Electric motor

Abstract

Electric motor, comprising: a rotor (20) with a shaft (22), a rotor core (24) which is attached to the shaft (22) and has a plurality of teeth (26), a commutator mounted on the shaft (22) adjacent to the rotor core (24) and having a plurality of segments (30), and rotor winding units (36) wound around the teeth (26) and connected to the segments (30); a stator (40) magnetically coupled to the rotor (20); and brushes (60) in electrical sliding contact with the commutator, wherein each of the rotor winding units (36) comprises at least two sub-coils directly connected in series with each other and separated from each other by at least one tooth (26), and a first sub-coil and a last sub-coil of each rotor winding unit (36) are each directly connected to a pair of adjacent segments (30), wherein two rotor winding units (36) directly connected to the same segment (30) are wound in opposite directions.

Description

FIELD OF THE INVENTIONThe invention relates to an electric motor and more particularly to a wound rotor of an electric motor.BACKGROUND OF THE INVENTIONA single-phase series motor is also known as a universal motor since it can be operated with both alternating current and direct current. This is a single-phase motor with a wound stator and a wound rotor, the rotor windings of which are connected in series with the stator windings via brushes. Because of its characteristic high speed, small size, light weight, high starting torque, comfortable speed control and series excitation, the universal motor is often used to drive low power appliances that require high speed and must be small and light in weight, such as kitchen blenders, vacuum cleaners, household sewing machines, power tools, etc.In Figs. 5 and 6, a conventional universal motor of the known type has a rotor and a stator. The rotor includes a shaft 22, a commutator (not shown) fixed to the shaft, a rotor core 24 fixed 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 having a yoke in a rectangular configuration and two teeth 43 (stator poles) extending inward from two opposite sides of the yoke and on which stator windings 44 are wound. The motor also has brushes (not shown) that are 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 on the stator poles and 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 that the path is relatively long as shown in Fig. 6. Further, both ends of each rotor winding are normally 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.JP 2005-269 781 A discloses a rotor for an electric motor. JP S58-163 258 A discloses an electric motor having a commutator.It is an object of the present invention to shorten the stator magnetic circuit and improve the commutation performance of the motor in an electric motor.To achieve this object, the invention provides an electric motor according to claim 1.SUMMARY OF THE INVENTIONAccordingly, the present invention provides an electric motor comprising: a rotor having a shaft; a rotor core having a plurality of teeth fixed to the shaft; a commutator having a plurality of segments and rotor winding units wound around the teeth and connected to the segments and fixed to the shaft adjacent to the rotor core; a stator magnetically coupled to the rotor; and brushes disposed in sliding contact with the commutator; wherein each of the rotor winding units has at least two sub-coils directly connected in series and separated from each other by at least one tooth, and wherein a first sub-coil and a last sub-coil of each rotor winding unit are each connected to a pair of adjacent segments, wherein two rotor winding units connected to the same segment are wound in opposite directions.Preferably, each of the rotor winding units has two sub-coils which are connected directly in series with each other and which are separated from each other by a tooth.Preferably, the at least two secondary coils of a rotor winding unit have the same number of turns.Alternatively, the at least two secondary coils of a rotor winding unit have a different number of turns.Preferably, the at least two secondary coils of a rotor winding are wound in the same direction.Preferably, the ratio of the number of segments to the number of teeth is 1, 2 or 3.Preferably, the stator is configured to form 2P magnetic poles, the commutator has m segments, and the rotor core has n teeth, where P is an integer greater than 1, and where m and n are even integers greater than P. Ideally, P is equal to 2.Preferably, the stator comprises a stator core having a yoke having at least two first portions each having a main pole extending and at least two second portions each having an auxiliary pole extending, wherein the at least two main poles and the at least two auxiliary poles are alternately arranged 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 of the motor, main magnetic poles of the same polarity are formed on the at least two main poles and induced magnetic poles of the same polarity opposite to the polarity of the main magnetic poles are formed on the at least two auxiliary poles.Preferably, the first portions are narrower than the second portions.Preferably, the auxiliary poles do not carry any stator windings.Alternatively, the auxiliary poles carry stator windings, the number of turns of the stator winding at the auxiliary poles being less than the number of turns of the stator winding at the main poles.Preferably, the ratio of an outer diameter of the rotor to a minimum outer dimension of the stator is greater than 7:10.BRIEF DESCRIPTION OF THE DRAWINGSA 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 throughout the figures in which they appear. The dimensions of components and features illustrated in the figures are generally chosen for clarity 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 pattern 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 EMBODIMENTSA single phase in-line motor according to a preferred embodiment of the present invention is shown in Fig. 1. FIG. 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.The rotor 20 includes a shaft 22, a rotor core 24 fixed to the shaft 22, and a commutator (not shown) fixed to the shaft 22 adjacent to the rotor core 24. The rotor core 24 is formed by laminations stacked 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 each two adjacent teeth 26. The commutator has m segments (also referred to as 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.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 having two first portions 46 and two second portions 48. the first portions 46 and the second portions 48 are alternately arranged in the circumferential direction of the stator core 42. Two main poles 50 and two auxiliary poles 52 extend inwardly from the first portions 46 and from the second portions 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 four brushes 60 fixed with respect to the stator 40 so as to be 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 has a neck 54 extending inward from the yoke and a pole shoe 56 extending circumferentially from the neck 54. Each pole shoe 56 forms a continuous curved surface facing the rotor 20. Together, the pole shoes form a discontinuous cylindrical wall.In the radial direction, the necks of the main poles 50 are narrower than the necks of the auxiliary poles 52, the first portions 46 are narrower than the second portions 48, and the distance between the first portions 46 and the shoes 56 of the main poles 50 is larger than the distance between the second portions 48 and the shoes 56 of the auxiliary poles 52, thereby forming a larger space for accommodating the stator windings 44 between the first portions 46 and the main poles 50. It is understood that the aforementioned circumferential direction and radial direction not only apply to round or cylindrical structures, but also other shapes such as square or oval shapes are included in the scope of the present invention.When the stator windings 44 are energized, two main magnetic poles having the same polarity are formed on the two main poles 50 by the stator windings 44, and two induced magnetic poles having the same polarity opposite to the polarity of the main magnetic poles are formed on 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.