Gearless wheel motor drive system

Abstract

Methods and apparatus are provided for an electric vehicle embodying an axial flux traction motor directly coupled to a wheel thereof. The traction motor comprises a stator having coils for producing a magnetic field, an annular rotor magnetically coupled to the stator and mechanically to an output shaft. Permanent magnets of alternating polarity are mounted on the annular rotor. Magnetic shunts bridge a portion of the stator slots above the coils. The magnets are arranged in groups with group-to-group spacing exceeding magnet-to-magnet spacing. Adjacent edges of the magnets diverge. The method comprises, looking up d- and q-axis currents to provide the requested torque and motor speed for the available DC voltage, combining at least one of the d- and q-axis currents with a field weakening correction term, converting the result from synchronous to stationary frame and operating an inverter therewith to provide current to the coils of the motor.

1 . A method of operating a permanent magnet axial flux motor, comprising:
 receiving a requested torque and motor speed;   determining commanded d- and q-axis currents corresponding to the requested torque and motor speed;   combining at least one of the commanded d-axis current or the commanded q-axis current with a field weakening correction to provide a corrected output;   converting the corrected output to a stationary current frame of reference output; and   operating an inverter based on the stationary frame of reference output to provide current to coils of the motor, thereby causing the motor to operate substantially at the requested torque and motor speed.   
     
     
         2 . The method of  claim 1 , wherein the combining step comprises:
 for M* index <M low , omitting the field weakening correction;   for M low <M* index <M high , setting M ref =M* index ; and   for M* index >M high , setting M ref =M high .   
     
     
         3 . The method of  claim 1 , wherein the determining step includes determining the commanded d- and q-axis currents from an optimized current command table. 
     
     
         4 . The method of  claim 3 , wherein the combining step comprises:
 adding or subtracting an incremental d-axis current to accommodate differences between data stored in the optimized current command table and actual behavior of the motor.   
     
     
         5 . The method of  claim 3  further comprising, prior to the obtaining step,
 measuring operating parameters of the motor under different combinations of torque and speed; and   determining, based on the operating parameters, the commanded d- and q- axis current expected to produce the requested torque and speed for a given DC supply voltage at optimum operating efficiency, then storing these values in the optimized current command table.   
     
     
         6 . A method of controlling an electric motor, comprising:
 receiving a requested torque and speed associated with the electric motor;   determining commanded currents corresponding to the requested torque and speed;   adjusting at least one of the commanded currents with a field weakening correction to provide a corrected output; and   operating an inverter associated with the electric motor based on the corrected output, thereby causing the electric motor to operate substantially at the requested torque and speed.   
     
     
         7 . The method of  claim 6 , wherein the determining step includes determining commanded d- and q-axis currents. 
     
     
         8 . The method of  claim 7 , further comprising determining a commanded modulation index (M* index ), and wherein the adjusting step includes adjusting the at least one of the commanded currents with the field weakening correction based on the M* index . 
     
     
         9 . The method of  claim 8 , wherein the adjusting step comprises:
 for M* index <M low , omitting the field weakening correction;   for M low <M* index <M high , setting M ref =M* index ; and   for M* index >M high , setting M ref =M high .   
     
     
         10 . The method of  claim 7 , wherein the determining step includes determining the commanded d- and q-axis currents from an optimized current command table. 
     
     
         11 . The method of  claim 10 , wherein the combining step comprises:
 adding or subtracting an incremental d-axis current to accommodate differences between data stored in the optimized current command table and actual behavior of the motor.   
     
     
         12 . The method of  claim 10  further comprising, prior to the obtaining step,
 measuring operating parameters of the motor under different combinations of torque and speed; and   
       determining, based on the operating parameters, the commanded d- and q-axis current expected to produce the requested torque and speed for a given DC supply voltage at optimum operating efficiency, then storing these values in the optimized current command table.