Motor having a rotor with interior split-permanent-magnet

Abstract: A motor includes a rotor with interior permanent magnets and a stator with teeth wound by concentrated windings. The permanent magnet is split along a plane oriented to the stator, and an electrically insulating section is set between the spilt magnet pieces. This structure allows the permanent magnet to be electrically split thereby restraining the production of eddy current. As a result, heat-production is damped thereby preventing heat demagnetization of the permanent magnet.

Description: Field of the Invention The present invention relates generally to a motor having a rotor with interior permanent magnets, more particularly it relates to a motor with interior split-permanent-magnets, thereby restrains eddy-current from occurring and prevents heat-demagnetization. Background of the Invention Fig. 6 illustrates a rotor with interior permanent magnets of a conventional motor. The motor has rotor 310 in which permanent magnets 312 are embedded, and rotor 310 is disposed in a stator (not shown) with concentrated windings, so that the motor can driven by not only magnet torque but also reluctance torque. This rotor is hereinafter referred to as a "rotor with interior permanent magnets". However this conventional motor has the following problems: Comparing with a motor with a distributed-winding stator, a motor with a concentrated-winding stator subjects itself to greater changes of magnetic flux interlinked with rotor 310 when the motor rotates. As a result, a large eddy-current occurs in magnets 312 embedded in the rotor, and thus the motor with a concentrated-winding stator is vulnerable to irreversible heat demagnetization. Meanwhile, the distributed-winding stator is structured in the following way: A slot is formed between two stator-teeth, and a plurality of teeth thus form a plurality of slots. Windings striding over at least one slot are provided, and part of a winding of a phase exists between pitches of another phase winding. The concentrated-winding stator, on the other hand, is structured by providing a winding of one phase to one stator tooth respectively. The reason why the motor having the concentrated-winding stator is vulnerable to heat-demagnetization is detailed hereinafter. It is well known that eddy current loss "W e " is proportionate to a square of maximum operable magnetic-flux-density "B m ", and this relation can be expressed in the following equation. W e = P t /t = {1/(6ρ)} π 2 f 2 B m 2 t 2 [W/m 3 ] where P t = power consumption t = plate width interlinking with the magnetic flux ρ = resisting value proper to the permanent magnet f = exciting frequency Since the motor having the concentrated-winding stator is subjected to greater changes in magnetic flux running through the rotor, the maximum operable magnetic-flux-density "B m " in the above equation becomes greater and thus eddy-current-loss "W e " grows larger. If a motor has the concentrated winding stator, and yet, the permanent magnets are stuck onto an outer wall of the rotor, the changes in magnetic-flux-density is not so large that the heat-demagnetization due to the eddy-current-loss is negligible. In the motor having the concentrated winding stator and a rotor in which the permanent magnets are embedded, the space between the magnet and the outer circumference of rotor core 314 forms a path for the magnetic-flux to flow. The density of magnetic-flux from the stator changes depending on the position of stator teeth with regard to the magnets, so that magnitude of changes in the magnetic-flux-density at the path is increased. As a result, eddy-current occurs in magnets 312 embedded in rotor 310, thereby heating the magnet to produce irreversible heat-demagnetization. US 5, 684, 352 describes a permanent magnet field-type rotating machine having an internal magnet-type rotor with permanent magnet pieces in a rotor yoke. In the embodiment of FIG. 3, two permanent magnets are provided for each magnetic pole with an inclination to each other and to the radial direction of the rotor. US 5, 936, 323 describes a motor with a rotor core having insertion holes for the permanent magnets. The magnets are split into two pieces in a plane perpendicular to the rotor axis and inclined against each other. A fill-up agent is used for providing a clearance between the magnet pieces and the rotor core. EP 0 823 771 A1 relates to a motor comprising a stator core having plural teeth and slots with windings on the teeth. The rotor incorporates plural permanent magnets. The magnets are single piece magnets. Summary of the Invention The present invention addresses the problems discussed above and aims to provide a motor having a rotor with interior-permanent-magnets. This rotor produces the less eddy-current and can prevent the heat-demagnetization in the permanent magnets embedded in the rotor. The motor of the present invention is defined in claim 1. Brief Description of the Drawings Fig. 1 is a cross section illustrating a motor, having a rotor with interior permanent magnets. Fig. 2 is a perspective view of the permanent magnets to be embedded into the rotor of the motor shown in Fig. 1. Fig. 3 is a perspective view of permanent magnets to be embedded into a rotor of a motor. Fig. 4 is a cross section illustrating a rotor of a motor, in which "I" shaped permanent magnets are embedded. Fig. 5 is a block diagram of an electric vehicle in which the motor of the present invention is mounted. Fig. 6 is a cross section illustrating a conventional motor having a rotor with interior permanent magnets. Detailed Description of the Invention Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. Fig. 1 is a cross section