Patent Application
20110273047
The subject matter disclosed herein relates to the art of electric machines and, more particularly, to a rotor lamination assembly for a permanent magnet electric machine.
Electric machines include a rotor that sets up a magnetic field. Electrical current passing though a stator is influenced by the magnetic field creating an electro-motive force that causes the rotor to spin. Certain electric motors/generators employ permanent magnets in the rotor. The permanent magnets are mounted in magnet slots formed in the rotor, which is typically formed from a plurality of laminations. Generally, the permanent magnets are mounted near an outside edge of the rotor, as close to the outside edge as possible, in order to maximize torque and minimize losses. Mounting the permanent magnets in this manner creates a thin bridge area between the magnet slots and the outside edge of the rotor.
During high speed operation, centrifugal forces on the rotor create stress in the thin bridge area. If operated at too high a speed, the stress can exceed the yield strength of the laminations. In such a case, the rotor could fail. Accordingly, there is a trade off between maximizing torque and high speed operation. That is, maximizing torque by mounting the permanent magnets as close to the outside edge of the rotor limits the operational speed of the electrical machine.
According to one aspect of the invention, an electric machine includes a stator, and a rotor lamination assembly configured and disposed to rotate relative to the stator. The rotor assembly includes at least one lamination members that includes a body having an inner diametric edge that extends to an outer diametric edge, and at least one magnet receiving member. The at least one magnet receiving member is formed in the body and includes a first end that extends to a second end. The second end is spaced from the outer diametric edge to form a bridge portion. The at least one lamination member includes at least one stress concentration element positioned between the inner diametric edge and the outer diametric edge. The at least one stress concentration element is configured and disposed to reduce stress in the bridge portion.
According to another aspect of the invention, a method of forming a high speed rotor lamination member includes forming a lamination member that includes a body having an inner diametric edge that extends to an outer diametric edge. At least one magnet receiving member is created in the body. The at least one magnet receiving member includes a first end that extends to a second end. The second end is spaced from the outer diametric edge to form a bridge portion. At least one stress concentration element is constructed in the lamination member. The at least one stress concentration member is arranged between the inner diametric edge and the outer diametric edge. The at least one stress concentration element is configured and disposed to reduce stress in the bridge portion.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Referring to
Reference will now be made to
As each magnet receiving member is similarly formed, a detailed description will follow with reference to
Second end 105 of magnet receiving member 44 is spaced from outer diametric edge 34 forming a bridge portion 130. Bridge portion 130 is typically formed to be as thin as possible so as to reduce magnetic flux losses from lamination assembly 16. However, the thickness of bridge portion 130 places limits on an overall operational speed envelope of electric machine 2. More specifically, if bridge portion 130 is formed to be so thin as to reduce most if not all loses; electric machine 2 cannot be operated at speeds above, for example 5,000 rpm. When operated at such speeds, centrifugal forces on rotor lamination assembly 16 create stress in bridge portion 130. If the stress bridge portion 130 exceeds a yield strength of body 30, lamination member 20 could fail. In order to mitigate the stress in bridge portion 130, and enable electric machine to operate at speeds above 5000 rpm, lamination member 20 includes a plurality of stress concentration elements 140-147 arrayed about web portion 40.
Stress concentration elements 140-147 are positioned between adjacent ones of pairs 64-71 and include a continuous edge, such as shown at 151 on stress concentration element 140 that defines an opening in web portion 40. In the exemplary embodiment shown, continuous edge 151 defines an irregular, i.e., non-circular opening. However, it should be understood that continuous edge 151 could also define a circular opening. Stress concentration elements 140-147 extends about web portion 40 along a radial axis. However, it should also be understood that in accordance with other aspects of the exemplary embodiment, stress concentration elements can be staggered over web portion 40. For example, in
During high speed operation, stresses resulting from centrifugal force on lamination member 20, shift away from bridge portion 130 and toward stress concentration elements 140-147. In this manner, electric machine 2 can be operated at high speeds without experiencing stress cracking at the bridge portions, or magnetic flux losses at the outer edge. At this point it should be understood that the particular number, size, disposition, and shape of the stress concentration elements could vary without departing from the scope of the claims. Also, it should be understood that exemplary embodiments do not require that all lamination members include stress concentration elements.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
