DESIGN OF AN IN-WHEEL MOTOR

FOR A SOLAR-POWERED ELECTRIC VEHICLE

This report has detailed a motor design, which has been optimised for high-efficiency, in-wheel operation. The design uses an axial field, air-gap winding in order to produce 1800 W at 1060 rev/min, with 6.0 kg of active mass at an efficiency of more than 97.5%. The design is thermally suitable for twice the required rating.

The paper reports the design of a very efficient, lightweight, direct-drive, in-wheel motor, ultimately placed in the single front wheel of the Aurora solar car (Figure 1). The efficiency of the motor was optimised, incorporating a mass penalty to allow for tyre rolling resistance. Cost was not considered in the design.

Direct-drive motors were first used in the World Solar Challenge in 1993 by 3 teams: Honda, Engineering College of Biel, and Northern Territory University (Storey et. al [1]). Several other teams followed in 1996. Aurora Vehicles Association introduced a novel “wing section” car shape in 1993, which required a single front wheel and two rear wheels, rather than the more common reverse configuration (Baddeley et. al [2]). The mass of the front wheel had to be low to avoid lifting on rough roads at high speed. The specification given below calls for a continuous torque per unit active mass of at least 3.24 Nm/kg, more than double the value previously achieved by the earlier direct-drive motors, or by a typical induction motor of similar rating (but of much lower efficiency), and a torque per unit volume of at least 3700 Nm/m3, also at least double that of a typical induction motor.

To win the race a car needs to convert the maximum amount of solar energy, and use this energy well. The solar cell efficiency (24% from the University of New South Wales) and the race rules limit the available power to about 1.8 kW for a single-seater car. In decreasing order, the losses are: aerodynamic drag, rolling resistance, controller and motor. A well designed car can potentially average 100 km/h. A typical internal combustion engine road car at this speed uses about 35 kW from the engine and about 140 kW from the fuel.