illustrating a motor, having a rotor with interior permanent magnets, in accordance with the first exemplary embodiment of the present invention, and Fig. 2 is a perspective view of the permanent magnets to be embedded into the rotor of the same embodiment. In Fig. 1, motor 10 includes rotor 14 with interior permanent magnets 12, and stator 15 facing to rotor 14 via annular space. Respective teeth 17 of stator 15 are wound by windings 18 in a concentrated manner, i.e. concentrated windings are provided to respective teeth. Rotor 14 comprises the following elements: a rotor core laminated with a plurality of steel plates; permanent magnets 12 embedded into slots axially provided; and a rotating shaft 16 extending through a center of the rotor core. Respective magnets 12 have a shape protruding toward the center of rotor core. As such, the magnets are embedded into the rotor so that rotor 4 can produce respective directions for magnetic flux to flow with ease and with difficulty. An inductance ratio in respective directions can be thus obtained, and it is called a salient pole rate. A rotor polarity is formed between magnet 12 and an outer wall of the rotor core to which magnets 12 face. The magnetic-flux from the permanent magnet flows with ease through the section covering the rotor polarity, and this flowing direction is called "d axis". On the other hand, the magnetic-flux flows with difficulty through a section covering a boundary between two adjacent magnets, and this flowing direction is called "q axis". Stator 15 is formed by linking twelve stator-blocks 19 to each other in an annular shape. Each stator block 19 comprises teeth 17 wound by winding 18 in the concentrated manner, and the blocks are welded to form a ring. In the case of a three-phase and eight-pole motor, for instance, windings provided to a first four teeth every three teeth out of 12 teeth are coupled with each other thereby forming phase "U". In the same manner, the windings provided to the second four teeth on the right side of the respective first four teeth discussed above are coupled with each other thereby forming phase "V". Further, the windings provided to the third four teeth on the left side of the first four teeth are coupled with each other thereby forming phase "W". Stator 15 thus forms three-phase with concentrated windings. In motor 1 constructed above, the magnetic flux generated by magnet 12, i.e. the magnetic flux produced by the rotor-magnetic-poles, travels to teeth 17 of the stator via the annular space thereby contributing to the torque production. This motor has the salient-pole-rate and controls the current-phases to be optimal by current thereby driving itself not only by the magnet torque but also by the reluctance torque. One of the features of the present invention is a method of embedding the permanent magnets into the rotor. Magnets 12 to be embedded into rotor 14 in the first exemplary embodiment are detailed hereinafter. As shown in Fig. 2, each magnet 12 is split into two magnet pieces 13 in the axial direction of rotor 14. Each two magnet pieces 13 are embedded into one single hole provided to rotor 14, thereby forming each magnet 12. Epoxy resin of electrically insulating, used as a coating material, is applied to the overall surface of each magnet piece 13. If magnet pieces 13 are stacked-up, each piece is electrically insulated and they can form an independent circuit. A space between respective stacked-up magnet pieces 13 is not less than 0.03 mm corresponding to the thickness of coating material applied to the magnet pieces. The two magnet pieces 13 are embedded adjacently with each other into the hole of the rotor core so that magnet 12 is split into two sections facing to stator 15. Respective magnet pieces 13 are arranged in the following way: Respective magnetic-fluxes generated from two magnet pieces embedded in one hole flow in the same direction with regard to the outer wall of the rotor to which these two magnet pieces face. Another pair of magnet pieces embedded into a hole adjacent to the hole discussed above generate the magnetic flux in the direction reversed to the direction of the magnetic flux discussed above. For instance, two magnetic pieces embedded into one hole face to the outer wall of the rotor with poles "N", then another pair of magnet pieces embedded into the hole adjacent to this hole should face to the outer wall with poles "S". The space between the two magnet pieces is not necessarily resin, and it can be any electrically-insulating-material. Magnet 12 is split by a plane facing toward stator 15, thereby reducing the eddy current produced in magnet 12. The plane extends from the rotor center toward the stator. This is because of the following reason: Since teeth 17 are wound by concentrated windings 18, stator 15 receives greater changes in the density of magnetic-flux supplied from teeth 17. The maximum operable magnetic-flux-density B m expressed in the equation discussed previously thus grows greater. This change in the magnetic-flux-density produces the eddy current in each magnet 12. In this first exemplary embodiment, each magnet 12 embedded in rotor 14 is split into two magnet pieces 13, and epoxy resin-which is non-magnetic material-is put between these two pieces, thereby dividing magnet 12 not only physically but also electrically. As a result, the production of eddy current is restrained by narrowing the width "t" of plate interlinking with the magnetic flux in the equation discussed previously. Fig. 3 is a perspective view of permanent magnets to be embedded into a rotor of a motor in accordance with the second exemplary embodiment of the present invention. This second embodiment differs from the