Method of fabricating a rotor for an electric traction motor

#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.

#In today'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 and corresponding power electronics. Brushless DC motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to DC motors and AC induction motors. Brushless DC 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 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 machine attractive for vehicle propulsion applications.

#Permanent magnets buried inside a rotor for a brushless DC 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. Reluctance in the d-axis can be created by one magnet such as found in a single barrier rotor design.

#FIG. 1 is a diagrammatic cross-sectional drawing of a permanent magnet motor;

#FIG. 2 is a diagrammatic cross-sectional drawing of a permanent magnet motor with magnets buried near the surface of the rotor;

#FIG. 3 is a diagrammatic cross-sectional drawing of a permanent magnet motor with a spoke-type rotor configuration of magnetic material;

#FIG. 1 is a diagrammatic drawing of a permanent magnet motor 10 having a wound stator 12 and permanent magnet rotor 14. 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 and tooth set, and back emf detection. The motor 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 diagrammatic cross-sectional drawing of a permanent magnet rotor 14 with magnetic material 19 buried near the surface of the rotor 14. The magnetic material because of its position proximate to the rotor 14 surface may be magnetized by a magnetizing fixture or the wound stator 12 during a post-magnetization process. The post-magnetization process in one embodiment of the present invention includes positioning a magnetizing fixture around the rotor 14 to magnetize the magnetic material 19 in the rotor 14. Magnetizing fixtures similar to the stator 12 contain windings which are used for the magnetization process. The stator 12 may also be used to magnetize the rotor 14 instead of a magnetizing fixture in alternate embodiments of the present invention.

#In a preferred embodiment of the present invention, magnetic powder mixed with plastic may be injected into rotor 14 cavities under high temperatures and pressure, allowing the material to bond and form inside the rotor 14 cavity. This process is desirable for large scale production. As mentioned earlier, post-magnetization of the magnetic material is currently only practical if the magnetic material is buried near the rotor surface.