This invention relates to electromechanical machines, and in particular, to a permanent magnet electromechanical machine incorporating a high speed rotor design.
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 xe2x80x9csurface mountedxe2x80x9d 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 xe2x80x9cembeddedxe2x80x9d 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 xe2x80x9ctongue and groovexe2x80x9d 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 xe2x80x9coff-designxe2x80x9d 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, xe2x80x9cfir-treexe2x80x9d 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 thereof in cases of stator short circuits and pole slips during operation.
It is a further 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 have higher power ratings than prior permanent magnet electric motors and/or generators.
It is a further object and feature of the present invention to provide a rotor assembly for use in high speed, permanent magnet electric motors and/or generators which is simpler and less expensive to manufacture than prior permanent magnet rotors.
In accordance with the present invention, a rotor assembly is provided for an electromechanical machine. The rotor assembly includes a rotor connectable to a shaft for rotational and movement therewith. The rotor extends along an axis and has first and second circumferentially spaced lobes projecting radially therefrom. First and second sets of laminated pole pieces are provided. Each set of laminated pole pieces is receivable on a corresponding lobe. A magnet is disposed between the sets of pole pieces.
The rotor assembly includes a magnet retention ring for preventing radial movement of the magnet. The magnet retention ring has a radially outer edge and includes a backing plate and a magnet retention element. The backing plate has first and second cutouts therein for receiving corresponding lobes therethrough. The magnet retention element projects from a first side of the backing plate and extends between the first and second sets of laminated pole pieces. Each lobe projecting from the rotor includes a neck terminating at an enlarged head. Each laminated pole piece is generally c-shaped and includes first and second ends separated by a predetermined distance for accommodating the neck of a corresponding lobe therebetween. The magnet retention element includes a radially outer surface extending between the first and second sets of laminated pole pieces and an inner surface directed towards the magnet. A shim may be positioned between the inner surface of the magnet retention element and the magnet for preventing radial movement of the magnet during rotation of the rotor.
Each set of laminated pole pieces includes a plurality of first pole pieces having a first radial thickness and a plurality of second pole pieces having a second radial thickness. The plurality of first pole pieces of a corresponding set of laminated pole pieces are positioned adjacent each other to form a first stack and the plurality of second pole pieces of the corresponding set of laminated pole pieces are positioned adjacent each other to form a second stack. It is contemplated that the first radial thickness be greater than the second radial thickness to control end leakage of flux.
In accordance with a further aspect of the present invention, a rotor assembly is provided for an electromechanical machine. The rotor assembly includes a rotor connectable to a shaft for rotational movement therewith. The rotor extends along an axis and has a plurality of circumstantially spaced lobes projecting radially therefrom. A plurality of ring assemblies are supported on the rotor. Each ring assembly includes a plurality of circumferentially spaced poles supported on corresponding lobes. A plurality of magnets are circumferentially spaced about the rotor and extend through the ring assemblies. Each magnet is generally parallel to the axis of the rotor and is disposed between corresponding pairs of poles of each ring assembly.
Each ring assembly includes a magnet retension ring for preventing radial movement of the plurality of magnets. Each magnet retention ring has a radially outer edge and includes a backing plate and a plurality of circumferentially spaced magnet retention elements projecting from a first side of the backing plate. The backing plate has a plurality of cutouts therein for receiving corresponding lobes therethrough. It is contemplated that each magnet retention element extends between corresponding pairs of poles and has a retaining bar projecting from the terminal thereof. Each backing plate includes a second side having a plurality of circumferentially spaced retaining bar receipt cavities formed therein. Each retaining bar receipt cavity is adapted for receiving a corresponding retaining bar of an adjacent magnet retention ring in a mating relationship.
Each of the poles of each ring assembly includes a plurality of laminated pole pieces. The rotor includes first and second ends wherein one of the plurality of ring assemblies is positioned adjacent the first end of the rotor. The laminated pole pieces of each of the poles of the one of the plurality of ring assemblies positioned adjacent the first end of the rotor includes a plurality of first pole pieces having a first radial thickness and a plurality of second pole pieces having a second radial thickness. The first radial thickness of the first pole pieces is greater than the second radial thickness of the second pole pieces and are positioned adjacent the first end of the rotor. It is contemplated that all of the laminated pole pieces include a generally arcuate, radially outer edge. In addition, all of the laminated pole pieces include a leading edge and a trailing edge which are asymmetrical.
In accordance with a still further aspect of the present invention, an electromagnetic machine is provided. The machine includes a stator extending along a longitudinal axis and having an inner surface defining a rotor receipt cavity. A rotor is positioned within the rotor receipt cavity. The rotor extends along and is rotatable about the longitudinal axis. A plurality of ring assemblies are supported on the rotor. Each ring assembly includes a plurality of circumferentially spaced poles. A plurality of magnets are circumferentially spaced about the rotor and extend through the ring assemblies. Each magnet is generally parallel to the axis of the rotor and is disposed between corresponding pairs of poles of each ring assembly.
The rotor of the electromagnetic machine includes a plurality of circumferentially spaced lobes projecting radially therefrom. Each ring assembly includes a plurality of circumferentially spaced poles supported on corresponding lobes. Each of the poles of each ring assembly includes a plurality of laminated pole pieces. The rotor includes first and second ends and one of the plurality of ring assemblies is positioned adjacent the first end of the rotor. The laminated pole pieces of each of the poles of the one of the plurality of ring assemblies positioned adjacent the first end of the rotor includes a plurality of first pole pieces having a first radial thickness and a plurality of second pole pieces having a second radial thickness. It is contemplated that the first radial thickness of the first pole piece be greater than the second radial thickness of the second pole pieces. The first pole pieces are positioned adjacent the first end of the rotor. Each laminated pole pieces includes a leading edge and a trailing edge which are asymmetrical.
The stator of the electromagnetic machine includes a plurality of laminated stator pieces laminated along an axis generally parallel to the longitudinal axis. The laminated stator pieces are radially spaced from the poles of the rotor assemblies. The stator may include a plurality of radially extending cooling channels extending therethrough. The cooling channels communicate with the rotor receipt cavity in order to cool the rotor.