first one in the way of splitting the magnet, and others stay the same. In the first embodiment, the magnet is split into two pieces in the axial direction, however magnet 22 in this second embodiment is split into five pieces in the axial direction, and this produces the same advantage as the first embodiment has done. Fig. 4 is a cross section illustrating a rotor of a motor, in which "I" shaped permanent magnets are embedded, in accordance with the fourth exemplary embodiment of the present invention. This fourth embodiment differs from the previous embodiments 1 - 3 in the shape of magnet. In the previous embodiments, the magnet is in a "V" shape; however, magnet 42 in the fourth embodiment is shaped in a letter of "I". In Fig. 4, each magnet 42 formed by two magnet pieces aligned in an "I" shape is inserted into each hole provided in rotor 44. Electrically insulating material is put between the two pieces. The fourth embodiment can produce the same advantage as the first embodiment has done. When rare-earth-sintered magnet is used as interior permanent magnets in the rotor, splitting the magnet effects the advantage distinctly because the rare-earth-sintered magnet has less electrical resistance and is easier for current to run through comparing with a ferrite magnet. (The specific resistance of the ferrite magnet is not less than 10 -4 Ω · m, and that of the rare-earth-sintered magnet is ca. 10 -6 Ω · m.) In other words, when the same magnitude of change in the magnetic-flux-density is applied from outside to the magnet, the rare-earth-sintered magnet allows the eddy current to run through more than 100 times in volume than the ferrite magnet does. Thus the split of such magnet effectively restraints the production of eddy current. A driving control of the motor is demonstrated hereinafter, this motor includes the rotor with the interior magnet of the present invention. A motor with a stator wound by concentrated windings produces greater changes in the magnetic-flux-density when the motor is driven under weakening-magnetic-field control. Because in the motor having a rotor with interior permanent magnets, the magnetic-flux runs through the space between the magnets and the outer circumference of the rotor core, and thus the magnetic-flux is distributed unevenly between the rotor and the stator. The weakening-magnetic-field control applies inverse magnetic-filed to the motor so that the magnetic-flux produced by the magnet can be counteracted, therefore, this control method produces greater changes in the magnetic-flux than a regular control method does. Further the inverse magnetic-field narrows tolerance for irreversible demagnetization, and this produces a possibility of heat demagnetization at a temperature which is a matter of little concern in a normal condition. The weakening-magnetic-field-control thus produces distinctly an advantage of damping the heat generated by the eddy current. It is preferable to restrain the production of eddy current as well as the heat-generation from the eddy current by splitting the magnet, and this shows distinctly its effect when the motor is under weakening-magnetic-field-control. The motor used in the embodiments discussed above is an inner-rotor type, i.e. a rotor is disposed inside a stator, however, an outer-rotor type, i.e. a rotor is disposed outside a stator, and a linear motor, i.e. a rotor moves linearly with regard to a stator, produce the same advantages. As the exemplary embodiments discussed previously prove that the motor with interior permanent magnets of the present invention can restrain the production of eddy current and damp the heat demagnetization because the Fig. 5 is a block diagram of an electric vehicle in which the motor of the present invention is mounted. Body 80 of the electric vehicle is supported by wheels 81. This vehicle employs a front-wheel-drive method, so that motor 83 is directly connected to front-wheel-shaft 82. Motor 83 includes a stator being wound by concentrated windings and having interior permanent magnets as described in the exemplary embodiments previously discussed. Controller 84 controls the driving torque of motor 83, and battery 85 powers controller 84 and further powers motor 83. Motor 83 is thus driven, which then rotates wheels 81. In this embodiment, the motor is employed to drive the wheels of the electric vehicle. The motor can be employed also to drive wheels of an electric locomotive.

Claims: An electric motor comprising: a stator (15) having teeth (17) wound by concentrated windings (18) and a rotor (14, 44) having a rotating shaft (16, 46) and interior permanent magnets (12, 22, 42) in cross-sectional V- or I-shape embedded in respective recesses of corresponding shape at the poles of the rotor, characterized in that the permanent magnets embedded within the respective recesses are split in the axial direction of the rotor (14), along at least one plane orientated toward said stator into a plurality of magnet pieces (13), the magnet pieces (13) being applied with electrically insulating coating material to their overall surface to provide a space between the stacked-up magnet pieces (13) of not less than 0.03mm corresponding to the thickness of the coating material. The motor as defined in claim 1, wherein the electrically insulating material comprises epoxy resin. The motor as defined in claim 1 or 2, wherein the permanent magnet comprises a rare-earth-sintered magnet.

Inventors: IKKAI YASUFUMI, KONDO YASUHIRO, NAKAMURA TOMOKAZU, NISHIYAMA NORIYOSHI, OGUSHI MASAKI, IKKAI, YASUFUMI, KONDO, YASUHIRO, NAKAMURA, TOMOKAZU, NISHIYAMA, NORIYOSHI, OGUSHI, MASAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H02K21
IPC codes: H02K21, H02K1