1. Field of the Invention
The present invention relates to a rotor including a magnetic holding structure and an electric motor including the rotor.
2. Description of the Related Art
A known synchronous electric motor includes a rotor having a plurality of magnets arranged on a surface thereof in a circumferential direction. When a higher speed of rotation is desired in this type of electric motor, it is necessary to provide sufficiently high holding strength for holding the magnets, in order to prevent the magnets from being detached from the rotor due to centrifugal force.
JP-A-11-089142 discloses a rotor of an electric motor, which includes a rotational axis, a sleeve provided in an outer circumference of the rotational axis, a cylindrical permanent magnet provided on an outer circumference of the sleeve, and a holding member made of carbon-fiber-reinforced plastic and provided on an outer circumference of the permanent magnet, so as to cover the permanent magnet.
JP-A-2013-169103 discloses a rotor of an electric motor, which includes a cylindrical protective tube provided so as to surround a plurality of magnets. The protective tube is attached to the rotor so as to have an increased diameter with filling pressure of resin, and therefore the protective tube is pressed against the magnets on a radially inside due to restorative force of the protective tube. According to this related art, the magnets can be held in position without an adhesive.
In order to rotate a rotor at a high speed, even greater holding strength on the magnets is necessary. For example, when adopting the holding member made of carbon-fiber-reinforced plastic as disclosed in JP-A-11-089142, the holding strength can be increased by thickening the holding member. However, since stress is generated concentrically on an inner diameter side, simply increasing the thickness of the holding member is not effective. In addition, when a thickened holding member is adopted, a distance between the magnets of the rotor and the stator (magnetic gap) becomes greater, resulting in the decreased torque. Further, it is expensive and a high level of skill is necessary to form a thickened fiber-reinforced plastic member, possibly causing defects in the internal structure and impairing the holding strength.
In order to improve the function of holding the magnets, an interference of the holding member may be increased so as to increase an amount of extension in the circumferential direction. However, if in particular, the holding member is made of FRP formed from a sheet-like material wound around a cylindrical jig, a terminal end portion of the sheet material or a portion nearby may not be able to extend enough that these portions may get loose or come off from the holding member, impairing the reliability of the rotor. In addition, as a result of increasing the interference of the holding member, great restorative force acts on the holding member, a fiber on an outer circumferential side may get into a gap between fibers on an inner circumferential side, and therefore the intended holding strength may be lost.
According to a known art, the magnets and the protective pipe are held by the internal pressure of a filing material as disclosed in JP-A-2013-169103. In this case, a filling material is thermoplastic resin injected by injection molding. The internal pressure generated by the resin is small enough relative to tensile strength of the resin that the resin is not deformed. In other words, this known art can be only applied to the case where it is only necessary to provide fixing force which can be borne by plastic. If the rotor having such a structure is operated at high speed, the following problems arise. (1) The protective tube is expanded due to centrifugal force corresponding to the own weight of the protective tube, but the filling material cannot follow the expansion of the protective tube after reaching a certain limit. As a result, the fixing force acting on the protective tube is lost, and the protective tube starts to run idle. The protective tube scrapes the filling material with its inner wall, and is eventually detached from the rotor with noise. (2) As compared to the centrifugal force corresponding to the weight of the magnets having comparable density to iron, the fixing force by the filling material of resin is very weak, and may not be enough to fix the magnets against the centrifugal force. Therefore, when the rotor reaches a certain rotational speed, the magnets start to move to a radially outside and in a rotational direction, running idle and destroying the filling material. Accordingly, the structure disclosed in JP-A-2013-169103 is not appropriate to increase rotational speed.
Therefore, there is a need for a rotor which can increase efficiency and reliability of an electric motor.