EP0303703A1

EP0303703A1 - Rotor structure of motor having permanent magnet

Info

Publication number: EP0303703A1
Application number: EP88901645A
Authority: EP
Inventors: Masatoyo Sogabe; Kazushi Kumagai
Original assignee: Fanuc Corp
Current assignee: Fanuc Corp
Priority date: 1987-02-16
Filing date: 1988-02-16
Publication date: 1989-02-22
Also published as: KR880701178A; KR930007668B1; EP0303703A4; WO1988006374A1; JPS63202247A

Abstract

Rotor structure of a motor. Yokes (13) each equipped with a permanent magnet (14) fixed to its outer circumference of a rotor core (12) with predetermined gaps between them in such a manner as to form recesses (20) between the yokes (13). The air layer of this recess (20) act as a large magnetic resistance to a main path (24) of the magnetic flux generated from the side of a stator (30). Therefore, the drop in permeability (µ) of each of the rotor core (12), the yoke (13) and the stator core (36) can be restricted and the output torque of the motor can be increased while minimizing the magnetic resistance in the main path (22) of the main magnetic flux generated from the permanent magnet (14).

Description

TECHNICAL FIELD

The present invention relates to a type of rotor structure having magnets fixed on the outer surface thereof, in an electric motor.

BACKGROUND ART

Two kinds of magnetic fluxes exist in an electric motor provided with a rotor having magnets fixed on the outer surface thereof; a main magnetic flux forming a path from one magnet to another magnet, and a magnetic flux caused by an electric current flowing in an coil of a stator. The latter is hereinafter called an armature reaction magnetic flux. An electric current must flow in a coil in each slot of the stator in response to a change of a main flux caused by magnets on the rotor, to ensure the performance of an electric motor. In this case, a phase of the armature reaction magnetic flux caused by an electric current flowing in a coil, is shifted by r/2 on the base of a main magnetic flux generated on the rotor.
An electric current flowing in a coil of the stator must be increased in order to generate a larger output torque in an electric motor, and thus the density of the armature reaction magnetic flux becomes high. On the other hand, a magnetic permeability of an electromagnetic steel is suddenly lowered, due to the characteristics thereof, when a density of a magnetic flux passing therethrough becomes high. Consequently, a magnetic reluctance of an electromagnetic steel plate of which a stator is constructed, becomes large when a large electric current flows in a coil of the stator, thereby reducing a density of a main magnetic flux passing through the stator. This means that it is difficult to increase an output torque of an electric motor, and in practice, the output torque is saturated when a large electric current flows in a coil.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to eliminate the saturation of the output torque in an electric motor, and thus solve the above problem.
In view of the above-mentioned object, the present invention provides a rotor structure of an electric motor, characterized in that a plurality of yokes are fixed on an outer surface of a rotor core and separated by an equi-distance from each other, and that a magnet is fixed on a surface of each yoke.
A main magnetic flux and an armature reaction magnetic flux pass through the rotor along different paths, respectively, because each flux is shifted by «/2 in phase from the other. A recessed portion of the rotor is formed by a pair of adjacent yokes, and because a high density portion of the generated armature reaction magnetic flux passes through the recessed portion which has a large magnetic reluctance, due to the presence of air therein, the magnetic flux density of the armature reaction magnetic flux is reduced. Consequently, a reduction of a magnetic permeability of an electromagnetic steel plate constructing a rotor can be prevented, to thereby improve an output torque of the motor.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional view taken along the line I-I in Fig. 2, and showing a rotor structure according to the present invention with a part of a stator shown by an imaginary line and widely separated from a rotor;
Figure 2 is a side view of the rotor shown in Fig. 1; and,
Figure 3 is a graph showing a curve of a magnetic permeability ^p of an electromagnetic steel in relation to a magnetic flux density B.