Abstract:
A rotor for an interior permanent magnet machine has a rotor body having an output shaft and a first cavity filled with magnetic material. Second cavities are disposed inboard of the first cavities and are not filled with magnetic material. Non-magnetic rods extend through the second cavities and protrude beyond end faces of the rotor body. The rods are press-fit in blind bores formed in non-magnetic end plates disposed adjacent the end faces of the rotor body. Shrink disks are shrunk around projecting ends of the output shaft in abutting relation with the end plates. In order to prevent induced voltage from generating current in the cage formed by the rods and end plates, an oxide layer is disposed between the rods and blind bores in the end plates.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electric or hybrid electric vehicle propulsion systems. More specifically, the present invention relates to the design of electric traction motors or machines for use in electric or hybrid vehicles. 
     BACKGROUND OF THE INVENTION 
     In today&#39;s automotive market, there exists a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors, permanent magnet synchronous motors (PMSM) and corresponding power electronics. PMSM motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to regular DC motors and AC induction motors. PMSM motors typically operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. An interior permanent magnet (IPM) motor or machine has performance attributes, when compared to regular DC motors and AC induction motors, that include relatively high efficiency, relatively high torque, relatively high power densities, and a long constant power operating range which make an IPM motor attractive for vehicle propulsion applications. 
     Permanent magnets buried inside a rotor of a PMSM motor exhibit high reluctance directly along the magnetic axis or the d-axis due to the low permeability of the permanent magnets. While along the q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier, and reluctivity to magnetic flux is very low. This variation of the reluctance around the rotor creates saliency in the rotor structure of an IPM machine. Therefore, the IPM rotors have reluctance torque in addition to the permanent magnet torque generated by the magnets buried inside the rotor. 
     The magnets in the motor are arranged in several layers creating a multi-barrier design. The multi-barrier design reduces leakage and improves the rotor saliency. Accordingly, motors having multi-barrier rotors have numerous performance advantages over a single barrier rotor designs, including relatively high overall efficiency, extended high-speed constant power operating range, and improved power factor. Reducing dependency on magnetic torque helps lower the number of magnets or amount of magnetic material in an IPM machine, as compared to a single barrier IPM machine or surface mounted permanent magnet machine. The amount of magnetic material needed to generate a specific torque and wattage rating depends on the level of saliency of the rotor. The higher the rotor saliency, the lower the magnet material usage for the same overall machine performance. Electric motors having a multi-barrier rotor design, as compared to single barrier design, generate higher rotor saliency. 
     The reduction of magnetic material in an electric motor rotor is desirable from a cost standpoint. Consequently, deep cavities are generally left empty as their contribution to the rotor magnetic field should they be filled is relatively small. The reason for this is the relative distance to the motor airgap. A pure synchronous reluctance motor that has similar rotor geometry to the multi-barrier permanent magnet (PM) design, but no magnetic material in the rotor, is a relatively low performance machine. Multi-barrier IPM electric motors have the beneficial attributes of both synchronous reluctance machines and the permanent magnet machine and are therefore excellent candidates for vehicle propulsion. A major difficulty involved with IPM machines is the design and manufacture of the rotor. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned considerations, the present invention is directed to a rotor for an interior permanent magnet machine comprising a cylindrical body of non-magnetic material having an axial shaft projecting therefrom, the cylindrical body having first and second ends and a cylindrical outer surface. An array of first cavities are disposed in the rotor, which first arrays contain permanent magnets, and array of second cavities are disposed in the rotor that do not contain magnets. The cavities are separated by rotor bridges comprised of the rotor material. An array of rods of non-magnetic material extend through the second cavities in the rotor and are fixed to first second end plates that are disposed adjacent to the first and second ends of the rotor. 
     In a further aspect of the rotor, at least one of the end plates has an insulating oxide layer between the rods and end plate. 
     In still a further aspect of the rotor, the rods and end plates are made of stainless steel or titanium. 
     In still a further aspect of the rotor, the rods have end protrusions that are received in blind bores in the end plates, which end plates are held adjacent to the ends of the rotor. 
