Redesigning the Electric Motor

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Electric car efficiency can be improved. Current designs use transmissions and differentials. Improved motor design with lower loss allows direct drive without transmissions and differentials. Cost and complexity are reduced and performance is increased. Commercially available lithium-air batteries is a major obstacle to the feasibility of electric vehicles but new discoveries promise to correct this in the near future. Integrate circuit controllers have reduced the cost of electronics making electric cars practical. The benefit of electric cars displacing petroleum-fueled cars could be a paper in itself. The Volt and all hybrids carry the cost burden of a fuel engine. The Leaf does not but is greatly limited by size and battery storage.

The new design motors are ceramic magnets rotor, 3? brushless DC operating on 500V and limited to 120Amp by the controller or a maximum of 60kw per motor. Eight motors are used, two for each wheel. The teeth are minimized for lower flux density and hysteresis loss, which also allows increased slot area for windings. Twenty-four pole design allows lower cost, high volume magnets with tighter tolerance, higher torque and no demagnetization risk, and can still be switched with IC controllers to allow 10,000 rpm.

Four motors are mounted on the gearbox with inboard disk brakes. A benefit is the minimum unsprung to sprung weight ratio for efficiency, handling and ride. The Design is four-wheel drive with NO differential gears. Wheel speeds are independent to assist steering by slowing the inside wheels. Gearbox and motors leave adequate room for steering knuckle, suspense, and shocks. The same design may be used with steering for front and locked or no steering for rear. Gearbox and motor are designed for high volume with looser tolerances and lower complexity compared to engine manufacturing.

The magnets are TDK FB12B for high Br and adequate Hc and low cost. The magnets are bars for low cost and tighter tolerances than arc segments and either pole by flipping. Rare earth magnets provide twice the flux density but only marginal gain in performance dew to increased steel area and less slot area for copper. The RE design increases hysteresis loss and resistance. The RE performance increase to 10%. Cost makes the decision to not use RE.

The torque constant for each motor is 1.16ft-lb /amp. The speed constant is 19.5rpm/volt. The gear ration provides speed reduction of 1 : 7.5. A 120amp per motor controller allows 139ft-lb at the motors. With the gear reduction and 8 motors this provides 8340 ft-lbs of torque for accelerating (2085 at each wheel). With a 5000 lb. vehicle with batteries and load this will provide an adequate acceleration. The batteries will deliver at 450V with some controller loss and batteries down to minimum usable limit. The speed constant is 19.5 rpm/V or 8775rpm for full voltage. With 7.5 : 1 drop the wheel speed will be 1170rpm. With a 265/55R16 tire the diameter is 26". This would produce 90mph. Fully charged batteries would have a top speed of 98mph.

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  • ABOUT THE ENTRANT

  • Name:
    David Wilcox
  • Type of entry:
    individual
  • Profession:
    Student
  • Number of times previously entering contest:
    never
  • David's favorite design and analysis tools:
    AutoCAD, Autodesk Inventor, SolidWorks, Catia, X-Plane
  • David's hobbies and activities:
    RC Aircraft flying and construction, sports
  • David belongs to these online communities:
    AIAA, rcgroups.com, rcuniverse.com
  • David is inspired by:
    Nature: I model my designs out of getting the most efficient use out of a material while limiting the overall consumption of that material.
  • Patent status:
    none