High speed rotor

Details High speed rotor High speed rotor

2003-04-07

2004-06-15

Mullins, Burton S. (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S216055, C310S156380, C310S156480, C310S156610, C310S156790

Reexamination Certificate

active

06750584

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electromechanical machines, and in particular, to a permanent magnet electromechanical machine incorporating a high speed rotor design.
BACKGROUND AND SUMMARY OF THE INVENTION
In order to meet the constant demand for efficient, power dense drivers for industrial and commercial applications, high speed, permanent magnet electric motors and generators are required. Presently, however, there are very few permanent magnet electric motors or generators that are rated over several hundred kilowatts (kW) and that provide high speed shaft rotation. While shaft speeds of up to approximately 100,000 revolutions per minute (rpm) have been achieved in permanent magnet electric motors and generators having low power ratings, higher rated machines are typically limited to shaft speeds of several thousand rpms or less. In order to provide high speed, permanent magnet electric motors and generators, a rotor designed for high speed rotation is required.
Permanent magnet electric motors and generators typically incorporate a drum-shaped rotor having permanent magnets located thereon to establish magnetic poles. In a first rotor construction, the permanent magnets are fastened on the outer surface of the rotor drum. This type of rotor construction is known as a “surface mounted” permanent magnet rotor. Alternatively, the permanent magnets may be embedded below the surface of the motor. This type of rotor construction is known as an “embedded” permanent magnet rotor. Both types of rotor constructions utilize rare earth magnets. As is known, rare earth magnets typically have poorer mechanical properties than the other elements of the rotor, and as such, cannot be used as load bearing elements in the rotor design. Further, rare earth magnets exhibit a weak resistance to corrosion, as well as, to the flow of electricity. Consequently, rare earth magnets can be de-magnetized by exposure to corrosive environments or high temperatures caused by eddy currents flowing in the magnets, or any other heat generating mechanism of the machine's operation.
Surface mounted permanent magnet rotors are conceptually simple, and therefore, perceived to be less costly. Typically, the magnets are retained on the outer diameter of the rotor in one of four ways. First, the magnets may be enclosed in a non-ferromagnetic holder that is attached to the rotor by mechanical means such as fasteners, a version of “tongue and groove” geometry, or a combination of both. Second, the magnets may be glued directly to the outer surface of the rotor. Third, the magnets may be glued directly to the outer surface of the rotor, and thereafter, a non-ferromagnetic, metal sleeve is shrink-wrapped around the magnets. Fourth, the magnets may be glued directly to the outer surface of the rotor, and thereafter, the rotor assembly is wrapped with a high strength, high modulus composite fiber/epoxy.
Each of the prior designs for surface mounted permanent magnet rotors has certain shortcomings. For example, in the designs wherein the magnets are shielded by a metallic sleeve, the metallic sleeve is subjected to higher order harmonics in the stator due to the power supply and the stator slot geometry. As a result, eddy currents are generated in the metallic sleeve so as to cause heating of the rotor and the magnets. At very high frequencies, such as those experienced in machines running significantly faster than approximately 3600 rpms, the heating of the metallic sleeve can damage the magnets. As such, rotor thermal management is a significant design consideration for any high speed, permanent electric motor or generator using such a magnet retention means.
In the designs wherein the magnets are glued to the rotor or wherein a composite fiber/epoxy wraps is used to retain the magnets on the rotor, the electrical properties of the magnetic material allow eddy currents to flow, thereby heating the magnets directly. It can be appreciated that a composite wrap over the magnets makes the cooling of the magnets a greater challenge since the composite wrap also acts to thermally insulate the magnets. Alternatively, simply gluing the magnets to the rotor is not feasible for high speed applications as the mechanical properties of the magnets are not up to the task of holding together when subjected to the tensile loads that results from high rotational speeds. Further, finding a suitable adhesive for gluing the magnets on the rotor may be difficult.
An additional drawback to surface mounted permanent magnet rotors is the cost of the magnets. The surface mounted magnets are necessarily shaped to closely fit the outer surface of the rotor. Shaping the surface mounted magnets involves the precision grinding of each magnet at its interface with the rotor, usually before magnetization, followed by the use of special tooling to energize the magnets after they are installed on the rotor. These manufacturing steps can add significantly to the overall cost of the final product. Finally, surface mounted rotors are more susceptible to damaging the magnets in “off-design” operating conditions, such as pole slips or stator short circuits.
While rotors that incorporate magnets embedded below the surface of the rotor are more complex in appearance, this type of rotor constructions has proved to be relatively simple to design, manufacture and assemble. In such embedded magnet rotor configuration, the rotor is made of a non-ferromagnetic material and the magnets are arranged so that the direction of magnetization is perpendicular to an axial point passing through the middle of each installed magnet and the rotor center line. Laminated pole pieces are installed on the sides of each magnet, with the polarity of the magnets arranged to have the same polarity on each side of a particular pole piece. As a result, a magnetic pole is formed on the outside diameter of the rotor. The embedded magnet rotor configuration has the advantage of shielding the magnets from the stator harmonics that can cause eddy current heating in the magnets, as well as, damage to the magnets from the high flux transients and reversals resulting from stator short circuits or pole slipping during operation. In addition, the laminated pole pieces effectively limit eddy currents in the poles, and thus, the heating of the rotor in total. Further, in embedded magnet rotor configurations, the magnets are usually simple rectangular shapes and are installed magnetized. As a result, a manufacturer does not have to invest in unique magnetizing tooling for each rotor diameter being produced. This, in turn, significantly reduces the cost of the final product. In view of the foregoing, it can be appreciated that the embedded magnet rotor configuration offers greater design freedom since the burden of cooling the rotor is limited and/or eliminated.
Heretofore, in embedded magnet rotor configurations, the magnets are restrained from movement in the radial direction by the pole pieces. For example, wedges or other blocking features may be used to restrain radial movement of the magnets. These wedges or blocking features are attached to the rotor by keyed tangs, “fir-tree” tongue and groove geometry and composite fiber/epoxy materials wound around the outside diameter, or any combination of the above. Alternatively, the magnets may have a trapezoidal cross section with the pole pieces being in contact with the magnets. If the magnets move radially away from the rotor center, the magnets and the pole pieces are loaded in compression by their respective geometries. In most circumstances, these arrangements for embedding the magnets within the rotor are adequate. However, the mechanical properties of the magnet materials and pole pieces limit the surface speeds such machines can achieve, making them most suitable for low RPM, high torque/power design.
Therefore, it is a primary object and feature of the present invention to provide a rotor assembly for use in high speed, permanent magnet electric motors and/or generators that maximizes protection for the magnets ther

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