     In still a further aspect of the rotor, annular shrink disks are shrunk over ends of the output shaft outboard of the end plates to retain the end plates. 
     In another aspect of the rotor, the first cavities are arranged in groups adjacent to the second cavities with a plurality of first cavities extending obliquely away from the second cavities toward the outer cylindrical surface of the rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
         FIG. 1  is a diagrammatic cross-sectional drawing of a permanent magnet motor; 
         FIG. 2  is a perspective view of a rotor having empty cavities inboard of cavities having magnetic inserts; 
         FIG. 3  is a perspective exploded view similar to  FIG. 2  but showing rods inserted into the rotor for fixing to annular end plates rings; 
         FIG. 4  is an elevation of a portion of the annular end plate with a blind hole aligned with an end of a rod, and 
         FIG. 5  is a perspective view partially cut away of the rotor of  FIG. 3  assembled. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic drawing of a permanent magnet motor  10  having a wound stator  12  and permanent magnet rotor  14 , which has an output shaft  15  integral therewith. A power supply and inverter  16  commutate and control the speed and torque of the motor  10  in response to feedback including, but not limited to, an encoder, resolver, tachometer, proximity switch tooth set, and back emf detection. The motor  10  may be characterized as a brushless DC motor with square wave or sinewave excitation provided by the power supply and inverter  16 . 
       FIG. 2  is a permanent magnet rotor  14  of non-magnetic material with magnetic material  17  buried in cavities  18  near the surface  19  of the rotor  14 . The magnetic material  17  because of its position proximate to the rotor  14  surface may be magnetized by a magnetizing fixture or the wound stator  12  ( FIG. 1 ) during a post-magnetization process. 
     Inboard of the first cavities  18  that are filled with magnetic material  17  to establish rotor barriers, are second rotor cavities  23  which are empty. The presence of empty cavities (or barriers)  23  inside rotor structure can create structural problems inside the rotor  14  as the rotor spins at high angular speeds that result in a high levels of material stress mostly in non-magnetic rotor material bridges  27  between rotor barriers  18  and empty cavities  23 . 
     A simple solution to the mechanical stress problem would appear to make the rotor bridges  27  thicker. However, this approach diminishes the torque producing performance of the motor  10  in two ways. First, the permanent magnet flux is reduced as the bridges  27  “short-circuit” the magnets  17  residing in the adjacent cavity  18 , and secondly, the saliency characteristics of the rotor  14  are degraded. Moreover, motor torque density suffers because permanent magnet flux “leaks” through the thick bridges  27  at magnet edges. Consequently, if the same motor torque density is desired, more expensive, higher strength magnet material  17  is needed to compensate for the flux leakage. 
     Multi-layer or barrier geometry for an IPM rotor improves the rotor  14  saliency. Accordingly, the rotor  14  geometry of  FIG. 2  has the advantage of having relatively high saliency, which improves machine torque density and lowers magnetic material volume requirements for a specific torque or wattage. Lower magnetic material volume requirements lower the motor cost and also alleviate problems associated with high flux PM machines, such as short circuit and open circuit fault problems, and spin losses (eddy current induced losses) due to the presence of a permanent magnet field. Multi-barrier rotor geometries also have the advantage of favorable torque speed profile, with extended constant power range, for vehicle propulsion application. 
     Magnetic field strength is defined by the magnet remnant flux density B r  and the magnet coercivity H c . B r  is the measure of the magnetic flux density when the two ends of the magnet (magnet north and magnet south) is shorted using a highly permeable magnetic material. H c  is the measure of the field strength that would be required to drive the total flux within the magnet to zero. Magnetic energy product (MEP) is proportional to the product of the magnet remnant flux density and the magnet coercivity, which has a unit of Joules per unit volume. MEP multiplied by the total magnet volume gives the total energy of the magnet formed by the magnetic material. In order to keep the magnet flux unaltered in the air-gap, the MEP is increased by the same proportion as the volume of magnetic material is reduced. Thus, the MEP times the magnet volume will remain substantially unchanged for the rotor geometries. 
