first sub-coil W 4a the coil W4 is around the teeth between the winding slots S7 and S 11 wound, and the second sub-coil W 4b the coil W4 is around the teeth between the winding slots S8 and S 12 wound. The first sub-coil W 22a the coil W 22 is around the teeth between the winding slots S16 and S2, and the second sub-coil W 22b the coil W 22 is around the teeth between the winding slots S 17 and S 13 wrapped, etc.

[0036] The winding scheme of the rotor winding units in the preferred embodiment can be summarized as follows. 1) If 1≤k≤35 and k≠m / P or 18, the force applied to the rod Z k and Staff Z k+1 connected rotor winding unit R k formed from two coils connected in series. The two coils are also connected to a common rod Z y which has the same polarity as the rod Z k has. For 1≤k≤17, y is equal to k+m / P or k+18 and for 19≤k≤39, y is equal to k-(m / P-1) or k-17. The first sub-coil of each coil of each rotor winding unit R kis wound with the same number of teeth q as the second sub-coil of the coil, where q is an integer that satisfies the equation |qn / 2P |<1. This is 4 or 5 in the embodiment. Preferably, q is equal to 4 to reduce the consumption of winding material. On the other hand, when 1≤k≤(m / P-1), the two coils of the rotor winding unit R k the same winding direction and are spaced apart by n / P or 9 teeth. For 1≤k≤(m / P-2), R k+2 and R k the same winding direction and are spaced apart by one tooth, and R k+1 and R k have opposite winding directions and are spaced apart by one tooth. With the formation of the rotor winding units R1-R 17 the rotor winding units R 19 -R 35 This is preferably done using a double-slot winding machine. 2) If k is equal to m or 36, the one with the bars Z36 and Z1 connected rotor winding unit R 36 from a coil W 36 formed. The first secondary coil W 36a the coil W 36 is wound around the teeth between the winding slots S5 and S9, and the second sub-coil W 36b the coil W 36 is around the teeth between the winding slots S6 and S 10 wrapped. 3) If k is equal to m / P or 18 is the one with the bars Z 18 and Z 19 connected rotor winding unit R 18 formed from three coils, where the first coil W 18 with the bars Z 18 and Z 36 , the second coil W 36 with the bars Z 36 and Z1 and the third coil W1 with the rods Z1 and Z 19 The first secondary coil W 18a the coil W 18 is around the teeth between the winding slots S 14 and S 18 wound, and the second sub-coil W 18bthe coil W 18 is around the teeth between the winding slots S 15 and S1. The other two coils W 36 and W1 have been described above, so their description will not be repeated here.

[0037] Preferably, the first sub-coil of each coil has the same number of turns (also referred to as the number of turns) as the second sub-coil of the coil. Alternatively, the second sub-coil may have a different number of turns than the first sub-coil.

[0038] In the conventional universal motor, each pair of commutator segments is connected via a coil having a single sub-coil, and the electromotive induction force generated in the commutator coil is directly proportional to the square of the total number x of turns of the rotor winding unit. In the present invention, almost every pair of segments is connected via a coil having two sub-coils, and the electromotive induction force generated in the commutator coil is directly proportional to (x1 2 +x2 2), where x1 and x2 are the number of turns of the two sub-coils, respectively. Given that x is equal to (x1 + x2), the electromotive induction force in the commutation coil is lower in the present invention, which can improve the commutation performance and extend the expected service life of the motor. Furthermore, since two sub-coils of each two-segment connected coil are wound around multiple winding slots, the initial winding imbalance in the winding process can be reduced.

[0039] Furthermore, since in the embodiments of the present invention each magnetic flux circuit passes through adjacent main and auxiliary stator poles, the magnetic flux path is shortened and optimized. This allows the stator core and winding material of the universal motor according to the present invention to be reduced compared to a known universal motor of the same power, thereby saving costs.

[0040] Verbs such as "comprise," "comprise," "contain," and "have," as well as their variations, in the description and claims of this application are to be understood in an inclusive sense. They indicate the presence of the stated element, but do not exclude the presence of further elements.

[0041] For example, the number of brushes in a motor is not limited to two. Each rotor winding unit can have more than two sub-coils, and the sub-coils of the same rotor winding unit can have different numbers of turns.

Claims

[1] Electric motor, comprising: a rotor (20) with a shaft (22), a rotor core (24) attached to the shaft (22) and having n teeth (26), a commutator, which is attached to the shaft (22) adjacent to the runner core (24) and m segments Z1-Z m (30) exhibits, and rotor winding units (36), which are wrapped around the teeth (26) and connected to the segments (30); a stator (40) configured to form 2P magnetic poles and magnetically coupled to the rotor (20); and brushes (60) arranged to be in sliding contact with the commutator, where P is an integer greater than 1 and where m and n are even integers greater than P, wherein each of the rotor winding units (36) is connected to a pair of adjacent segments (30) and at least one of the rotor winding units (36) comprises at least two coils connected in series, and wherein each such coil comprises at least two auxiliary coils connected directly in series and separated from each other by at least one tooth (26), and wherein a first auxiliary coil and a last auxiliary coil of the coil are each connected to a pair of segments (30), wherein the rotor (20) m rotor winding units R1-R m (36) and each rotor winding unit R k (36) is connected to a pair of adjacent segments Z k and Z k+1 (30) connected, is formed from P+1 coils if k is an integer multiple of m / P, and is formed from P coils if k is not an integer multiple of m / P, where 1≤k≤m-1. [2] Motor according to claim 1, wherein at k≠m / P a pair of segments (30) passing through each coil of a rotor winding unit R k (36) are connected, essentially having the same polarity. [3] Motor according to claim 1 or 2, wherein a rotor winding unit R m (36), which is connected to a pair of adjacent segments Z m and Z1 (30) is connected, formed from P-1 coils. [4] Motor according to claim 1, 2 or 3, wherein at 1≤k≤m / P-1 the coils of the rotor winding unit R k (36) have the same winding direction. [5] Motor according to claim 4, wherein at 1≤k≤(m / P-2) the rotor winding units R k and R k+2 (36) the same winding direction and the rotor winding units R k and R k+1 (36) have opposite winding directions. [6] Motor according to any of the preceding claims, wherein the ratio of the number of segments (30) to the number of teeth (26) is 1, 2 or 3. [7] Motor according to one of the preceding claims, wherein the at least two auxiliary coils have the same winding direction. [8] Motor according to any of the preceding claims, wherein the stator (40) has a stator core (42) with a yoke having at least two first sections (46) with two main poles (50) extending from them and at least two second sections (48) with two auxiliary poles (52) extending from them, wherein the at least two main poles (50) and the at least two auxiliary poles (52) are arranged alternately in the circumferential direction of the stator core (42) and the at least two main poles (50) carry stator windings (44) wound on them; and wherein the stator windings (44) are configured such that, when they are electrified, at least two main magnetic poles of the same polarity are formed at the two main poles (50) and at least two induced magnetic poles of the same polarity, which is opposite to the polarity of the main magnetic poles, are formed at the at least two auxiliary poles (52). [9] Motor according to claim 8, wherein no stator winding is wound around the auxiliary poles (52). [10] Motor according to claim 8, wherein the auxiliary poles (52) carry stator windings and the stator windings at the auxiliary poles (52) have a smaller number of turns than the stator windings (44) at the main poles (50). [11] Motor according to one of the preceding claims, wherein the ratio of an outer diameter of the rotor (20) to a minimum outer size of the stator (40) is greater than 7:10.

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