1. Technical Field of the Invention
The present invention relates to the structure of a motor that can employ magnets, such as ferrite magnets, having a tendency of demagnetization, and in particular to a double-stator motor that includes an annular rotor in which circumferentially magnetized permanent magnets are individually sandwiched between segment poles, and inner and outer three-phase stators which are arranged radially inside and outside, respectively, of the rotor.
2. Related Art
Mainstream motors in recent years are synchronous motors embedding neodymium magnets as permanent magnets. For such neodymium magnets, a heavy rare earth metal, such as dysprosium, is indispensable as an additive material. However, with the soaring prices of heavy rare earth metal, future use of such rare-earth magnets, when applied to mass-produced motors, will face a risk of not being suitable for mass production. Accordingly, for the departure from using rare-earth motors, research and development are accelerated for the production of motors which use magnets made of ferrite that is a readily-available material.
However, ferrite magnets have characteristics inferior to neodymium magnets in that the magnetic flux density and the coercive force are only a fraction of the neodymium magnets. Thus, the technical issue at the moment is how the magnets having such inferior characteristics are proficiently used.
In a synchronous motor embedding permanent magnets, replacement of the neodymium magnets that are embedded in the motor with ferrite magnets will deteriorate the output characteristics in accordance with the low magnetic flux density of the ferrite magnets. The deterioration may be accepted by compromise but may raise a problem. The problem is that, having only a small coercive force, the ferrite magnets are not able to withstand the large magnetomotive force of the stator windings, unlike the neodymium magnets. Accordingly, the ferrite magnets are permanently demagnetized, allowing the motor to lose its functions. In order to take measures against the problem of demagnetization of the ferrite magnets, a system or a structure is sought for, which allows less magnetomotive force from the stator windings to be applied to the magnets or which reduces the magnetomotive force of the stator windings.
For the problem set forth above, the structure of a double-stator motor disclosed in JP-A-H03-139156 is referred to, which structure may be informative to solve the problem. Specifically, the magnetomotive forces of the stators as the sources of the magnetomotive forces applied to the permanent magnets may be reduced to a half by providing a structure that divides the magnetomotive forces into two, i.e. inner and outer forces.
However, well-known double-stator motors suffer from the problems as set forth below under items (1) to (3).                (1) Due to the double structure (double stator), the number of components of the stator is doubled. In particular, the number of the stator windings requiring a complicated manufacturing process becomes two times that previously. Therefore it is difficult to manufacture double-stator motors.        (2) Due to the double structure, a good relationship between an inner stator's winding space and that core's cross-sectional area is hardly established. In other words, since a magnetic path of the inner stator's core becomes narrow, it is difficult to exert the characteristics as a double-stator motor.        (3) The magnets are arranged so as to be magnetized in the radial direction of the rotor, while the magnetomotive forces of the inner and outer stators are configured to cooperate with each other and unidirectionally work on the magnets. Therefore, demagnetization of the magnets cannot be reduced.        
Because of these problems, use of the system disclosed in JP-A-H03-139156 as it is does not lead to practical realization of a double-stator motor having ferrite magnets. Moreover, even though the disclosed system is used as it is, no effect of reducing demagnetization can be expected.