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 D of the rotor 20 is slightly smaller than the diameter of a circle defined by the curved surfaces of the pole shoes of the 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. When the two distances are different, the curved surfaces of the shoes of the main poles 50 and the curved surfaces of the shoes of the auxiliary poles 52 define two circles having different diameters. The circle diameter in this configuration is called the diameter of the smaller circle.In the preferred embodiment, the auxiliary poles 52 project inwardly from the inside of the second portions 48 and are thus formed as salient poles. Alternatively, the auxiliary poles 52 may be formed as non-salient poles and may be buried relative to the inner side of the second portions 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 50The preferred winding scheme of the rotor winding units in the present invention will now be described generally. For convenience of description, the segments (or bars) 30 of the commutator are indicated by Z1-Zm, the winding slots 28 of the rotor core 24 are indicated by S1-Sn, the rotor winding units 36 of the rotor 20 are indicated by W1-Wm, and the number of stator magnetic poles is 2P. The rod Zk is electrically connected to the rod Z(k+1) via a rotor winding unit Wk, and the rod Zm is electrically connected to the rod Z 1 via the rotor winding unit Wm. Each winding unit Wi includes a first sub-coil Wia and a second sub-coil Wib that is directly connected in series with the first sub-coil Wia and that is separated from the first sub-coil Wia by at least one tooth. The first sub-coil Wia and the second sub-coil Wib are each connected to a pair of adjacent rods. The number of teeth q is the number of teeth around which the coil is wound. The number of teeth q of the first sub-coil Wia is equal to the number of teeth q of the second sub-coil Wib. The number of teeth q is an integer and satisfies the equation: |q-n / 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, and the ratio of m to n may be 1, 2, or 3. Preferably, the ratio of m to n is 2, and q is an integer less than n / 2P. The winding scheme of the rotor winding units in the preferred embodiment will now be described in detail with reference to FIGS. 3 and 4. Fig. 3 is a winding diagram in which the upper row illustrates four brushes C1-C4 carried by the stand. The second row represents the thirty-six segments ZU1-Z36 of the commutator. The third row represents the eighteen teeth of the rotor core and the eighteen winding slots S1-S18 formed by the teeth, and the fourth row represents the four poles of the stator. FIG. 4 is a table of the winding scheme 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, as viewed in the winding direction of the coil.The rod Z 1 is electrically connected to the rod Z 2 via a rotor winding unit W 1. The rotor winding unit W 1 has a first sub-coil W 1 aand a second sub-coil W 1 bconnected directly in series with each other. The first sub-coil W 1 ais wound around the teeth between the winding slots S 5 and S 1, and the second sub-coil W 1 bis wound around the teeth between the winding slots S 6 and S 2. The first sub-coil W1a has the same winding direction as the second sub-coil W1b, namely, in the clockwise direction as shown in FIG. 3, and both sub-coils span four teeth.The rod Z 2 is electrically connected to the rod Z 3 via the rotor winding unit W 2. The rotor winding unit W 2 has a first sub-coil W 2 aand a second sub-coil W 2 bconnected directly in series with each other. The first sub-coil W 2 ais wound around the teeth between the winding slots S 6 and S 10, and the second sub-coil W 2 bis wound around the teeth between the winding slots S 7 and S 11. The first sub-coil W 2 has the same winding direction as the second sub-coil W 2 bbut opposite to the winding direction of the sub-coils W 1 aand W 1 bof the winding unit W 1.Similar to the electrical connection of the rod Z 1 and Z 2, the rod Z 3 is connected to the rod Z 4 via a rotor winding unit W 3. The rotor winding unit W 3 has a first sub-coil W 3 aand a second sub-coil W 3 bconnected directly in series with each other. The first sub-coil W 3 ais wound around the teeth between the winding slots S 6 and S 2, and the second sub-coil W 3 bis wound around the teeth between the winding slots S 7 and S 3. Like the sub-coils W 1 aand W 1 b, the sub-coils W 3 aand W 3 bare also wound in the clockwise direction.Similar to the electrical connection of the bars Z2 and Z3, the bar Z4 is electrically connected to the bar Z5 via a rotor winding unit W4. The rotor winding unit W 4 has a first sub-coil W 4 aand a second sub-coil W 4 bconnected directly in series with each other. The first sub-coil W 4 ais wound around the teeth between the winding slots S 7 and S 11, and the second sub-coil W 4 bis wound around the teeth between the winding slots S 8 and S 12. Like the sub-coils W2a and W2b, the sub-coils W4a and W4b are also wound counterclockwise.Other rotor winding units 36 of the rotor 20 are formed by repeating the above-described process. That is, the two sub-coils of the same rotor winding unit 36 are separated from each other by one tooth and wound in the same direction, two rotor winding units 36 directly connected to the same rod are wound in opposite directions, and the rotor winding unit W(j+2) is separated from the rotor winding unit W j by one tooth, where 1≤j≤m-2.In the conventional universal motor, each pair of commutator segments is connected to each other via a winding having a single sub-coil, the electromotive induction force generated in the commutation coil being directly proportional to the square of the total number x of turns of the rotor winding unit. In the present invention, each pair of segments is connected to each other via a winding unit having two sub-coils, the induction electromotive force generated in the commutation coil is directly proportional to (x 12+ x 22), where x 1 and x 2 are respectively the number of turns of the two sub-coils. Provided that x is (x 1+ x 2) the induction electromotive force in the commutation coil in the present invention is smaller, and therefore the commutation performance can be improved and the life of the motor can be prolonged. Incidentally, since two sub-coils of each rotor winding unit connected to two segments are wound around a plurality of winding slots, the initial imbalance in the winding process can be optimized.Further, in embodiments of the present invention, since each magnetic flux circuit passes through adjacent main and auxiliary poles of the stator, the magnetic flux path is shortened and optimized. Thereby, the stator core and the winding material in the universal motor according to the present invention can be reduced in performance as compared with a known universal motor, thereby saving cost.Verbs such as "comprise", "comprise", "contain" and "have", as well as their variations in the specification and claims of the present application, are to be understood in an inclusive sense. They indicate that said element is present, but do not exclude that still further elements are present.For example, the number of brushes of the motor is not limited to four. Each rotor winding unit may have more than two secondary coils, and the secondary coils of the same rotor winding unit may have a different number of turns.