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to Figs. 1 and 2, a rotor core 12 formed by stacked electromagnetic steel plates is fixed on a rotor shaft 10, and yokes 13 are fixed on the outer surface of the rotor core 12 and separated by an equi-distance from each other in a circular direction, each of the yokes 13 having the same thickness and the same configuration. Also, preferably each of the yokes 13 is made of stacked electromagnetic steel plates such as the rotor core 12, in view of the performance of an electric motor, and in this embodiment, each of the stacked electromagnetic steel plates forming each of the yokes 13 is formed integrally with each electromagnetic steel plate constructing the rotor core 12. Of course, each yoke 13 may be welded to the outer surface of the rotor core 12 when made of a ferromagnetic substance as a separated part of the rotor core 12. A magnet 14 is fixed to the outer surface of each yoke 13 by an adhesive. An outline of the magnet 14 has sine curves or close curves thereto at both sides thereof in a longitudinal direction of the rotor shaft 10, and that of each yoke 13 is the same as that of the magnet 14. Consequently, a pair of yokes 13 adjacent to each other define a recessed portion 20 between side faces 18 of the yokes 13. Most preferably, the outline of each yoke 13 is the same as a magnet 14, but another outline may be used. Moreover, although preferably the outline of each magnet 14 is a sine curve, another outline also may be used.
The reason why the above-mentioned structure according to the invention is adopted will be explained hereinafter. In an electric motor having a rotor having magnets 14 fixed on the outer surface thereof, two kinds of magnetic fluxes exist; a main magnetic flux forming a path thereof from one magnet to another magnet, and an armature reaction magnetic flux caused by an electric current flowing in a coil 32 of the stator 30. An electric current must flow in a coil 32 in each slot 34 of the stator 30 in response to a change of the main magnetic flux generated by magnets on the rotor, to ensure the performance of the electric motor. In other words, a peak electric current flows in the coil 32 when a center of a magnet 14 arrives at a position facing the coil 32, as shown in Fig. 1, during a rotation of the rotor. Accordingly, the armature reaction magnetic flux caused by the electric current flowing in the coil 32 is shifted by ir/2 in phase on the base of the main magnetic flux on the rotor. In Fig. 1, a main path of the main magnetic flux is represented by a solid line circle 22, and a main path of the armature reaction magnetic flux is represented by a broken line circle 24. The density of the main magnetic flux is high near the circle 22, and low near the center of the circle. The density of the armature reaction magnetic flux is high near the circle 24, and low near the center of the circle.
An electromagnetic steel plate material constructing the rotor core 12, the yokes 13, and the stator core 36 has characteristics such that a magnetic permeability p decreases as a magnetic flux density B increases, as shown in Fig. 3. Therefore, in an electric motor not having a recessed portion 20 on a rotor of the prior art, the density of the armature reaction magnetic flux becomes high when an large electric current flows in a coil 32, to increase an output torque, and thus the magnetic permeability ^p of an electromagnetic steel is reduced. Consequently, a magnetic reluctance is increased for a magnetic path of the main magnetic flux, so that the output torque is not increased as intended.
On the other hand, a recessed portion 20 is defined by a pair of yokes 13 adjacent to each other at a position between adjacent magnets 14, in the present invention, and exists in the circle 24 which is a main path of the armature reaction magnetic flux. As the magnetic permeability of air is small, namely, the reluctance is large for a magnetic flux, the density of the armature reaction magnetic flux caused by an electric current flowing in the coil 32 can be reduced due to the existence of the recessed portion 20, so that the reduction of the magnetic permeability of each electromagnetic steel plate can be prevented, and the output torque can be increased.
A reduction of a magnetic permeability of each electromagnetic steel plate constructing a rotor core, yokes, and a stator core, through which a main magnetic flux passes, respectively, can be prevented by an air layer which exists in the recessed portion between yokes in a main path of the armature reaction magnetic flux for increasing a magnetic reluctance for the armature reaction magnetic flux, and therefore, an output torque can be increased. Namely, saturation of an output torque can be eliminated.

Claims

1. A rotor structure of an electric motor, characterized in that a plurality of yokes are fixed on an outer surface of a rotor core while separated by an equi-distance from each other, and that a magnet is fixed on a surface of each of said yokes.

2. A rotor structure of an electric motor according to claim 1, wherein said rotor core is formed integrally with said yokes.

3. A rotor structure of an electric motor according to claim 1, wherein each of said yokes has the same outline as that of said magnet.

4. A rotor structure of an electric motor according to claim 2, wherein each of said yokes has the same outline as that of said magnet.

5. A rotor structure of an electric motor according to claim 3, wherein said each yoke and said magnet have a sine curve outline at both end portions in a longitudinal direction thereof.

6. A rotor structure of an electric motor according to claim 4, wherein said each yoke and said magnet have a sine curve outline at both end portions in a longitudinal direction thereof.