Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-01-18
2001-03-20
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S261100, C310S06700R, C310S049540, C310S216006
Reexamination Certificate
active
06204584
ABSTRACT:
The present invention relates to the art of electric motors and more particularly to a rotor for a low cogging torque, permanent magnet type, brushless motor.
The invention finds particular utility in connection with such permanent magnet DC motors and will be described with reference thereto; however, it will be appreciated that the present invention can be applied to other types of permanent magnet rotating machines.
BACKGROUND OF THE INVENTION
Brushless motors commonly include permanent rotor magnets with one or more pairs of poles having opposite magnetic polarity. In such motor configurations, the magnetic attraction between the rotor poles and the stator poles changes as the rotor rotates with respect to the stator. This variance in magnetic attraction as a function of rotor position is known as cogging. Several problems are caused by cogging, as discussed in the various patents above. Among these are a variable torque characteristic while the rotor is rotating, and the existence of preferred relative positions between the rotor and stator. In the first case, cogging results in decreased operational or steady-state efficiency. In the latter case, cogging reduces the ability to stop the motor in a desired position because the rotor tends to align itself into one of a limited number of positions relative to the stator poles. The ability to stop a motor at precise positions may be critical in certain control applications. In such situations where the desired rotational stopping position is not at a point of polar alignment, a motor subject to cogging effects may be inadequate or less desirable. Cogging results in torque and speed variations in permanent magnet electric motors due to the magnetic flux variations as the rotor poles move past the stator poles. In this regard, the cogging appears as a variable AC torque component. The reluctance of the motor air gap is significantly higher at the stator slots than at the pole teeth, thus causing cogging. A reduction in the reluctance variation will thus result in a reduction in cogging torque and its associated problems.
In a typical permanent magnet rotor, a single piece cylindrical magnet is mounted on a cylindrical shaft and charged to include at least one pair of poles having opposite polarity. This configuration, while economical to produce, suffers from high cogging torque. Heretofore, several different attempts have been made to reduce permanent magnet electric motor cogging effects, as discussed in the foregoing patents incorporated by reference. One method includes skewing the stator winding slots with respect to the permanent magnet pole edges, as shown in Musil U.S. Pat. No. 4,424,463. Another method involves skewing the permanent magnet pole pieces with respect to the stator winding slots. Cogging torque may also be reduced by providing a motor where the number of stator salient poles is less than the number of rotor permanent magnet poles as shown in Kawasaki, et al. U.S. Pat. No. 3,860,843. These practices, while reducing cogging effects, greatly complicate the manufacturing process, and correspondingly increase manufacturing costs. Another approach is shown in Sievert U.S. Pat. No. 4,341,969, wherein the edges of the permanent magnet pole pieces are cut or machined to form a series of notches on the leading and trailing edges of the stator poles. Similarly, in Mulgrave U.S. Pat. No. 5,783,890, the leading and trailing edges of the rotor permanent magnets are magnetized in a longitudinally varying magnetization strength pattern, while the central portion of each magnet is uniformly magnetized. Selective magnetization, like selective machining or forming of permanent magnets, increases the cost of manufacturing. Still another attempt at reducing cogging torque is where a rotor includes a square shaft and four section-shaped magnet pieces are attached to the flat outer surfaces of the square shaft. As discussed further hereinafter, this method significantly increases the manufacturing cost of permanent magnet rotors due to the specialized shape of the shaft and the use of separate magnet pieces. Consequently, there remains a need for an improved permanent magnet rotor which reduces cogging and adds little or no manufacturing costs.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a low cogging torque permanent magnet rotor by which the foregoing and other problems and disadvantages are minimized or overcome. More particularly, and in accordance with a principle aspect of the invention, there is provided a permanent magnet rotor including a longitudinal shaft with an outer surface, and an overlying magnet with an inner surface engaging at least a portion of the shaft outer surface and with at least two magnetic poles of opposite polarity adjacent one another at longitudinal pole boundaries. A gap is provided between the shaft and the magnet near at least one of the pole boundaries. In this regard, the gap between the shaft and the magnet effectively reduces the reluctance variation of the finished motor at the corresponding pole boundary. The lowered reluctance variation operates to reduce the cogging torque effects as the rotor magnetic poles rotate past the poles of the stator. Varying the gap dimensions allows control of the amount of cogging.
In accordance with another aspect of the invention, the shaft outer surface includes a contoured portion providing the gap between the shaft and the magnet near at least one pole boundary. In addition, the magnet may include multiple pairs of opposite polarity magnetic poles with pole boundaries therebetween and a gap between the magnet and the shaft near one, some, or all such pole boundaries.
In accordance with still another aspect of the invention, there is provided a method of manufacturing a low cogging torque permanent magnet rotor. The method includes providing a shaft with an outer surface and a magnet with an inner surface and at least two poles of opposite magnetic polarity with a pole boundary therebetween; and interengaging the shaft within the magnet with a gap therebetween near at least one of the pole boundaries. In this regard, the gap can be provided by contours in the shaft outer surface and/or the magnet inner surface to reduce the reluctance variations near the pole boundary.
It is accordingly a primary object of the present invention to provide an improved permanent magnet rotor by which electric machine cogging torque can be reduced.
Another object of the present invention is the provision of a rotor of the character described above which adds little or no manufacturing costs.
Yet another object of the present invention is the provision of a rotor of the character described above which can be easily customized according to desired cogging torque reduction and other motor performance characteristics.
Still another object of the present invention is the provision of a rotor manufacturing method by which a variety of low cogging torque rotors can be made quickly, at minimal cost, and with simple tooling.
REFERENCES:
patent: 3164735 (1965-01-01), Lichowsky
patent: 3860843 (1975-01-01), Kawasaki et al.
patent: 4091300 (1978-05-01), Lynch et al.
patent: 4216400 (1980-08-01), Lynch et al.
patent: 4341969 (1982-07-01), Sievert
patent: 4424463 (1984-01-01), Musil
patent: 4477744 (1984-10-01), Gerber
patent: 4504755 (1985-03-01), Semones et al.
patent: 4585967 (1986-04-01), Mayer et al.
patent: 4707645 (1987-11-01), Miyao et al.
patent: 4748359 (1988-05-01), Yahara et al.
patent: 5065063 (1991-11-01), Watanabe
patent: 5498917 (1996-03-01), Ninomiya et al.
patent: 5783890 (1998-07-01), Mulgrave
patent: 5936322 (1999-08-01), Yamaguchi et al.
patent: 63-121438 (1988-05-01), None
patent: 6-038416 (1994-02-01), None
patent: 7-322576 (1995-12-01), None
patent: 10-201152 (1998-07-01), None
Cleveland Motion Controls, Inc.
Mullins Burton
Vickers Daniels & Young
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