Claims

An electric motor comprising: a rotor (20) having a shaft (22); a rotor core (24) fixed to the shaft (22) and having a plurality of teeth (26); a commutator fixed to the shaft (22) adjacent to the rotor core (24) and having a plurality of segments (30); and rotor winding units (36) wound around the teeth (26) and connected to the segments (30); a stator (40) magnetically coupled to the rotor (20); and brushes (60) in electrical sliding contact with the commutator, each of the rotor winding units (36) comprising at least two secondary coils connected directly in series to each other and separated from each other by at least one tooth (26), and a first secondary coil and a last secondary coil of each rotor winding unit (36) being respectively directly connected to a pair of adjacent segments (30), two rotor winding units (36) directly connected to the same segment (30) being wound in opposite directions.The motor of claim 1, wherein each of the rotor winding units (36) comprises two auxiliary coils connected directly in series with each other and separated from each other by a tooth (26).The motor according to claim 1 or 2, wherein the at least two sub-coils of a rotor winding unit (36) are wound in the same direction.The motor according to any one of the preceding claims, wherein the stator (40) is configured to form 2P magnetic poles, the commutator has m segments (30) and the rotor core (24) has n teeth (26), wherein P is an integer greater than 1, especially 2, and m and n are even integers greater than P.The motor according to any one of the preceding claims, wherein the stator (40) has a stator core (42) with a yoke having at least two first portions (46) each of which has a main pole (50) extending therefrom and at least two second portions (48) each of which has an auxiliary pole (52) extending therefrom, the at least two main poles (50) and the at least two auxiliary poles (52) being arranged alternately in the circumferential direction of the stator core (42) and the main poles (50) carrying stator windings (44) wound thereon; and wherein the stator windings (44) are configured such that, upon electrification of the motor, main magnetic poles having the same polarity are formed on the at least two main poles (50) and induced magnetic poles having the same polarity opposite to the polarity of the main magnetic poles are formed on the at least two auxiliary poles (52).The motor of claim 5, wherein the first portions (46) are narrower than the second portions (48).The motor of claim 5 or 6, wherein no stator windings are wound around the auxiliary poles (52).A motor according to claim 5 or 6, wherein the auxiliary poles (52) carry stator windings and the stator windings on the auxiliary poles (52) have a smaller number of turns than the stator windings (44) on the main poles (50).The motor of any preceding claim, wherein the ratio of an outer diameter of the rotor (20) to a minimum outer dimension of the stator (40) is greater than 7:10.

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