     Actual cost of the magnetic material  17  is a complex function of many factors, such as the type of magnets, chemical composition of the magnetic material, the MEP, magnetic material volume, magnetic material processing requirements, etc. However, by reducing the total magnetic material volume while keeping the MEP multiplied by the magnetic material volume substantially the same, the overall magnetic material cost can be reduced. 
     IPM geometries have reluctance torque in addition to magnetic torque due to the permanent magnet field. The reluctance torque is produced due to the variation of rotor  14  saliency with rotor position. This variation of the rotor  14  reluctance with the rotor position produces rotor saliency, which is the source of reluctance torque. When magnetic material  17  is absent from cavities  23  and replaced with non-magnetic material or kept empty, the reluctance of rotor  14  in general does not change. Therefore, the reluctance torque is mostly unchanged. When the removal of magnetic material  17  is compensated by increasing the MEP of the rotor magnets or barriers  27  formed by the magnetic material  17 , the motor torque is substantially unaltered. 
     Referring now to  FIG. 3  where a rotor structure  30  in accordance with the present invention assembly is shown, an array  32  of rods  34  extends through the previously empty cavities  23  of  FIG. 2 . The rods  34  have protruding end portions  36  and  38  which protrude from first and second ends  40  and  42  of the rotor  14 . The rods  34  are made of a non-magnetic material, such as but not limited to stainless steel or titanium, which is not permeable to magnetic flux and therefore do not affect the magnet state of the rotor  14 . 
     First and second annular end plates  46  and  48 , each having blind holes  50  therein aligned with the protruding end portions  36  and  38  of the rods  34 . The end plates  46  and  48  each have center openings  52  that receive the output shaft  15  of the rotor  14 . 
     The rods  34  are placed at a location in the rotor  14  where the magnetic flux is low compared to the rest of the rotor  14 , however there is a small component of the magnetic flux at the fundamental electrical frequency of the motor  10 . This flux is enough to induce a voltage in the circuit formed by two adjacent rods  34  and the end plates  46  and  48 . According to calculations performed on a 400 mm long rotor  14  with a 250 mm OD spinning at 12,000 rpm, a voltage of about 2V peak is produced. 
     Because of the low impedance of the circuit formed by the two adjacent rods  34 , the current generated by the induced voltage can be very large. This current will not generate any torque in the motor  10  because it has the same frequency as the main motor field, however it can generate unwanted loses in the cage formed by the rotor rods  34 , losses which in turn will reduce the efficiency of the motor  10 . For this reason, at least one of the end plates  46  or  48  has an insulating oxide layer  56  applied in the region  58  mating with the rods, as is seen in  FIG. 4 . In this way, a closed loop of two adjacent rods  34  is open and very little or no loss-generating current is able to circulate. 
     A pair of annular shrink disks  62  and  63  having central openings  64  align with opposite ends of the output shaft  15  to help hold the end plates  46  and  48  in press-fit relation with ends  36  and  38  of the rods  34 . 
     Referring now to  FIG. 5 , the rotor structure  30  is shown assembled upon press-fitting the end portions  36  and  38  of the rods  34  into the blind holes  50  in the end plates  46  and  48  and upon shrink fitting the shrink disks  62  and  64  on the output shaft  15 . 
     By inserting the rods  34  in the rotor empty barriers or cavities  32  and connecting them to the end pates  46  and  48 , a structural cage is developed inside the rotor  14 . Consequently, the stress levels in the rotor bridges  27  are reduced and deformations of the rotor bridges are diminished as well. 
     As is seen in  FIG. 5  as well as  FIGS. 2-4 , the first cavities  18  filled with magnetic material  17  are arranged in groups that extend outwardly from the second cavities  23  containing the rods  34  in directions extending obliquely toward the outer peripheral surface  19  of the rotor  14 . 
     While this invention has been described in terms of specific embodiments, it will be appreciated that other forms can readily be adapted by one skilled in the art. Accordingly, the scope of this invention is to be considered limited only by the following claims. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing form the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.